Arachnoid cysts, Ependymal cysts, Benign intracranial developmental cysts

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 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, 7^th edition, Thieme