Ventricular diverticulum in posterior fossa

Clinical History

A 75-old-female patient presented with mild memory impairment, abnormal gait, decreased mobility and increased falls. Her neurological examination revealed left sided intention tremor and dysdiadochokinesia more prominent on the left side. Her motor and sensory examination was normal. Her mini-mental state examination (MMSE) was 21/30.

Imaging Findings

A non-contrast CT scan of the brain showed an infratentorial 3 x 3 x 3.5 cm cystic area just posterior to the fourth ventricle and above the superior aspect of the cerebellar vermis. The lateral and third ventricles were very dilated in keeping with severe hydrocephalus associated with prominent fourth ventricle and a patent aqueduct. Multiplanar reconstruction showed a possible communication of the cyst with the atrium of right lateral ventricle.
An MRI scan showed a posterior fossa cystic abnormality, in the infra-tentorial compartment, posterior to the quadrigeminal plate. It was noted to cause significant compression of the cerebellar hemispheres and the superior vermis and was seen extending up to the superior medullary velum. MRI also confirmed communication of this cystic structure via the tentorial hiatus, to the atrium of right lateral ventricle through the choroid fissure. This feature confirmed the structure to be a large atrial diverticulum.

Discussion

Ventricular diverticulum has been described for many years although less often remembered and correctly recognised on imaging. These are focal dilatations of ventricular system resulting from longstanding, severe obstructive hydrocephalus. Occasionally it is described in association with communicating hydrocephalus.
The raised intraventricular pressure increases the ventricular wall tension and weakens its surface. The affected area, between the choroid plexus and fornix may stretch, dilate and herniate into the subarachnoid space, thus forming the diverticulum. It most commonly originates from the inferomedial wall of the atrium where the lumen is usually widest and the distance separating the ependymal surface from the pial surface is short [1-2]. Subsequently it herniates through the tentorial incissura into the superior cerebellar and quadrigeminal plate cisterns and can cause compression of the tectal plate and cerebellum, and can clinically present as ataxia. The commonest differential diagnosis includes a posterior fossa arachnoid cyst and other cystic lesions of posterior fossa.
On CT, the communication with the lateral ventricle is often difficult to see as the neck of diverticulum can be quite narrow, but it is usually well identified on MRI. Invasive techniques such as ventriculography are rarely required [3].
Correct identification of the ventricular diverticulum is of great therapeutic importance as it is usually the result rather than the cause of hydrocephalus and tends to resolve with shunting or correction of the primary cause.
In this particular case, following neurosurgical assessment, a decision was made to pursue a conservative approach considering her clinical state and other general co-morbidities.
Ventricular diverticulum is a focal dilatation of the ventricular system resulting from longstanding, severe obstructive hydrocephalus and it is most commonly mistaken as an arachnoid cyst, the management of which is different. The current report is intended as a reminder of this condition.

Differential Diagnosis List

Final Diagnosis

Ventricular diverticulum

URL:	https://www.eurorad.org/case/9178
DOI:	10.1594/EURORAD/CASE.9178
ISSN:	1563-4086