Introduction
Cushing’s syndrome (CS) is a rare disease caused by chronic exposure to
inappropriately high levels of glucocorticoids. The clinical phenotype of CS is
characterized by a wide spectrum of severity that varies from mild to severe
presentation. If untreated, CS is potentially fatal, given the high incidence of
cardiovascular events and opportunistic infections. One of the major goals of
precision medicine is a correct and early diagnosis [1]
[2]. According to the pathogenic mechanism, endogenous CS is traditionally
classified into adrenocorticotropic hormone (ACTH)-dependent and ACTH-independent
forms (respectively 70-80% and 20-30% of CS); in the latter ACTH is suppressed and
cortisol is secreted directly from the adrenals [3]
[4].
ACTH-dependent hypercortisolism is further classified into a pituitary source of ACTH
secretion (from a corticotroph adenoma or hyperplasia), defined as Cushing’s disease
(CD), or ectopic ACTH secretion (EAS) characterized by the paraneoplastic
corticotropin secretion in a neuroendocrine tumor (NET). ACTH-independent forms are
primarily caused by unilateral adrenal cortisol hypersecretion and bilateral adrenal
hypersecretion due to primary bilateral macronodular adrenal hypercortisolism
(PBMAH) or primary pigmented nodular adrenocortical disease (PPNAD). The term PBMAH
has been proposed because the dogma of “ACTH-independent” cortisol secretion has
been questioned after the discovery of adrenal ACTH synthesis and paracrine
ACTH-cortisol stimulation [5].
Furthermore, in some cases, non-neoplastic hypercortisolism (NNH, previously known as
pseudo-Cushing) should be considered in the differential diagnosis of
hypercortisolism before the characterization of ACTH, especially in milder forms. In
clinical practice, peculiar clinical features of hypercortisolism in some patients
are commonly suspected to be secondary to a sustained or intermittent activation of
the hypothalamic-pituitary-adrenal (HPA) axis caused by psychological diseases
(major depression, eating disorders), metabolic conditions (obesity, polycystic
ovary syndrome, poorly controlled diabetes mellitus), as well as physical
(starvation/chronic intense exercise) or chemical (alcohol) stimuli [6]
[7]. In these cases of NNH, the HPA activation is not secondary to
neoplasia; nonetheless, their resulting biochemical features after first-line
screening tests can be indistinguishable from neoplastic hypercortisolism.
After the exclusion of exogenous glucocorticoids for therapeutic purposes, the
initial diagnosis of endogenous hypercortisolism is often challenging because many
other common conditions (metabolic syndrome, osteoporosis, depression) overlap with
CS in their clinical presentation. The clinical practice guidelines of the Endocrine
Society and recent consensus of the Pituitary Society recommend CS screening with
three first-line tests: 24-h urinary free cortisol (UFC), serum cortisol after 1-mg
overnight dexamethasone suppression test (DST), and late-night salivary cortisol
(LNSC) [1]
[8]. All these tests present a high
diagnostic accuracy to detect CS [8];
however, UFC is the less sensitive, especially in a mild form of hypercortisolism
(as in the case of recurrent CD after surgery [9]). On the contrary, a false positive result of a screening test must be
considered because it can capture the NNH form [10].
Therapy is based mostly on surgical options (in case of pituitary or adrenal adenoma,
as well as in those localized NET with EAS) in addition to medical treatment
(steroidogenesis inhibitors could be used in all forms of CS, pituitary-directed
drugs only for CD) [11]. Therefore, when
overt CS is established, the current differential diagnosis among subtypes of CS is
of utmost importance. A correct diagnosis is the crucial crossroad in the pathway of
CS and should never be delayed, except in those rare cases of extremely severe
presentation of hypercortisolism (such as sepsis, fungal infection, pulmonary
embolism, malignant hypertension, and acute psychosis) when the prompt reduction of
cortisol excess with medical therapy and management of comorbidities is mandatory,
but ideally should not delay the diagnostic process [12].
In our narrative review, we will discuss the most challenging situations in the
diagnosis of hypercortisolism, from neoplastic CS (differential diagnosis of ACTH
dependent and adrenal hypercortisolism) to non-neoplastic forms.
Tools available for the subtyping of different forms of Cushing’s
syndrome
The differential diagnosis of CS may be challenging. Many strategies have been
proposed, but no single test reaches 100% of diagnostic accuracy alone.
