An external ventricular drain (EVD) is the gold standard for measuring intracranial pressure (ICP) because it is both diagnostic and therapeutic.1 Elevated ICP can be quickly ascertained and subsequently relieved by draining cerebrospinal fluid (CSF) through the EVD catheter. Occasionally, placement of an EVD represents an emergent lifesaving procedure that needs to be done within minutes in a patient who is deteriorating rapidly from a neurologic perspective.2 This chapter will discuss some of the situations when this procedure may be considered, other therapeutic options, and an explanation of how to perform an emergent ventriculostomy.
ANATOMY AND PATHOPHYSIOLOGY
The cranium is a fixed space after infancy that has little capacity for added volume or mass. This concept is the fundamental principal behind the Monro-Kellie doctrine which states that the fixed space of the skull must accommodate brain tissue, blood, and CSF. An increase in volume of any one of these substances without a compensatory decrease in the amount of the other will lead to an increase in ICP. All states of increased ICP can be understood within this framework. Examples include tumors that may increase ICP either by adding volume to the cranium (i.e., the mass itself), blocking CSF outflow (i.e., increasing the amount of CSF), causing vasogenic edema (i.e., expanding the amount of brain tissue), hemorrhaging into the tumor cavity (i.e., increasing the amount of blood in the cranium), or any combination of these mechanisms. Other pathologic conditions including edema, infection, intracranial hemorrhage, and massive cerebral infarctions can increase ICP.
The patient with increased ICP may display the classic clinical signs of headache, vomiting, and papilledema.2 Vomiting is particularly associated with acute increases in ICP. Other signs include a cranial nerve VI (i.e., the abducens nerve) palsy that causes diplopia, decreased consciousness, and an elevated blood pressure with bradycardia (i.e., Cushing’s phenomenon). An increase in ICP may eventually progress to herniation which occurs when there is a force applied to a part of the brain great enough to push other parts of the brain into different cranial compartments. The cranial contents are divided into compartments by invaginations of the dura mater (Figure 147-1).3 The supratentorial space is separated from the infratentorial space by the tentorium cerebelli. The right and left hemispheres are separated by the falx cerebri.
The falx cerebri and tentorium cerebelli divide the skull into compartments. A. Sagittal view. B. Coronal view. C. Top of the skull removed with a section of tentorium cerebelli also removed.
When a unilateral supratentorial mass exerts enough force, the ipsilateral cerebral hemisphere is pushed medially toward the opposite hemisphere (Figure 147-2A). A tentorial herniation is when the medial aspect of the temporal lobe is pushed down toward the brainstem and over the edge ...