Purpose or Case Report: Background:
AVID is an acronym describing a triad of findings including 1) asymmetric ventriculomegaly, 2) interhemispheric cyst, and 3) dysgenesis of the corpus callosum. This entity accounts for one of the presentations of callosal dysgenesis along a wide spectrum. Because midline anomalies occur with many processes, including holoprosencephaly and aqueductal stenosis, it may appear to have overlapping features on initial glance. However, by focusing attention on the secondary findings, a specific diagnosis may be determined.

Objectives:
By the end of this presentation the learner will: 1) Become familiar with the imaging characteristics of AVID. 2) Describe the differential diagnosis of AVID and the important distinguishing features. 3) Understand the clinical implications of interhemispheric cysts and similar diagnoses.

Cases/Differential Diagnosis:
Through several case examples of mistaken diagnoses, we will describe key findings to differentiate brain disorders with midline anomalies including AVID, holoprosencephaly, and aqueductal stenosis. Holoprosencephaly creates a monoventricle, but may also be associated with a dorsal midline cyst which can be confused for an interhemispheric cyst. Features that differentiate holoprosencephaly from AVID are the presence of fused cerebral hemispheres, thalamic fusion, and a true monoventricle. Aqueductal stenosis may also show severe hydrocephalus, but lacks the cystic component which is seen with the other mentioned entities. Aqueduct stenosis usually causes symmetric ventriculomegaly of the lateral and 3rd ventricles as well as upward displacement of anterior cerebral arteries and inferior displacement of internal cerebral veins. The hydrocephalus from all of these entities may be treated with ventricular shunt placement, but AVID is an important diagnosis to consider because the wall of the interhemispheric cyst could be imperceptible by imaging and may not improve if the tip of the drainage catheter is not within the cyst.

Conclusion:
When evaluating cases of true ventriculomegaly it is important to consider a differential including AVID, holoprosencephaly, aqueductal stenosis, among other less common congenital syndromes. Careful attention to additional imaging findings is necessary to distinguish the correct diagnosis from look-alikes. Making an accurate diagnosis is important as there are differences in medical decision making, treatment outcomes, and long-term prognosis.
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