Contributors: Rodney Bagley, Laurent Garosi

 Species: Feline   |   Classification: Diseases

Introduction Pathogenesis Diagnosis Treatment Outcomes Further Reading


  • Cause: not known, developmental abnormality.
  • Signs: dullness, neurological signs, seizures.
  • Diagnosis: signs, imaging, eg MRI or CT.
  • Treatment: medical to reduce intracranial pressure.
  • Prognosis: guarded to poor.

Presenting Signs

  • Skull deformity (dome-shaped) and persistent fontanelles often observed.
  • Poor development of intelligence.
  • Altered mentation.
  • Inappropriate behavior.
  • Seizures.
  • Cortical blindness and ventral/lateral strabismus.
  • May be asymptomatic in milder case.

Acute Presentation

Age Predisposition

  • Usually <1 year.
  • Rarely present later in life in less severe cases.

Breed Predisposition

  • Some breeds are predisposed to congenital hydrocephalus.
  • Siamese Siamese have hereditary hydrocephalus as an autosomal recessive trait.

Cost Considerations

  • Medical therapy is relatively inexpensive.

Special Risks

  • Hypoxia due to seizure activity and compromise of airways will   →   cytotoxic brain edema and, possibly, raised intracranial pressure.



  • Hydrocephalus 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.


  •  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 pathogenesis 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.
  • 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 specialized 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 at 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.
  • Feline cerebellar hypoplasia may be caused by intrauterine panleukopenia infection, which affects the external germinal layer of the cerebellum, and prevents formation of the granular layer.
  • Congenital malformations of the cerebellum are occasionally associated with hydrocephalus.
  • Some affected cats have concurrent hydrocephalus and hydranencephaly.
  • 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.


  • Weeks to months.


Client History

  • Many animals affected congenitally may appear to be less intelligent than normal.
  • Abnormal shape of the skull.
  • Circling.
  • Pacing.
  • Seizures.
  • Paresis.

Clinical Signs

  • Clinical signs of hydrocephalus reflect the anatomical level of disease involvement.
  • Forebrain, vestibular, and cerebellar signs are most common.
  • Severity of clinical signs is not necessarily dependent upon the degree of ventricular dilation, but rather on a host of concurrent abnormalities including the underlying disease process, associated intracranial pressure changes, intraventricular hemorrhage, and the acuity of ventricular obstruction.
  • A ventral and/or lateral strabismus has been noted in humans and animals with hydrocephalus. This may be referred to as the 'setting-sun sign'. Confusion remains as to the underlying reason for these clinical findings. Some have suggested that this appearance is associated with the skull deformity and distortion of the orbits. Others suggest that because this abnormality can be improved with shunting on the lateral and third ventricles that this strabismus is associated with pressure on the mesencephalic tegmentum.
  • As the forebrain structures are often involved with hydrocephalus, alterations in awareness and cognition are common.
  • Congenital hydrocephalus may be asymptomatic in milder cases.

Diagnostic Investigation


  • Ultrasound can be used to diagnosis hydrocephalus.
  • This is most readily accomplished when a fontanelle is present providing an 'acoustic window' as ultrasound waves do not usually penetrate the skull well enough.
  • Optimal resolution is provided by a high-frequency probe (7-12 MHz).
  • If the bone is intact a craniotomy defect can be created.
  • Depending upon the size of the fontanelle the lateral and third ventricles, as well as the mesencephalic aqueduct, are usually easily identified.
  • Enlarged lateral ventricles are easily seen as paired anechoic regions.
  • With marked ventricular enlargement, the septum pellucidum that normally separates the lateral ventricles is absent and the ventricles appear as a single, large anechoic structure. 


  • Computed Tomography (CT)  Computed tomography (CT)  :
    • Computed tomography, as a non-invasive intracranial imaging modality, is often useful in defining ventricular size.
    • As CSF has a lower CT number than brain parenchyma, the ventricular system is usually readily identified due to its relative blackness in comparison to parenchyma.
    • Computed tomographic evaluation also affords one the ability to examine the majority of the ventricular system, as well as additional intracranial structures.
    • As stated previously, however, ventricular size does not correlate with clinical signs, and the clinical significance of the ventricular enlargement is difficult to predict.
  • Magnetic resonance imaging (MRI)  Magnetic resonance imaging: brain  :
    • Magnetic resonance imaging also affords evaluation of the ventricular system.
    • It enables better assessment of the presence of any focal lesion that may account for the hydrocephalus.
    • This modality provides better for better parenchyma resolution than CT, and is especially useful for evaluation of the caudo-tentorial structures.


