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Background: Traditionally, hydrocephalus is divided into communicating or non-communicating (obstructive) based on the identification of a blockage of cerebrospinal fluid (CSF) flow through the ventricular system. Hydrocephalus ex vacuo refers to ventricular enlargement as a consequence of neuroparenchymal loss. Hydrocephalus related to obstruction of the lateral apertures of the fourth ventricles has rarely been described., Case Report: The clinicopathologic findings in two dogs with hydrocephalus secondary to obstruction of the lateral apertures of the fourth ventricle are reported. Signs were associated with a caudal cervical spinal cord lesion in one dog and a caudal brain stem lesion in the other dog. Magnetic resonance imaging (MRI) disclosed dilation of the ventricular system, including the lateral recesses of the fourth ventricle. In one dog, postmortem ventriculography confirmed obstruction of the lateral apertures. Microscopic changes were identified in the choroid plexus in both dogs, yet a definitive cause of the obstructions was not identified. The MRI findings in both dogs are similar to membranous occlusion of the lateral and median apertures in human patients., Conclusion: MRI detection of dilation of the entire ventricular system in the absence of an identifiable cause should prompt consideration of an obstruction of the lateral apertures. In future cases, therapeutic interventions aimed at re-establishing CSF flow or ventriculoperitoneal catheterisation should be considered., (© 2016 Australian Veterinary Association.)

Two young Miniature Dachshunds were presented with abnormal gait. Magnetic resonance imaging showed, hydrocephalus with expanding fourth ventricle, and syringohydromyelia in the cervical spinal cord. These dogs underwent ventricle-peritoneal shunting, after which hydrocephalus, syringohydromyelia, and their clinical signs, improved.

A 10-year-old spayed domestic shorthaired cat was presented for behavioural changes, signs suggestive of visual deficits and aimless circling. Neuro-ophthalmological examination suggested the cat had central blindness. CT scans following administration of iohexol demonstrated a contrast-enhancing mass in the vicinity of the third ventricle resulting in obstructive hydrocephalus. Following rostral tentorial craniotomy and incision through the cerebral cortex, the third ventricle was approached via the dilated left lateral ventricle. An ependymoma was seen through a dorsocaudolateral incision into the third ventricle, and removed by gentle manipulation and suction. The cat recovered unremarkably, regaining normal vision and behaviour.

Sixty-seven defects of the nervous system of lambs were examined during a 3-year investigation. Thirteen (19.4 percent) were single defects, and 54 (80.6 percent) were associated with defects of other body systems. Twice as many male lambs as females were affected. The various defects were: hydrocephalus 15, spina bifida 9, meningocoele 8, perosomus elumbis 8, cyclopia 6, cerebellar agenesis 3, aprosopia 3, cranium bifidum 3, anencephaly 2, cerebellar fusion 2, meningo-encephalocoele 2, congenital ataxia 2, micrencephaly 2, cerebellar hypoplasia 1, and acrania 1. A number of lambs had more than 1 defect of the nervous system. Congenital defects of the nervous system were found in 1.5 percent of the 4,417 lambs autopsied.