Intracranial Benign Developmental
Cysts
Arahnoid cysts and Ependymal cysts
It is very common observation in brain
imaging to find small cysts inside the brain. These cystic areas resemble and
look like areas of the brain with normal cerebrospinal fluid. Such incidental
finding increase anxiety in the person in whom brain imaging was usually done for some other purpose. Such cysts are
usually arachnoid cysts or ependymal cysts.
Arachnoid Cysts
Arachnoid cyst is a benign intracranial
developmental cyst which is between two split layers of arachnoid and it
usually contains clear colurless CSF.
So, arachnoid cyst is a cyst containing CSF
and it forms due to splitting of the archnoid membrane. These are benign
congenital malformation.
It is usually an incidental finding.
They are commonly seen in Sylvian fissure,
cerebellopontine angle, supracollicular area, vermian area, sellar and
suprasellar area, etc. This may appear at any age from infancy and adolescence
to adults.
Arachnoid cyst may remain asymptomatic
throughout life, only to be diagnosed incidentally by a neuroimaging study.
Imaging often shows remodelling of bone, and imaging characteristics exactly
mimic CSF on CT and MRI in most cases.
Symptoms and signs of arachnoid cyst depend
on its size and location inside the brain and spinal cord.
Recommendation for incidentally discovered
arachnoid cyst in adults: a single follow up imaging study in 6-8 months is usually
adequate to rule-out any increase in size. Subsequent studies only if
concerning symptoms develop.
Sylvian fissure archnoid
cyst may present with headache, seizures,
dysarthria (speech problem), focal bulge
in temporal region, exophthalmos, papilloedema, and hemiparesis. X-ray skull or CT scan may show evidence of
expansion of the middle cranial fossa, elevation of the lesser wing of the
sphenoid, forward protrusion of the greater wing the sphenoid bone and outward
expansion and thinning of the temporal bone. But expansion of skull in relation
to arachnoid cyst is not an indication for surgery , but mass effect,
displacement of midline structures and presence of obsructive hydrocephalus are
indications for surgical intervention for middle fossa arachnoid cysts.
On CT, arachnoid cyst appear as low
density, smooth bordered lesions having attenuation values similar to that of
CSF. The cyst wall has well-defined margins and does not enhance after
intravenous injection of contrast agent.
MRI is better at demonstrating multiplanar
relationship and characteristics of the lesion on T1, T2, FLAIR, Diffusion
weighted images and contrast, MTR ( Magnetization transfer ratio), MRS ( MR
spectroscopy), MRA ( MR angiography)study. It helps in differentiating arachnoid
cyst from epidermoid, dermoid, lipoma, ependymal cysts, tumors like low grade
glioma & metastasis, old hemorrhage, cavernoma, hydatid cyst, hemangioma,
and infective granulomas.
On T1 weighted image arachnoid cyst appears
hypointense and on T2 weighted image it appears hyperintense like CSF. On T1
weighted MRI , lipomas will appear
hyperintense as fat appears hyperintense on both T1 and T2 weighted images.
The high protein content of a
nonhemorrhagic tumour cyst will cause the cyst to appearslightly hyperintense
to CSF on proton density images. The associated peritumoural oedema of cystic
astrocytoma will look hyperintense on FLAIR image of MRI.
Ependymal cysts and epidermoid tumors
appear isointense or slightly hyperintense to CSF on proton-density images.
Epidermoid tumors are more likely to be lobulated, have less distinct margins,
and encase rather than displace neighbouring structures. Diffusion-weighted
imaging reflects the amount of Brownian motion of proteins which is greater in
cystic than in solid lesions.
Sylvian fissure is the most common site for
intracranial arachnoid cysts. Sylvian fissure arachnoid cyst may be of three
types. It may be a small lenticular lesion at the anterior pole of the middle
cranial fossa without any mass effect (Type 1) or quadrangular in shape
reflecting a completely open insula (Type II) .
Type III sylvian fissure arachnoid cyst presents as large rounded area
with significant compression of the brain. Displacement of the midline
structures in type III cysts is an indication for surgical decompression.
Treatment of archnoid cyst
Adults with asymptomatic arachnoid cyst should
be treated conservatively, even for large cysts without symptoms and signs or
with only a complaint of headache. Only arachchnoid cysts which cause a mass
effect or neurological deficit should be treated surgically.
In children, decompression of sylvian
fissure archnoid cyst is more likely to lead to decreased parenchymal
compression, cyst collapse, and subsequent resolution if intracranial
hypertension and neurological deficits.
Ventriculoperitoneal shunting
Cystoperitoneal shunting
Cyst fenestration
Cyst excision
Skull may be very thin and may be even
eggshell-like, so care must be taken in placing burrhole during surgery. The
dura is bluish because of presence a large pool of fluid underneath. The
exposed cyst wall may be clear and transparent; in some areas
a web of milky thickening may be noted as a result of collagen
reinforcement. The forntal lobe appears widely separated from the temporal lobe
because of the failure of opercula to develop. So, the insula and branches of
middle cerebral artery may be completely exposed after excision of the sylvian
fissure arachnoid cyst. When the outer wall of the cyst is excised, clear CSF
escapes. Long bridging veins may be observed either on the surface of the cyst
or within the cyst. Bridging veins that traverse the cavity of the cyst do not
have much support. Rupture of such unsupported veins account for high incidence
of subdural hematoma associated with these cysts.
