external hydrocephalus

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ex·ter·nal hy·dro·ceph·a·lus

1. accumulation of fluid in the subarachnoid spaces of the brain;
2. accumulation of fluid in the subdural space owing to a persistent communication between the subarachnoid and subdural spaces.

benign external hydrocephalus

A condition that occurs in infancy and early childhood, which is characterised by enlargement of the subarachnoid space, typically in the frontal areas and interhemispheric fissure with raised intracranial pressure without significantly enlarged ventricles. It is attributed to an absorption deficiency and typically resolves within a year.

external hydrocephalus

An accumulation of fluid in subdural spaces.
See also: hydrocephalus


a condition characterized by abnormal accumulation of cerebrospinal fluid within the cerebral ventricular system. As a consequence, the ventricles are enlarged and the brain is diminutive. Called also water on the brain.
Although hydrocephalus occurs occasionally in adults, it is usually associated with a congenital defect in offspring.
There are two types of hydrocephalus, the distinction being based on whether there is abnormal absorption of the cerebrospinal fluid or an obstruction to its flow. In communicating hydrocephalus there is some abnormality in the capacity to absorb fluid from the arachnoid space. There is no obstruction to the flow of fluid between the ventricles. In noncommunicating hydrocephalus there is an obstruction at some point in the ventricular system. The cause of noncommunicating hydrocephalus usually is a congenital abnormality, such as stenosis of the aqueduct of Sylvius, or congenital atresia of the foramina of the fourth ventricle. Infections, intraventricular hemorrhage, trauma and tumors can produce acquired communicating hydrocephalus.
There are three forms that occur in cattle: in one there is gross distention of the cranium with normal facial bones; in the second there is a similar enlargement of the cranium with an accompanying achondroplastic dishing of the face and foreshortening of the maxilla and a shortening of the limb bones—these are the classical 'bulldog' calves; in the third the cranium is normal in size but there is internal hydrocephalus and the calves are blind and imbecile. There are a number of inherited hydrocephalitides in cattle. The disease also occurs in pigs but the inheritance is complex in that it is exacerbated by a concurrent hypovitaminosis A.
In dogs and cats, hydrocephalus is common in some toy breeds such as the Chihuahua in which a domed cranium is a desirable feature of conformation. It also occurs less often in adults in association with brain tumors and from infections such as toxoplasmosis and feline infectious peritonitis.
Enlarge picture
Hydrocephalus in a calf. By permission from Parkinson TJ, England GCW, Arthur GH, Arthur's Veterinary Reproduction and Obstetrics, Saunders, 2001

compensatory hydrocephalus
cerebrospinal fluid occupies space vacated by brain parenchyma because of malformation or degeneration. Examples are seen in fetal infection by bluetongue virus or bovine virus diarrhea virus, severe polioencephalomalacia in cattle, and cerebral infarction in cats.
external hydrocephalus
the excess fluid is in the arachnoid space; rare in animals.
hypertensive hydrocephalus
accompanied by increased cerebrospinal fluid pressure.
internal hydrocephalus
the excess fluid is within the ventricular system; common in domestic animals and may be congenital or acquired.
normotensive hydrocephalus
accompanied by normal cerebrospinal fluid pressure.