The condition known as hydrocephalus, also known as water on the brain, is most common in toy dogs and brachycephalic breeds. In Maltese, hydrocephalus is more likely to be seen in puppies with very round skulls and short muzzles. The dog can either be born with this condition (congenital), or, less likely, it can be acquired from either a disease process or the result of trauma. Dogs with this condition have abnormal accumulation of cerebrospinal fluid (CSF) in the ventricles or cavities of the brain.

Puppies with severe hydrocephalus often die at a very early age due to pressure from the increased fluid in the brain. In less severely affected puppies, the signs gradually become apparent over the first few months of life, and in mild cases the condition is not diagnosed until later in life.

The symptoms of hydrocephalus include depression, severe loss of coordination, restlessness, aimless walking, eye abnormalities, seizures, vision problems or blindness and skull enlargement. Young affected puppies often show slow growth as compared to their littermates and are very slow to learn.

Hydrocephalus can be difficult to diagnose since the neurological signs can be similar to other conditions. The condition is usually seen in a puppy that is less than four months of age. The head can take a dome-shaped appearance, which gradually becomes more pronounced, and the skull bones fail to close; hence, an open fontanel. (Note: An open fontanel is not always an indication of hydrocephalus.) When older dogs are affected, the physical signs are not as evident since the bones of the skull are already fused.

The veterinarian will consider the combination of physical, behavioral and neurological abnormalities in the dog. The diagnosis can be confirmed by MRI, CT or ultrasound. Different levels of severity exist.

Treatment depends on the severity of clinical signs. Medical treatment involves the use of diuretic drugs to remove excess fluid, together with corticosteroids to reduce the swelling and inflammation. Surgical treatment involves draining of the excess fluid by inserting temporary or permanent drain tubes. The prognosis, even with treatment, is not good, and most cases are progressive once serious neurological symptoms occur.

Affected dogs should not be bred. Even though little is known about the heritability of this condition, breeding of dogs that are affected or have familial history should be avoided.