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Dog hydrocephalus clasification

Hydrocephalus is an active distention of the ventricular system of the brain caused by an inadequate movement of the cerebrospinal fluid (CSF) from its production site in the ventricular system to the location where it is absorbed. The more common presentation is the congenital one however, it can also be acquired after birth, generally being secondary to an obstruction of the ventricular system. Both congenital and acquired forms can occur in dogs and cats.

The CSF behaviour in hydrocephalus is important for its classification. It is also important to understand that it is paramount to make clinical decisions.

  • Internal hydrocephalus: occurs when the increased CSF is within the ventricular system. For example, in congenital or secondary to a tumour or meningitis.
  • External hydrocephalus: occurs when there is communication between the ventricular system of the brain and the subarachnoid space (generally owing to an obstruction of the 4th ventricle). For example in bacterial meningitis.
  • Communicating hydrocephalus occurs when the CSF flow is blocked after leaving the ventricles towards the subarachnoid space. It is called communicating because the CSF can still flow between the open ventricles. The reabsorption of the CSF is altered within the arachnoid villus owing to infection or haemorrhage. In this type of hydrocephalus, the distension of the ventricular cavities of the brain is located in front of the obstruction site. These changes can be observed in congenital hydrocephalus, but also in inflammation and tumours.
  • Non communicating hydrocephalus :also called obstructive hydrocephalus occurs when the CSF flow is blocked at some point along the narrow pathways that connect the ventricles. One of the most common causes is aqueduct stenosis. It can be observed in cats with feline peritonitis.
  • Compensated hydrocephalus or ex vacuo is secondary to a lack of cerebral matter that is refilled by CSF. The destruction of brain matter can occur in uterus or after birth by trauma or vascular lesions. It is generally observed in puppies that had a difficult birth. In these cases, a prolonged hypoxia or vascular lesion causes the lack of an important quantity of cortical tissue (hydranencephaly).
  • Normotensive hydrocephalus occurs following the progressive block of the CSF drainage pathways. The ventricles slowly increase volume leading to chronic compression and damage of cerebral tissue. The name normotensive hydrocephalus comes from the fact that the cerebral ventricles grow without increasing the intracranial pressure. Congenital hydrocephalus can be normotensive hence the difficult decision of medical versus surgical treatment.
  • Hypertensive hydrocephalus occurs when there is an acute obstruction of the CSF circulation causing acute intracranial hypertension. Compression and damage to the cerebral tissue can occur if the obstruction is not corrected as dilatation of the ventricular system can occur. It presents in tumours and inflammations but also in congenital hydrocephalus.

These definitions describe the hydrocephalus that presents in companion animals. As an example, congenital hydrocephalus tends to be internal, communicating and normotensive/hypertensive.

Fontanela Boston terrier
Acute obstruction of the CSF circulation occurs, producing a picture of acute intracranial hypertension. If not resolved, compression and damage to brain tissue also occurs due to dilation of the ventricular system. It is typically seen in tumors and inflammations, but also in congenital hydrocephalus.

Clinical presentation

The clinical presentation shares signs that are common to all types of hydrocephalus, however the speed of progression of the symptoms depends on the underlying cause (i.e. congenital, neoplastic, inflammatory etc). The clinical signs are secondary to the intracranial pressure alteration, secondary periventricular oedema (white substance) and the loss of cortical neurons (grey matter) function.

The more commonly affected breeds in congenital hydrocephalus are the small or brachycephalic breeds such as Bichon Maltes, Yorkshire terrier, English Bulldog, Chihuahua, Lhasa Apso, Pomeranian and Toy Poodle. However congenital hydrocephalus can also present in large breeds even though it is less frequent.

The more common clinical signs are head enlargement or convex head and ventrolateral strabismus. It is common to find open fontanelas in small breeds. The neurological signs show a supratentorial dysfunction being the more common ones mental status and behaviour alterations -for example a difficult to train puppy-, blindness, convulsions and circling. When the hydrocephalus is severe and hypertense a secondary syringomyelia can develop and this can complicate the presentation with a multifactorial localization (i.e. proprioceptive ataxia, cervical pain and tetraparesia).

Typical eye separation with a widened forehead and a more prominent stop. In this type of breed, physical changes can go unnoticed.

In the following post we'll explain more about this topic.