These rare but interesting lesions comprise 1
per cent of all brain 'tumors'. The spectrum of developmental cysts in CNS
include, arachnoid cysts, ependymal cysts, colloid cysts, dermoid and
epidermoid cysts, Dandy walker cysts ,epithelial cysts, and porencephalic
cysts. They are mostly intracranial, but can occur intraspinally as well.
1) ARACHNOID CYSTS (leptomeningeal cysts):
They were referred by Richard Bright in 1831
as ‘serous cysts of the arachnoid’; he described cystic lesions with
liquid, clear, contents and intra-arachnoidal localization. About 127
years later, Starkman described the intra-arachnoid localization of these
cysts, giving an explanation for their etiology.
Arachnoid cysts correspond to about 1% of all
intracranial lesions in the general population and about 3% in the
pediatric population. Although 60-90% of reported cases have been found
in young people (less than 20 yeas old), it is possible to find them in
The sex distribution shows a preference for
the males, in the ratio of 3:1 and for left side of the CNS structures.
They are congenital lesions that probably
arise during development from splitting or duplication of this membrane.
The etiology of these lesions is extremely
Intra-arachnoid cyst theory theory suggests a
congenital origin for the etiology of the arachnoid cysts; at some time
during development, there is a duplication of the arachnoid membrane,
proliferation of the arachnoid cells, forming a cavity which later fills
with CSF through several mechanisms. This was defended by Starkman and
suggested by Bright in 1831. Frequently these cysts are associated with
venous anomalies and the developmental anomaly probably occurs between the
sixth and eighth week of fetal life, precisely when the vascular
structures begin their development.
Shreiber, suggested that most arachnoid cysts
were caused by incomplete canalization of the subarachnoid space with the
formation or pouch like spaces which become distended with CSF consequent
upon arterial pulsation.
Dott and Gillingham, also postulated that
such pouches usually form along the axis of a main cerebral artery, that
the leptomeningeal space at the distal boundary of a cistern becomes
occluded and that the fluid propelled into the proximal portion cannot
escape over the convexity of the brain.
The striking and nearly invariable
association of arachnoid cysts with normal subarachnoid cisterns has led
to hypothesize that arachnoid cysts represent a congenital anomaly of the
developing subarachnoid cisterns in early intrauterine life. It is
postulated that, during the process of the complex folding of the
primitive neural tube and the formation of normal subarachnoid cisterns,
an anomalous splitting of the arachnoid membrane occurs.
Cysts of the sellar region (supra, para and
intrasellar) are considered to be an extension of the Lillequist membrane,
which divides the chiasmatic cistern of the interpeduncular cistern, being
associated with alterations of the normal CSF flow.
Cysts of the posterior fossa are probably
caused by various degrees of obstruction of the foramens of Luschka or
Magendie. Intraventricular cysts are explained by mesenchymatous
invagination through the choroidal tissue that is formed by digitations of
pia-mater with arachnoidal stroma.
Spinal intradural cysts are related to the
septum posticum at a thoracic level, and thoracolumbar extradural cysts
are explained as invaginations of the arachnoid through small pre-existent
The fact that the arachnoid cysts occur
mainly in young people (less than 20 years old) with a strong incidence in
the first years of life as well as the occurrence of case reports with a
familiar incidence and the frequent association of these cysts with
diseases related to chromosome alterations (polycystic kidney,
neurofibromatosis, chromosome 12 trisomy) reinforce the possibility of a
congenital origin for these lesions.
Apart from the structural alterations there
is an alteration of the normal CSF flow, as confirmed in some cases, by
the persistence of symptoms in patients whose cysts were surgically
An arachnoid cyst is by definition, a
benign lesion, with well defined outlines, within the arachnoid
membrane or covered by layers of arachnoid cells supported by collagen
fibres, having liquid contents similar to CSF. It is a cavity, whose
walls are formed by arachnoid cells (simple or multiple layers),
supported by a stroma, rich in collagen fibres.
