Hydrocephalus, also known as water on the brain, is a very serious and usually fatal condition in many of the toy breeds of dogs. It can also occur in almost any other breed due to toxin ingestion by a pregnant female, through trauma or injury to the head, brain or spinal column or through several different disease and genetic conditions. In toy breeds the condition is often congenital which means the puppies have the condition from birth. Any breeding dogs that produce litters where hydrocephalus is diagnosed should be spayed or neutered or restricted from any further use in breeding programs. The breeds that have the highest incidence of hydrocephalus include the Chihuahua, Pomeranian, Maltese and Yorkshire Terrier.

Hydrocephalus occurs when the fluids in the brain are either produced in too great a quantity or when the natural draining systems in the nervous system do not function properly. The normal brain has a cushion of liquid to provide protection from injury plus the fluid acts as a medium to transmit brain chemicals in normal body reactions. In hydrocephalic puppies and dogs the fluid builds up in the ventricles which are the fluid rich spaces in the brain. This then puts pressure on the brain as well as outwards on the skull resulting in physical deformation or malformation of the head. In many hydrocephalic puppies the skull does not properly close and the soft spot remains open in what is known as an open fontanel. Often this area has a noticeable bulge outwards which becomes worse as the puppy ages. This in itself can pose a problem as the area is prone to trauma and injury, especially if there are other neurological and movement problems associated with the condition.

Typically puppies with hydrocephalus rarely live to an adult age due to the damage to the brain and central nervous system from the pressure of the fluid. Often these puppies are blind or deaf and may have a great deal of difficulty in movement and feeding. Many puppies die very suddenly and shortly after birth before the physical signs of hydrocephalus are noted. In cases where the puppies do survive past the first few days there are some notable physical signs of the condition that is usually diagnosed before the puppy reaches about four months of age. A dome shaped head that is much more pronounced than other puppies in the litter is often the first physical sign. The puppies may be very uncoordinated and have an inability to move or coordinate their gait as per the development of the littermates. They may have seizures as well as other neurological impairments. The puppies may have a marked inability to learn which may be very evident when basic obedience training or housetraining is attempted.

While there are some options for draining the fluid and using medications to reduce the swelling and fluid build up on the brain, typically there is little that can be done for the puppy or young dog with the condition. There can be various degrees of hydrocephalus from very severe to very mild, so some puppies may live into adulthood, but few with the condition live past the second year.