Therefore, in clinical practice, the combination of at least two tools is used
to define the subtyping of CS (as the combination of imaging and dynamic tests).
The diagnostic tools available are mentioned in [Table 1]. After the exclusion of NNH
(reassumed in a dedicated paragraph), the first step in patients with neoplastic
hypercortisolism is to differentiate between ACTH-dependent and independent
forms ([Fig. 1]).
Fig. 1 Diagnostic flow chart of Cushing syndrome subtyping. After
first-line screening tests, characterized by high evidence, none of the
tools proposed on the right half of the panel is able to alone confirm the
final diagnosis of hypercortisolism. Therefore, a skillful clinical
interpretation is required to perform a correct diagnosis. Created with
Biorender.com [rerif].
Table 1 Diagnostic tools for the differential diagnosis of
endogenous hypercortisolism (references in brackets).
|
Test
|
Suspected condition
|
Expected results for the suspected condition
|
Remarks (references in brackets)
|
|
CRH test
|
CD
|
Positive response: ACTH and/or cortisol response above
baseline
|
|
|
EAS
|
Negative response
|
|
|
PBMAH
|
Negative response (ACTH always suppressed)
|
|
|
NNH
|
No ACTH or cortisol response (after dexamethasone
suppression)
|
|
|
HDDST
|
CD
|
Positive response: cortisol suppression
|
-
Overnight test with serum cortisol in Western countries
[22]
-
Dexamethasone levels
-
Concerns: high dexamethasone levels can exacerbate
hypertension, diabetes, psychosis
-
No standardized cut-off
|
|
EAS
|
Negative response
|
|
|
PPNAD
|
Paradoxical urinary cortisol rise
|
|
|
Desmopressin test
|
CD
|
Positive: ACTH and/or cortisol response above baseline
|
|
|
EAS
|
Negative
|
|
|
NNH
|
No ACTH or cortisol response
|
|
|
Pituitary CEMRI
|
CD
|
Adenoma detection
|
|
|
EAS
|
Negative
|
|
|
NNH
|
Negative
|
|
|
BIPSS
|
CD
|
Central to peripheral gradient (at baseline and after
stimulation)
|
|
|
EAS
|
No gradient
|
|
|
Conventional imaging
|
EAS
|
|
|
Adrenal CS
|
Single adenoma, bilateral macronodular or micronodular
hyperplasia
|
-
Consider ACTH-dependent bilateral enlargement
-
Rule out those rare bilateral malignancies (adrenal
cancer)
-
Differential diagnosis between MACS and overt CS
|
|
Functional imaging
|
EAS
|
Positive 18-FDG or 68Ga-derivate uptakes
|
|
|
PBMAH
|
Unilateral adrenalectomy in case of differential uptake
|
|
|
Genetic evaluation
|
PBMAH
|
ARMC5 or KDM1A mutation
|
|
|
PPNAD
|
PRKR1A
|
|
ACTH, adrenocorticotropic hormone; CD, Cushing’s disease; CS, Cushing’s
syndrome; EAS, ectopic ACTH secretion; NET, neuroendocrine tumor; MACS, mild
autonomous cortisol secretion; PBMAH, primary bilateral macronodular adrenal
hypercortisolism; NNH, non-neoplastic hypercortisolism; PPNAD, primary
pigmented nodular adrenocortical disease.
Morning unstimulated adrenocorticotropic hormone
Morning ACTH concentrations below detection levels or 10 pg/mL (2 pmol/L),
combined with normal or elevated cortisol levels, suggest adrenal
hypercortisolism [13]. Immediately
after collection, blood should be stored in chilled tubes containing
ethylenediaminetetraacetic acid, placed inside a container with ice, and quickly
transported to the laboratory for ACTH measurement, as ACTH is rapidly degraded
by plasma protease. However, ACTH may not be entirely suppressed in some
patients with adrenal CS with mild or intermittent cortisol secretion; 11% of
cyclic forms are of adrenal origin [14]. ACTH levels+>+20 pg/mL (4 pmol/L) indicate an ACTH-dependent
CS. Commercially available ACTH immunoassays may be imprecise in patients with
normal-low ACTH levels, leading to pre-analytical errors [15]; therefore, we suggest a
corticotropin-releasing hormone (CRH) test to check neuroendocrine
responsiveness and exclude ACTH-independent CS [16].