They can develop anywhere along the
cerebro-spinal axis, but almost all occur in relation to an arachnoid
The distribution of arachnoid cysts in
two hundred and eight reported cases is as follows:
Sylvian fissure, 49%; cerebellopontine
angle, 11%; supracollicular area, 10%; the vermis, 9%; sellar and
suprasellar area, 9%; interhemispheric fissure, 5%; cerebral
convexity, 4%; and the clival and interpeduncular area, 3%.
Lobulated arch.cyst(H&E)- lining of
cells supported by collagen fibres.
A lesser percentage occurs within the
ventricular system. Intrasellar cysts are the only intracranial arachnoid
cysts that are extradural. Intraspinal cysts are rarer.
The structural features of the arachnoid cyst
wall that distinguish it from the normal arachnoid membrane are
(1) splitting of the arachnoid
membrane at the margin of the cyst,
(2) a very thick layer of collagen
in the cyst wall,
(3) the absence of traversing
trabecular processes within the cyst, and
(4) the presence of hyperplastic
arachnoid cells in the cyst wall, which presumably participate in collagen
Most of the small arachnoid cysts do not
increase in size and can be asymptomatic, but the larger cysts can become
bigger and produce symptoms.
The possible reasons for the increase in
The active secretion through the cyst wall
inside the cavity
Unidirectional valve mechanism (ball valve)
with CSF flow from the sub-arachnoid space into the cyst
The active osmosis through the wall by a
similar mechanism to that which occurs at the Pacchionian granulations
Pulsations of the CSF and of the intracranial
arteries transmitted to the cyst cavity through a wide opening into the
Transudation through the wall cyst from the
choroids plexus (in the intraventricular cysts).
It is not known which mechanism is
responsible for the increase of the cyst volume, which is probably
multifactorial. However in intraventricular cysts related to the choroid
plexus, the principal mechanism is the transudation through the cyst wall
from plexus secretion. For cysts that are in communication with the sub-arachnoid
space, the principal mechanism is likely to be the ‘bell valve’ effect and
in non-communicating cysts, active secretion mechanism or osmosis may be
A classification based on cisternography with
metrizamide and CT isotopic cisternography and, more recently cine MRI,
defines the existence of communication with the subarachnoid space and the
cavity of the cyst. The cyst can then be as slowly or rapidly
communicating or non communicating (real cysts).
A classification based on etiology divides
the cysts into primary (congenital) or secondary (traumatic or infectious
Finally a classification which is based on
the morphology, volume and effect on the CNS structures and bone structure
as suggested by Galassi divides the cysts into 3 types. Type 1 are small
asymptomatic cysts, type II have some mass effect, with bone erosion and
type III have deformation of large areas of the CNS and gross bony
In many instances, an arachnoid cyst is an
incidental finding. The clinical presentation will depend on the location
and size of the
achnoid cyst, and the symptoms often are mild
considering the large size of some cysts. Most patients will come to
medical attention in the first two decades of life, often in the first 6
These lesions cause symptoms and signs of
increased ICP by compressing the normal tissue and obstructing the CSF
The symptoms depend on the cyst localization.
Symptoms and signs include cranial enlargement, localized cranial bulging,
especially the large cysts, can present acutely with sudden deterioration.
Suprasellar cysts may also present with
endocrine symptoms, head bobbing, and visual disturbances. Either
communicating or obstructive hydrocephalus is often present. In the
elderly, dementia has been described.
Intraspinal cysts may produce a tetra or para
paresis, with abnormal reflexes, sphincter dysfunction, sensibility
alterations and radicular pain according to the level of the lesion.
Arachnoid cysts are usually diagnosed by CT
or MRI. Further evaluation with CSF contrast flow studies is only
occasionally necessary for the diagnosis of midline suprasellar and
posterior fossa lesions.
CT shows a smoothly bordered noncalcified
extraparenchymal cystic mass with density similar to CSF and no contrast
enhancement and also the bony remodeling.
MRI may show the arachnoid membrane and also
differentiates the CSF in the arachnoid cysts from the neoplastic cysts
and ependymal ( usually, intraparenchymal) cysts. Porencephalic cyst
usually communicate into the ventricle. In addition associated cerebral
and cerebellar hypoplasias are well studied.
