Hydrocephalus in cats is an abnormal buildup of cerebrospinal fluid (CSF) inside the brain’s ventricles, the small chambers that normally produce and circulate this fluid. As fluid accumulates, it puts pressure on surrounding brain tissue, causing neurological problems that range from mild behavioral changes to seizures and vision loss. The condition can be present at birth or develop later in life, and while it’s more commonly discussed in dogs, cats are affected too.
How CSF Buildup Damages the Brain
Your cat’s brain constantly produces cerebrospinal fluid, a clear liquid that cushions the brain and spinal cord. Normally, this fluid flows through a series of connected chambers (ventricles), circulates around the brain and spinal cord, and gets reabsorbed into the bloodstream. Hydrocephalus develops when something blocks that flow or prevents proper reabsorption, causing fluid to pool and the ventricles to swell.
As the ventricles expand, they compress the surrounding brain tissue, particularly the white matter that connects different brain regions. Research in hydrocephalic cats has shown that resistance to CSF flow increases significantly when the condition develops. Interestingly, that resistance tends to decrease over time even as the condition worsens, which means the progressive damage can’t be explained by poor absorption alone. The sustained pressure itself reshapes and injures the brain.
Congenital vs. Acquired Hydrocephalus
Congenital hydrocephalus is present at birth or develops shortly after. The most common cause is a narrowing or complete blockage within the ventricular system that prevents CSF from draining properly. Viral infections during the neonatal period can trigger inflammation that scars and narrows these passages, sometimes leaving no trace of the original infection by the time symptoms appear. In dogs, certain breeds carry a strong genetic predisposition, and while breed-specific data in cats is limited, Siamese cats have been noted as potentially more susceptible.
Acquired hydrocephalus develops later in life when a disease process blocks CSF flow. Brain tumors, hemorrhage, and infections that cause brain inflammation are the most common culprits. Feline infectious peritonitis (FIP), a viral disease, is one well-known cause of brain inflammation in cats that can lead to secondary hydrocephalus.
Signs to Watch For
The symptoms of hydrocephalus depend on how much pressure has built up and which parts of the brain are affected. In kittens born with the condition, you may notice a visibly domed or enlarged skull, since the skull bones haven’t fully fused and can expand under pressure. The soft spot on top of the head (fontanelle) may remain open longer than normal or feel bulging.
Neurological signs are often the first thing owners notice, especially in acquired cases where the skull is already fully formed. These can include:
- Unsteady walking or circling, sometimes favoring one direction
- Seizures, which may appear suddenly or increase in frequency over time
- Vision problems or blindness, even though the eyes themselves look normal
- Behavioral changes like dullness, confusion, or a cat that seems “not quite right”
- Head pressing, where the cat pushes its head against walls or furniture
- Difficulty learning or being trained, particularly in kittens
Some cats with mild hydrocephalus show very subtle signs or compensate well enough that owners don’t notice a problem for months. Others deteriorate rapidly, especially when an underlying tumor or active infection is driving the fluid buildup.
How Hydrocephalus Is Diagnosed
MRI is the gold standard for diagnosing hydrocephalus in cats. It clearly shows the size of the brain’s ventricles and can reveal whether a tumor, inflammation, or structural abnormality is responsible for the blockage. A study examining feline brain scans found moderately enlarged ventricles in 16 symptomatic cats, and importantly, both low-field and high-field MRI machines produced comparable measurements, meaning the diagnosis doesn’t require the most expensive equipment available.
In very young kittens with an open fontanelle, ultrasound through that soft spot can provide a quick, non-invasive look at the ventricles. It’s less detailed than MRI but can confirm the diagnosis without sedation. CT scans are another option that shows ventricular enlargement well, though MRI is better at identifying the underlying cause. Your veterinarian may also recommend blood work and analysis of the cerebrospinal fluid itself to check for infection or inflammation.
Medical Treatment
Medical management aims to reduce CSF production and lower the pressure inside the skull. This approach works best for cats with mild to moderate symptoms or as a bridge while deciding whether surgery is appropriate.
The main medications fall into two categories. Steroids like prednisolone reduce inflammation and decrease CSF production. Treatment typically starts at a higher dose given twice daily, then gradually tapers over weeks to the lowest effective amount. Diuretics, which pull fluid from the body, are the second tool. Some work by directly reducing how much CSF the brain produces, while others act more broadly to decrease fluid volume throughout the body.
For cats in crisis with rapidly rising brain pressure, an osmotic diuretic can be given intravenously to pull fluid out of the brain quickly. This is an emergency measure, not a long-term solution. The reality of medical management is that it controls symptoms rather than curing the condition, and many cats need ongoing medication for life. In a retrospective study of 12 dogs treated with medication alone (the closest available data, since feline-specific studies are scarce), half improved and half deteriorated.
Surgical Treatment With a VP Shunt
When medication isn’t enough, the most common surgical option is a ventriculoperitoneal (VP) shunt. This is a thin tube placed into one of the brain’s enlarged ventricles, running under the skin down to the abdominal cavity, where excess CSF drains and gets absorbed by the body. A one-way valve in the tube controls how much fluid flows through, preventing the ventricles from draining too quickly.
The overall success rate for VP shunts is approximately 80%. In a study of 26 dogs that received shunts, 14 improved, 1 stabilized, and 11 deteriorated. These are encouraging but imperfect numbers, and the outcome depends heavily on the underlying cause and how much brain damage occurred before surgery.
Shunt Complications
VP shunts are not set-and-forget devices. About 10% become blocked by cellular debris over time, requiring surgical revision. A more concerning complication is overshunting, where too much fluid drains too quickly, causing the brain to shift or bleed. One study found overshunting-related problems like brain collapse or bleeding between the skull and brain in 18% of cases. Infection is comparatively rare in cats and dogs, occurring in roughly 4% of cases, which is much lower than the infection rates seen in human shunt surgeries.
Shunts may need to be replaced or adjusted as a kitten grows, and mechanical failures can happen at any point during the cat’s life. Owners should watch for a return of neurological symptoms, which could signal a shunt malfunction.
What to Expect Long Term
Prognosis varies widely. Cats with mild congenital hydrocephalus that responds to medication can live comfortably for years, sometimes with minimal ongoing treatment. Cats that do well after shunt surgery often show significant neurological improvement within weeks, though some deficits from prior brain damage may be permanent.
The outlook is less favorable when hydrocephalus is caused by an aggressive brain tumor or a progressive infection. In these cases, treating the underlying disease matters as much as managing the fluid buildup itself. Cats whose symptoms are rapidly worsening before treatment begins also tend to have poorer outcomes, since prolonged pressure causes irreversible damage to brain tissue. Early detection, before the brain has been compressed for too long, gives cats the best chance at a good quality of life.