Hydrocephalus

TL;DR. Hydrocephalus is a serious neurological condition in young dogs and cats where cerebrospinal fluid builds up inside the brain, often causing a dome-shaped head, learning difficulties, and vision problems that require medical or surgical management.

A dome-shaped skull is a classic physical indicator of congenital hydrocephalus in young toy-breed dogs.

What is it?

Cerebrospinal fluid (CSF) is a clear, protective fluid that cushions the brain and spinal cord, delivers vital nutrients, and removes waste products. It is constantly produced within specialized cavities in the brain called ventricles. In a healthy animal, this fluid flows through a series of narrow pathways before exiting the brain to be absorbed back into the systemic circulation via tiny structures called arachnoid villi.

Hydrocephalus occurs when this delicate balance is disrupted. If there is an obstruction or blockage anywhere along the fluid's pathway, the CSF cannot drain properly. Because the fluid continues to be produced, it builds up inside the ventricles, causing them to stretch and enlarge. This fluid buildup puts pressure on the surrounding brain tissue, leading to progressive neurological damage.

In very young puppies and kittens, the bones of the skull have not yet fused together. The rising pressure from the expanding ventricles pushes the skull bones outward, resulting in a characteristic dome-shaped head and open "soft spots" (persistent fontanelles). In older animals whose skull bones have already fused, the skull cannot expand, meaning the pressure builds up directly against the delicate brain tissue, leading to rapid and severe neurological signs. While hydrocephalus can occasionally be acquired later in life due to brain tumors, severe inflammation, or internal bleeding, it is most commonly a congenital (present at birth) defect in dogs and cats.

Causes & risk factors

The vast majority of hydrocephalus cases in veterinary medicine are congenital. This means the animal is born with an anatomical malformation that blocks the normal flow of cerebrospinal fluid. The most common site of obstruction is the mesencephalic aqueduct, a narrow channel connecting the third and fourth ventricles of the brain.

Certain breeds are highly predisposed to congenital hydrocephalus, particularly toy breeds and brachycephalic (flat-faced) dogs. The breeds most frequently affected include the Maltese, Yorkshire Terrier, English Bulldog, Chihuahua, Lhasa Apso, Pomeranian, Toy Poodle, Cairn Terrier, Boston Terrier, Pug, Chow Chow, and Pekingese. While the condition is far more common in dogs, cats are occasionally affected as well. In felines, congenital hydrocephalus can sometimes be linked to prenatal infections or developmental abnormalities.

Acquired hydrocephalus, which develops later in life, can occur in any breed. This form is typically triggered by an external factor that blocks CSF flow or absorption, such as:

• Intracranial inflammation (such as meningitis or encephalitis)
• Brain tumors (neoplasia)
• Prior brain hemorrhage or trauma
• Infections or parasitic migrations

Signs to watch for

The clinical signs of hydrocephalus can vary depending on the severity of the fluid buildup and the age of the animal. Because the brain tissue is compressed, affected pets display a range of physical and behavioral changes.

Common Signs

• Enlarged, dome-shaped head: A highly visible physical sign in young animals.
• Persistent fontanelles: "Soft spots" on the top of the skull where the bones have failed to close.
• Divergent strabismus: A classic "setting sun" appearance where the eyes point outward and downward.
• Cortical blindness: Apparent blindness despite normal pupillary reflexes, caused by pressure on the visual processing centers of the brain.
• Slow learners and difficulty house-train: Affected puppies and kittens often take much longer to grasp basic training concepts due to cognitive impairment.
• Dullness or depression: A general lack of energy, alertness, or interest in their surroundings.
• Abnormal behavior: Compulsive behaviors, pacing, or aimless wandering.
• Circling: Walking in repetitive circles in one direction.

Occasional Signs

• Seizures: Sudden, uncontrolled electrical activity in the brain, which can range from mild twitching to full-body convulsions.
• Head tilt: Holding the head at an abnormal angle.
• Slow postural reactions: Delays in how the animal corrects its paw placement when gently challenged.
• Tetraparesis: Weakness or partial paralysis affecting all four limbs, making walking difficult or clumsy.
• Nystagmus: Involuntary, rapid, rhythmic eye movements.

Divergent strabismus, where the eyes point outward and downward, is a common clinical sign of hydrocephalus.

How vets diagnose it

Diagnosing hydrocephalus begins with a thorough physical and neurological examination. Your vet will evaluate your pet's skull shape, check for open fontanelles, assess their vision and reflexes, and observe their gait and behavior.