Deep invagination of an arachnoid cyst into
the cerebral hemisphere may simulate porencephaly to such an extent that
it has been termed pseudoporencephaly. However, the inferior aspect of the
arachnoid cyst shows a displaced but otherwise normal cerebral cortex,
while in porencephaly, the surrounding cortex and white matter are
The cysts, that do not cause mass effect and
have not changed in size need not be treated.
Many procedures for treatment of arachnoid
cysts have been proposed. Surgical options for arachnoid cysts include
drainage by needle aspiration, craniotomv with excision of the wall and
fenestration into the basal cisterns (open or endoscopically) with or
without a silastic shunt, and shunting of the cyst into the peritoneum or
The Fenestration and shunting are the two
most common options.
Shunting of the cyst material into the
peritoneum or into the vascular system is associated with low morbidity
and mortality and a relatively low rate of recurrence, but the patient
becomes shunt dependant. Long-term complications of a shunt occur in more
than one third of cases. Low pressure shunt is preferred.
Craniotomy with excision of the cyst wall and
fenestration into the basal cisterns permits direct inspection of the cyst
and avoids placement of a permanent shunt in some cases. However, it is
associated with reaccumulation of CSF at the cyst site. In addition,
significant morbidity and mortality may accompany; abrupt displacement of
brain structures following the rapid decompression that accounts for the
unexpected rapid deterioration.
Endoscopic approaches have been used with
good results, although the follow-up period has not been long. The goal of
fenestration is free communication with the basal cisterns.
Shunting the associated hydrocephalus alone
will only worsen the symptoms, and may increase the size of the arachnoid
Arachnoid cysts are benign lesions, with a
poorly defined natural history, sometimes with spontaneous disappearance.
Regression of cyst volume post-surgery,
occurs independently of the surgical technique. Sometimes the cyst volume
remains constant, leaving the patient without symptoms. Usually there is
a good clinical and imaging correlation of regression.
For the suprasellar cysts with hydrocephalus,
it is common to have persistent ventricular dilatation in spite of the
cyst having decreased in its volume, with normal intracranial pressure.
This has no consequences in the intellectual development of the patients.
For the sellar cysts it is also common to have persistence of
endocrinological alterations, even with cyst disappearance.
Arachnoid Cysts by location:
Sylvian fissure/Middle cranial fossa cysts:
The sylvian fissure is the most common site
for arachnoid cysts (50% of adult cases and 30% of pediatric cases). They
are usually small or medium, but they can become quite large and open up
the fissure to expose the insula and middle cerebral branches. They
usually manifest clinically in children or adolescents, but can present at
any age. Males predominate and the left hemisphere is more commonly
Global headache, seldom severe, is common.
Macrocephaly or an asymmetric macrocrania is often the presentation in the
Children may present with developmental delay. Acute symptoms of increased
ICP, hemiparesis, or seizures are common.
On CT and MRI, the large cysts extend
posteriorly and open up the sylvian fissure. The temporal lobe may be
underdeveloped. The greater wing of the sphenoid may be displaced
anteriorly and the lesser wing may be elevated. In most cases, the
associated hydrocephalus is due to compression of the 3rd ventricle.
Treatment is either shunting the cyst or
fenestration of the cyst into a cistern or both combined. Symptoms
typically improve after successful treatment. Results are poor when
behavioral abnormalities and mental retardation are present.
Suprasellar location accounts for
approximately 10% of arachnoid cysts and less than 1% of intracranial mass
lesions. Cysts in the suprasellar region appear to arise from the
suprasellar cistern. It has been suggested that the cysts may arise from
an imperforate membrane of Lillequist.
Typically, most patients are adolescents, and
present with hydrocephalus. Adults present with visual problems.
Hypopituitarism, especially the growth hormone deficiency, can occur.
Rarely, there may be ‘bobble head-doll’
syndrome. They experience an involuntary two or three times per second
vertical bobbing of the head, with compensatory horizontal movements of
the trunk, attributed to the abnormal pressure exerted by the cyst on the
3rd ventricle and on the dorsomedial nucleus of the thalamus.
