Approach to Risk Stratification
When applying dementia prevention methods in cognitively normal patients, proper risk
stratification is imperative to allow for pathology-specific interventions. [Table 1] summarizes the clinical domains considered in dementia risk stratification. Often,
family history of AD or dementia is the primary motivator for patients to seek risk
reduction management.[20]
[21] As AD is responsible for 60 to 80% of all dementia cases, and significant overlap
between clinical presentation exists, other less common forms of dementia are often
misdiagnosed as AD.[22]
[23]
[24] It is necessary to diligently investigate any family history of diagnosed AD, other
dementias, or undiagnosed symptoms of cognitive impairment—especially in first-degree
relatives—along with the age of onset and clinical presentation for any affected relatives.
For example, dementia onset at age <65 years (yet more likely with onset in 40s–50s)
in multiple relatives may suggest involvement of highly penetrant autosomal dominant
genes. These include amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) in the appropriate clinical context and may warrant genetic testing for these variants
after genetic counseling is performed. A family history prominent for psychiatric
disease, anterolateral sclerosis (ALS), or autism spectrum disorder (ASD) may suggest
frontotemporal dementia (FTD) is more likely.[25]
Table 1
Dementia risk stratification
Domain
|
Supports underlying, typical LOAD pathology
|
Supports underlying, typical vascular dementia pathology
|
Supports underlying, typical EOAD pathology
|
Supports underlying synucleinopathy (PD, DLB)
|
Supports underlying FTD pathology
|
Supports underlying atypical neurodegenerative disease (atypical AD, CBS, PSP)
|
Age
|
≥65[44]
|
≥65[310]
|
<65[311]
|
Variable but typically
≥60[312]
|
>40[313]
|
≥60[314]
|
Sex
|
Female > male[70]
[71]
|
Male > female[315]
|
Female > male[311]
|
Male > female[312]
[316]
|
Male > female[313]
|
Female = male[317]
[318]
[319]
|
Cognition
|
Amnestic
Impairments in episodic memory and semantic function[320]
[321]
|
Impairments on attentional/executive function, semantic memory, and visuospatial skills[320]
|
Amnestic, frontal/executive function and visual memory impairment
Apraxia/visuospatial dysfunction[322]
[323]
|
Non-amnestic (bradyphrenia, poor learning, logopenia)
Impairments in visuospatial function and letter fluency[312]
|
Non-amnestic (language, behavioral, visual, or dysexecutive)[324]
|
Non-amnestic (language, behavioral, visual, or dysexecutive)[325]
|
Medical history
|
Vascular and metabolic conditions (e.g., heart disease, stroke, high blood pressure,
diabetes, obesity)[209]
[225]
[226]
[227]
Developmental and learning disabilities, head injury[319]
|
Vascular and metabolic conditions (e.g., heart disease, stroke, high blood pressure,
diabetes, obesity)[310]
|
|
Cancers, especially melanoma; seborrheic dermatitis; constipation; apathy; visual
changes and vertigo; changes to handwriting; exposures to pesticides/solvents; well
water[312]
|
|
|
Family history
|
Amnestic-type dementia
Developmental and learning disabilities[319]
|
|
Early-onset dementia in multiple family members in autosomal dominant pattern[115]
|
Parkinsonian disorders
Psychiatric disorders especially suicide
Melanoma and other cancers[312]
[326]
|
ALS, ASD, substance use disorders, and other psychiatric disorders[324]
|
Learning disability or atypical dementia[327]
non-amnestic dementia
|
Sleep
|
Sleep inefficiency[264]
[265]
[266]
[267]
[268]
[269]
[270]
|
Sleep apnea
Sleep disturbances[328]
[329]
|
Sleep disturbances[330]
|
REM behavior restless leg syndrome
Periodic limb movements of sleep[312]
|
Somnolence, narcolepsy-like attacks, insomnia[331]
|
Irregular sleep/wake behavior[332]
|
Mood
|
Late-life depression/anxiety Irritability
Apathy[245]
|
Late-life depression, apathy, and psychosis[245]
[333]
|
Apathy, agitation, disinhibition, irritability[322]
[334]
|
Early-life depression/anxiety obsessive compulsive disorder
Visual hallucinations[312]
|
Emotional blunting, mood changes, disinhibition[335]
|
Apathy
Disinhibition
Loss of empathy
|
Motor
|
Slowed walking (without parkinsonism)[336]
|
Parkinsonian-like gait disorder, dysarthria, and autonomic dysfunction[310]
|
Aberrant motor behavior[334]
|
Parkinsonism (bradykinesia, rigidity, tremor, stooped posture, gait changes, imbalance/falls)
or autonomic dysfunction (constipation, urinary frequency, orthostatic hypotension,
impotence)[312]
|
Parkinsonism, dysphagia[324]
|
Muscle twitching
Muscle atrophy
Myoclonus
Dystonia
Gaze paralysis
Difficulty walking
Eye movement difficulty[325]
|
Genetic
|
APOE4, TOMM40, APOC1, mitochondrial haplotype
|
|
APP, PEN1, PEN2, APOB[337]
[338]
|
APOE, SNCA, LRRK2, VPS35, PRKN, PINK1, DJ-1, GBA[331]
|
C9orf72, GRN, MAPT
[339]
|
MAPT, C9orf72
[340]
|
Abbreviations: AD, Alzheimer's disease; ALS, amyotrophic lateral sclerosis; APOC1,
apolipoprotein C1; APOE, apolipoprotein E-4; APP, amyloid precursor protein, PSEN1,
presenilin 1; ASD, autism spectrum disorder; C9orf72, chromosome 9 open reading frame 72; CBS, corticobasal syndrome; EOAD, early-onset
Alzheimer's disease; FTD, frontotemporal dementia; GRN, progranulin; LOAD, late-onset Alzheimer's disease; LRRK2, leucine-rich repeat kinase
2; MAPT, microtubule-associated protein tau; PD, Parkinson's disease, DLB, dementia with
Lewy bodies; PSEN2, presenilin 2; PSP, progressive supranuclear palsy; REM, rapid
eye movement; SNCA, alpha synuclein; TOMM40, translocase of outer mitochondrial membrane
40; VPS35, vacuolar protein sorting-35, PINK1, PTEN-induced kinase-1.
A dementia-prevention–focused clinical history including age, biological sex, medical
history, and factors related to early life brain development (academic performance,
class ranking, standardized test scores, and career achievement) should be performed.
This expanded history should also include assessment of past and current lifestyle
habits (exercise, diet, sleep, stress management) that would be targetable risk factors.