Dynamic tests
Blood samples for ACTH and cortisol levels are collected before (usually twice in
15 min) and 15, 30, 45, 60, 90, and 120 min after intravenous bolus injection of
100 μg ovine/synthetic human CRH. The diagnostic accuracy of ovine CRH seems
superior to that of the human peptide [17]; nonetheless, the recent relative shortage of human CRH requires
additional considerations [18]. The
majority of corticotropinomas and a small number of NETs respond to synthetic
CRH, with increased ACTH and cortisol concentrations. There is no consensus
about the interpretation criteria, which vary according to the type of parameter
considered (35-50% increase in ACTH or 14-20% increase in cortisol, compared to
baseline). According to the literature, the sensitivity and specificity of ACTH
peak are around 90%. Ritzel et al. [19] reported that the duration of CRH test can be shortened: ACTH
rise+≥+43% 15 min after CRH injection is the strongest predictor for CD, with a
sensitivity of 83% and a specificity of 94%. A study in a larger series
confirmed that a test duration longer than 1 h did not improve diagnostic
performance [20]. We recently
reported that an ACTH and cortisol increase of+>+31% or 20% after human CRH,
respectively, resulted in 91-86% of sensitivity and 80% of specificity [16].
Desmopressin is a long-acting arginine-vasopressin analog, which binds
preferentially to V2 and V3 receptors, whose expression is increased in
ACTH-secreting pituitary adenomas. The intravenous administration of 10 μg
desmopressin stimulates an exaggerated ACTH or cortisol response in 80-90% of
patients with CD; nonetheless, those without ACTH-secreting tumors usually have
a minimal (or absent in case of dexamethasone suppression) ACTH and cortisol
response to desmopressin, suggesting NNH [6]. ACTH and cortisol are measured during the CRH test. The test is
easily available, inexpensive, and can be useful also in the post-operative
evaluation of corticotropinoma’s recurrence [21]. Its application in the
differential diagnosis of ACTH-dependent CS is, however, limited: 20-50% of NETs
express vasopressin receptors as well, and, therefore, can respond to
desmopressin.
High dexamethasone (HD) concentrations can partially suppress ACTH secretion of
most corticotroph adenomas with consequent reduction of cortisol levels. NET, on
the contrary, are usually resistant to this negative feedback. It is the most
reported test in literature (3057 tests in 43 studies, followed by the CRH test
with 1715 patients and desmopressin with 1038 tests) [22]. There are different versions of
the high-dexamethasone suppression test (HDDST): 8 mg of dexamethasone overnight
and serum cortisol measurement is the most used in Western countries; on the
contrary, 2 mg of dexamethasone every 6 h (for eight doses) with urinary
cortisol is the most used in the Eastern world. Overnight HDDST comprises oral
administration of 8 mg of dexamethasone between 23:00 and 24.00 and measurement
of plasma cortisol at 8:00 the next morning. Using a cut-off for cortisol
suppression+>+80% compared to the baseline levels, the sensitivity and
specificity in differentiating pituitary disease from ectopic form varies
between 60-90%. The measurement of dexamethasone levels [23], available only in selected
referral centers, may enhance the diagnostic accuracy of HDDST. The relative
shortage of human CRH, combined with the low specificity of the desmopressin
test, can improve the role of HDDST in the differential diagnosis of
ACTH-dependent hypercortisolism.
Bilateral Inferior Petrosal Sinus Sampling (BIPSS)
During BIPSS, the ACTH gradient between the right and left petrosal sinuses and a
peripheral vein is assessed. Being an invasive procedure, we suggest that its
use should be limited to patients with ACTH-dependent CS whose clinical,
biochemical, and imaging data are discordant or doubtful [24], conversely to other authors that
consider the sampling in all patients with ACTH-dependent form or in those with
a pituitary lesion+<+6 mm [25]
[26]. An unstimulated
gradient between the central and peripheral ACTH+>+2, or+>+3 after CRH or
desmopressin, indicates CD. The reliability of BIPSS in localizing the
left/right side of the adenoma within the pituitary gland is poor; a recent
meta-analysis reported 69% correct tumor lateralization with BIPSS, 53%
concordant with magnetic resonance, without association with postoperative
remission [27].
The sensitivity of BIPSS for the diagnosis of CD is very high, either at baseline
or after stimulation [28].
False-negative results include corticotroph adenomas with poor responsiveness to
CRH, cyclic CS, or anomalous venous drainage. Occasional false positive results
are reported in cerebral NET with venous drainage into the cavernous sinuses.
