Etiology

• Hydrocephalus can result from obstruction of the ventricular system, irritation of the ventricle (from inflammation or hemorrhage), increased size of the ventricles due to loss of brain parenchyma (hydrocephalus ex vacuo), be present without an obvious cause (congenital), or rarely, be the result of overproduction of CSF associated with a choroid plexus tumor.
• Ventricular obstruction can occur due to intraventricular or extraventricular obstruction.
• The most commonly identified cause of congenital hydrocephalus is stenosis of the mesencephalic aqueduct associated with fusion of the rostral colliculi.
• In many cases, an obvious site of obstruction is not apparent and some authors prefer to use the term congenital hydrocephalus to describe cases of hydrocephalus in young animals for which no underlying cause is identified.
• Diffuse ventricular enlargement suggests congenital ventricular dilation or obstruction at the level of the lateral apertures or foramen magnum.
• Focal ventricular enlargement suggests focal obstruction or parenchymal cell loss.
• It is not uncommon to have bilateral lateral ventricle enlargement that is asymmetric. Animals with asymmetric appearance of the ventricles should be critically evaluated for focal obstruction of or impingement on the ventricular system due to mass effect.

Pathophysiology

• Hydrocephalus is the term commonly used to describe a condition of abnormal accumulation of cerebrospinal fluid within the ventricular system of the brain.
• External hydrocephalus is a rare condition in which the accumulated CSF is primarily in an extra-axial location, rather than within the lateral ventricles. It is usually associated with an abnormally large cranium (ie macrocephaly). The pathogeneses is unknown but most theories propose either a congenital or acquired deficiency of the arachnoid villi in their ability to absorb CSF. An alternate theory is that external hydrocephalus is a sequela to severe internal hydrocephalus. In this theory, CSF accumulation within the lateral ventricle eventually leads to rupture of a region of the surrounding cerebral parenchyma with subsequent extra-axial accumulation of CSF.
• Ventricular dilation occurs with some frequency in dogs due to a variety of intracranial disease processes.
• With the aid of modern imaging studies, diagnosis of the condition is usually not difficult, however, the clinical ramifications of intracranial ventricular dilation vary widely.
• For a better understanding of the pathophysiology of hydrocephalus, an understanding of normal cerebrospinal fluid physiology is advantageous:
  • The brain normally contains areas that are devoid of cells but filled with cerebrospinal fluid (CSF).
  • These areas are collectively known as the ventricular system.
  • From rostral to caudal the components of this system include the lateral ventricles, the third ventricle, the mesencephalic aqueduct, and the fourth ventricle.
  • The fourth ventricle is continued into the spinal cord via the central canal.
  • The ventricular system is lined by specialised columnar cell with microvilli known as ependymal cells.
  • These cells are important as a partial barrier between the CSF and the brain parenchyma.

• If the ventricular system is obstructed, CSF will be trapped behind the level of obstruction. This may also be referred to as a non-communicating hydrocephalus.
• As some, but inadequate, amounts of CSF may pass the level of the obstruction, this may not always be the most appropriate description of the pathophysiological state.
• Anatomically smaller areas of the ventricular system are common sites of obstruction. These include the interventricular foramen and the mesencephalic aqueduct.
• The cause of congenital hydrocephalus is not always apparent. Speculation suggests that this abnormality may be due to an obstruction of the ventricular system during a critical stage during development and subsequent damage to the vulnerable maturing nervous parenchyma: the obstructive lesion later resolves, leaving only the ventricular enlargement. Another possibility is obstruction at the level of the subarachnoid space or arachnoid villi, which are difficult to detect..
• Many of these breeds have asymptomatic ventricular dilation which may not result in clinical disease.
• Congenital malformations of the cerebellum are occasionally associated with hydrocephalus.
• Caudal vermian hypoplasia and cysts associated with the fourth ventricle are described, with some dogs having associated ventricular dilation (Dandy Walker malformation).
• Rarely, abnormalities of ependymal cilia function can result in ventricle dilation, probably due to poor or absent CSF flow. Immotile cilia syndrome and Kartagener's syndrome would be examples.
• Hydrocephalus can result in clinical signs due to loss of neurons or neuronal function, alterations in intracranial pressure and associated pathophysiological effects of intracranial disease.
• Interstitial edema, for example, is increased water content of the periventricular white matter due to movement of CSF across the ventricular walls in instances of hydrocephalus.
• Periventricular white matter is reduced due to the disappearance of myelin lipids secondary to increases in white matter hydrostatic pressure or decreases in periventricular white matter blood flow.
• Increased CSF pressure may contribute to intracranial disease through alterations in intracranial pressure. (Consequences of increased intracranial pressure are described above.)
• If formation of CSF equilibrates with absorption, a compensated hydrocephalic state may occur.
• In some instances, CSF production may decrease, possibly due to pressure damage to the choroid plexus or ependyma.

Timecourse

• Weeks to months.