A detailed review of systems for symptoms that may indicate alternate pathology should
be included.[21] For example, the presence of REM sleep behavior disorder is strongly associated
with the development of Parkinson's disease (PD) or other synucleinopathy (e.g., LBD).[26] The identification of conditions such as hypertension, hearing impairment, periodontal
disease, obesity, depression, diabetes, alcoholism, and traumatic brain injury is
of the utmost importance, as these conditions are some of the strongest risk factors
for developing AD.[11] Physical and neurological exam should follow. Physical exam findings such as bradykinesia,
myoclonus, orthostatic hypotension, and loss of the optokinetic reflex may suggest
atypical AD or alpha-synuclein pathology. Although it is nonspecific, olfactory impairment
is a common and early sign of neurodegenerative diseases corresponding to emerging
pathologic processes in the olfactory system. While patients may report hyposmia or
anosmia, objective testing is necessary to accurately determine olfactory function.[27]
Vital signs and anthropometric measurements are also essential. Collecting objective
measurements of body composition including lean dry mass, skeletal muscle mass, and
body fat percentage using tools such as bioimpedance devices or other more rigorous
means (e.g., a dual-energy X-ray absorptiometry [DEXA] scan) should be performed in
a standardized manner (e.g., fasting, same device, same time of the day) for more
accurate longitudinal comparisons. Measurements such as body mass index (BMI) are
imprecise and not as informative, whereas waist-to-hip circumference—which is associated
with hippocampal volume—may be a better proxy.[28]
Additionally, blood biomarkers are an essential component of the risk stratification
process and can aid in identifying modifiable risk factors. Since hypercholesterolemia
and hypertriglyceridemia are associated with cognitive decline, advanced lipid and
inflammatory panels more routinely performed by the preventive cardiology field should
be considered.[29] Although clinical guidelines in the United States advise on the use of low-density
lipoprotein cholesterol (LDL-C) and non–high-density lipoprotein cholesterol (HDL-C)
to guide cholesterol management decisions, the 2019 European Society of Cardiology/European
Atherosclerosis Society Guidelines concluded that apolipoprotein B (apoB) is a more
accurate measure of risk, especially in younger patients. Obtaining advanced lipid
panels to better guide risk stratification and to monitor adequacy of lipid lowering
therapy has been successfully deployed in AD prevention clinical practice over the
last decade.[20]
[21]
[30]
[31] In apolipoprotein E (ApoE4) carriers, Apo-A1 (a component of HDL cholesterol) is a marker for increased AD risk,
whereas N-terminal probrain natriuretic peptide (NT-proBNP) is a plasma protein associated
with VaD.[32]
[33] Insulin resistance is associated with poor cognitive function and can promote neuroinflammation
and amyloid deposition.[34]
[35] As such, metabolic metrics (e.g., glycosylated hemoglobin [HgbA1C%], fasting insulin,
fasting glucose, and HOMA-IR) should be evaluated. Nutritional measures such as omega-3/6
fatty acids, vitamin levels (B12 and D), and homocysteine should be considered as they may guide the need for dietary
modification or supplementation directly related to brain health outcomes.[21]
Neurocognitive evaluation using both computerized and more traditional individual
tests (as well as preclinical focused batteries) to quantify global cognitive status,
executive function, processing speed, language, verbal knowledge, and learning and
memory may demonstrate early presymptomatic changes.[36]
[37]
[38] Although AD typically presents with episodic and associative memory dysfunction,
language and executive dysfunction are seen in early FTD and visuospatial deficits
are common in early LBD.[24]
[39]
[40] See [Fig. 1] for a comprehensive model of AD prevention management.
Fig. 1 ABCs of Alzheimer's prevention management. (Seifan A, Isaacson R. The Alzheimer's
Prevention Clinic at Weill Cornell Medical College/New York - Presbyterian Hospital:
Risk Stratification and Personalized Early Intervention. J Prev Alzheimers Dis 2015;2:254-266.
Reprinted with permission of John Wiley and Sons.)
Though genetic information must be interpreted in the context of family and complete
clinical history, it may be valuable for risk assessment in the appropriate clinical
context and with adequate patient counseling. The ApoE gene—whose E4 allele is the most significant and well-studied genetic risk factor
for AD—affects lipid homeostasis, especially in the central nervous system (CNS),
and impedes Aβ clearance at the blood–brain barrier (BBB).[41]
ApoE4 carriers account for an estimated 40 to 65% of AD cases despite making up only around
20% of the general population.[42] While estimates vary among different studies, the ApoE4 genotype incurs a 2- to 3-fold increased risk of AD in heterozygotes and an 8- to
12-fold increased risk in homozygotes as compared to ApoE3 homozygotes.[15] The ApoE2 genotype—present in only 5% of the population—is protective with carriers being two
times less likely to develop AD.[43]
[44]
ApoE is not only an important mediator of AD risk, but it also may account for specific
disease heterogeneity.[45] For example, ApoE4 carriers may have greater memory impairment due to more tau deposition and brain
atrophy in the medial temporal lobe, whereas AD patients without an ApoE4 variant demonstrate greater executive, visuospatial, and language dysfunction due
to more tau accumulation and atrophy in the frontal and parietal lobes.[45]
However, being a carrier of ApoE4 does not ensure that an individual will develop the disease—its influence is variable
across age, race, ethnicity, and biological sex, and can change once modifiable risk
factors are targeted.[21] It is further complicated by specific gene–gene interactions including other somatic
and mitochondrial genetic variances/haplogroups that may alter the impact of ApoE by neutralizing, enabling, or synergizing its expression.[46]
[47]
[48] For example, KL-VS heterozygosity (KL-VShet)—having one copy of the Klotho gene—incurs a protective effect on ApoE4 carriers and has been linked higher cognitive performance in adulthood.[49]
[50]
[51]
[52] Recent findings showed that a substantial portion of the approximately 15% of Americans
carrying the ApoE4 allele are protected by their KL-VShet status.[49]
[50] Given the complex genetic architecture of AD, a risk stratification methodology
needs to be developed accounting for polygenic risk scores as well as additional clinical
and genetic components that may directly modify ApoE expression or independently modify
overall AD risk.
Until recently, invasive cerebrospinal fluid (CSF) biomarkers (Aβ1-42, T-tau, and
P-tau181) and costly imaging studies (amyloid-PET, tau-PET, fMRI, etc.) were the only
clinically available methods to diagnose preclinical AD. Advancements in blood-based
biomarkers have opened the door to a new era of early detection and intervention.[53] PrecivityAD, a mass spectrometry-based plasma Aβ42/Aβ40 assay, is not only highly
concordant with CSF biomarkers and amyloid PET scan in cognitively normal individuals,
but it may be more sensitive in detecting preclinical AD. “False positives” were highly
likely to convert to positive by CSF biomarkers (13-fold higher risk over an average
follow-up period of 7.3 years) or amyloid PET (9-fold higher risk over an average
follow-up period of 6.2 years), suggesting their superiority in detecting preclinical
AD.
Risk Reduction and Prevention Overview
The pathophysiological changes of AD and other neurodegenerative disorders begin decades
before clinical apparent symptoms, providing the opportunity for early intervention.
Observational studies have identified several modifiable risk factors for AD that,
if addressed, may prevent—or at minimum delay—a significant portion of AD cases.[11] A 2020 report by the Lancet commission added three modifiable risk factors—excessive alcohol consumption, traumatic
brain injury, and air pollution—to the nine previously identified in 2017—less education,
hypertension, hearing impairment, smoking, obesity, depression, physical inactivity,
diabetes, and low social interaction—which together account for approximately 40%
of global dementia cases.[11] Other prospective studies have evaluated the effectiveness of AD risk reduction
via addressing modifiable risk factors. The Finnish Geriatric Intervention Study to
Prevent Cognitive Impairment and Disability (FINGER) trial, a large 2-year randomized
clinical trial, demonstrated a decrease in AD risk at 24 months using multidomain
lifestyle interventions including dietary and exercise recommendations, social activity,
cognitive training, and metabolic/vascular monitoring when compared to general medical
advice. The Exercise and Nutritional Interventions for Cognitive and Cardiovascular
Health Enhancement (ENLIGHTEN) trial, which focused on aerobic exercise and dietary
approaches to stop hypertension (DASH) diet, saw the greatest improvement in executive
function in participants who received the combined intervention of aerobic exercise
and the DASH diet rather than aerobic exercise alone, DASH alone, or general health
education.[54] The Comparative Effectiveness Dementia and Alzheimer's Registry (CEDAR) study demonstrated
that an individualized multidomain risk reduction approach improves cognitive function
related to both AD pathology and changes more so characterized by cognitive aging,
as well as reduced both AD and cardiovascular risk scores in both high- and low-compliance
primary prevention and preclinical AD patients and high-compliance patients with MCI
due to AD at 18 months.[55]
However, multidomain risk reduction interventions may be less effective in older patients
and those with more advanced pathology. The nurse-led Prevention of Vascular Dementia
by Intensive Care (PreDIVA) trial did not demonstrate a reduction in incident dementia
with a multidomain cardiovascular risk factor reduction intervention in individuals
aged 70 to 78 years.[56] The authors concluded that the intensity of care delivered might have been insufficient
to induce the necessary effects on lifestyle change. The Multidomain Alzheimer Preventive
Trial (MAPT) failed to demonstrate benefit on cognitive decline at 3 years with nutritional
counseling, physical exercise, and cognitive training, omega-3 fatty acid supplementation,
or both, though a post hoc analysis performed on participants with positive amyloid
scans showed a significant benefit in favor of the interventions.[57] Defining appropriate recommendations based on the totality of evidence is imperative
and may be in part responsible for these conflicting results. The French National
Nutrition and Health Program (PNNS) guidelines were used to guide dietary recommendations
instead of individualizing interventions based on more recent evidence (e.g., Mediterranean–DASH
Intervention for Neurocognitive Delay [MIND] diet). The necessity of individually
tailored recommendations may also be supported by the Lifestyle Interventions and
Independence for Elders (LIFE) study, which failed to demonstrate positive cognitive
benefit from moderate-intensity physical activity interventions (walking, resistance
training, flexibility).[58] It is thought that the lack of benefit in this 2-year study stems from a higher
mean age of participants and interventions that were not individually tailored.
