Hydrocephalus in dogs – symptoms, treatment and follow-up

Overview

Fluid-filled cavities enlarge within the brain

Cerebrospinal fluid is produced within the brain's ventricular system and must reach its normal site of absorption into the bloodstream. If circulation is disrupted, fluid volume in the ventricles increases and they stretch. The disease can impair brain function even without a noticeably enlarged head.

Hydrocephalus is a recognised developmental disorder particularly in toy and short-nosed dogs. Even in the congenital form, age at onset and speed of progression vary. The dog's condition may also fluctuate between worse and better periods. In some dogs, even extensive imaging changes are associated with only mild observable signs.

An unusually large brain ventricle alone does not establish that hydrocephalus explains all the dog's signs. No single ventricular size threshold reliably distinguishes all dogs with signs from those without them.

Causes

Fluid flow is obstructed or circulation disrupted

Congenital narrowing between the ventricles can obstruct normal cerebrospinal fluid flow. Developmental brain cysts and additional partitions between ventricles can also be responsible. Bleeding into a ventricle around birth can leave adhesions interfering with fluid circulation. Brain inflammation in a newborn can cause later narrowing even after the original signs of inflammation have disappeared.

Hydrocephalus can be divided into an obstructive form and a communicating form without such an obstruction. In the communicating form, fluid can flow from the ventricles into the subarachnoid space around the brain. Alongside this classification, investigations seek the original disease causing the disruption of fluid circulation.

Hydrocephalus developing later

A brain tumour can block a fluid pathway in an adult dog as well. Other causes of acquired hydrocephalus include bleeding and inflammation within the skull. In some cases, the exact cause remains unidentified despite investigation, and the condition is called idiopathic hydrocephalus. A congenital form does not automatically mean that the same single inheritance pattern explains all breeds.

Risk factors

Hydrocephalus occurs particularly in toy breeds and short-nosed dogs. Recognised predisposed breeds include the Chihuahua, Boston Terrier and Cavalier King Charles Spaniel. An acquired obstruction of fluid flow can result from brain disease regardless of breed. Hydrocephalus is among the neurological diseases recognised in Chihuahuas.

Selecting for very small size and an exaggerated rounded skull is associated with Chihuahua health risks, including hydrocephalus. An inherited contribution to the developmental abnormality is suspected in some dogs of predisposed breeds. The exact general mechanism of inheritance is unknown.

A young dog's signs may become apparent only as movement and learning increase. In an older dog, skull shape may not reveal enlarged brain ventricles.

Typical symptoms

Typical signs involve brain function and may gradually intensify. You may notice:

• A puppy learns more slowly than usual, with house training not progressing as expected, for example.
• Behaviour changes, circling or barking without an obvious reason.
• A dog is more subdued than usual and responds poorly to their surroundings.
• Wobbly walking and increasing difficulty coordinating movements.
• Poor vision or apparent blindness.
• Eyes deviating outwards and downwards.
• A markedly domed skull or a larger head than usual.
• Seizures in a puppy.

Appetite may decrease and a puppy become drowsy. Head tilt and involuntary eye movements can also accompany enlargement of the rearmost ventricle. An open fontanelle, or soft spot, is common in small dogs but does not by itself confirm hydrocephalus.

Complications and consequences

Brain damage and limits to recovery

An expanding ventricular system can cause permanent brain damage. If visual pathways have already been destroyed, blindness arising in the central nervous system is likely to be permanent. Smaller ventricles after surgery do not always reverse existing brain tissue atrophy. The prognosis for congenital hydrocephalus with severe signs is guarded.

After shunt surgery, around 70% of dogs and cats with congenital hydrocephalus show improvement in signs. This figure concerns improvement, rather than complete neurological recovery in every animal.

A shunt can stop working

Possible shunt complications include displacement, mechanical failure, infection and obstruction. Complications occur most often during the first 3 months after surgery. In a follow-up study of 100 dogs published in 2026, the catheter placed in the ventricle became obstructed in 9 dogs, or 9%. Obstruction was detected 8–210 days after surgery. The median time to obstruction was 38 days. A later return of signs is therefore also significant.

Overly rapid drainage can also cause a serious complication. It can draw brain tissue away from the skull wall and rupture small veins. The resulting haematoma can compress the cerebral hemispheres. This complication may cause an entirely sudden neurological deterioration.

Inflammatory cells and other cellular material accumulating inside the tube can obstruct the catheter. Brain tissue re-expanding around the ventricle can also compress and obstruct it. In the 2026 study, histological examination of 3 obstructed catheters revealed inflammatory cells, fibrin and choroid plexus cells.

Shunt infections have been reported in around 4.1% of operated dogs and cats. Bacterial shunt infection can cause obstruction and a return of neurological signs. Device infection does not always mean inflammation has spread into the brain tissue itself.

Diagnosis at the vet

Signs are localised before treatment decisions

Your vet assesses alertness, walking, proprioception and cranial nerve function. The neurological examination guides the choice of further investigations. Blood and urine tests look for other diseases that may resemble hydrocephalus. Suitability for anaesthesia is assessed at the same time.

