Aventriculi associated with holoprosencephaly in a dog

CASE DESCRIPTION: A 10‐month‐old neutered male mixed breed dog was presented for assessment of poorly controlled seizures. CLINICAL FINDINGS: Magnetic resonance imaging of the brain disclosed complete absence of the lateral and third ventricles and mesencephalic aqueduct. Postmortem computed tomographic (CT) imaging and positive contrast cisterno‐ventriculography confirmed the lack of a contiguous ventricular system. However, histopathology identified the presence of vestigial lateral and third ventricles with hypoplastic choroid plexus, atresia of the third ventricle, and fused thalami, consistent with a diagnosis of lobar holoprosencephaly (HPE). CLINICAL RELEVANCE: To our knowledge, this report is the first case of radiographically confirmed aventriculi associated with HPE, a rare congenital malformation previously reported in people, to be described in veterinary medicine.