Past studies suggest that personalized multidomain interventions may offer the greatest
opportunity to decrease the incidence of, delay, or prevent AD and other dementias.[59] Individualizing risk reduction interventions requires accurate risk stratification
based on both nonmodifiable and modifiable risk factors. Examples of risk factor and
intervention considerations are described in [Table 3].
Table 2
Example considerations for ApoE4+ carriers
Domains
|
Considerations
|
Benefits for ApoE4+ carriers
|
Dietary patterns[133]
|
Low-glycemic index/Low-carbohydrate diets
|
Regular dietary pattern
|
• Protects against insulin resistance
• Lessens risk of Aβ degradation impairment by insulin
• Reduces GSK3β-mediated hyperphosphorylation of tau
• Ameliorates reduced cerebral blood flow
• Protects against AGE formation and ApoE4 glycation, with consequences on cerebral
lipid metabolism
• Overall associated with reduced AD risk
|
Organic food
|
Regular substitution of non-organic foods
|
• Reduces body burden of toxins
• Reduced exposure to certain pesticides that tend to effect ApoE4 carriers[341]
|
Extra virgin olive oil
|
1-2 Tbsp/day
|
• Anti-amyloid and anti-tau properties induced through phenolic compounds, oleocanthal
and hydroxytyrosol
• Increases levels of LRP1
• Inhibits CypA-NFκB-MMP9
• Increases ABCA1
|
Fatty fish
|
2–4 servings/week
|
• Inhibits NLRP3 formation
• Inhibits CypA-NFκB-MMP9
• Improves amyloid and tau pathologies in animal models
• Overall consumption associated with reduced AD risk
|
Alcohol
|
Limit alcohol consumption
|
• Reduced consumption overall associated with reduced AD risk
|
Biomarkers
[133]
|
Low RBC omega-3 (EPA/DHA)/omega-3 index
|
Nutritional counseling + consider supplementation
|
• Possible improvements in memory reaction time in males
• Improved attentional measures of cognition
• Improved composite cognition scores
|
Low serum vitamin D (<30 nmol/L)
|
Consider supplementation and increase exposure to natural sunlight
|
• Possible improvement in cognitive function
|
Insulin resistance
|
Nutritional counseling, low-glycemic index or low carbohydrate diet; consider early
referral to endocrinology; consider Zone 2 training, supplementation with cocoa flavanols
|
• Possible reduction in age-related cognitive dysfunction[173]
|
Hyperlipidemia (elevated ApoB, LDL-C, and/or LDL-P)
|
Early referral to preventive cardiology
|
• Management of dementia-related cardiovascular risk factors
|
Exercise
|
Aerobic/Resistance training
|
Increased overall amount and intensity of exercise
|
• Helps counteract the increased clinical decline and brain atrophy in ApoE4 carriers
|
HIIT
|
Implementation of HIIT in regular exercise routine[195]
|
• Higher intensity exercise may protect against cognitive decline for ApoE4 carriers[342]
|
Abbreviations: ABCA1, ATP binding cassette subfamily A member 1; AD, Alzheimer's disease;
AGE, advanced glycation end; ApoB, apolipoprotein B; Aβ, amyloid-beta; CypA-NFκB-MMP9,
cyclophilin A-nuclear factor kappa B-matrix metallopeptidase 9; DHA, docosahexaenoic
acid; EPA, eicosapentaenoic acid; GSK3β, glycogen synthase kinase-3β; HIIT, high-intensity
interval training; LDL-C, low-density lipoprotein cholesterol; LDL-P, low-density
lipoprotein particles; LRP1, LDL receptor-related protein 1; NLRP3, NOD-, LRR-, and
pyrin domain-containing protein 3; RBC, red blood cell.
Table 3
Example risk reduction interventions
Risk factor
|
Individual factor
|
Intervention considerations
|
Servings
|
Diet[129]
|
|
|
Green leafy vegetables
|
6 or more servings/week
|
Other vegetables
|
6 or more servings/week
|
Berries
|
2 or more servings/week
|
Other whole fruit (low glycemic)
|
7–14 servings/week
|
Plant-based fats (i.e., avocado, seeds)
|
5 or more servings/week
|
Legumes
|
5 or fewer servings/week
|
Whole grains
|
1–2 servings/day
|
Fish (not fried or shelled)
|
2–4 servings/week
|
Poultry (not fried)
|
4 or fewer servings/week
|
Eggs
|
4–8 or fewer/week
|
Unprocessed red meat and pork (grass-fed when possible)
|
2 or fewer servings/week
|
Coffee, tea
|
2 or more servings/day
|
Dark cocoa powder
|
5–7 servings/week
|
Wine, other alcohol
|
Women: 1 serving/day
|
Men: 1–2 serving/day
|
Extra virgin olive oil
|
Preferred over other monounsaturated fats
|
Evidence of insulin resistance, elevated visceral body fat, or elevated waist:hip
ratio
|
Time-restricted eating pattern
|
12–16 h ∼5 d/wk
|
|
Total carbohydrates
|
≤120 g/d daily depending on amount of regular exercise
|
Fast fried foods
|
Limit intake
|
Desserts, pastries, sweets
|
Limit intake
|
Nutritional biomarkers
|
Low RBC omega-3 (EPA/DHA)/omega-3 index
|
|
Nutritional counseling +/− supplementation[343]
[344]
|
|
Vitamin D deficiency
|
|
Cholecalciferol +/− daily, brief exposure to sunlight[345]
[346]
|
|
Vitamin B12 deficiency
|
With absorption deficit[347]
|
Oral vs. IM B12 supplementation; nutritional counseling[163]
|
|
With absorption and/or potential genetic methylation deficit (e.g., MTHFR)
|
Methylated B12 or B complex
|
With elevated homocysteine
|
Consider methylated B-complex vitamins (folate, pyridoxine, cobalamin); TMG; NAC[347]
[348]
|
Exercise
|
|
Female
|
Greater ratio of cardiovascular to resistance training[87]
[88]
[89]
[90]
[91]
[92]
[93]
[94]
|
|
|
Male
|
Equal-to-greater ratio of resistance to cardiovascular training
|
|
|
Elevated visceral body fat
|
Cardiovascular exercises, such as fast walking (Zone 2, 65–70% of maximum heart rate),
running, swimming, and cycling; HIIT
|
|
|
Low muscle mass
|
Resistance training to improve strength and endurance, often with use of weights
|
|
|
Insulin resistance
|
Zone 2 or steady-state cardiovascular training; HIIT
|
|
Sleep
|
Insufficient sleep/sleep disturbances
|
|
Maintain a consistent sleep schedule and optimize sleep environment
|
|
Careful review to identify sources of impaired sleep
|
Limit use of electronics and restrict food and alcohol use before bed
|
Avoid caffeine after ∼1 pm
|
Avoid sleep aids such as Benadryl
|
Consider low-dose melatonin
|
Sleep apnea
|
Type of apnea
|
CPAP or other treatment as indicated
Refer to sleep specialist
|
|
Education
|
Suboptimal cognitive activity or reserve