  • Survey radiographs of the skull Radiography: skull (basic) may suggest the presence of hydrocephalus but are usually not helpful for definitive diagnosis.
  • Findings associated with congenital hydrocephalus include loss of gyral striations, separation of cranial sutures (diastasis), and persistent fontanelles.
  • Cranial vault may have a ground-glass appearance; also may reveal calvarial thinning.
  • If hydrocephalus develops after the skull has formed, radiographic abnormalities are rarely encountered.

Electroencephalography (EEG)

  • Electroencephalography has been used to diagnose hydrocephalus, primarily prior to advanced imaging.
  • Classically, low frequency (1-7 Hz), high amplitude (25-300 µV) activity is noted.
    This pattern can be seen with other encephalopathies that destroy cortical parenchyma. So EEG is rarely used as the sole means of diagnosis of hydrocephalus.

Cerebrospinal fluid analysis (CSF)

  • Collection of CSF Cerebrospinal fluid: sampling can aid in diagnosis of underlying disease, especially in cases of suspected meningoencephalitis (eg feline infectious peritonitis (FIP) Feline infectious peritonitis).
  • Should be done with care due to potential for brain herniation secondary to elevated intracranial pressure (ICP) Intracranial pressure measurement.
  • Aspiration of CSF from an enlarged ventricle (via a fontanelle or hole made with a trochar), rather than from the cerebellomedullary cistern, may help to reduce this risk. Use of hyperosmolar agents immediately prior to collection may also be beneficial in this regard.

Gross Autopsy Findings

  • The gyri and sulci may be flattened, and cortex may appear soft.
  • If CSF inadvertently leaks from the brain, the cerebral hemispheres may collapse.
  • On transverse section, enlargement of the ventricles is present.
  • With severe disease, brain herniation may occur.

Histopathology Findings

  • The ventricular space is enlarged.
  • Early changes consist of loss of the integrity of the ependymal surface, followed by edema of the periventricular white matter as a result of lack of an effective ependymal barrier. Enlargement of the ventricles leads to compression of the white matter with demyelination, axonal degeneration and astrocyte proliferation.
  • Thinning of the cerebral cortex with neuronal vacuolation and loss of neurons, rupture of the septum pellucidum, and atrophy of other adjacent structures.
  • Periventricular inflammation, hemorrhage, or tumor may be noted with acquired hydrocephalus.

Differential Diagnosis

  • Cerebrovascular disease Cerebrovascular disease.
  • Metabolic or toxic diseases resulting in diffuse cerebral dysfunction.


Standard Treatment

  • The choice of treatments is generally dictated by physical status, age of the animal, and cause of the hydrocephalus, if known.
  • Medical treatment may include general supportive care, and medications to limit CSF production, reduce intracranial pressure and treat underlying cause if one can be identified.
  • Surgical treatment is designed to provide drainage of CSF from the brain to another site for absorption.

Medical Treatment

  • Medical therapy usually does not provide long-term resolution of the clinical signs.
  • Treatment is generally used for peri-operative stabilization of the patient, and for management of cases with limited financial resources.
  • Medications are grouped into two categories: glucocorticoids, and diuretics.


  • Acetazolamide Acetazolamide, a carbonic anhydrase inhibitor, reduces CSF pressure by decreasing CSF production:
    • Especially useful for short-term management of animals with elevated intracranial pressure.
    • A dose of 10 mg/kg is given orally every 6-8 hours.
    • Frusemide Furosemide can be added at 1 mg/kg orally once daily.
    • Electrolyte status should be monitored closely because acetazolamide, especially when used with corticosteroids, may cause potassium depletion.
  • Mannitol Mannitolis an osmotic diuretic which decreases CSF production and reduces intracranial pressure:
    • A dose of 1-2 g/kg IV over 15-20 min.
    • This may be repeated 2-4 times over the following 48 hours.
    • The patient should be monitored closely for dehydration.
    • Short-term treatment only, helpful for acute treatment of severe cases.