If the patient is a young animal with open fontanelles, your vet can use ultrasound to look directly into the brain. As noted in a leading veterinary internal medicine reference:

"Hydrocephalus is suspected on the basis of characteristic signs and physical examination findings in a young animal of a typical breed. If fontanelles are open, ultrasound examination of the brain can be performed through the openings, and this can determine the size of the lateral ventricles and confirm the diagnosis..."

If the fontanelles are small or completely closed, ultrasound is much more difficult to perform. In these cases, or to obtain a highly detailed view of the brain's internal structures, advanced imaging is required. Magnetic Resonance Imaging (MRI) is the gold standard for diagnosing hydrocephalus, as it provides clear, high-contrast images of the brain tissue, ventricles, and any underlying causes of obstruction (such as tumors or inflammatory lesions). Computed Tomography (CT) scans can also be used, particularly to evaluate the bony structures of the skull and the size of the fluid cavities.

Ultrasound can confirm a diagnosis of hydrocephalus in young animals by visualizing the enlarged lateral ventricles through open fontanelles.

Treatment options

Treatment for hydrocephalus is aimed at reducing the volume of cerebrospinal fluid, lowering intracranial pressure, and managing secondary symptoms like seizures. Depending on the severity of the disease, your vet may recommend medical management, surgical intervention, or a combination of both.

First-Line Medical Therapies

• Glucocorticoids (Corticosteroids): These anti-inflammatory medications, such as prednisone, are highly effective at temporarily reducing CSF production and easing brain swelling. According to a leading veterinary internal medicine reference:

  "Some animals improve with glucocorticoid treatment (prednisone, 0. 5 mg/kg, administered orally daily, tapered weekly until 0. 1 mg/kg q48h)."

• Diuretics: Medications like furosemide (a loop diuretic) and acetazolamide (a carbonic anhydrase inhibitor) help reduce the overall volume of fluid in the body and decrease the rate of CSF production in the brain.

Second-Line Medical Therapies

• Proton Pump Inhibitors (PPIs): Medications like omeprazole, while typically used for gastrointestinal issues, have been shown to help decrease the production of cerebrospinal fluid in the brain's ventricles.
• Anticonvulsants: If your pet experiences seizures as a result of the pressure on their brain, your vet will prescribe specific seizure-control medications to stabilize brain activity.

Surgical Therapy

For long-term management, surgery is often the most effective option. The gold-standard surgical procedure is the placement of a ventriculoperitoneal (VP) shunt. This involves surgically inserting a specialized, one-way valve and catheter system that drains excess cerebrospinal fluid from the brain's ventricles directly into the abdominal cavity, where the body can safely reabsorb it. While this is an aggressive and specialized procedure, it is associated with a sustained positive outcome in more than 50% of patients.

Prognosis

The long-term outlook for pets with hydrocephalus depends heavily on the severity of their neurological signs and how early the condition is diagnosed and treated. If an animal is already showing significant neurological deficits, the prognosis for a normal life is poor without aggressive intervention.

Medical therapy can provide temporary relief and improve quality of life for some pets, but it rarely cures the underlying structural blockage. For pets undergoing ventriculoperitoneal shunt surgery, the prognosis is much more encouraging, with over half of these patients experiencing long-term, positive outcomes and a good quality of life. However, owners must be prepared for the potential of shunt complications, such as infections or mechanical blockages, which may require follow-up veterinary care.

Prevention

Because congenital hydrocephalus is an inherited developmental defect, there is no way to prevent it from occurring in an individual puppy or kitten once they are conceived. The primary method of prevention is responsible breeding. Dogs and cats diagnosed with congenital hydrocephalus, as well as their parents and siblings, should not be used for breeding. This helps reduce the genetic transmission of the anatomical abnormalities that lead to fluid blockages.

When to call your vet

Hydrocephalus is a progressive neurological condition that requires close monitoring. You should contact your veterinarian immediately if you notice any of the following warning signs:

• Seizures or sudden tremors
• Sudden loss of vision or bumping into walls
• Extreme lethargy, stupor, or inability to wake up
• Loss of coordination, falling over, or weakness in all four limbs (tetraparesis)
• A rapid increase in head size or a bulging soft spot on the skull

For specific breeds

If you own a toy breed or a brachycephalic dog—such as a Chihuahua, Maltese, Yorkshire Terrier, Pomeranian, Toy Poodle, Pug, or English Bulldog—it is crucial to monitor them closely during puppyhood. Watch for signs of slow learning, difficulty house-training, or an unusually rounded, dome-like skull. Early detection and intervention are key to giving these pets the best possible chance at a comfortable, happy life.

Sources

• Internal Medicine, 5th Edition, pages 1037-1038.

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