Introduction
Arachnoid cysts are the most common cystic congenital abnormality of the brain.[1] They comprise 1% of intracranial masses[1] and are found in approximately 1.7% of the adult population.[2] Studies of their true prevalence in the general population must be made carefully
because they carry a selection bias toward those patients undergoing neuraxis imaging.
Autopsy studies demonstrated a lower prevalence of cysts at 0.1% of individuals.[3] As pathologic entities, they are more frequently diagnosed in the pediatric population
in which their prevalence is 2.6%.[4] A male predominance (male-to-female ratio: 2:1) is noted in both adults and children.[2]
[4]
[5] In this study also, there is a slight male predominance (adult males [6] to females
[5] is 1.2:1) (male children [3] to female children [2] is 1.5:1). Most arachnoid
cysts appear to be congenital anomalies.[4] They are usually discovered during workup for headache, increasing head circumference,
or developmental delay, or after trauma.[4] In this study, seven (43.75%) patients had headache, one (6.25%) had hydrocephalus,
and two (12.5%) had developmental delay. Ten (62.5%) patients were discovered during
workup for headache, increasing head circumference, and developmental delay.[4] Rarely, they cause weakness and cranial neuropathies or have been associated with
seizures and psychiatric disorders.[6]
[7]
[8]
[9] In this study, five (31.25%) patients had seizures, one (6.25%) had weakness, and
one (6.25%) had neuropsychiatric manifestation. In this study, other symptoms that
led to the diagnosis of arachnoid cyst are numbness of limbs in one (6.25%) patient,
visual obscuration in one (6.25%), and hard of hearing in one (6.25%). Arachnoid cysts
secondary to trauma, hemorrhage, or meningitis have also been described.[10]
[11]
Epidemiology
Retrospective studies of arachnoid cyst prevalence do not distinguish between congenital
and acquired arachnoid cysts, but acquired cysts are thought to represent a minority
of cases. In the era before computed tomography (CT) and magnetic resonance imaging
(MRI), a literature review demonstrated 49% of cysts in the middle fossa or sylvian
fissure; in the cerebellopontine (CP) angle, 11%; quadrigeminal plate area, 10%; vermis,
9%; suprasellar area, 9%; inter-hemispheric fissure, 5%; cerebral convexity, 4%; and
interpeduncular area, 3%.[12] More recent prevalence studies have shown a slightly different distribution.[2]
[4] Among adults, middle fossa and retrocerebellar cysts show roughly the same prevalence
(34% and 33%, respectively). Although middle fossa or sylvian fissure cysts remain
the most common in all age groups, it is possible that some of the differences in
prevalence reflect the likelihood of cysts in each location to cause symptoms. In
their study of adults with radiographically diagnosed arachnoid cysts, Al-Holou and
colleagues found a statistically significant increased rate of symptomatic cysts for
cysts located in the CP angle, quadrigeminal cistern, sellar, and ambient cisterns.[2] Middle fossa cysts in adults were associated with a significantly lower rate of
symptoms.[2] In this study, 7 out of 16 patients with symptoms had middle fossa cyst (43.75%),
which is contrary to the study by Al-Holou et al in which middle fossa cysts were
associated with lower rate of symptoms. Of these seven, four were adults. Two were
male and two were female.
Pathology
The most general definition of arachnoid cyst implies a locculated cavity within the
arachnoid mater without discrimination of the wall or cyst content. This definition
allows for a variety of etiologies. As proposed by some,[5] a more precise definition would exclude entities that have an appearance similar
to arachnoid cysts, such as leptomeningeal cysts secondary to infection or trauma,
porencephalic cysts, and neuroepithelial cysts. This definition makes some presumptions
on the etiology of congenital cysts but also highlights important confounding factors
in the diagnosis of arachnoid cysts.
Using a somewhat broader definition, a recent pathologic analysis of a series of arachnoid
cysts demonstrated varying cyst wall composition. Although most cysts had walls of
normal arachnoid cells, several walls demonstrated fibrosis and some demonstrated
microvilli and cilia.[13] This suggests the possibility that there may be more than one patho physiologic
origin. From a practical perspective, because only select cases undergo operation
and pathologic examination, it may not be possible to distinguish between all types
of cysts arising from the arachnoid. In this study, four patients underwent marsupialization,
and histopathologic examination (HPE) was conclusive of arachnoid cyst in all four
cases.
