CONDITION

Hydrocephalus

Hydrocephalus describes an abnormal accumulation of cerebrospinal fluid within the brain's ventricular system. The fluid, which normally cushions and nourishes the brain, builds up when production exceeds absorption, or when the normal circulation pathways become obstructed. This can lead to increased pressure inside the skull and, in young animals with open skull sutures, an enlarged, dome-shaped head. Owners most often notice hydrocephalus in puppies or kittens, where a disproportionately large head may be accompanied by behavioural changes, difficulty learning, altered gait, or seizures. In some cases the condition is identified incidentally on imaging performed for other reasons, and the animal may show few or no outward signs. Certain toy and brachycephalic breeds are seen more frequently, though hydrocephalus can occur in any dog or cat. This page explores the signs that may be observed, the mechanisms and causes underlying fluid accumulation, how hydrocephalus is investigated through imaging and clinical examination, and the medical and surgical approaches that exist to manage it.

Why this matters now

Congenital hydrocephalus tends to be recognised in young animals, often between two months and two years of age, with toy-breed dogs such as Chihuahuas, Yorkshire Terriers, Maltese, and Pomeranians showing higher occurrence, as well as brachycephalic breeds including Bulldogs, Pugs, and Boston Terriers. Acquired hydrocephalus can develop at any age when cerebrospinal fluid drainage becomes obstructed by inflammation, infection, bleeding, or masses within or around the brain. In cats the condition is less common overall, though congenital cases may occur in Siamese lines.

The rate at which signs appear varies widely between individuals. Some animals with congenital hydrocephalus show behavioural or learning changes within the first few months of life, whilst others remain outwardly normal until adulthood, when subtle difficulties or gait changes become more apparent. Acquired hydrocephalus may evolve over days to weeks, depending on the speed at which the underlying cause develops. In both forms, progression is not uniform; some animals stabilise and live with mild, persistent signs, whilst others experience worsening neurological changes over months to years.

Signals & patterns

Early signals

Difficulty with house training

A young dog may struggle to learn where to toilet, appearing slow to form the association even with consistent routine. This reflects broader learning difficulty rather than stubbornness, and is a commonly reported early sign in affected puppies.

Unusually domed head shape

The skull may look rounder or more apple-shaped than expected for the breed, sometimes with a soft spot that persists on the top of the head. In toy breeds, this fontanelle can remain palpable well beyond the age at which the skull bones would normally close.

Slow learning or delayed responses

The animal may take longer to learn basic commands, seem less engaged with surroundings, or appear mentally quieter than littermates. Owners often describe a puppy or kitten as the smallest or least responsive in the litter.

Eyes positioned downward or outward

The eyes may appear to gaze toward the floor, sometimes called the setting-sun sign, or drift outward slightly. This reflects pressure effects on the structures that control eye position and is often noticed when looking at the animal's face from the front.

Circling or veering to one side

The animal may walk in tight circles, lean consistently in one direction, or drift off course when moving in a straight line. This pattern suggests asymmetric pressure or damage affecting the pathways that coordinate movement and balance.

Later signals

Head pressing into objects

The animal may stand with its head pushed firmly against a wall, furniture, or the floor for prolonged periods. This behaviour is thought to reflect discomfort from raised intracranial pressure or altered perception of the surroundings.

Loss of vision or blindness

Vision may deteriorate gradually or be absent from early life, often without obvious changes to the appearance of the eyes themselves. The loss reflects compression or damage to the optic pathways or the visual-processing areas of the brain.

Seizure activity

Seizures are a recognised but less common sign in affected animals and may become more frequent over time. They reflect abnormal electrical activity in brain tissue that has been compressed, stretched, or inadequately nourished by disrupted blood flow.

Worsening gait or loss of coordination

An animal that once walked fairly normally may begin to stumble, fall to one side, or lose the ability to rise smoothly. This progression suggests ongoing or increasing pressure on the parts of the brain that govern motor control and spatial awareness.

Click to read about the biological mechanisms

How this is usually investigated

The investigation of suspected hydrocephalus typically begins with a detailed history of the animal's age at onset, the pattern of neurological signs, and any relevant breed predispositions or past head trauma. A thorough neurological examination follows, assessing gait, mental state, cranial nerve function, and in young animals the presence and size of a persistent fontanelle. Imaging of the brain is then used to confirm ventricular enlargement and identify any structural cause, with the choice of technique depending on availability, the animal's stability, and the level of anatomical detail required.