On CT and MRI images, a suprasellar arachnoid
cyst appears as a smooth, oval or round lesion in the region of the third
Multiple approaches to suprasellar cyst
fenestration have been attempted, including transfrontal removal of the
anterior portion of the cyst, a transcallosal approach to communicate the
cavity of the cyst into the ventricular system, and insertion of catheters
between the cyst and ventricle or chiasmatic cisterns. Any one of these
operations has a poor chance of prolonged success and may fail to decrease
either the size of the cyst or the ventricular system.
Shunting the associated hydrocephalus will
only worsen the symptoms, and may increase the size of the arachnoid cyst.
A combination of cyst fenestration and ventricular shunting may be more
Convexity and Interhemispheric fissure cysts:
Convexity lesions differ from arachnoid cysts
in other locations due to their lack of contact with cisternal spaces.
Determining the origin of interhmispheric cysts can be difficult. Imaging
of the corpus callosum, third ventricle, and collicular plate is
essential, because the third ventricle can herniate into the
interhemispheric fissure, mimicking an arachnoid cyst.
Usually, they are incidental findings. Adults
can present with raised ICP, epilepsy, and focal neurological deficit.
Children may present with localized macrocrania.
CTand MRI demonstrate rounded lesions with
CSF density, overlying the cerebral cortex or interhemisheric area.
Typically, they are treated with cysto-peritoneal
shunting, often with excision of the outer cyst wall.
Most of these cysts have been detected in
patients aged less than 15 years. Symptoms frequently arise from
hydrocephalus secondary to compression of the posterior third ventricle or
Quadrigeminal plate arachnoid cysts:
Pupillary reactivity or eye movements are
disturbed due to compression of the quadrigeminal plate or stretching of
the 4th nerve.
CT and MRI show an ovoid collection
posterior to the 3rd ventricle.
Definitive surgical treatment can be
difficult because the region is not easily accessible. Fenestration
followed by shunting may have a better success. Recurrence is high.
Intraventricular arachnoid cysts:
The differential diagnosis includes
ependymal cyst, epidermoid cyst, dermoid cyst, infectious cyst, and
MRI is the definitive study. Asymptomatic
cavum septi pellucidi, and septum cavum veli
interpositi may be considered normal
Simple drainage is usually followed by
reaccumulation. Endoscopic fenestration, craniotomy and cyst excision,
cyst-peritoneal shunting are the surgical options.
Posterior fossa archnoid cysts:
They are less common, about 25% of all
intracranial cysts and usually occur in the midline, superficial to the
vermis or in the CP angle. Less frequent locations include the cerebellar
convexities, or pre pontine areas. The posterior fossa is a common site of
benign intracranial cysts, especially in children.
The differential diagnosis of a posterior
fossa include, arachnoid cysts, Dandy-Walker malformation, and mega
Post. fossa arch. cyst- MRI
Mega cisterna magna-MRI
An arachnoid cyst results in anterior
displacement of the fourth ventricle, but normal cerebellar
Dandy-Walker malformation is a cystic
dilatation of the fourth ventricle or a cyst in communication with 4th
Mega cisterna magna is an anatomic
variant with normal fourth ventricle and small cerebellum.
Most pediatric patients present with macro
crania. In adults, there are intermittent symptoms or influenced by
posture. CP angle lesions can cause tinnitus or hearing loss.
Fenestration, cyst shunting, and combination
of shunting and fenestration are the treatment options. A ventricular
shunting is often necessary, as the hydrocephalus often fails to resolve
with cyst decompression. Deeper cysts are often multiloculated, and
probably would not respond to shunting and are usually treated with
Spinal arachnoid cysts:
Arachnoid cysts also occur within the
spinal canal, in which arachnoid cysts or arachnoid diverticula may be
located subdurally or in the epidural space, respectively. They are
rarer than intracranial cysts. Extramedullary location is common. They
are mostly congenital in origin.
