The problem in demonstrating an advantage to monitoring may partly be a matter of the level of ICP at which treatment is instituted and the proper selection of patients for treatment. Contributing to the decision to institute monitoring are the prospects of ameliorating the underlying lesion, the patient’s age, and associated medical disease(s). Despite the several discouraging trials noted above, our practice has been to measure ICP with an indwelling fiberoptic monitor or intraventricular catheter in otherwise salvageable patients who are stuporous or comatose, and in whom a traumatic or other intracranial mass has caused either a shift of intracranial structures or severe generalized brain swelling. Published guidelines suggest that monitoring should be undertaken in patients with severe traumatic brain injury if they are over 40 years of age, and have a Glasgow Coma Score below 9 (see Chap. 34). The monitor is generally placed on the same side as a mass lesion.
The emergency management of raised ICP is considered in greater detail in Chaps. 33 and 34, where the subject is discussed in relation to stroke and cerebral trauma.
This is a condition of ventricular enlargement as a result of an obstruction to the normal flow of CSF. The sites of obstruction may be at the third ventricle, aqueduct of Sylvius, at the medullary foramina (Luschka and Magendie, Fig. 29-1), or in the basal or convexity subarachnoid spaces. Because of the obstruction, CSF accumulates within the ventricles under increasing pressure, enlarging the ventricles, and expanding the hemispheres. As noted earlier, in the infant or young child, the head increases in size because the expanding cerebral hemispheres separate the sutures of the cranial bones.
Regarding terminology, the term hydrocephalus (literally, “water brain”) is frequently, but incorrectly, applied to any enlargement of the ventricles, even if consequent to cerebral atrophy, that is, hydrocephalus ex vacuo, or to ventricular enlargement because of failure of development of the brain, a state known as colpocephaly. Reference to these conditions as hydrocephalic is so common that it is unlikely to change; the use of “tension hydrocephalus,” although not widely adopted, may be useful to separate enlargement caused by pressure from passive enlargement of the ventricles.
In 1914, Dandy and Blackfan introduced the also somewhat ambiguous but now well-established terms communicating and noncommunicating (obstructive) hydrocephalus. The concept of communicating hydrocephalus was based on the observations that dye injected into a lateral ventricle would diffuse readily downward into the lumbar subarachnoid space and that air injected into the lumbar subarachnoid space would pass upward into the ventricular system; in other words, the ventricles were in communication with each other and the spinal subarachnoid space. If the lumbar spinal fluid remained colorless after the injection of dye into the ventricles, the hydrocephalus was assumed to be obstructive, or noncommunicating. In actuality, the distinction between these two types is not fundamental, because all forms of true hydrocephalus are obstructive at some level, and the obstruction is virtually never complete.
Pathogenesis of Hydrocephalus
There are several sites of predilection of obstruction to the flow of CSF. One foramen of Monro may be blocked by a tumor or by horizontal displacement of central brain structures from a large cerebral mass. An essential, perhaps self-evident aspect of hydrocephalus is that the accumulation of CSF and expansion of the ventricular system is unidirectional, by which is meant that the ventricle(s) proximal to the obstruction expands, not the ones distal to obstruction. A useful adage, attributed to Ayer, is that the ventricle closest to the obstruction enlarges the most; meaning, for example, that occlusion of the basal CSF pathways causes a disproportionate enlargement of the fourth ventricle, and a mass within the fourth ventricle leads to a greater dilatation of the third than of the lateral ventricles.