Hydrocephalus in Dogs and Cats

of an excessive accumulation of cerebrospinal fluid (CSF) within the cranial cavity with subsequent dilation of the ventricular system. Hydrocephalus has been characterized by the anatomic relationship of the underlying disease process and abnormal pressure differences. Internal and external hydrocephalus refer to increased fluid accumulation within the ventricular and subarachnoid spaces, respectively. Obstructive (i.e., noncommunicating) hydrocephalus is characterized by occlusion of CSF flow within the ventricular system rostral to the site of obstruction. Conversely, communicating hydrocephalus occurs when there is extraventricular impedance to normal CSF circulation and absorption. Communicating hydrocephalus may also be a result of excessive CSF production, but this is rare. Compensatory hydrocephalus (i.e., hydrocephalus exvacuo) occurs when CSF occupies space in the cranial cavity that normally would be occupied by brain parenchyma. For example, CSF replaces the area of parenchymal destruction secondary to an ischemic event or senile atrophy of the brain. Although CSF pressure measurements are rarely done in veterinary medicine, normotensive and hypertensive are descriptive terms used to further classify cases of hydrocephalus in which the CSF pressure is normal or increased, respectively.