The skull contains brain, cerebrospinal fluid, and blood. At normal intracranial pressures of 10–15 mm Hg (120–180 mm H2O). Increased volume of one component will elevate intracranial pressure unless the volume of the other two components decreases proportionately (Monro-Kellie doctrine).
The compensatory properties of the intracranial contents follow a pressure-volume exponential curve. Increased volume of any of the three components can be accommodated to a certain point without change in intracranial pressure. Once that critical volume is reached, however, additional volume increase produces an increase in intracranial pressure.
Common examples of a significant volumetric change in one or more of the three normal intracranial components are cerebral edema (brain), hydrocephalus (cerebrospinal fluid), and cerebral venous occlusion (blood).
An intracranial mass (tumor, hematoma) represents a fourth component, and its presence initiates compensatory adjustments of the other three:
Intracranial vessels are compressed, reducing the amount of intracranial blood;
cerebrospinal fluid volume is reduced by increased absorption or reduced production
intracranial bulk is reduced by brain creeping out of adjacent foramina (eg, transtentorial herniation, tonsillar herniation).
Children with expandable skulls due to unfused sutures have an additional compensatory mechanism to accommodate expanding intracranial volume and are thereby partially protected from extreme rises of intracranial pressure.
Most brain insults, whether from trauma, ischemia, poisoning, or other sources, are accompanied by raised intracranial pressure. Following head trauma, intracranial pressure may rise quickly to very high levels as a result of vascular congestion, extravasation, and cerebral edema. Intracranial pressure may also rise substantially when a neoplasm occupies the intracranial cavity. Intracranial hypertension may occur after cerebrovascular occlusions (stroke), during central nervous system infections, and following cerebral hypoxia. Intracranial pressure by itself is rarely a clinical problem when coma is the result of a metabolic disorder (eg, uremia, hepatic coma).
Specific Signs of Raised Intracranial Pressure
While any of the following signs may result from causes other than raised intracranial pressure, most will appear during raised intracranial pressure if the elevation is severe or prolonged.
Blood pressure elevation accompanied by bradycardia and respiratory slowing classically results from raised intracranial pressure. This “Cushing response,” however, usually appears only when intracranial hypertension is severe.
Hemorrhage from gastric ulcerations (Cushing’s ulcer) may accompany raised intracranial pressure.
Hemorrhagic pulmonary edema may result from severe elevation of intracranial pressure as well as from other brain insults. The lung lesion is the end product of a pathophysiologic sequence mediated by the sympathetic nervous system. Few patients survive this hemodynamic storm of neurogenic origin unless intracranial pressure is reduced.
Papilledema, abducens nerve paresis (unilateral occasionally; bilateral often), and depressed consciousness are the most common signs associated with generalized intracranial pressure elevations. In children, decreased upward gaze is common.
Specific syndromes may appear when intracranial pressure is raised by the presence of an intracranial mass.
A laterally placed supratentorial mass may push the uncus and hippocampus medially into the tentorial incisure. The oculomotor nerves, the cerebral peduncles, the cerebral aqueduct, and the midbrain (containing the reticular formation) are vulnerable to compression from the displaced temporal lobe. Transtentorial herniation may then appear clinically.
Clinical Manifestations of Transtentorial Herniation
This sign is a consequence of herniation and does not indicate the localization of the primary process; in this sense, the sign falsely localizes the primary lesion.
Herniation causes compression of the medulla. The hallmark of medullary compression is respiratory failure: slow and irregular breathing followed by apnea. Earlier signs of herniation are nuchal rigidity, intermittent opisthotonos, and depressed gag and cough reflexes. Consciousness may be retained until the patient becomes severely hypoxic.
Although raised intracranial pressure usually becomes obvious clinically, it may go undetected for months. Patients with benign intracranial hypertension often have no symptoms despite severe papilledema and intracranial hypertension. Similarly, patients with obstruction of cerebrospinal fluid pathways may tolerate intracranial pressure elevation for weeks or months without developing overt clinical signs. Failing mentation may provide the only clue to progressive hydrocephalus in the latter circumstance.
Treatment of Elevated Intracranial Pressure
Management of specific causes of raised intracranial pressure is effective treatment. Removal of intracranial masses, shunting of obstructed cerebrospinal fluid, and removal of toxins (eg, lead, in lead encephalopathy) are examples of specific forms of treatment.
When raised intracranial pressure as such must also be managed, the following nonspecific measures are useful:
Control of Body Temperature
Hypothermia reduces cerebral metabolism and lowers intracranial pressure. Hyperthermia increases intracranial pressure. Thus, fever must be controlled.
Mannitol can reduce intracranial pressure by cerebral dehydration.
Cerebrospinal Fluid Drainage
Reduction of cerebrospinal fluid volume by repeated spinal taps or by shunting may control raised intracranial pressure from pseudotumor. Ventricular drainage of cerebrospinal fluid reduces intracranial pressure transiently in severe head injury.
This nonspecific method of reducing intracranial pressure may be employed when other measures fail.