|
May pose higher risk for females[96]
[97]
|
Adult education and/or cognitive training
Second language training or musical training
Puzzles, card games, mind games[98]
[99]
|
|
Social interaction
|
Social isolation
|
May pose higher risk for males[349]
[350]
[351]
|
Referral to social worker or geriatric care manager
Encourage activity programs and social engagement[243]
[352]
|
|
Mental health/stress
|
Chronic stress, depression, and/or anxiety
|
Depression may pose higher risk in males[252]
|
Mindfulness training, meditation, periodic vacations, cognitive behavioral therapy,
exercise counseling +/− medication[105]
[106]
[107]
[108]
[110]
[111]
[112]
|
|
Stress and anxiety may pose higher risk in females;
Females may be more vulnerable to depression risk in the perimenopausal period[100]
[101]
[102]
[103]
[104]
|
As above
May also benefit more from vacation and meditation[110]
[113]
|
Management of other medical conditions
|
Hearing loss
|
|
Management of hearing loss
Introduction of hearing devices as per recommendation of primary care physician
|
|
Periodontal disease
|
|
Consistent oral hygiene through teeth brushing and flossing
|
|
|
Regular dental check-ups and teeth cleanings
|
Perimenopause (<5–7 y after start of menopause transition)
|
Symptomatic age <65[353]
[354]
|
Careful consideration of risks/benefits of type, route, and dose of HRT for an individualized
period of time[82]
[84]
|
|
Induced menopause[355]
|
Hypertension
|
Midlife[173]
|
Systolic BP target of 120 mm Hg or lower and diastolic BP target of 70 or lower mm
Hg.[212]
[213]
Consider ARBs (especially candesartan and telmisartan) or ACE inhibitors[212]
[213]
[214]
[215]
Avoid CNS-active beta-blockers[216]
|
|
Insulin resistance/type 2 diabetes mellitus
|
With unhealthy lifestyle[173]
|
Lifestyle counseling
|
|
Already healthy lifestyle
|
Consider referral to endocrinology; exercise as above, supplementation with cocoa
flavanols; TRE[173]
|
May pose higher risk for females[86]
|
As above
Tighter targets for control
|
Hyperlipidemia
|
Due to elevated sterol absorption
|
Ezetimibe
Refer to preventive cardiologist[356]
|
|
Due to elevated sterol production
|
Hydrophilic statin therapy
Refer to preventive cardiologist[357]
|
Refractory to monotherapy
|
May benefit more with combination therapy (e.g., statin and ezetimibe)
Refer to preventive cardiologist to discuss PCSK9i[356]
|
Elevated cystatin or reduced GFR
|
|
Early referral to internal medicine and/or nephrology[358]
|
|
Elevated systemic inflammatory markers
|
|
Careful review of potential etiologies; consider supplementation with curcumin
|
|
Abbreviations: ACE, angiotensin-converting enzyme; ARBs, angiotensin II receptor blocker;
BP, blood pressure; CNS, central nervous system; CPAP, continuous positive airway
pressure; DHA, docosahexaenoic acid; EPA, eicosapentaenoic acid; GFR, glomerular filtration
rate; HIIT, high-intensity interval training; IM, intramuscular; MTHFR, methylenetetrahydrofolate
reductase; NAC, N-acetyl cysteine; PCSK9i, proprotein convertase subtilisin/kexin
type 9 inhibitor; RBC, red blood cell; TMG, trimethylglycine; TRE, time-restricted
eating.
Risk Factors and Interventions
Age
The most significant risk factor for AD is advanced age.[44] In fact, AD risk doubles every 5 years after the age of 65 years, and the number
of Americans older than 65 years is projected to increase from 40.2 million in 2010
to 88.5 million in 2050.[60]
[61] Although the aging process itself contributes to the risk of neurodegeneration,
dementia is not a normal part of aging. As the brain ages, white matter volume decreases
much more significantly than grey matter volume, but in those with AD, the rate of
neuronal dysfunction, neuronal loss, and cognitive decline is accelerated.[62]
[63] There are many molecular and systemic hallmarks of aging that correlate with susceptibility
to neurodegenerative disease such as genomic instability, telomere attrition, epigenetic
alterations, loss of proteostasis, and mitochondrial dysfunction.[64] Aging is also characterized by cellular senescence and chronic, low-grade systemic
inflammation—a process colloquially referred to as “inflammaging”—which is strongly
associated with AD.[65]
[66] In acknowledgment of this theory, a recent study demonstrated the benefit of low/anti-inflammatory
diet plans for older at-risk patients.[67] Other hallmarks of aging—deregulated nutrient sensing, stem cell exhaustion, altered
intercellular communication—also correlate with susceptibility to neurodegenerative
disease.[64]
Biological Sex
Research has suggested that the second most significant risk factor for AD is female
sex. In fact, two-thirds of all AD patients are females, and postmenopausal women
comprise over 60% of affected females.[68]
[69] Research continues to discern the reasons for this disparity, but statistical models
have shown that women still possess twice the risk of men even when accounting for
gender-dependent mortality rates, age at death, and differences in lifespan.[70]
[71] A growing body of evidence suggests differences in brain anatomy, function, and
age-related morphological changes between women and men are involved.[71] The “estrogen hypothesis” posits that sex hormones, 17β-estradiol specifically,
exert a neuroprotective effect on female brains. Estrogen dysfunction in the years
leading up to and after menopause seems to exacerbate (or precipitate) the AD process
in women in contrast to men of the same age.[72] Such research implicates the menopause transition (MT)—a midlife neuroendocrine
transition state unique to females—in their higher risk for AD.[73] During the MT, the estrogen network uncouples from the brain's bioenergetic system,
and the resulting hypometabolic state may serve as the substrate for neurological
dysfunction. Women experiencing menopause may present neurological symptoms (disruption
of estrogen-regulated systems like thermoregulation, sleep, and circadian rhythms),
depression, and impairment in multiple cognitive domains.[74] Such unique circumstances for developing dementia warrant sex-specific recommendations
when pursuing personalized risk reduction.