Common minor complications after BIPSS are hematomas in the site of vascular
access; significant adverse events are rare, including deep venous thrombosis,
pulmonary embolism, and brain injury [29]. To avoid pitfalls in results interpretation, cortisol-lowering
medications should be withdrawn at least 48-72 h before the procedure and overt
hypercortisolism must be assured before BIPSS [30].
Desmopressin is an alternative to CRH during BIPSS [31], with the same procedure and
diagnostic accuracy; however, it is less expensive [32]. Nonetheless, desmopressin is a
pro-coagulant agent; its use during BIPSS needs extreme caution given the
increased risk of thromboembolic events in CS [33]. Moreover, the number of patients
with EAS evaluated after desmopressin-stimulated BIPSS is relatively small,
making it difficult to calculate the effective specificity of the test.
Recently, studies have reported that the simultaneous measurement of prolactin
might further improve the diagnostic performance of human CRH-stimulated BIPSS.
The normalized ACTH to prolactin ratio was able to correctly detect ACTH source
in 31 out of 32 confirmed ACTH-dependent CS [34].
Imaging
A contrast-enhanced magnetic resonance imaging (CEMRI) with gadolinium should be
performed in all patients with ACTH-dependent CS [26]. CEMRI reveals a pituitary adenoma
in up to 70% of patients [35]
[36]. In authors’ opinion, the presence
of a focal lesion (at least 2-3 mm) on CEMRI in a patient with a characteristic
clinical presentation and concordant biochemical data in defining the
ACTH-dependent form after dynamic tests suggests a definitive diagnosis and does
not require any further investigation (as BIPSS) [35]. A microadenoma is hypointense and
sometimes isointense compared to the surrounding normal tissue on a T1-weighted
image without a contrast agent. After administration of gadolinium the lesion
appears still hypointense because it is less vascularized compared to normal
pituitary. Unilateral elevation of the sellar diaphragm or pituitary stalk
deviation are indirect radiological signs that may indicate the presence of a
microadenoma.
If the BIPSS does not show a significant ACTH gradient between the lower sinus
and periphery, a total-body computed tomography (CT) is recommended in the
suspicion of an EAS. Although the thorax is the most likely site of
ACTH-secreting tumors [37]
[38]
[39], the identification of these
lesions is often difficult and they are detected only in 65% of cases during the
first assessment. Nuclear medicine improves the sensitivity of conventional
radiology when the NET is not identified; however, its high diagnostic accuracy
(75% of patients with initial occult EAS with conventional imaging) is not
confirmed in large series. A positive octreoscan was described in 67% of CS (50
patients) and positive 18FDG-PET in 60% (32 patients) [40]. There was no single diagnostic
imaging technique with optimal accuracy [40]; in 23% of EAS the NET can remain occult for years after the
diagnosis of CS [38], and in 22% of
EAS, the detection of the primary tumor occurred in median 16 months after the
diagnosis of hypercortisolism (defined as covert EAS) [37]. PET/CT using
68Ga-conjugated somatostatin receptor targeting peptide
(68Ga-SSTR-PET/CT) reported by Goroshi et al. in 12 patients was
useful in increasing the specificity of the suggestive CT-positive lesions [41] (in other words, to confirm the
neuroendocrine origin and suggest ACTH secretion in a CT-detected node). In
their series, the number of positive CT scans was higher than that of nuclear
imaging. Similarly, Wannachalee et al. reported that 68Ga-SSTR-PET/CT is
sensitive to detect primary and metastatic neoplasms in EAS, achieving a
significant clinical impact in the diagnostic-therapeutic management of patients
in 65% of cases [42]. A careful
interpretation of nuclear imaging with 68Ga derivates is of utmost
importance, because PET/CT presents a considerable number of indeterminate/false
positive images [43]. In case of high
cortisol levels, somatostatin receptors (especially type 2) are down-regulated
[44], and a medical treatment
with steroidogenesis inhibitors before nuclear imaging with somatostatin-based
radioligand can enhance the identification of the NET [45].
Differential diagnosis of non-neoplastic hypercortisolism (formerly known as
pseudo-Cushing’s syndrome)
NNH is a functional and often mild form of hypercortisolism, characterized by a
partial or complete recovery of the HPA axis after the treatment of the
underlying condition. It is caused by chronic non-tumorous activation of the HPA
axis by heterogenous conditions such as psychiatric disorders, uncontrolled
diabetes mellitus, alcoholism, and severe obesity, with a consequent clinical
presentation suggestive of CS [10].