Typically, spinal arachnoid cysts
occur at the midthoracic level and, less frequently, at the
lumbosacral or sacral level. Commonly located dorsal to the cord. A
cyst in this location is usually secondary to a congenital or acquired
defect and is situated in an extradural location. Intradural spinal
arachnoid cysts are secondary to a congenital deficiency within the
arachnoid or are the result of adhesions resulting from previous
infection or trauma.
Spinal Archnoid cyst
They cause symptoms indistinguishable from
cord compression due to other causes. Patients with spinal arachnoid cysts
may become symptomatic as a result of local cord displacement or cord
compression. Epidural arachnoid cysts often are associated with
kyphoscoliosis in juveniles. Arachnoid cysts also are associated with
myelodysplasia in spinal dysraphic lesions. Pain produced by intraspinal
arachnoid cysts typically is aggravated by the Valsalva maneuver, which
increases pressure within the cyst.
Remission of symptoms is not uncommon.
Surgical excision is curative.
Tarlov cysts (Spinal perineurial cyst):
Many consider this a degenerative cyst and
Tarlov described this cyst in 1938, while
conducting an anatomic study of the filum terminale. It is a cystic
dilatation of the subarachnoid space around a nerve root (between the
perineurium of the nerve root and the outer surface of its pia). The cyst
can dissect into the nerve and can contain nerve fibers within it. These
cysts typically involve the sacral nerve roots. It is found on as many as
5% of lumbosacral MRI.
It is usually diagnosed incidentally.
Approximately, 4 out of 5, are asymptomatic. Some may present with
radicular pain, frequently occurring in attacks with pain free
intervals. Many patients get relief while by assuming Trendelenberg
position in which the patient is on an elevated and inclined plane,
usually about 45°, with the head down and legs and feet over the edge of
the table. Incontinence.& pain on moving the sacrum are seen in some
Although a spinal perineurial cyst involves a
single nerve root at first, it may enlarge to the point where it compress
adjacent nerve roots as well.
It is difficult to prove that Tarlov cysts
cause symptoms in many cases because other findings that can cause these
symptoms (disc herniation, stenosis.) are usually found along with the
cyst on MR images or at surgery.
Surgery indicated only if they cause
progressive or disabling symptoms.
Percutaneous CT-guided drainage provides good
relief for several months in most patients but cysts recurs in almost all.
Although the cysts re-pressurize and the patients' symptoms returned in
most cases, this technique seems to be a quick and simple way of at least
attaining a pain-free interval and possibly a complete cure in some
Sacral laminectomy with microsurgical cyst
fenestration and closure with reinforced epidural fat or muscle grafts and
fibrin glue application is widely employed; lumbar drains for
cerebrospinal fluid diversion for several days postoperatively has also
Secondary (False) arachnoid cysts:
Arachnoid cysts that are not congenital have
been termed secondary or false cysts. I feel a brief mention of these
acquired cysts is appropriate in this section of developmental cysts.
Such cysts represent accumulations of CSF
resulting from postinflammatory loculation of the subarachnoid space in
patients with head injury, infection, or ICH. The cyst membrane may also
be composed of arachnoid, but in addition, inflammatory cells and
hemosiderin deposits may be present and it is difficult to fenestrate
Lepto meningeal cysts associated with growing
fractures in children are rare. The physiologic brain growth and CSF
pulsations contribute to cyst herniations through a dural rent. Treatment
includes primary repair of the dural rent and the bony defect.
2) EPENDYMAL CYSTS:
These rare cysts arise due to the
inclusion of ependymal cells in the substance of the brain. They may
also originate from ectopic glial tissue present in the subarachnoid
space. Ependymal cysts are common in oral-facial-digital syndromes.
These later take up a secretory function
resulting in the formation of a cyst. These are intracerebral and
usually adjacent to the ventricles with which they may occasionally
The gross appearance is usually
indistinguishable from an arachnoid cyst. Microscopically, the cyst
may be lined by cells resembling ependyma in some places. They may
bear cilia. Ultrastructural studies confirm the neuroepithelial origin
of these cysts.