Research has been mixed as to the efficacy of hormone-replacement therapy (HRT) in
protecting women from cognitive decline. Earlier studies generally showed a lack of
benefit and even a potential harmful effect, whereas recent investigations indicate
that the effectiveness of estrogen depends on multiple factors, such as the time of
initiation of HRT, the individual's cognitive functioning at the time of HRT initiation,
and the forms of hormones used.[75]
[76]
[77]
[78]
[79]
[80]
[81]
[82]
[83]
[84] Hypertension—a major risk factor for both cognitive decline and cardiovascular diseases—increases
significantly in women after menopause, which suggests an additional need for female-specific
cardiovascular interventions.[85]
Furthermore, insulin resistance and/or type 2 diabetes mellitus (DM2) may pose a higher
risk for females than for males, thus warranting more aggressive management.[86] In patients with added risk due to physical inactivity, women seem to benefit more
from a greater ratio of cardiovascular-to-resistance training than men, and both sexes
should receive sex-specific nutritional counseling.[87]
[88]
[89]
[90]
[91]
[92]
[93]
[94]
[95] Women also face an added risk from suboptimal cognitive activity or reserve, for
which they can be recommended adult education and cognitive training or second language
or musical training.[96]
[97]
[98]
[99] While depression generally poses a higher risk for men, stress and anxiety pose
a higher risk in women, and women may be more vulnerable to depression risk in the
perimenopausal period.[100]
[101]
[102]
[103]
[104] For mental health purposes, both women and men may be recommended mindfulness training,
meditation, periodic vacations, cognitive behavioral therapy, exercise counseling,
and/or medication.[105]
[106]
[107]
[108]
[109]
[110]
[111]
[112]
[113] In a follow-up analysis to the CEDAR study, it was found that individualized multidomain
interventions may offer equal cognitive benefits to both women and men when such sex-specific
factors are employed, as well as better mitigation of calculated AD and CV risk in
women compared to men.[114]
Genetics
The genetic architecture of AD is complex and an area of ongoing active investigation.
While highly penetrant autosomal dominate mutations exist, they account for less than
1% of all AD cases and are associated with early-onset AD.[115] Late-onset AD has a more complicated etiology influenced by lifestyle factors, polygenetic
heterogeneity, and epigenetic interactions.[116]
[117]
[118] Early risk reduction interventions, guided by risk stratification that considers
genetics, allows for more personalized care.
As discussed, the ApoE gene is the most significant genetic risk factor. ApoE encodes a surface peptide found on plasma lipoproteins and is responsible for the
transport, metabolism, and redistribution of lipids in the body, especially in the
CNS.[41] The three ApoE variants have different affinities for lipoproteins and receptors, with ApoE2 and ApoE4 having a significant influence on lipid and lipoprotein levels.[119] In general, ApoE4 is associated with an increase in LDL-C and ApoB and, therefore, a higher cardiovascular risk.[120]
[121]
[122]
[123]
[124]
[125]
[126]
[127]
ApoE2, on the other hand, is associated with a three- to fourfold decrease in LDL-C compared
to ApoE4. ApoE4 carriers may also be more susceptible to certain modifiable lifestyle factors than
noncarriers. For example, individuals with the ApoE4 genotype who live a sedentary lifestyle, smoke, or consume alcohol possess a higher
risk for developing AD than those who do not carry the ApoE4 allele.[128]
[129] Given the propensity of ApoE4 carriers to develop hyperlipidemia and atherosclerosis, early referral to a preventive
cardiology specialist may be warranted based on individual factors, such as family
history, serum lipid markers, and other evidence of early atherosclerotic changes.
Statin therapy may provide a greater therapeutic efficacy in ApoE4 homozygotes.[130]
Although ApoE4 increases risk of AD, research indicates that ApoE4 magnifies the impact of certain modifiable factors such as physical inactivity, obesity,
tobacco use, alcohol consumption, and insulin resistance compared to those without
ApoE4.[128]
[131]
[132] Addressing these factors may lead to a greater potential for success. Due to its
differential effect on lipid metabolism, neuroinflammation, and glucose metabolism,
ApoE4 may provide a highly targeted and distinct approach to AD prevention interventions
(see [Table 2]).[133] Optimizing insulin sensitivity through low-glycemic index or low-carbohydrate diets
should be a priority. Additionally, fatty fish is rich in the omega-3 fatty acids
eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), and consumption and/or
supplementation is especially critical for ApoE4 carriers. Higher doses of DHA may be required due to ApoE4-induced metabolic alterations and BBB dysfunction.[133]
[134]
[135] Studies show that the phenolic compounds oleocanthal and hydroxytyrosol found in
extra virgin olive oil display anti-amyloid and anti-tau properties and may act on
specific pathways adversely affected in ApoE4 carriers.[136]
[137]
[138]
[139]
ApoE4 carriers may be more susceptible to pesticide exposure, specifically dichlorodiphenyldichloroethylene
(DDE), and thus should be educated on the need to wash all produce properly as well
as the potential benefits of buying organic. Other factors including hypertension,
vitamin D supplementation, cognitive engagement, and physical activity may also warrant
greater attention and more specific recommendations (see [Table 2]).[21]
[128]
[140]
[141]
Dietary Patterns
Several epidemiologic studies have reported an association between dietary patterns
and risk for various forms of dementia, including AD.[142]
[143] There are sufficient data for the recommendation of a Mediterranean diet (MeDi)
and Mediterranean–DASH Intervention for Neurocognitive Delay (MIND) diet to support
cognitive function and reduce dementia risk.[144] The MIND diet, similar to the Mediterranean diet but with further focus on lowering
blood pressure (BP), emphasizes plant-based foods and encourages eating green leafy
vegetables, berries, whole grains, fish, poultry, beans, nuts, wine, and olive oil.
Research supporting the MIND diet consists of evidence from 900 dementia-free older
adults, showing an association with reduced AD risk and slower rate of cognitive decline.[145] Certain aspects of a MeDi, such as its emphasis on an intake of lower glycemic carbohydrates,
healthy fats, leafy vegetables, and lean proteins, are associated with delay in the
onset of dementia and a decrease in AD risk.[146]
[147]
[148]
[149] Higher adherence to the MeDi is associated with reduced risk in developing MCI and
reduced risk for conversion of MCI to AD, while lower adherence to the MeDi is associated
with progressive AD biomarker abnormalities in middle-aged adults.[150]
[151] A meta-analysis of 26 different studies investigating different dietary habits and
their subsequent AD risk showed that a healthy diet can decrease oxidative stress,
inflammation, and accumulation of Aβ, therefore lowering AD risk.[152]
Inflammation is a risk factor for dementia and may be modulated by various nutritional
patterns/interventions. As referenced prior, low inflammatory/anti-inflammatory diets
are associated with a lower incidence of dementia.[67] Though restrictive and often difficult to adhere to, the ketogenic diet has been
shown to decrease systemic inflammation and may have the potential to delay the progression
of dementia.[153]
[154]
Time-restricted eating and intermittent fasting may also be neuroprotective by promoting
the process of autophagy in the brain.[155]
[156] Studies report that intermittent fasting may attenuate the progression of dementia
by improving cognitive dysfunction, inhibiting hippocampal neuronal damage against
oxidative stress, reducing neuroinflammation, and increasing the production of neurotrophic
factors.[157]
[158]
[159] However, the decision to recommend time-restricted eating or intermittent fasting
needs to be based on individual factors such as comorbid conditions, body composition,
and other personal needs and considerations.[160] Dietary recommendations are summarized in [Table 3].