A recent study analyzing the fluid contained within a series of middle fossa cysts
determined that arachnoid cyst contents are very similar, but not identical, to cerebrospinal
fluid (CSF). The contents have some unique characteristics, particularly with regard
to protein, lactate dehydrogenase, and phosphate concentration.[14]
[15] In this study, one patient underwent cystoperitoneal shunt and one underwent ventriculoperitoneal
(VP) shunt. However, owing to logistical difficulties, lactate dehydrogenase and phosphate
concentrations could not be assayed.
Pathogenesis
Owing to a wide range of presenting symptoms, anatomic locations, age at presentation,
and comorbidities, the possible etiologies for arachnoid cysts are several. Because
most surgical cases present in the pediatric population, a congenital or genetic etiology
is hypothesized. Evidence for a genetic cause is growing. Certain hereditary syndromes,
such as Marfan's syndrome, neurofibromatosis, glutamic aciduria type I, autosomal
dominant polycystic kidney disease, and tuberous sclerosis, have demonstrated a higher
incidence of arachnoid cysts than the general population. A clearer genetic linkage
was recently described in a consanguineous family with a high prevalence of arachnoid
cysts. The authors found an autosomal recessive inheritance of the familial arachnoid
cysts that linked to chromosome 6q22.31–23.2. A pattern in the location or size of
the cysts did not appear.
Molecular Biology and Genetics
The prevailing theory is that cystogenesis is brought about by a splitting or duplication
of arachnoid membrane during embryogenesis. Cyst fluid may thereafter accumulate either
by active water transport across the cyst membrane by cells lining the cystic cavity
or by a one-way mechanical valve. In both cases, net flow of water is into the arachnoid
cyst. There are several features of arachnoid cyst that are not fully explained by
these theories alone.
These features include a significant predilection for middle cranial fossa (in this
study 7 out of 16 had middle cranial fossa arachnoid cyst), a significant male preponderance
(9 males out of 16 patients), and the left sidedness for cysts in this location (6
out of 7 middle fossa arachnoid cysts were left sided). For cysts in CP angle, there
is female preponderance (both patients with CP angle arachnoid cyst were female in
this study) and right sidedness (one was right side and one was left sided in this
study). These peculiarities and several reports documenting a familial occurrence
of arachnoid cyst, either as a separate entity or coexisting with other hereditary
disorders, indicate a genetic mechanism underlying the development of arachnoid cyst.
Owing to congenital nature of most intracranial arachnoid cysts, it is possible that
altered gene expression in neural crest cells at the time of leptomeningeal development
may contribute to cyst formation.
There is a limited amount of research on molecular biology of arachnoid cyst. In their
publications, Go et al demonstrated that the morphologic features of the cells lining
arachnoid cyst were consistent with fluid secretion capacity. Moreover, enzyme cytochemistry
demonstrated a structural organization of (Na+-K+)-ATPase and alkaline phosphatase indicating fluid transport toward the lumen.
In “pure” arachnoid cyst family, that is, a family with familial arachnoid cyst occurrence
without association to any other syndromes, genome-wide linkage analysis localized
the linkage interval to chromosome 6q22.31–23.2. Other mutations described in connection
with arachnoid cyst include SOX2 mutations and GPSM2, as well as other gene alterations.
Investigations of gene expression profiles in arachnoid cyst tissue have further identified
potential candidate genes underlying cyst formation such as SHROOM3 and SOX9. The differential expression of these genes might be important for the development
of arachnoid cyst, but owing to low number of cases studied, these findings must be
further studied in model systems and replication studies containing more samples before
firm conclusions can be made about causality in cyst development.
Age Distribution
In this observational study over 6 months, the authors encountered 16 patients with
arachnoid cysts. In this study, prevalence in pediatrics is 31.25% (5) and in adults
is 68.75% (11).
[Table 1] shows prevalence of arachnoid cysts in various locations.