Physical examination

Purpose: A neurological examination identifies abnormalities in gait, posture, cranial nerve responses, mental status, and proprioception that localise the problem to the brain. In young toy-breed dogs, palpation of the skull may reveal a persistent open fontanelle or an abnormally domed cranium.
Considerations: The examination can suggest hydrocephalus and guide further testing, but it cannot confirm the diagnosis or distinguish between congenital malformation, inflammation, or mass lesions obstructing fluid flow. Findings are often subtle in animals with mild or slowly progressive disease.

Magnetic resonance imaging (MRI)

Purpose: MRI provides detailed cross-sectional images of the brain, showing the size and shape of the ventricles, the presence of obstruction or malformation in the fluid pathways, and any masses, inflammation, or bleeding that may have triggered acquired hydrocephalus. It is the most sensitive modality for assessing soft-tissue anatomy within the skull.
Considerations: MRI requires general anaesthesia and access to specialist imaging centres, which may limit availability and add cost. It is particularly useful when surgical intervention is being considered, as it maps the precise site of obstruction and reveals concurrent structural abnormalities.

Computed tomography (CT)

Purpose: CT scanning produces cross-sectional images that demonstrate ventricular enlargement and can identify large masses, haemorrhage, or skull abnormalities. It is faster than MRI and may be more readily available in some referral settings.
Considerations: CT offers less soft-tissue contrast than MRI, so subtle inflammation, small obstructions, and detailed white-matter changes may be harder to appreciate. It remains a useful option when MRI is unavailable or when rapid imaging is needed in an unstable patient.

Cerebrospinal fluid analysis

Purpose: Sampling cerebrospinal fluid from the base of the skull or lower spine allows laboratory assessment of cell counts, protein concentration, and microbial culture, helping to identify inflammatory or infectious causes of acquired hydrocephalus. Cytology may reveal abnormal cells if a tumour is shedding into the fluid.
Considerations: The procedure carries a small risk of herniation if intracranial pressure is very high, so imaging is often performed first to assess safety. Normal cerebrospinal fluid does not rule out hydrocephalus, as the fluid may be clear even when flow is obstructed.

Ultrasound through open fontanelle

Purpose: In young animals with a persistent fontanelle, transcranial ultrasound can demonstrate enlarged ventricles at the bedside without anaesthesia. It is a quick, low-cost screening tool in neonatal and juvenile patients.
Considerations: Ultrasound is only possible when a skull opening exists, limiting its use to very young animals or those with congenital skull defects. It provides less anatomical detail than MRI or CT and cannot reliably identify the cause of obstruction or assess the entire brain.

Options & trade-offs

Management of hydrocephalus aims to reduce intracranial pressure, slow or halt the progression of neurological signs, and address any treatable underlying cause such as infection or inflammation. The approach taken depends on the severity of signs, the animal's age and general health, the rate of progression, and the owner's practical and financial circumstances. Some animals respond to medical therapy alone, while others benefit from surgical diversion of excess fluid, and a subset with mild, stable signs may be monitored without active intervention.

Medical management with diuretics and corticosteroids

Drugs such as furosemide or acetazolamide reduce cerebrospinal fluid production by decreasing the activity of the choroid plexus, the tissue that generates the fluid within the brain's ventricles. Corticosteroids such as prednisolone are also used, primarily to reduce cerebrospinal fluid production by decreasing choroid plexus transport activity, a similar effect to the diuretics, with possible additional anti-inflammatory benefit. Treatment is usually lifelong, with doses adjusted according to the animal's clinical response.

Trade-offs: Medical therapy can stabilise or improve signs in some animals, particularly those with mild to moderate disease, but it does not correct the underlying obstruction and may lose effectiveness over time as fluid accumulation continues. Side effects such as increased thirst, urination, and appetite are common with long-term corticosteroid use, and some animals show little or no improvement despite treatment.

Surgical placement of a ventriculoperitoneal shunt

A ventriculoperitoneal shunt is a narrow tube inserted into one of the brain's ventricles, tunnelled under the skin, and directed into the abdominal cavity, where excess cerebrospinal fluid can be absorbed. The shunt contains a one-way valve that opens when pressure exceeds a set threshold, allowing controlled drainage. This procedure is performed by specialist neurologists or neurosurgeons.

