Hydrocephalus is the excessive accumulation of cerebrospinal fluid (CSF) within the brain.  It occurs when there is an obstruction along the fluid pathway, which prevents the fluid from being absorbed.  When this happens, the continuous production of CSF eventually leads to excess fluid.

The most common form of hydrocephalus is congenital hydrocephalus. In congenital hydrocephalus, the excess fluid accumulates before or soon after birth.  We are not sure why this happens in all cases, but possibilities include an inherited malformation of the fluid pathway or infection or injury around the time of birth.  Less commonly hydrocephalus occurs in adult dogs, usually due to a tumor or infection in the brain that obstructs the CSF pathways.

Congenital hydrocephalus occurs in young puppies. Sporadic cases can occur in any breed, but it is most common in small and toy breeds.  If hydrocephalus occurs in a puppy there is usually an abnormally large, dome shaped head.  The fontanel (morula, "soft spot") on the top of the skull is usually abnormally large and can be felt.  If hydrocephalus occurs after the skull has stopped growing, the head will not enlarge.

Hydrocephalus can cause a variety of neurological signs, including seizures, incoordination, abnormal behavior, a tendency to walk in circles, and blindness.  These signs may come and go or become worse over time.  Diagnosis is based on the signs in conjunction with techniques to image the brain.  In dogs with a fontanel, ultrasound can be performed by scanning through the fontanel to detect the excessive accumulation of fluid within the brain.  Computed tomography (CT) and Magnetic Resonance Imaging (MRI) are also useful.

Young dogs with mild signs may improve without treatment, as they get older. Medications, such as corticosteroids, are sometimes helpful.  In some cases, surgery is performed to implant a ventriculoperitoneal shunt.  This is a device that transfers the excess fluid from within the brain to the abdomen, where it is absorbed.