NEURORADIOLOGY

Discussion
Interhemispheric dorsal cyst is usually associated with anomalies such as agenesis of corpus callosum, holoprocencepaly and Dandy Walker malformation

Callosal agenesis results in several anatomical features that are distinct from the normal brain. Axons that usually cross the interhemispheric fissure within the corpus instead form fiber bundles that run medial to the medial walls of the lateral ventricles, parallel to the interhemispheric fissure. These white matter fiber bundles are known as the longitudinal callosal bundles, or the bundles of Probst. The bundles of Probst invaginate the medial borders of the lateral ventricles to give them a crescentic shape when viewed in the coronal plane. The third ventricle and foramina of Monro are usually widened in callosal agenesis; moreover, the third ventricle often extends superiorly between the bodies of the lateral ventricles. Occasionally, the third ventricle extends high into the interhemispheric fissure, forming a cerebrospinal fluid (CSF) collection that is generally referred to as an interhemispheric cyst. Although arachnoid cysts can occur in the interhemispheric fissure in association with agenesis of the corpus callosum, more commonly this CSF collection is lined by ependymal cells. It may communicate with the third ventricle or one or both of the lateral ventricles.

Examination of the brain in holoprocencepaly reveals fused thalami, absence of the interhemispheric fissure and falx cerebri, a large crescent-shaped holoventricle, absence of the septum pellucidum, and, most often, a large dorsal cyst. This cyst will usually occupy more than half of the volume of the calvarium. When the lobes of thalami separate and the third ventricle can be identified, the diagnosis of semilobar or lobar holoprosencephaly should be entertained. The recognition of a sagittal sinus within the falx and a torcula make lobar holoprosencephaly most likely. If a dorsal cyst is seen, alobar or semilobar prosencephaly should be given first consideration.

Dandy-Walker malformation can be associated with hypogenesis of the corpus callosum and interhemispheric cyst. MRI will shows an enlarged posterior fossa containing a huge cyst with the cerebellar vermis absent. A large CSF-intensity region can be present dorsal and superior to the genu. Both lateral ventricles can be identified and are separate from the cyst.

References

1. Fitz C R . Holoprosencephaly and related entities.Neuroradiology 1983;25:225-238

2. Kendall B E. Dysgenesis of corpus callosum. Neuroradiology 1983 ;25:239-257

Dr. Ashok Raghavan, Manipal Hospital, Bangalore