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Unilateral cerebellar hypoplasia (UCH) is a rare pathological condition characterized by the loss of volume in cerebellar hemispheres ranging from mild asymptomatic to severe symptomatic cases. As the designation of UCH remains problematic, the underlying etiopathogenesis also lacks explanation. We investigated the patients admitted to Departments of Child Neurology, Neurology, and Genetics between the years 1992 and 2010 and detected 12 patients with unilateral cerebellar volume loss, with the exclusion of all other cerebellar pathologies. The ages of patients ranged between 6 months to 55 years. Five patients had a delay in developmental milestones, and one of these was diagnosed with neurofibromatosis type 1. Two patients had epileptic seizures, one patient had peripheral facial paralysis as a component of Moebius syndrome, and four patients were incidentally diagnosed during etiological work-up for headache. The clinical outcomes of patients varied from healthy subjects to marked developmental impairment. Radiologically, five patients had severe disproportionate UCH, six had moderate disproportionate, and one had mild proportionate UCH. Cerebellar peduncles were affected in all, and vermis was partly hypoplastic in eight patients. Brainstem was involved in four patients, and seven patients showed involvement of white matter and/or corpus callosum. Imaging features supported that patients with severe disproportionate UCH also displayed additional cerebral and commissural changes, which were related to ischemic or vascular injuries, implying a prenatally acquired disruption. In the presence of such a wide spectrum of clinical and radiological features, a prenatally acquired lesion and, thus, a disruption seem to be more explanatory rather than a primary developmental process or malformation in the etiopathogenesis of unilateral cerebellar hypoplasia.

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