Frontal ependymal cyst
Colloid cysts of the third ventricle have also been termed
neuroepithelial cysts. Cysts in the fourth ventricle which were lined by
cells similar to ependymal cells and cuboidal cells resembling the
epithelium of the choroid plexus have been reported. Similar choroid
plexus cysts may also occur in the third ventricle and block the foramen
Excision is required in only the symptomatic
cases. The rest may be followed up periodically.
RATHKE'S CLEFT CYSTS:
These cysts were first described by Martin
Heinrich Rathke (1793–1860), a German anatomist. Rathke's cleft cysts are
benign, nonneoplastic lesions that are believed to be remnants of Rathke's
pouch, which is the superiorly directed evagination from the stomadium of
the 4 week old human embryo; all but the cranial portion of the pouch
becomes obliterated by week 7 of gestation. The anterior wall of the
remaining cavity becomes the anterior pituitary, while the posterior wall
becomes the pars intermedia of the gland.
Rathke’s cleft cysts are primarily
intrasellar and are found in 13% to 23% of postmortem examinations.
Rathke's cleft cysts are uniloculate and
thin-walled and contain watery to mucinous fluid. Light microscopy of the
lining shows goblet, ciliated and, to a lesser extent, secretory cells of
anterior pituitary type. The finding of ciliated epithelial and mucous
secreting cells in a pituitary gland are pathgnomonic for Rathke's cleft
Radiologically, they may mimic
craniopharyngiomas or as cystic pituitary adenoma.
Plain skull radiographs do not, usually,
reveal a enlarged sella turcica. In patients with symptomatic Rathke's
cleft cysts , plain skull radiographs commonly demonstrate findings of an
abnormally configured sella, which varies from slight asymmetry of the
sellar floor to massive erosion. In some patients, intrasellar and/or
suprasellar calcification is observed.
CT reveals an hypodense cystic mass lesions
arising from the pituitary fossa, without enhancement.
MRI shows cystic lesions with long T1 and
long T2 although sometimes intrinsic paramagnetic substances may produce
T1 and T2 shortening resulting in hyperintense T1 and hypointense T2
lesions. Rathke's cleft cysts usually have a thin wall that may enhance
with gadolinium-based contrast material. Variability in the gadolinium
enhancement among individual cysts may reflect squamous metaplasia in the
wall or a peripherally displaced rim of pituitary tissue.
Rathke's cleft cysts almost always are
homogeneous in signal intensity, whereas other lesions, such as cystic
craniopharyngiomas and hemorrhagic adenomas, more frequently have
heterogeneous signal intensity.
These cysts are rarely symptomatic and are
usually incidental findings on imaging studies. Occasionally, they cause
symptoms by causing pressure on adjacent structures such as the optic
nerves and the pituitary gland causing visual problems, loss of pituitary
function, hypothalamic dysfunction and headache when they grow in the
suprasellar space. A Rathke's cyst may occasionally be associated with a
pituitary adenoma. Partial excision of the cyst wall and drainage should
result in cure: only a small percentage of such cysts recur.
4) PORENCEPHALIC CYSTS:
More appropriate term may be 'hole in the
Porencephalic cysts, characteristically,
communicate with the ventricles or subarachnoid space and are covered
on the outside by arachnoid.
These are congenital (primary)
intracranial cysts and may arise as a leptomeningeal cyst.
It is possible that a failure of
development of a part of the cerebral mantle may result in a cyst.
In addition to their congenital origin,
porencephalic cysts may also arise as a result of trauma specially
during birth or infancy when following loss of cerebral tissue
adjacent to the ventricles a cyst forms and communicates with the
ipsilateral dilated ventricle.
Puncture porencephaly is the development
of a cystic cavitation along the track of a ventricular needle,
manifesting in course of time, following rise in intracranial pressure
due to nonfunctioning of the shunt in cases of hydrocephalus.
This may also follow prolonged ventricular
drainage or repeated ventricular punctures.
Surgical excision may be considered in
cases where these cysts are found to be the cause of intractable epilepsy.
The area of excision should include the surrounding gliosed cerebral
tissue as well.
5) Other developmental cysts are
Colloid cyst of the third
ventricle, Dandy Walker cyst,
and Epidermoids, Dermoids, &