The number of patients with obesity, cardiometabolic dysfunction, low bone
density or fractures, adrenal nodules, and psychiatric disorders is increasing,
therefore the diagnosis of NNH has emerged as an important topic [6]. The overlap with these
disorders/conditions, that may also be associated with abnormalities in the HPA
axis, increases the uncertainty in making the correct diagnosis. Imaging studies
should not be used alone to confirm the presence or absence of neoplastic
hypercortisolism [6] due to the large
number of patients with pituitary or adrenal incidentaloma [46]
[47].
Most CS patients present a criterion for major depressive disorder (MDD), while a
small part can present other behavioral and cognitive disorders: MDD and
hypercortisolism are mutually associated. In patients with MDD, some
stress-related mediators over-activate the HPA axis, resulting in persistent
hypercortisolism, which is non-suppressed in the dexamethasone suppression test
(DST) and can be resolved by the administration of anti-depressive drugs.
Several HPA axis alterations are reported in patients with eating disorders,
such as anorexia or bulimia nervosa. The mechanisms involved in HPA deregulation
and consequent hypercortisolism are increased CRH levels with normal ACTH,
enhanced adrenal response (fasting/binge eating induced stress, but the
regulation does not disappear after achieving a normal weight), reduced cortisol
clearance, increased affinity versus corticosteroid-binding globulin (CBG), and
resistance of glucocorticoid receptor (explaining the hypercortisolism without
the increased lipogenesis necessary for the features suggestive of CS) [48]
[49]. Likewise, alcohol abuse can
present features suggestive of CS in patients with negative DST and circadian
rhythm alterations due to chronic alcoholism or abstinence that lead to HPA axis
activation (increase in CRH, ACTH, and cortisol), new-onset of
alcohol-associated disorders in psychiatric (depression), and neurological
(hippocampus neurotoxicity) spheres that can alter HPA axes regulation, as well
as 11β-HSD1 induction secondary to alcoholic liver disease with increased
conversion of cortisone to cortisol [6]
[50]. Diabetes mellitus
type 2 is associated with hypercortisolism due to central HPA axis activation,
increased 11β-HSD1 expression in adipose tissue, and hippocampus damage due to
glycemic decompensation. Obese patients present HPA axis activation due to
increased responsiveness to neuropeptides and dietetic factors and increased
11β-HSD1 expression in adipose tissue. During pregnancy, physiological
modifications in the HPA axis include estrogen excess that increase the levels
of CBG (an increase of total cortisol with a constant free fraction), ACTH
(explained by different CBG levels, placental synthesis, cortisol feedback
desensitization, pituitary sensitization to CRH), and UFC [6].
In patients with NNH, the diagnostic accuracy of first-line screening tests for
hypercortisolism is limited due to the mild activation of the HPA axis [10]. Therefore, several second-line
tests have been proposed to distinguish NNH from neoplastic CS: although there
is no agreement on the gold standard, CRH test after dexamethasone suppression
and desmopressin test can be valid tools [51]. In authors’ opinion, one of the critical factors in the
differential diagnosis between CD and NNH is time. If we are approaching a
patient with mild hypercorticism, in the suspicion of NNH form, a re-evaluation
3-6 months after the first diagnosis could be an optimal choice, especially if
the underlying factor (e. g. alcohol abuse, depression, pregnancy) can be
adequately managed.
Differential diagnosis of adrenocorticotropic hormone-dependent
hypercortisolism (Cushing’s disease vs. ectopic adrenocorticotropic hormone
secretion)
CD is the most common cause (60-70%) of CS and it is characterized by a pituitary
ACTH hypersecretion. In most cases, it is due to a pituitary corticotroph
basophilic adenoma and rarely due to diffuse corticotroph cell hyperplasia.
Pituitary corticotroph adenoma is often+<+10 mm in diameter (microadenoma),
but in 10% of the cases, it exceeds 10 mm (macroadenoma). Pituitary corticotroph
adenomas present mild histological alterations compared to normal pituitary
cells. The hallmark of pituitary corticotropinoma is partial resistance to the
cortisol feedback, resulting in an ACTH hypersecretion and chronic
hypercortisolism that is not suppressible after a low dose of dexamethasone (the
screening test) but retains the glucocorticoid feedback and is almost completely
inhibited after a high dose of dexamethasone. While the presence of a 10 mm
adenoma is considered sufficient to diagnose CD [26], endocrinologists should remember
that attenuated response to dynamic tests is reported in macroadenoma [52].