Dietary Components, Vitamins, and Supplements
The use of vitamins and other nutritional supplements may also be considered using
a precision medicine approach. An additional risk for dementia is elevated total plasma
homocysteine, an amino acid metabolite which induces oxidative stress. Those with
elevated homocysteine levels (generally >10 μmol/L) may benefit from a B-complex vitamin
which contains B6, B12, and folate. Additionally, higher levels of both vitamin B6 and B12 have been associated with improved cognitive function in midlife and attenuated brain
atrophy for those with elevated omega-3 fatty acid levels.[133]
[161] Methylenetetrahydrofolate reductase (MTHFR) plays a critical role in folate metabolism,
and two polymorphisms in the MTHFR gene, MTHFR-C667T and -A1298C, are associated with high serum homocysteine levels.[162] Those with MTHFR polymorphisms and elevated homocysteine may benefit from supplementation with a methylated
B vitamin complex to allow for better metabolism of homocysteine, though further investigation
is needed.[21]
[163]
[164]
Furthermore, vitamin D deficiency is a known risk factor for AD and potential supplementation
for AD risk reduction may be considered.[59]
[133]
[165]
[166] Generally, a vitamin D serum level of 30 nmol/L is considered a target for treatment,
though recent studies have suggested a range of 50 to 70 nmol/L may be more beneficial.[167]
Omega-3 fatty acids (DHA and EPA) are essential polyunsaturated fats that support
cardiovascular function, lower triglycerides, and reduce inflammation. By increasing
cell membrane fluidity, they promote the synaptic plasticity essential for learning,
memory, and other cognitive processes.[168] Regular consumption of fatty fish including salmon, albacore tuna, sardines, mackerel,
lake trout, anchovies, and herring provides a rich source of omega-3 fatty acids,
but for some supplementation may also be necessary. Most western diets are much higher
in omega-6 compared to omega-3 due to consumption of plant oils like canola. Serum
fatty acid balance tests can be used to guide treatment decisions with a target of
a 4:1 ratio omega-3 to 6. Before recommending an omega-3 supplementation, risks and
benefits should be discussed, and all patients on supplementation should be monitored
for potential adverse effects, including bleeding and possible increased risk of atrial
fibrillation based on some recent studies.[169]
[170]
Oxidative stress can contribute to neurodegeneration and, hence, dietary consumption
of antioxidants including green leafy vegetables and berries is recommended.[165]
[171] Flavanols are another group of compounds with both antioxidant and anti-inflammatory
properties that may have a role in delaying or preventing the onset of dementia.[172] Some dietary sources of flavanols include cocoa, kale, tomatoes, apples, blueberries,
and tea. In a 2020 study, 921 cognitively normal elders were followed up for 6 years
after which 220 participants (24%) developed AD.[172] Results showed that a higher flavanol intake was associated with a 48% decrease
in risk for AD. Cocoa flavanols may also support memory function, BP regulation, and
glucose metabolism, and supplementation may be considered based on these factors.[173]
[174]
[175]
[176]
[177]
[178]
[179]
[180]
[181]
Curcumin, also known as turmeric, is a polyphenolic compound with antioxidant, anti-inflammatory,
and neuro- and chemoprotective properties that have traditionally been used as a remedy
for many illnesses in India and China.[182]
[183] Animal studies have definitively shown beneficial effects of curcumin on a molecular
and behavioral level, but clinical studies remain mixed as to its cognitive benefits.[182]
[184] Nevertheless, some evidence has shown that curcumin can improve cognitive functioning
in patients with AD.[185] This benefit is thought to stem from curcumin's ability to cross the BBB and interfere
with cellular signaling pathways and various molecular targets. Its antioxidant properties
come from its ability to scavenge free radicals such as reactive oxygen species and
reactive nitrogen species. Its anti-inflammatory benefits and other properties—decreasing
Aβ plaques, delaying degradation of neurons, decreasing microglia formation—may also
be responsible for any identified cognitive benefits.[185]
Alcohol
Light-to-moderate alcohol consumption has been associated with a decreased risk of
AD, whereas heavy consumption may increase risk, and this relationship appears to
be true for VaD as well.[186]
[187]
[188]
[189] Surprisingly, some evidence has also found abstaining from drinking to be associated
with a higher risk of AD. In fact, one longitudinal study found nondrinkers and heavy
drinkers both be twice as likely to have MCI in old age as light drinkers, and another
saw an increased risk of dementia only in nondrinkers and those who consumed more
than 14 servings per week.[190]
[191] A meta-analysis conducted on participants with AD, VaD, and other types of dementia
found drinkers to have a reduced risk of AD and dementia in general—but not a reduced
risk of cognitive decline—when compared to nondrinkers.[186] A variety of positive effects of alcohol on the CNS have been suggested, most having
to do with reduction of cardiovascular risk factors.[189] Further research is required, however, to identify the effects of alcohol on dementia
pathogenesis and progression.
Despite the detrimental effects of heavy alcohol consumption on cognition being widely
agreed upon, the benefit of light-to-moderate drinking does continue to receive debate.[189] Varying dosage definitions, ages of participants, and lengths of studies may lead
to different outcomes in epidemiological studies concerning low-to-moderate alcohol
consumption and dementia risk, so more standardized investigation is necessary.[189]
Future investigation should also focus on differing effects of alcohol consumption
on dementia risk between women and men, across socioeconomic statuses, and between
other subgroups. For instance, the relationship between alcohol consumption and dementia
risk seems to be modified by ApoE genotype, as consumption of any amount or type of alcohol has been found to increase
AD risk for ApoE4 carriers.[132] One study conducted in individuals aged 65 and older without dementia at baseline
found up to three servings of wine per day to be associated with a decreased risk
of AD—but only for individuals without ApoE4.[188] This further emphasizes the importance of personalized interventions guided by accurate
risk stratification.
Exercise
Several reviews and meta-analyses have concluded that regular physical activity has
general therapeutic potential, but substantial variation exists in exercise benefits
on cognitive health and dementia risk reduction at the individual level. As such,
clinical evaluation and analysis of anthropometrics, biomarkers, and cognitive testing
should be used to guide individualized recommended physical activity. For example,
studies have shown that ApoE4 carriers see greater long-term benefits from increased
exercise.[128] Such exercise interventions have shown to be more beneficial during the preclinical
stage of AD than in later clinical stages.[192]
High-intensity interval training (HIIT), which entails short bursts of extreme exertion
(85–95% maximum heart rate, depending on the protocol[193]) followed by recovery periods, has received increased attention in recent years
thanks to its capacity for metabolic, cardiovascular, and pulmonary benefits coupled
with the attraction of less time spent exercising.[194] It has been shown that HIIT may be more beneficial for ApoE4 carriers than noncarriers, and it is especially beneficial for individuals with elevated
homocysteine due to MTHFR mutations.[129]
[195] Furthermore, HIIT has the potential to decrease LDL, increase HDL, and improve insulin
sensitivity more effectively than moderate-intensity aerobic training, and these factors
carry influence on individual AD risk.[196]
[197]
Compared to HIIT, lower-intensity steady-state cardiovascular training (also called
zone 2 training) is traditionally defined as a more moderate intensity (∼65% maximum
heart rate), longer duration aerobic exercise, but, on a cellular level, is defined
as the range at which maximum ATP is produced in the mitochondria under aerobic conditions.
The evidence seems to support the benefit of HIIT in improving aerobic capacity, but
findings have been mixed as to whether HIIT or steady-state cardiovascular is more
effective in improving VO2max (maximum rate of oxygen).[198]
[199]
[200]
[201] It has been shown, though, that steady-state cardio is associated with more substantial
decline in resting heart rate and body weight than HIIT due to a resultant increase
in mitochondrial density and efficiency.[202] Therefore, while it should be a part of all exercise plans, it may be especially
important on a regular basis (e.g., three times a week) and for longer durations (e.g.,
at least 45–60 minutes per session) in those with elevated visceral body fat. Other
benefits include decrease in BP, lower risk of injury, and improvement in insulin
resistance.[203]
Another commonly recommended type of exercise is resistance training (also called
strength and weight training), which best improves lean muscle mass and metabolism.
Resistance training has shown to reduce cardiovascular risk factors—which are associated
with cognitive decline and dementia—and to promote neurotrophic factors that are beneficial
for the brain.[204] The SMART (Study of Mental and Resistance Training) trial supported the positive
impact of resistance training on cognition and occurrence of white matter lesions.[205] According to additional evidence, males may benefit from an equal-to-greater ratio
of resistance to cardiovascular training, while females may benefit from a greater
ratio of cardiovascular to resistance training, further highlighting the need for
individualized and sex-specific exercise interventions.[206] The correct dose or amount of exercise, as determined by frequency, duration, and
intensity, per patient should be informed by their comprehensive clinical evaluation.