  • Glucocorticoids Therapeutics: glucocorticoids are used to decrease CSF production, thereby, limiting intracranial pressure and further neurologic injury.
  • Prednisolone Prednisolone:
    • At 0.25-0.5 mg/kg is given orally twice daily.
    • The dose is gradually reduced at weekly intervals to 0.1 mg/kg every other day.
    • This dose is continued for at least one month.
    • Then the medication is discontinued if possible.
  • Alternatively, dexamethasone Dexamethasone may be given:
    • Orally at 0.25 g/kg every 6-8 hours.
    • The dose can be gradually reduced over two to four weeks.
  • Some animals can be adequately managed with long-term glucocorticoid administration at low doses.
  • If no clinical benefits are observed within two weeks, or if side effects develop, other forms of therapy should be tried.

Surgical Treatment

  • CSF drainage is generally required for those animals that do not improve within 2 weeks, or if deterioration occurs during corticosteroid therapy.
  • Ideally patients need to be treated before permanent neurologic deficits develop and before the patient becomes debilitated by the disease.
  • The surgical procedures are designed to provide controlled CSF flow from the ventricles of the brain to either the peritoneal cavity or the right atrium.
  • Shunt systems which have been designed for use in humans seem to work well for animals.
  • The shunts have not been proven to be more effective than medical management, but only surgical treatment offers the possibility of long-term control of the clinical signs.
  • Complications include shunt blockage, infection and over-shunting which results in collapse of the cerebral mantle.


  • For improvement in demeanor.
  • Repeat imaging studies may be performed but do not correlate well with clinical condition.

Subsequent Management


  • Recurrence of clinical signs.



  • Good to poor - depends on cause and severity.
  • Mild congenital form - good prognosis, may require only occasional medical treatment.

Expected Response to Treatment

  • Medical treatment will only provide temporary relief of signs.

Reasons for Treatment Failure

  • Medical management unlikely to be successful in long-term.

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Thomas W B (2010) Hydrocephalus in dogs and cats. Vet Clinic North Am Small Anim Pract 40 (1), 143-159 PubMed.
  • Dewey C W, Coates J R, Ducoté J M et al (2003) External hydrocephalus in two cats. JAAHA 39 (6), 567-572 PubMed.
  • Wright L C, McAllister J P 2nd, Katz S D et al (1990) Cytological and cytoarchitectural changes in the feline cerebral cortex during experimental infantile hydrocephalus. Pediatr Neurosurg 16 (3), 139-155 PubMed.
  • Whittle I R, Johnston I H & Besser M (1985) Intracranial pressure changes in arrested hydrocephalus. J Neurosurg 62 (1), 77-82 PubMed.
  • Bruinsma D L (1983) Acquired hydrocephalus in an adult cat. Vet Med Small Anim Clin 78 (12), 1857-1858 VetMedResource.
  • Rosenberg G A, Saland L & Kyner W T (1983) Pathophysiology of periventricular tissue changes with raised CSF pressure. J Neurosurg 59 (4), 606-611 PubMed.

Other sources of information

  • De Lahunta A, Glass E (2009) Veterinary Neuroanatomy and Clinical Neurology. 3rd edn. Philadelphia, Saunders.
  • Greenberg M S (1991) Treatment of Hydrocephalus. In: Handbook of Neurosurgery. Lakeland, FL, Greenberg Graphics. pp 200-218.
  • Adams R D &Victor M (1989) Disturbance of cerebrospinal fluid circulation, including hydrocephalus and meningeal reactions. In:Principle of Neurology. 4 th ed, New York: McGraw Hill. pp 501-515.
  • Simpson S T (1989) Hydrocephalus. In:Current Veterinary Therapy X.Ed R W Kirk. Philadelphia: W B Saunders. pp 842-47.
  • deLahunta, A (1983) In:Veterinary Neuroanatomy and Clinical Neurology.2nd edn. Philadelphia:W B Saunders.

Other Sources of Information