Hydrocephalus is an expansion or abnormal dilation of the ventricular system due to an increased volume of spinal fluid. In this case, the ventricles that are affected are those connected with the spinal cord. The abnormal dilation may affect only on one side of the brain, or both sides. It may involve the entire ventricular system (a set of hollow structures in the brain continuous with the central canal of the spinal cord), or only elements next to a site of ventricular system obstruction.

There are two types of hydrocephalus – obstructive and compensatory. Both compensatory and obstructive hydrocephalus can be congenital (present at birth) or acquired.

In the case of obstructive hydrocephalus, spinal fluid accumulates due to an obstruction along the normal circulatory pattern (noncommunicating hydrocephalus), or the fluid accumulates at the fluid resorption site near the meningeal arachnoid villi (communicating hydrocephalus). The meninges are composed of three membranous envelopes – the pia mater, which lies against the brain; the arachnoid, the middle layer; and the dura mater, the outer, thicker layer closest to the skull – that surround the brain and spinal cord. Intracranial (within the skull) pressure may be high or normal, however, clinical signs may be noted when intracranial pressure is normal.

Congenital obstruction causes primary obstructive hydrocephalus. The most common site of obstruction is at the level of the mesencephalic (middle brain) aqueduct. Prenatal (before birth) infections may cause aqueductal stenosis(narrowing) with subsequent hydrocephalus. This may result in considerable disruption of the architecture of the brain.

Acquired obstruction results in secondary obstructive hydrocephalus. It is caused by tumors, abscesses, and inflammatory diseases (including inflammation resulting from hemorrhage that has been caused by traumatic injuries or other causes of bleeding). The sites of obstruction include the interventricular foramina (channels that connect the paired lateral ventricles with the third ventricle at the midline of the brain), the mesencephalic aqueduct, or the lateral apertures of the fourth ventricle.

With compensatory hydrocephalus, spinal fluid fills the space where the nervous system’s functional parts have been destroyed and/or failed to develop. Intracranial (within the brain) pressure is a normal result. This is ventricular dilation incidental to the primary disease
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The congenital form of hydrocephalus is more likely to occur in small and brachycephalic dogs: Bulldogs, Chihuahuas, Maltese, Pomeranians, Toy Poodles, Yorkshire Terriers, Lhasa Apsos, Cairn Terriers, Boston Terriers, Pugs, and Pekingese. It is an inherited disease in Yorkshire terriers. Additionally, there is a high incidence of normal adult Beagles that are found to have enlarged ventricular systems and yet are clinically without symptoms. Acquired hydrocephalus can occur in all breeds.

A large dome-shaped head (due to intracranial swelling)
Gait abnormalities

A diagnosis is typically made with advancing imaging such as computed tomography (CT) and magnetic resonance imaging (MRI).

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Medical and surgical treatments exist depending on the type and severity. Medical treatments slow the production of CSF, however the response is variable and often does not work. Cases that are refractory to medication may require surgery to shunt the fluid to the abdomen.  The goal of treatment overall is to reduce the symptoms and halt the progression of the new symptoms. When the disease is chronic and significant brain destruction has occurred, treatment will not allow regrowth of the brain. Therefore, some symptoms such as blindness and seizures may remain.