One randomized controlled trial conducted showed that resistance training three times
per week for a total of 24 weeks significantly improved cognitive function among male
seniors aged 65 to 75 years.[93] Another study found that once- and twice-weekly resistance training significantly
improve executive functions in senior women.[92] Patients should be counseled on the need to consume adequate amounts of protein
and carbohydrates surrounding strength training to mitigate the risk of losing rather
than gaining muscle mass.
In all, physical activity reduces key cardiovascular and metabolic risk factors for
dementia, such as hypertension (HTN) and insulin resistance, on top of providing benefits
to cognitive health.[207]
[208] The recommended dose and type of exercise should depend on longitudinal anthropometric
assessments, and the individual abilities and responses of each patient.[129]
Hypertension
Midlife stage 1 and stage 2 systolic HTN is associated with an 18 and 25% increased
risk of AD, respectively, though HTN has also been linked to worse cognitive function,
behavioral symptoms, and hippocampal glucose metabolism.[209] Additionally, elevated BP and pulse pressure is associated with Aβ and p-tau deposition.[210] There are eight completed randomized controlled trials to date that have examined
various approaches to BP lowering on dementia outcomes.[211] As of 2018, the data more collectively support a significant reduced risk of dementia
with aggressive systolic BP lowering (at or below 120 mm Hg) compared to a standard
systolic treatment goal (<140 mm Hg) with the results from the Systolic Blood Pressure
Intervention Trial - Memory and Cognition in Decreased Hypertension (SPRINT-MIND)
study.[212] Diastolic BP should also be targeted at or below 70 mm Hg based on this study. Although
further study evaluating cerebrovascular-specific pathological endpoints such as MRI
brain is needed, the totality of current evidence supports targeting a systolic BP
of 120 mm Hg or lower without evidence of significant harm.[212]
[213] Optimal pharmacological agents are unknown and may depend on personal factors, but
data support use of angiotensin II receptor blockers (ARBs) such as candesartan and
telmisartan and ACE inhibitors over diuretics, and avoidance of CNS-penetrating beta
blockers.[212]
[213]
[214]
[215]
[216]
Hyperlipidemia
The identification of ApoE4 as the major genetic risk factor for AD helped elucidate the relationship between
lipids and cholesterol and AD. Literature has confirmed the association between high
LDL and dementia risk independent of the effects of ApoE status and some research suggests that midlife hyperlipidemia may be more impactful
on AD risk later in life.[217]
[218]
[219]
[220] However, studies also show low total cholesterol in late life can be detrimental
for neural function and memory suggesting a delicate cholesterol balance in the brain.[221]
[222] Desmosterol, a precursor of cholesterol, can be metabolized in the brain, and is
important in the degradation of APP and has been shown to inhibit formation of Aβ.
Lower desmosterol levels are found in plasma, brains, and CSF of AD patients compared
to controls and this supports the emerging hypothesis that desmosterol may serve as
a marker for cholesterol metabolism in the brain.[223] Statin use is associated with decreased risk of dementia in many studies and while
inconclusive, some demonstrate a differential effect depending on pharmacodynamic
properties. Few report that lipophilic statins may increase risk of dementia. At this
stage, the mechanism of this increased risk is unknown but possibly may be due to
increased penetration of the BBB, thereby blocking cholesterol production in the brain.[224] As statins therapy reduces desmosterol production, low levels of desmosterol may
suggest that cholesterol synthesis is over-suppressed in the brain and could potentially
guide treatment decisions.
Metabolic Comorbidities
The brain is a very highly metabolic tissue and depends heavily on glucose metabolism
for neuronal synaptic transmission. Thus, alterations in glucose availability and
utilization as found in conditions such as obesity, insulin resistance, and DM2 are
postulated to lead to neurodegeneration and Aβ and tau protein phosphorylation.[225]
[226]
[227] The idea of glucose-mediated neurodegeneration has inspired investigation into the
use of antidiabetic agents for treating AD. Specific interventions for those with
insulin resistance and/or DM2 may include lifestyle counseling, pharmacological treatment
with evidence supporting semaglutide and metformin, and supplements like cocoa flavanols.[173]
[228]
Education and Cognitive Engagement
Having more years of education is associated with lower risk of dementia.[229]
[230] This association is possibly due to the improvement of cognitive reserve with increased
education but is also impacted by early life cognitive development (which is related
to genetics, socioeconomic status, psychosocial, and other variables). Early life
educational attainment was found to have a protective effect on total brain volume,
specifically during the MCI stage of AD.[231] Consistent cognitive engagement throughout life such as learning a new language
or how to play an instrument is associated with improved cognitive function.[98]
[99]
[232]
[233] Studies also show that those with a history of reading, writing, and playing games
had lower levels of amyloid deposition on PET imaging suggesting that cognitive engagement
may influence the onset or progression of AD.[234]
Social Interaction
Social isolation and loneliness—major sources of mental and psychosocial stress—are
also risk factors for dementia. Research shows that social isolation may be associated
with neuronal degeneration, while a rich social life may slow cognitive aging and
be neuroprotective against AD.[235] Though the exact mechanisms linking isolation to dementia are not completely understood,
it has been proposed that production of Aβ, the p-tau protein, and increases in oxidative
stress and inflammation are involved.[236]
[237]
[238] Social enrichment in older adults has been associated with increased levels of BDNF,
which increases neuroplasticity and promotes synaptic consolidation.[239] Several studies on the association between social support and dementia have shown
that this association may differ between women and men. In one study, social activities
were associated with less memory decline in men in their midlife and old age, but
this relationship was not found in women.[240] In another, support from co-residing with family members and being married was protective
among men, while community engagement was protective for women.[241] Social enrichment interventions may be most beneficial when they are also mentally
engaging, associated with physical activity, or provide a sense of purpose and can
even be technology-based.[242]
[243]
[244]
Mental Health and Stress
Findings suggest that depression, anxiety, and chronic stress are associated with
dementia and AD development among asymptomatic patients. Stress raises cortisol and
acutely may have a positive effect on cognition. However, prolonged cortisol elevation
due to chronic stress and rumination leads to a detrimental effect on memory, may
accelerate brain aging, and may contribute to neurodegeneration.[245] Depression-related symptoms in AD adults have been shown to affect dementia risk
through neurodegeneration in AD-related brain regions; however, this is shown independently
of amyloid and p-tau risk.[246] Anxiety has shown to predict individual risk of both AD and VaD, and prolonged stressful
experiences have been associated with conversion from MCI to dementia.[247]
[248] In one 35-year longitudinal study, risk of dementia increased in women who reported
frequent or constant stress, and reporting stress at one, two, or three examinations
correlated with a sequentially higher dementia risk in participants.[249] The relationship between stress and dementia is made even more evident by the possible
twofold higher risk of dementia incurred by posttraumatic stress disorder (PTSD).[250]
[251] Research also shows depression-associated risk may be more substantial in males,
while stress and anxiety-associated risk may be more substantial in females.[100]
[101]
[102]
[252]
[253] Decreasing the threat posed by these psychological conditions is yet another avenue
for preventing or delaying the onset of dementia.[254]
[255] In addition to positive effects on mood, evidence suggests that some therapies (such
as Lexapro) carry anti-amyloid benefits.[256] Individualized recommendations may include mindfulness training, meditation, periodic
vacations, cognitive behavioral therapy, exercise counseling, and medication.[98]
[107]
[108]
[110]
[111]
[112]
[113]
[257]
[258]
[259]
[260]
Sleep
Evidence from several observational studies demonstrates that both too short and too
long sleep duration are associated with cognitive decline and risk of dementia.[261]
[262]
[263] In older adults, changes in sleep duration and patterns are also associated with
increased risk of dementia, but are also a marker for early AD pathology.[264]
[265]
[266]
[267]
[268]
[269]
[270] In a study of more than 2,800 participants, those who slept less than 5 hours per
night were twice as likely to develop dementia—and twice as likely to die—compared
to those who slept 6 to 8 hours per night.[271] Data from a 30-year Europe cohort of almost 8,000 participants showed a 30% increase
in dementia risk in those who slept for 6 hours or less at ages 50, 60, and 70 years
independent of other confounding factors and ApoE genotype.[272] Clearance of Aβ is ascribed to the glymphatic system which operates most efficiency
during slow wave sleep.[273] In fact, in one study, Aβ accumulation imaging increased by 5% in healthy controls
after a single night of sleep deprivation.[274] Others have demonstrated similar findings in the CSF.[275] Additionally, rapid eye movement (REM) stage sleep is essential for memory consolidation
and reduction in REM sleep is associated with dementia risk.[276]
While sleep need is highly individualized, most healthy adults require 7 to 8 uninterrupted
hours of sleep.[277] Guidance on good sleep hygiene practice should be provided including avoiding caffeine
consumption after 1 pm; restricting food, alcohol, and the use of electronics before
bed; maintaining a consistent sleep schedule every day; and optimizing sleep environment.[21] Anticholinergics, benzodiazepines, and z-hypnotics should be limited due to their
associated dementia risk.[278]
[279]
Referral for management of comorbid conditions that affect sleep quality such as nocturia
due to benign prostate hypertrophy may be required. Another highly prevalent and treatable
condition is obstructive sleep apnea (OSA).[280] OSA is associated with increased AD risk with women suffering the adverse effects
more than men despite the less frequent presentation of moderate-to-severe OSA in
women.[280] In one study, patients with OSA were found to have a 1.70 times greater risk for
dementia within 5 years of diagnosis compared to age- and sex-matched patients without
OSA.[281] Another study conducted in 2021 found significant associations between OSA and brain
white matter hyperintensities (WMHs)—commonly observed in older people and significantly
associated with AD—which indicates a novel pathological mechanism linking OSA and
dementia.[282] As such, screening for symptoms of OSA should be included as a part of a comprehensive
AD risk reduction plan.