EAS is diagnosed in 5-10% of CS cases and derives mostly from NETs as bronchial
or thymus or gastrointestinal carcinoids, small-cell pulmonary carcinoma,
medullary thyroid carcinoma or pheochromocytoma [37]. EAS associated with malignant
aggressive tumors is usually characterized by extremely high ACTH and cortisol
circulating levels, producing a rapid and severe clinical presentation with
hypokalemia, catabolic presentation, and opportunistic infections, rather than
an insidious and mild outset CS that is more typical of small or occult tumors
[53].
In some cases, the differentiating CD and EAS can be challenging. Aggressive
pituitary adenomas with a large amount of cortisol secretion, rapid onset of
catabolic signs of hypercortisolism, and acute severe comorbidities (such as
hypertension with hypokalemia, mainly due to 11β-hydroxysteroid dehydrogenase
type 2 saturation [54]) mimic a
paraneoplastic cortisol excess. On the contrary, some patients with indolent and
small, well-differentiated (mainly bronchial) EAS present only with the common
signs of cortisol excess [39]. In the
context of ACTH-dependent forms, ACTH levels tend to be higher in the case of
ectopic secretion compared to those from a pituitary origin; however, there is
considerable overlap, so the ACTH measurement alone is not sufficient to
distinguish the two conditions [37]
[54].
Although the evidence of a+>+6 mm pituitary adenoma in the diagnostic work-up
for ACTH-dependent hypercortisolism is highly suggestive of the pituitary source
of ACTH secretion [26], pituitary
incidentalomas are not uncommon in the context of EAS (reported in 5 out of 26
patients with EAS [55]) and can
further complicate the diagnosis in case of discordant dynamic tests. Moreover,
in clinical practice, 65.5% of patients with CD displayed a+<+6 mm pituitary
adenoma at CEMRI [36]. CRH test has
emerged as the most reliable non-invasive test for the differential diagnosis of
ACTH-dependent forms in terms of diagnostic odds ratio [22]. However, it cannot guarantee an
absolute differentiation between the pituitary and ectopic origin, since some
ectopic ACTH-secreting tumors may respond to CRH. Similarly, some
well-differentiated NETs (especially bronchial carcinoids) may express
glucocorticoid receptors and, therefore, be sensitive to the suppression by
HDDST. High-resolution conventional imaging is now the technique of choice in
patients with EAS, because it is the most used approach to localize the source
of ectopic ACTH secretion (sensitivity is 98% for CT and 93% for MRI) [40]. By definition, an occult EAS is
not detected during the initial management of hypercortisolism: in more than 30%
of EAS, the ACTH source was detected during follow-up [40], with 22% occult tumors after at
least two years of follow-up [38].
A skillful combination of dynamic tests, BIPSS, and imaging is required to
differentiate ACTH-dependent CS. The diagnostic accuracy of BIPSS is high and
has long been the gold standard to reliably exclude EAS [26]. Nonetheless, several authors
proposed diagnostic strategies to reduce the number of invasive procedures.
Vilar et al. documented an increased diagnostic accuracy by combining the
results of different dynamic tests; ACTH response to CRH or desmopressin (+≥+35%
above basal) and cortisol suppression+>+50% after HDDST was found only in
patients with CD, with a sensitivity of 63.3% and a specificity of 100% [56]. Similar results were described in
a large Italian series, where none of the patients with EAS had positive
responses in HDDST and CRH tests [57]. A recent large European study in 205 patients (197 CD and 8 EAS)
reported that different combinations (ACTH and/or cortisol) of the human CRH
test and the overnight HDDST revealed similar diagnostic accuracy to the single
tests [20], even if the reduced
cohort of patients with EAS could partially limit the impact of the results.
During the evaluation of combined tests, overall performance depends on whether
discordant results can be interpreted as positive or negative; in the case of
the discordant test, the CRH test was more likely to be positive in the CD group
compared with EAS, while the HDDST was more often incorrect in both
ACTH-dependent CS [58], suggesting
that CRH performs better than the other tests [22]. The innovative combination
strategy proposed by Frete et al. [35], also reported in the consensus of the Pituitary Society [26], suggested that a noninvasive
diagnostic strategy that combined dynamic tests and imaging enabled to avoid the
BIPSS in half the number of patients. They excluded evident cases of cancer and
paraneoplastic ACTH secretion to reflect an “endocrinological” setting
recruitment and considered positive a 2 mm adenoma detected after CEMRI thanks
to the high expertise of pituitary-dedicated neuro-radiologist. Their strategy
detected all cases of CD with positive response of ACTH and cortisol to dynamic
tests (CRH and desmopressin) and positive CEMRI, as well as in those cases of CD
with negative CEMRI, or detected all EAS with negative test response and
positive CT (and negative CEMRI) [35].