MRI is the preferred method for more detailed investigation of hydrocephalus. It shows the ventricles, other possible developmental abnormalities, swelling and changes affecting brain tissue. Ultrasound can be performed through an open fontanelle. CT can also be used to assess the ventricles.

An enlarged ventricle does not tell the whole story

Small, short-nosed dogs can have proportionally larger brain ventricles than other dogs. Imaging results are therefore interpreted alongside signs and other findings. Brain inflammation can coexist with enlarged ventricles in the same dog. CT alone in particular may miss such inflammation. Where needed, further investigations address inflammation and other brain disease before a shunt decision.

Ventricular pressure in hydrocephalus can be high, normal or even lower than usual. A normal pressure result therefore does not automatically mean that a dog cannot benefit from a shunt.

Treatment

Medication relieves symptoms

The treating vet selects care according to your dog's neurological condition, age and underlying cause. Medication may aim to reduce pressure within the skull and cerebrospinal fluid production. Antiseizure medicines are used if seizure control is needed.

The long-term clinical benefit of medicines used to reduce fluid production is, however, limited or uncertain. Corticosteroids have not been shown reliably to correct internal hydrocephalus itself. In some situations, their effect may mainly involve swelling or inflammation. Possible relief of signs with medication therefore does not mean that the structural problem with fluid circulation has disappeared.

A shunt diverts fluid into the abdomen

A ventriculoperitoneal shunt directs excess fluid from a brain ventricle into the abdominal cavity. The system includes a flexible silicone catheter placed in the ventricle. A valve regulates drainage according to the pressure difference. The shunt is intended to remain in place for life.

The surgical decision is based on ventricular enlargement causing signs and investigation of other brain diseases that could explain them. A growing puppy's system needs sufficient slack to accommodate growth. Very young puppies aged 2–4 weeks have a higher anaesthetic risk than usual. A minimum age of 6–8 weeks has been proposed for planned imaging and surgery. The specialist team determines timing according to the dog's condition; a puppy with acute signs is not left waiting to reach an age threshold.

Follow-up continues after surgery

A neurological reassessment and repeat MRI are recommended at around 3 months. This assesses changes in signs, shunt function and recovery of brain tissue. Returning signs may require X-rays along the entire shunt route to identify disconnection, kinking or a break. An obstructed ventricular catheter may require replacement surgery.

Confirmed shunt infection may require device removal to treat the infection. MRI can also look for inflammatory changes in the brain, meninges or ventricles.

Prevention

Not every cause of congenital hydrocephalus is known, so not every case can be prevented. Exaggerated small size and skull roundness are avoided in Chihuahua breeding. A dog with hydrocephalus and neurological signs is unsuitable for breeding.

Careful surgical planning and selection of appropriate equipment help prevent shunt complications. A concurrent ear canal or skin infection is treated before planned shunt surgery to reduce infection risk. Follow-up after surgery helps identify device malfunction and the need for corrective treatment.

Sufficiently loose loops of tubing beneath the skin reduce catheter displacement with neck movement. An overly tight loop can instead kink the tube and prevent flow.

Important for dog owners

Record changes in learning, walking, vision, alertness and any seizures before and after treatment. The disease can involve abnormal learning and behaviour, so a slow-learning puppy is not simply considered difficult to train. Finding an open fontanelle does not establish the diagnosis without other investigations.

A shunt may need repair later even if initial recovery went well. Keep the surgical report and shunt details you receive for follow-up visits. Follow the surgical team's device-specific instructions for handling the shunt reservoir. If previous signs return, the treating service reassesses shunt function.

A new sign is not always explained by ventricular size alone, because another brain disease can coexist. Treatment is therefore monitored using both your dog's ability to function and examination findings.

Frequently asked questions

Does an open fontanelle mean a puppy has hydrocephalus?

An open fontanelle is common in many toy breeds and does not by itself indicate hydrocephalus. Diagnosis is based on signs and brain imaging. Ultrasound through the fontanelle can help assess the ventricles, but MRI gives a more detailed picture of the brain.

Can an adult dog develop hydrocephalus?

Yes. Acquired hydrocephalus can develop when a tumour, bleeding or inflammation obstructs cerebrospinal fluid circulation. An adult dog's head does not necessarily become domed. Signs and imaging investigate both the fluid circulation problem and the underlying disease causing it.

Does medication cure hydrocephalus in dogs?

Medication generally relieves symptoms and may aim to reduce pressure and fluid production. Its long-term benefit for internal hydrocephalus itself is limited or uncertain. A dog with signs may need a shunt to divert fluid.

How often does a shunt become obstructed?

In a follow-up study of 100 dogs published in 2026, the ventricular catheter became obstructed in 9 dogs, or 9%. Obstruction occurred 8–210 days after surgery, with a median of 38 days. No reliable predictive factor was identified for assessing an individual dog's risk.

Does vision return after surgery?

Recovery of vision depends on how much damage has occurred to brain tissue carrying visual information. Blindness associated with already destroyed visual pathways is likely to be permanent. Smaller ventricles do not always reverse previous brain damage, so improved imaging and recovery of the dog's ability to function do not mean the same thing.