Page 808 - Clinical Small Animal Internal Medicine

776 Section 8 Neurologic Disease

Causes of obstruction may be either congenital (steno- fourth ventricle and mesencephalic aqueduct may also
VetBooks.ir sis of the mesencephalic aqueduct, failure of opening of be enlarged. MRI may reveal the presence of increased
fluid signal (on T2‐weighted or FLAIR scans) in the
the lateral apertures of the fourth ventricle) or acquired
(secondary to inflammatory disease or neoplasia or other
cephalus is acute. The septum pellucidum which
cysts obstructing part of the system). Dilation of the ven- periventricular tissue if the development of the hydro-
tricular system upstream of the obstruction results in normally separates the lateral ventricles on midline may
clinical signs. In mature animals with acquired hydro- be absent.
cephalus, the cause may not be determinable and is often Treatment is either medical or surgical; reasonable
presumed to be the result of clinically inapparent inflam- long‐term management is possible with corticosteroid
mation or hemorrhage within the ventricular system. therapy at antiinflammatory doses. Carbonic anhydrase
Enlargement of the ventricular system can also occur inhibitors have also been used in acute stages of disease
with age – as the cerebral cortex tissue volume decreases, but cause significant electrolyte disturbances in the long
so ventricles get larger to take up the extra space availa- term so are not recommended for chronic management.
ble (this is also known as hydrocephalus ex vacuo). Recently, some authors have described omeprazole in
Clinical signs predominantly refer to the area affected, some cases of acquired hydrocephalus based on experi-
most commonly the forebrain but potentially also hind- mental studies that suggest it may reduce CSF produc-
brain if the fourth ventricle is affected. Seizures, tion by 26% but no evidence supports its use in dogs with
behavioral changes, cortical blindness, dullness, head hydrocephalus. Surgical treatment is also commonly
pressing, and circling are all reported in association performed which involves placement of a ventriculop-
with this condition. Occasionally vestibular signs are eritoneal shunt via a small craniotomy which transfers
also present. Diagnosis requires imaging – either MRI fluid via a one‐way, pressure‐sensitive valve into the
or CT, or ultrasound in dogs where an open fontanelle is abdominal cavity for absorption by mesentery.
present for placement of the ultrasound probe. This Long‐term prognosis is generally good, with approxi-
applies to congenital cases in particular, and often the mately 75% of cases improving or resolving following
skull is abnormally large and dome‐shaped with diver- shunt placement. However, up to 22% may require sub-
gent strabismus as a result of displacement of the orbits. sequent revision to deal with blockage or infection which
On all imaging modalities, the lateral ventricles appear are common complications
abnormally enlarged, and in congenital cases a very thin
rim of cerebral cortex may surround these (Figure 71.2).
Depending on the level of obstruction, the third and/or Metabolic Causes

Hepatic and Uremic Encephalopathy
The pathogenesis of hepatic encephalopathy (HE) is
poorly understood. Hepatic insufficiency from a number
of causes, including acute liver failure, chronic liver
failure (including cirrhosis), congenital portosystemic
shunts and congenital urea cycle enzyme abnormalities,
can all result in secondary prosencephalic disease. As a
consequence, various substances pass from the portal
circulation directly into the systemic circulation without
undergoing detoxification. These include ammonia
(probably integral to the pathogenesis of dysfunction),
various amino acids (especially the aromatic amino acids
phenylalanine, tyrosine, and tryptophan), short‐chain fatty
acids, mercaptan and various biogenic amines, indoles
and skatoles, incriminated in causing HE. Increased intra-
cellular osmolality from too‐rapid glutamine accumula-
tion (ammonia is metabolized by astrocytes to glutamine)
may result in cerebral edema, which also may play a role
in development of cerebral hyperemia and increased
intracranial pressure. Glutamine, short‐chain fatty acids,
Figure 71.2 Dorsal plane CT scan of the brain of a dog with
obstructive hydrocephalus. Note the extreme paucity of cerebral aromatic amino acids, and mercaptans are sodium/
cortex. potassium ATPase inhibitors.