From a surgical perspective, the identification of a pituitary adenoma at CEMRI
is of utmost importance, even if in referral centers, the outcome of surgery is
not affected by adenoma identification [36]. In clinical practice, whichever sequences are considered, the
most important “soft skill” is the expertise of the neuroradiologist supported
by a pituitary multidisciplinary team. Most corticotroph tumors are
microadenomas (defined also as “picoadenomas”), and up to 50% of them are not
readily visualized using lower field strength (up to 1.5 Tesla), especially if
image acquisition is performed using 2–3 mm slice thickness with gaps between
consecutive slices [59]. The core
protocol of a pituitary CEMRI for the detection of corticotropinoma should
consist of coronal and sagittal T1-weighted spin-echo pre- and post-gadolinium
and coronal T2w fast (turbo) spin echo [59], acquired with 1-2 mm slice thickness and minimal spacing, using
a 3 Tesla field. Suggested additional supplementary sequences (ideally
immediately in the same session) are volumetric 1 mm T1w gadolinium-enhanced
3D-spoiled gradient echo (useful to provide better soft tissue contrast and
improve detection of smaller adenomas [60]) and dynamic T1w gadolinium-enhanced acquisition every
10–20 seconds over 1–2 min starting with contrast injection (some authors
reported that the high number of false positive images after dynamic CEMRI
results in reduced final diagnostic accuracy [61]).
Recently, several radioligands have been developed. Functional pituitary imaging
can confirm a suspected microadenoma or reveal a previously unsuspected one
[62]. Modern functional pituitary
imaging is predominantly performed using PET due to the limited spatial
resolution of gamma cameras, reflecting its superior spatial resolution compared
to SPECT. Hybrid imaging techniques co-registered with volumetric magnetic
resonance have been successfully used with 11C-methionine in patients
with newly diagnosed CD [63]. The
same cellular pathway (peptide synthesis via the l-type amino acid transporter)
has been further studied in 9 patients with 18F-fluoroethyltyrosine
[64], characterized by a longer
half-life (~110 min vs. 20 min) and therefore the possibility to transport the
radioligand out of the on-site cyclotron. The detection of the CRH receptor on
pituitary adenoma with a 68Ga-DOTA was studied in 24 patients with CD
(17 microadenoma, 10 with+<+6 mm adenoma) [65], and should be further
investigated in patients with negative CEMRI.
Differential diagnosis of adrenal hypercortisolism
The mild autonomous cortisol secretion (MACS) that characterizes 20-40% of
incidentally discovered adrenocortical adenomas, defined as abnormal cortisol
suppression (+>+50 nmol/L) after 1mg-DST [66], is probably the most frequent
form of hypercortisolism. The prevalence of cortisol-related comorbidities is
increased in patients with MACS [67],
leading to increased cardiovascular mortality, especially in women younger than
65 years [46]. Only the signs and
symptoms of overt CS are able to differentiate adrenal hypercortisolism in MACS
because 1-mg DST is positive (unsuppressed) by definition; therefore, clinical
expertise and additional biochemical tests to assess the degree of cortisol
secretion should be used in clinical practice. In this scenario, we reported
that UFC achieved the highest diagnostic accuracy in detecting CS in a patient
with adrenal incidentaloma [68].
A Unilateral adrenal adenoma is the main cause of ACTH-independent CS; it can be
associated with alterations of cAMP-dependent or β-catenin pathways [69]. PBMAH is rare and characterized
by multiple bilateral adrenal nodules+>+10 mm in diameter, usually sporadic
or sometimes familial [70]
[71]. Steroidogenesis is dysregulated
in PBMAH: the aberrant expression of ectopic and/or eutopic G-protein coupled
receptors combined with autocrine non-CRH-dependent ACTH secretion (gastric
inhibitory polypeptide, β-adrenergic ligands, 5-hydroxytryptamine, luteinizing
hormone , and antidiuretic hormone) [72] enables cortisol secretion. Recently, a genetic landscape of
PBMAH has been reported. Inactivating bi-allelic mutations (first a germline and
then somatic) of the ARMC5 gene (armadillo repeat containing 5) have been
identified either in familial or sporadic PBMAH cases [73]. Patients with ARMC5 mutations are
characterized by increased cortisol-related comorbidities (especially arterial
hypertension and diabetes mellitus) and meningiomas [74]
[75]. Another recent acquisition is the
discovery that a two-hit inactivation of KDM1A (a tumor suppressor gene member
of the lysine demethylase family involved in human tumorigenesis) explains the
GIP receptor upregulation in food-dependent PBMAH with CS [76]
[77].