Other Medical Conditions
Other significant risk factors for dementia include hearing loss, past traumatic head
injuries, and smoking. Hearing impairment significantly increases the risk of AD and
other cognitive disorders (though this association is not causal and requires further
investigation), and the progression of dementia is faster in those with hearing loss
than without it.[283]
[284]
[285] As such, when applicable, audiometry evaluation should be considered. Furthermore,
robust evidence indicates that approximately 30% of patients who die from traumatic
brain injuries (TBIs) have Aβ plaques in their brains—the pathological hallmark of
AD—signaling an association between head injuries and neurodegenerative disease.[286] While accidents are unavoidable, risk may be mitigated by wearing a helmet during
high-risk activities and a seat belt in motor vehicles. Cigarette smoking also confers
an increased risk of AD—an effect that has tended to be stable over time—and heavy
consumption in middle-aged smokers can increase the risk of dementia by more than
100% compared to non-smokers.[287]
[288] For this and other adverse effects on overall health, smokers should be counseled
on smoking cessation and provided other cessation support resources.
Periodontal disease affects 70% of adults 65 and older and is another modifiable medical
condition that may be associated with dementia.[289]
[290]
[291]
[292]
[293] In a recent analysis by NIA scientists, gum disease was associated with the development
of all-cause dementia and especially VaD.[291] It has been proposed that Porphyromonas gingivalis, the bacteria that most commonly causes gum disease, can lead to the production
of Aβ. Inflammation may serve as another connecting link between periodontitis and
dementia.[294]
[295]
[296]
[297] Patients should be educated on the importance of proper oral hygiene and following
up regularly with their dentist. Bidirectional care integration between dental and
medical providers may best help mitigate patient's risk. Further clinical research
is needed in this emerging area, and this may be best served by collaborations among
medical and dental providers and oral systemic health experts.[298]
Future Directions and Conclusion
Although the evidence supporting dementia prevention is substantial, many areas need
further investigation. Individualized multidomain clinical care for AD prevention
has already demonstrated benefit, as can be seen with the Alzheimer's Prevention Clinic
(APC) at Weill Cornell Medical College/New York - Presbyterian, which was founded
in 2013. The APC framework applies evidence-based principles of clinical precision
medicine to tailor individualized recommendations.[20]
[21] The National Institutes of Health defines precision medicine as “an emerging approach
for disease treatment and prevention that takes into account individual variability
in genes, environment, and lifestyle for each person.”[299]
[300] In adaptation of this approach for use in a clinical setting, clinical precision medicine informs risk stratification and personalized intervention via
comprehensive and longitudinal assessment of patients' anthropometrics, blood biomarkers,
cognition, genetics, and family and clinical history.[21]
Moving forward, the utility of the clinical precision medicine approach will benefit
from the expansion of genomic sequencing and direct-to-consumer testing, as AD risk
and clinical course is thought to be impacted by the polygenetic factors.[301] The interest in broader-scale genetic data to guide clinical decision-making has
occurred in other medical specialties as well, and the comorbid risk factors for dementia
have their own environmental and genetic contributors.[301]
[302]
[303]
[304] It is hoped that self-provided genomic data, including whole genome sequencing and
other at-home commercial qualitative genotyping, can be incorporated more regularly
into medical practice to better identify disease risk and inform individualized risk
reduction strategies. It is important to recognize the limitations that persist in
the field of precision medicine, as progress and accessibility continues to be constrained
by high costs, fear of genetic discrimination, access and availability of genetic
testing, and inadequate and unprepared infrastructure.[305]
[306]
Already many AD risk factors cluster around inequalities and inequities, which occur
primarily in Black, Asian, Hispanic, and other marginalized ethnic groups and vulnerable
populations.[11] To continue to progress within the field of dementia prevention, we must address
these disparities. The differences in AD prevalence and pathophysiology between racial
and ethnic groups are due to various factors, such as genetic variants associated
with certain groups, but also with other environmental factors such as socioeconomic
status, diet, or medical comorbidities, which are heavily influenced by inequity.[307]
[308]
[309] For instance, chronic health conditions that are associated with higher AD risk,
such as cardiovascular disease and diabetes, disproportionately affect Black and Hispanic
populations.[15] Worldwide, multisite research collaborations are required to construct a large and
diverse cohort, and the APC framework may be applied. Further research is additionally
needed to determine the comparative effectiveness of prevention interventions and
ongoing follow-up is necessary to assess long-term effectiveness on the development
of AD and other dementias.[21]
[59]
Preventive care for dementia has not been routinely integrated into clinical practice
in the United States, largely because it is not covered by Centers for Medicare/Medicaid
Services and, therefore, private insurance policies. Considering the economic burden
of dementia, its public health impact, the emergence of blood-based amyloid biomarkers,
and growth of preventative practices, it is worthwhile to invest in and expand clinical
prevention efforts for the benefit of tens of millions of patients in need. In 2022,
a consortium of six clinical research sites in the United States (Weill Cornell Medicine/New
York - Presbyterian in New York City, NY; Florida Atlantic University in Boca Raton,
FL; Cleveland Clinic Lou Ruvo Center in Las Vegas, NV; Norton Neuroscience Institute
in Louisville, KY; Alzheimer's Prevention Clinic and Research Center in San Juan,
PR; Atria Medical in New York City, NY) and three globally (McGill University in Montreal,
Canada; Jersey Memory Service in Jersey, United Kingdom; University of New South Wales/University
of Notre Dame, St. Vincent's Hospital in Sydney, Australia) formalized plans to harmonize
measures collected and better accomplish this goal.