Trade-offs: Shunting can produce rapid and sustained improvement in animals with moderate to severe hydrocephalus, but it carries surgical risks including infection, shunt obstruction, over-drainage leading to collapse of the ventricles, and the need for revision surgery if the device fails. Long-term success varies, and some animals require multiple interventions over their lifetime.

Monitoring without active treatment

Animals with mild ventricular enlargement discovered incidentally, or those showing minimal neurological signs that do not progress, may be observed over time with periodic reassessment of behaviour, gait, and comfort. This approach avoids the risks and costs of medication or surgery in cases where quality of life remains good.

Trade-offs: Monitoring is appropriate when the condition is stable and signs are absent or subtle, but it requires vigilance for any deterioration that might prompt a change in strategy. It offers no protection against future progression, and some animals may worsen suddenly, particularly if an acquired cause such as bleeding or infection develops.

Treatment of underlying inflammatory or infectious disease

When hydrocephalus arises secondary to meningitis, encephalitis, or infection, targeted antimicrobial or immunosuppressive therapy may reduce inflammation, open obstructed pathways, and halt or reverse fluid accumulation. The choice of drug depends on the organism or immune process identified through cerebrospinal fluid analysis and other tests.

Trade-offs: Addressing the root cause can be highly effective in acquired cases, particularly when caught early, but response varies with the severity of scarring or tissue damage already present. Some infections or immune conditions prove difficult to control, and even successful treatment may leave residual ventricular enlargement that requires ongoing medical or surgical management.

Common misconceptions

Misconception:

"A domed skull or open fontanelle in a toy-breed puppy always means hydrocephalus is causing problems."

Reality:

Many toy and small-breed dogs have a persistent fontanelle and mildly enlarged ventricles that never produce neurological signs, and a rounded skull shape can be a normal breed characteristic. Hydrocephalus becomes clinically relevant only when fluid accumulation causes observable changes in behaviour, learning, coordination, or consciousness, and imaging is needed to distinguish benign anatomical variation from progressive disease.

Misconception:

"Once a shunt is placed, the problem is permanently solved and no further intervention will be needed."

Reality:

Ventriculoperitoneal shunts can become blocked, infected, or displaced over time, and some animals require one or more revision surgeries during their lives. The device does not cure the underlying obstruction but manages its consequences, and ongoing monitoring for signs of shunt failure—such as recurrence of neurological changes—remains important throughout the animal's life.

Misconception:

"Hydrocephalus only affects newborn or very young animals."

Reality:

While congenital hydrocephalus is most often recognised in puppies and kittens, acquired hydrocephalus can develop at any age following head trauma, intracranial bleeding, infection, inflammation, or the growth of a mass obstructing cerebrospinal fluid flow. The age at onset and the pace of progression reflect the cause rather than the diagnosis itself.

Related conditions

Brain Tumours

Brain tumours can present with signs that overlap with hydrocephalus—including altered behaviour, seizures, and changes in gait—because both involve space-occupying processes inside the skull that may raise intracranial pressure. In some cases, a tumour can also obstruct cerebrospinal fluid pathways directly, contributing to hydrocephalus as a secondary feature.

Meningioma

Meningioma, a growth arising from the membranes surrounding the brain, can obstruct the normal flow of cerebrospinal fluid and lead to secondary hydrocephalus. Both conditions may be identified on the same imaging study, and the signs observed—such as altered mentation or gait changes—can be similar, though the underlying cause differs.

Epilepsy in Dogs

Seizures are a feature that can occur in both epilepsy and hydrocephalus, though the mechanisms differ. In hydrocephalus, seizures may arise when increased pressure or structural distortion affects the brain's electrical activity, whereas in epilepsy the seizures reflect a primary disorder of brain signalling.

Portosystemic Shunt

Portosystemic shunt may produce neurological signs—such as episodes of confusion, head-pressing, or altered gait—that can prompt imaging, sometimes leading to the incidental discovery of hydrocephalus. Both conditions can occur in toy breeds and may be present from birth, though they arise through entirely different mechanisms.

If imaging has confirmed ventricular enlargement, it can be useful to understand whether any concurrent conditions—such as seizure disorders, congenital malformations elsewhere in the nervous system, or metabolic disturbances—may be influencing the clinical picture. For animals on long-term medical therapy, periodic reassessment of neurological function and quality of life helps track whether the current approach remains effective or whether adjustments are needed. The broader Cognitive and Behavioural Health pillar offers context on other structural and functional brain conditions that may present with overlapping signs or arise alongside hydrocephalus.