Table 1
Location and prevalence
|
Location
|
Recent studies (%)
|
This study
|
|
Middle fossa cyst
|
34
|
43.75% (7)
|
|
Retrocerebellar cyst
|
33
|
25.00% (4)
|
|
Cerebellopontine (CP) angle cyst
|
11
|
12.50% (2)
|
|
Convexity cyst
|
4
|
18.75% (3)
|
Clinical Presentation
The presenting symptoms for nonincidental pediatric arachnoid cysts are similar to
other childhood mass lesions. Progressive macrocephaly, intracranial hypertension,
headache, hydrocephalus, and developmental delay were the most common indications
for workup leading to treatment of the arachnoid cyst. In adults, headache was the
most common complaint found in patients considered to have symptomatic arachnoid cysts,
but hydrocephalus, ataxia, vertigo, hearing loss, and seizures were also common complaints
in this population.
Obstructive hydrocephalus was the presenting feature (6.25%) in a 2-year-old female
child with right CP angle arachnoid cyst that was managed with shunt surgery ([Fig. 1]).
Fig. 1 Axial section of T1-weighted MRI of right CP angle arachnoid cyst.
Global developmental delay (12.5%) was the presenting feature in an 8-year-old male
child and a 30-year-old man (this patient had neuropsychiatric manifestations in addition
to global developmental delay).
An 8-year-old boy had left temporal type III arachnoid cyst ([Fig. 2]). He was managed conservatively.
Fig. 2 Left temporal Galassi type lll arachnoid cyst.
A 30-year-old male patient with global developmental delay and neuropsychiatric manifestations
had posterior fossa arachnoid cyst ([Fig. 3]), which was managed conservatively.
Fig. 3 CT of the brain showing posterior fossa arachnoid cyst.
Seizure was the presenting symptom in two male children, one female child, and two
adult male, a total of five (31.25%) patients.
One pediatric patient (7 years) had right temporal arachnoid cysts and was managed
conservatively.
A 10-year-old boy with seizure had left temporal type II cyst. His seizures stopped
following marsupialization of arachnoid cyst. One pediatric patient with obstructive
hydrocephalus and seizure underwent shunt surgery.
A 54-year-old man with seizures had posterior fossa arachnoid cyst and was managed
conservatively.
One patient (34 years, male), who presented with giddiness (6.25%) and right-sided
weakness, had posterior fossa arachnoid cyst. He underwent cystoperitoneal shunt and
his symptoms improved.
Seven patients had headache (43.75%), and arachnoid cyst was detected during evaluation
for headache. Four of these were managed conservatively.
A 35-year-old woman with right high parietal arachnoid cyst ([Fig. 4]) was managed conservatively.
Fig. 4 CT of the brain showing right high parietal arachnoid cyst.
A 58-year-old woman with left temporal arachnoid cyst was managed conservatively.
A 23-year-old man with left temporal arachnoid cyst was managed conservatively.
A 40 years old man with right temporal arachnoid cyst was managed conservatively.
Six patients underwent surgery.
A 37-year-old woman with headache had left temporal arachnoid cyst and underwent marsupialization.
A 47-year-old woman with headache, who had left frontal arachnoid cyst, underwent
marsupialization.
A 10-year-old man with left temporal Galassi type II arachnoid cyst underwent marsupialization.
A 28-year-old woman with headache had left cp angle arachnoid cyst ([Fig. 5]) and underwent left retromastoid suboccipital craniectomy and marsupialization.
Fig. 5 CT of the brain showing left CP angle arachnoid cyst.
A 2-year-old female child with right cp angle arachnoid cyst ([Fig. 1]) and obstructive hydrocephalus was managed with shunt surgery.
A 34-year-old man with posterior fossa arachnoid cyst underwent cystoperitoneal shunt
and his symptoms improved.
Management
Ten patients were managed conservatively (62.50%). Six patients underwent surgery
(37.50%)—four underwent marsupialization, one underwent cystoperitoneal shunt, and
one underwent VP shunt. In this study, patients with arachnoid cyst of size larger
than 4 cm, cysts that produced mass effect and midline shift, and cysts that caused
seizures, were marsupialized. Patient with arachnoid cysts that caused hydrocephalus
underwent shunt surgery, and patient with arachnoid cyst of size larger than 8 cm,
which caused mass effect over brainstem, underwent cystoperitoneal shunt. All the
patients who underwent surgeries were relieved of their symptoms, whereas those managed
conservatively are being followed up regularly and their symptoms are under control
with medical management over the past 6 months.
