Pathophysiology of Neurologic Disease

The pathophysiology of critical neurologic disease includes increased ICP, abnormal electrical discharge (status epilepticus), and focal loss of functioning neurons (see Chapter 53 ).

Increased Intracranial Pressure

Increased ICP has been implicated as a potential source of morbidity and mortality in severe neurologic diseases such as head trauma, Reye's syndrome, hypoxic-ischemic encephalopathy, metabolic encephalopathies, intracranial space-occupying lesions, and hydrocephalus. Increased ICP occurs with an increase in intracranial volume: CSF, blood, brain, or supporting tissues. An increase in one compartment can occur at the same ICP only by displacing another compartment. When displacement is no longer possible, ICP rises proportionate to volume.^[192] In older children and adults with closed sutures and rigid calvaria, the cranial cavity is a closed receptacle whose contents are incompressible.

Intracranial Compliance Curve

Recognition of increased ICP is important. In addition to the underlying pathologic process, intracranial hypertension causes further neurologic injury by two mechanisms: (1) if ICP is sufficiently high to reduce the arteriovenous pressure gradient or cerebral perfusion pressure, brain ischemia occurs; and (2) ICP can cause compression and herniation of the brainstem or other vital structures if a pressure differential exists. This compression alters brain function acutely and may cause ischemia or infarction of the brainstem. During the past several years, monitoring of ICP and anticipatory treatment of intracranial hypertension have led to claims of improved outcome, particularly in patients with head trauma.

Head Trauma (also see Chapter 63 )

"Guidelines for the acute medical management of severe traumatic brain injury in infants, children, and adolescents" was published in spring 2003. A multidisciplinary task force met to review the available literature and present a set of evidence-based guidelines and options for treatment of pediatric head trauma. ^[193]

Determining who is at risk for intracranial hypertension is not clear. A normal initial cranial CT scan does not rule out the development of intracranial hypertension, and open fontanelle/sutures does not preclude it. One small study reported an ICP value greater than 20 mm Hg in 86% of children with a GCS score of 3 to 8.^[194] Though certainly not a substitute for careful neurologic assessment, ICP monitoring may provide evidence that further intervention is necessary. In head-injured patients, intracranial hypertension may occur with or without a surgical lesion, or it may only develop after a hematoma is evacuated. Although children have better outcomes than adults do after similar head injuries, significant damage frequently occurs.^[195] Evidence is accumulating that prompt recognition and treatment of increased ICP increase survival and improve neurologic outcomes.^[196]

Hypoxic-Ischemic Encephalopathy

There is little documented experience on the incidence of increased ICP in this group or on the ability to alter outcome. However, general clinical impressions suggest the following: (1) the outcome is less favorable in this group than in those with trauma or metabolic encephalopathy; (2) aggressive management of ICP, at best, prevents further damage; and (3) the GCS score provides a reasonable assessment of initial neurologic function in these patients.

Hydrocephalus

A massive increase in the CSF component of intracranial volume can result in increased ICP. The most common causes of hydrocephalus are obstructed ventricular shunts or aqueductal stenosis/compression from congenital malformations, infection, posterior fossa tumors, or intracranial bleeding. Decompression of the system by either external or internal shunts can be lifesaving.

Tumors

Brain tumors in children are common. They are characteristically found in the posterior fossa, which is a relatively clinically silent area. The initial manifestation may be focal deficits, ataxia, or symptoms of increased ICP either from the tumor mass or from secondary obstructive hydrocephalus.