Hydrocephalus is a neurological disease in which there is excessive accumulation of cerebrospinal fluid (CSF) within the ventricular system of the brain. The fluid in the brain (CSF) is normally formed in the brain. It bathes, protects, and circulates through the ventricular system within the brain and the coverings and is then absorbed into the circulatory system.

The production of CSF has an active and passive component; absorption is only a passive process. When the absorption of CSF is blocked or excessive fluid is produced, the volume of CSF increases. The increased CSF volume puts pressure on the brain, forcing it against the skull, damaging or destroying the tissues.

Symptoms of excess CSF volume vary with the cause, the age at presentation, the brain tissue being compromised, and the degree of tissue damage. In young animals, CSF can accumulate in the brain causing the fontanel (soft spot) to bulge. The bones of the skull are soft and can be enlarged due to the increased volume and pressure leading to a dome shaped cranium. The eye position within in the eye socket may be abnormally deviated where the white portion of the eye (sclera) is visible in both eyes towards the nose.

Causes

Canine distemper

Parvovirus

Parainfluenza virus

Bacterial meningitis

Aberrant parasitic migration

Fungal encephalitis

Ependymoma

Choroid plexus papilloma

Meningioma

Pharyngioma

Epidermoid & dermoid cysts
        
The most common cause of hydrocephalus in young animals is congenital defect. Toy breeds have the highest incidence. Some commonly affected breeds include:

Chihuahua

Maltese

Yorkshire terriers

English bulldogs

American bulldogs

Lhasa Apso

Pomeranian

Toy poodles