Fenestration of deep wall of cyst creates a
communication between the sylvian fissure cyst and the chiasmatic cistern.
A significant number of middle cranial
fossa arachnoid cysts are associated with bleeding hematomas which are usually
venous in nature and result from tearing of bridging veins within or external
to the cyst. It may precipitate symptoms
in a previously asymptomatic patient.
Arachnoid cyst in sella
turcica
Sella turcica arachnoid cyst may be
intrasellar or suprasellar. Suprasellar cysts are by far the more common. It
may present with hydrocephalus, visual impairment, endocrine dysfunction (
hypopituitarism, stunted growth, etc), gait disturbance. A curious head nodding
motion described as the “ bobble-head doll syndrome” has been described in
suprasellar arachnoid cyst. The nodding or bobbing consists of irregular
involuntary head motions in the anteroposterior direction occurring two to
three times per second.The motion is reminiscent of that seen in dolls with a
weighted head resting on a coiled spring; hence the name of this syndrome. Some
degree of mental retardation is associated with this syndrome.
Treatment of suprasellar sella turcica arachnoid
cysts are ;endoscopic ventriculostomy with concomitant fenestration of lamina
terminalis, subfrontal cyst excision with communication to the basal cisterns,
and transcallosal or transventricular cyst excision with concomitant
cystoperitoneal shunting.
Treatment of intrasellar sella turcica arachnoid
cysts is trans-sphenoidal approach with packing of sella with fat or fascia or
muscle tissue.
Arachnoid cysts in Interhemispheric Fissure
Two types of arachnoid cysts occur near the
midline in the supratentorial space.
1.
Interhemisheric cysts with associated
partial or complete corpus callosal agenesis.It straddles the falx and extends
equally on either side, compressing the medial surface of both hemispheres. A
coronal MRI shows a “ bat-wing” appearance of the lateral horns and dorsal
displacement of the third ventricle.
2.
Parasagittal cysts are usually
not associated with agenesis of the corpus callosum. The cyst is strictly
unilateral and is sharply limited by falx in the midline, thus it tends to be
wedge shaped. There is a marked bulging of the frontal and parietal bones in
the parasagittal area. The superior sagittal sinus and falx cerebri are
considerably off the midline.
Cerebral convexity arachnoid cyst
In infants, it may present with progressive
asymmetrical enlargement of head. MRI findings may mimic subdural hygroma, but
without an enhancing membrane
In adults, the lesion may present with
seizures, headache , papilloedemaand progressive contralateral
hemiparesis. Skull films may show erosion of the inner table of the
skull. CT scan shows biconvex or semicircular area of lucency over the
convexity without an enhancing membrane. Surgical therapy consists of excision
of the outer membrane of the cyst.
Quadrigeminal cistern arachnoid cysts
These cysts behave like pineal masses and
present with hydrocephalus and Parinaud’s syndrome. Therapy consists of
excision of the cyst wallthrough an occipital transtenorial approach, with or
without insertion of cystoperitoneal shunt.
Cerebellopontne angle arachnoid cysts
Clinical presentation of arachnoid cyst in
CP angle may mimic that of an acoustic neuroma.
Posterior fossa arachnoid cysts
Posterior fossa arachnoid cysts may present
in the midline near the fourth ventricle or the cistrna magna, or paramedian
area opposite the cerebellar hemisphere. X-ray skull may show a focal expansion
of the occipital bone. Diffrential diagnosis of midline posterior fossa
arachnoid cyst include mega cistern magna, Dandy-Walker malformation,
epidermoid cyst, cystic glioma and hemangioblastoma.
Clival region arachnoid cysts
The clival region is uncommon site for
intracranial arachnoid cyst. Although termed clival , the cyst may extend into
the interpeduncular cistern or the cerebellopontine angle. The cyst displaces
the midbrain and pons dorsally along with basilar artery.The cranial nerves are
stretched , elongated , and draped around the cyst.
Other rare locations of arachnoid cysts are
intraventricular, diploic space, etc.
Diagrammatic representation of 3 types of Sylvian fissure arachnoid cyts
Diagrammatic representation of 3 types of Sylvian fissure arachnoid cyts
Diagrammatic representation of probable mechanism of formation of ependymal cyst
Two types of interhemisheric arachnoid cysts
Ependymal cysts
Ependymal cysts may mimic arachnoid cyst
clinically and on imaging studies. They occur much less frequently than
arachnoid cysts. They occur in central white matter of the frontal and
temoporopatrietal lobes, causing progressive neurological deficits, seizures
and features of raised intracranial pressure. The protein content of the cyst
fluid is generally greater than that of the CSF; on MRI the cyst will typically
appear isointense or slightly hyperintense to CSF on proton density images. The
wall is lined by columnar or cuboidal cells with or without cilia.
Blepharoplasts may or may not be identifiable , These cysts never communicate
with the ventricular system. They are believed to arise by the sequestration of
a small segment of the primitive ependymal lining into either the cortical
mantle or the perimedullary mesh . treatment consists of drainage of the cyst
and excision of its wall.
Sources
Neurosurgery, second edition, volume III
Editors: Robert H.Wilkins and Setti S.Rengachary, McGraw –Hill, Chapter 374;
Intracranial arachnoid and ependymal cysts by Setti S. Rengachary and Jerome
D.Kennedy, pages 3709-3728
Wikipedia
Handbook of neurosurgery , Mark S
Greenberg, 7th edition, Thieme