Bilateral micronodular adrenal hypercortisolism is characterized by a nodule
diameter+<+10 mm. Familial cases are reported as part of a Carney complex
(CNC), a genetic syndrome characterized by endocrine tumors, atrial myxomas,
skin pigmentation anomalies, and peripheral nerve tumors. Isolated PPNAD and
Carney complex usually affect children and young adults and are characterized by
normal dimensioned adrenal glands with multiple nodular lesions. Germline
PRKAR1A gene mutations are detected in more than 80% of patients with CNC and
are transmitted with an autosomal dominant trait. Inactivating mutations cause
the haploinsufficiency of PRKAR1A, which encodes for the type 1α regulatory
subunit of PKA, leading to constitutive activation in cAMP/PKA signaling in the
affected tissues. Somatic mutations and loss of PRKAR1A locus have also been
identified in sporadic adrenal masses supporting a tumor suppressor role of
PRKAR1A in the adrenal cortex [78]
[79]
[80]. The HDDST leads to a paradoxical
increase in UFC in patients with PPNAD, with a high sensitivity [81].
The differential diagnosis of bilateral adrenal lesions is not always immediate.
First, endocrinologists must remember that ACTH-induced bilateral adrenal nodes
are detected in 37% of patients with ACTH-dependent CS, especially in older
patients with CD or a longer disease duration [82]. Furthermore, PRKACA somatic
mutations can be found in adrenal nodules of patients with CD [83]. Therefore, in the case of low
ACTH levels and bilateral adrenal nodes, a dynamic test (such as the CRH test)
is useful to assess adrenal autonomous CS [16]. CRH test has a predictive role in the management of patients
with PBMAH; an increase in ACTH+>+50% above baseline value was associated
with higher remission rates after unilateral adrenalectomy [84]. Specific dynamic tests can be
used to study the behavior of adrenal nodes further. In PBMAH, selective
provocative tests (with GnRH, TRH, desmopressin, metoclopramide, upright posture
test, oral glucose tolerance test or mixed meal) can suggest specific
treatments, such as octreotide LAR or propranolol in case of food-dependent CS
or positive postural test, respectively [85].
In subtyping bilateral adrenal lesions with imaging, each adrenal mass should be
assessed individually, considering that two different types of adrenal lesions
could be detected concomitantly [86].
Normal adrenal glands show an attenuation close to the liver on CT and have
homogeneous enhancement after contrast injection [87]. On unenhanced CT scans, adrenal
adenomas are characterized by attenuation values lower than 10 Hounsfield Units
(HU), secondary to a high lipid content [88]. However, almost 30% of adenomas are lipid-poor, thus presenting
HU+>+10, especially in the case of MACS [89]. MRI is a second-line imaging modality in the assessment of
adrenal lesions [87], and it is
fundamental when CT is contraindicated (pregnancy or allergy to iodine contrast
reagent) or in young people. In addition, MRI evaluates the fat content of
adrenal masses (called chemical shift), defined by a drop of signal on the
T1-weighted out-of-phase images compared with the T1-weighted in-phase images
(signal drop+>+16.5%) [90]. The
major role of metabolic imaging with fluorine-18 deoxyglucose positron emission
tomography/computed tomography (18-FDG PET/CT) regards the discrimination of
benign from malignant adrenal masses [91]
[92]. Regarding PBMAH,
18-FDG PET/CT value has not been assessed yet, but high standardized uptake
values have been reported, suggesting that cortisol-secreting masses have higher
FDG uptake than non-secreting lesions [93].
In opposition to its important role in primary aldosteronism, the use of adrenal
vein sampling in subtyping bilateral adrenal hypercortisolism has not proven to
be more accurate than conventional imaging. Given the need to use plasma
metanephrine as a marker for lateralization, it is associated with a consistent
risk of inadequate bilateral selectivity [94]
[95].
