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During a series of two articles we will tell you from the hand of Ane Uriarte, what is hydrocephalus in dogs, causes of its development, how it presents itself as well as the diagnosis and clinical treatment to be performed.

Author: Ane Uriarte

Dip ECVN, DVM, MRCVS, RCVS Recognized specialist in Neurology, EBVS, European Specialist in Veterinary Neurology Veterinary

Classification of hydrocephalus in dogs

Hydrocephalus in dogs is the active distension of the ventricular system of the brain due to inadequate movement of cerebrospinal fluid (CSF) from its site of production in the ventricular system to the site of absorption.Although the most common form is congenital, it can also occur in an acquired form after birth, usually secondary to obstructions of the ventricular system. Congenital and acquired hydrocephalus can occur in both dogs and cats.

The behavior of CSF in hydrocephalus in dogs is important for its classification and understanding that it is critical for clinical decisions:

  • Internal hydrocephalus: when the CSF accumulation is within the ventricular system. For example, in congenital hydrocephalus or, secondary to a tumor or meningitis.
  • External hydrocephalus: when there is communication between the cerebral ventricular system and the sub-arachnoid space (usually due to an obstruction of the 4th ventricle). For example in bacterial meningitis.
  • Communicating hydrocephalus: occurs when the flow of cerebrospinal fluid is blocked after exiting the ventricles into the subarachnoid space. This form is called communicating because CSF can still flow between the ventricles that remain open. The reabsorption of this fluid is impaired in the arachnoid villi by infection or hemorrhage. In this type of hydrocephalus the dilatation of the ventricular cavities of the brain is dilated in front of the site of the obstruction. This can be observed in congenital hydrocephalus, but also in inflammation or tumors.
  • Non communicating hydrocephalus:also called obstructive hydrocephalus. It occurs when the flow of CSF is blocked along one or more of the narrow pathways connecting the ventricles. One of the most common causes of noncommunicating hydrocephalus is aqueductal stenosis. This type of hydrocephalus may also be seen in cats with feline peritonitis.
  • Compensated or ex vacuo hydrocephalus is secondary to a lack of brain matter where CSF fills the space. This destruction of brain material may occur in utero or later, due to trauma or major vascular injury. This type of hydrocephalus is usually observed in puppies with a difficult birth in which prolonged hypoxia or vascular injury produces the absence of a good part of the cortical tissue (hydroanencephaly).
  • Normotensive hydrocephalus: resulting from progressive blockage of the CSF drainage pathways. Thus the cerebral ventricles slowly increase in volume, resulting in compression and chronic brain tissue damage. Normotensive hydrocephalus owes its name to the fact that the cerebral ventricles grow without increasing intracranial pressure. Congenital hydrocephalus can be normotensive and this is why the decision of surgical versus medical treatment can be difficult.
  • Hypertensive hydrocephalus: acute obstruction of CSF circulation occurs, producing acute intracranial hypertension. If left unresolved, compression and damage to brain tissue due to dilatation of the ventricular system also occurs. It is typically seen in tumors and inflammation, but also in congenital hydrocephalus.

These definitions complete and describe the hydrocephalus in dogs that we observe in the companion animal. For example, congenital hydrocephalus is usually internal, communicating and normotensive/hypertensive.

Fontanela Boston terrier
Fontanelle present in a 12-month-old Boston terrier.

Clinical presentation of hydrocephalus in dogs.

The clinical presentation of hydrocephalus in dogs shares certain signs common to all the different types of hydrocephalus, but the rapidity of evolution of the symptoms will depend on the underlying cause (congenital, tumor, inflammatory...). Signs directly related to hydrocephalus are secondary to altered intracranial pressure, secondary periventricular (white matter) edema, as well as loss in function of cortical neurons (gray matter).

The breeds most commonly affected with congenital hydrocephalus are small and/or brachycephalic breeds (Maltese, Yorkshire terrier, English bulldog, Chihuahua, lhasa apso, pomeranian, poodle toy, among others) but beware, congenital hydrocephalus can also occur in large breeds although it is less frequent.

The most common clinical signs are enlargement of the head or "bulging head" and ventro-lateral strabismus. The presence of open fontanelles in these small breeds is frequent. Neurological signs demonstrate supratentorial dysfunction: altered mental status and behavior (puppy difficult to train), blindness, seizures and circling gait are the most common. When hydrocephalus is severe and hypertensive, secondary syringomyelia may develop and complicate the case with multifocal localization: proprioceptive ataxia, cervical pain and tetraparesis.

Typical separation of the eyes with a widened forehead and a more prominent stop. In this type of breed the physical changes may go unnoticed.

In the next post we will continue to expand the information about hydrocephalus in dogs.