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130 Section II: Intracranial Procedures

Evidence of increased intraventricular pressure includes periven- Equipment
tricular edema, enlargement of the temporal horns, and efface- Ventriculoperitoneal shunts comprise three basic elements: a ven-
ment of sulci. These findings suggest acute, active hydrocephalus tricular catheter, valve, and peritoneal catheter. There are many
and are an indication for treatment, as compared with chronic, variations of these components available on the market. Pediatric or
relatively compensated hydrocephalus with normal intraven- low‐profile versions designed for small infants work well in small
tricular pressure, in which case surgery may not be beneficial. dogs and cats. Some systems also include a CSF reservoir that can
Periventricular edema starts at the dorsolateral angles of the lat- be pumped to check patency and an access port that can be aspi-
eral ventricles and spreads into the adjacent white matter. This is rated percutaneously to collect CSF. Antibiotic‐impregnated shunts
evident on CT as blurring or loss of the normally sharp ventricu- are available that have been shown to decrease the rate of shunt
lar margins. Periventricular edema is best appreciated on T2‐ infection in human patients [3] (Figure 14.2).
weighted MRI as increased intensity compared with normal white
matter. Heavily T2‐weighted fluid‐attenuated inversion recovery Ventricular Catheter
(FLAIR) sequences are useful in detecting subtle periventricular The ventricular catheter can be straight or right‐angled and usually
lesions. In doubtful cases, careful observation with serial imaging has multiple holes in the last 10 mm. Some systems consist of a sep-
is indicated [2] (Figure 14.1). arate ventricular catheter and distal tube connected at the time of
Analysis of CSF is helpful in cases of suspected meningoen- surgery while others use a single‐piece design. When the ventricu-
cephalitis. Imaging is performed first to identify any shifting of lar catheter is inserted, if bleeding occurs it is best to allow drainage
brain tissue, such as caudal cerebellar herniation, or other abnor- of fluid until it clears before attaching to the valve; otherwise the
malities that may increase the risk of CSF collection from the cere- blood can occlude the valve. For this reason, separate ventricular
bellomedullary cistern. In some cases it may be safer to collect CSF catheters are preferred [4].
from an enlarged lateral ventricle through a persistent fontanelle.
Removal of CSF is sometimes used as a temporary measure to Valves
decrease intraventricular pressure and to help predict which The most common valve is a differential pressure valve, which
patients will benefit from surgical shunting. In patients with a fon- opens when the pressure difference across the valve exceeds a pre-
tanelle, an enlarged lateral ventricle can be punctured with a 25G determined threshold. Most manufacturers provide fixed pressure
needle inserted at the lateral aspect of the fontanelle, avoiding the valves in ranges of three or four categories, for example very low (<1
sagittal sinus on the midline. Ultrasound is helpful in determining cmH O), low (1–4 cmH O), medium (4–8 cmH O), and high (>8
2
2
2
the depth of the center of the ventricle. Approximately 2 mL of CSF cmH O). Externally adjustable (“programmable”) valves are also
2
can be safely removed in most patients. available that allow the clinician to percutaneously adjust the open-
ing pressure as the patient’s clinical course changes. Flow rate
Indications for Surgery through the valve depends on the differential pressure and the
A young patient with clinical signs, ventriculomegaly, and evidence resistance in the shunt system. Two different valves may have the
of increased intraventricular pressure is a clear indication for a CSF same opening pressure and completely different resistance and
diversionary procedure. Progressive ventriculomegaly over time is therefore behave differently.
also an indication to treat unless it is secondary to cortical atrophy. Diaphragm valves are the most commonly produced and involve
Older patients with stable clinical signs and stable ventriculomegaly deflection of a silicone membrane in response to pressure. Some
are generally not considered for treatment [2]. shunts employ a slit valve, usually at the distal end. These valves

A B C

Figure 14.1 MRI of hydrocephalus. (A) T2‐weighted transverse image at the level of the midbrain. There is enlargement of the lateral ventricles and efface-
ment of the cerebral sulci. (B) FLAIR transverse image at the level of the third ventricle. There is hyperintensity adjacent to the dorsolateral aspects of the
lateral ventricles (arrows). (C) T2‐weighted image at the level of the midbrain 2 months after placement of a ventriculoperitoneal shunt. The lateral ventri-
cles are smaller and the cerebral sulci are more prominent compared with preoperative imaging. The ventricular catheter is evident in the lateral ventricle
(arrowhead).