The skull is a rigid box containing the brain parenchyma (80 per cent), blood (10 per cent), and cerebrospinal fluid (10 per cent). Intracranial pressure (ICP) in supine humans ranges from 5 to 13 mmHg (0.5-1.3 kPa) and is determined by the relationship between the volume allowed and the volume of the contents.
Raised ICP is diagnosed when pressure is persistently greater than 20 mmHg (2.6 kPa); it is the most common cause of death in patients with neurological conditions. Intracranial hypertension also complicates systemic conditions including post-hypoxic and metabolic encephalopathies. Since raised ICP may compromise cerebral blood flow, it is important to be familiar with its pathophysiology and diagnosis.
The most common cause of raised ICP is an expanding intracranial mass. When the mass volume exceeds the displaceable volume of venous blood or cerebrospinal fluid, ICP rises (Fig 1); this is known as the Monroe-Kelley doctrine. Initially ICP rises slowly in the compensation phase. The compensatory mechanisms include displacement of cerebrospinal fluid and reduced cerebral blood volume. As compensatory mechanisms are exhausted, ICP increases as a function of the relationship between ICP and volume (compliance) and of the rate of expansion of the intracranial mass.
Fig. 1 (a) Schematic representation of skull and normal intracranial contents with arrows indicating the direction of cerebrospinal fluid (CSF) flow. (b) Schematic representation of intracranial contents in the decompensated stage of raised ICP; cerebral vessels are compressed and abnormally dilated in different regions of the brain, and cerebrospinal fluid pathways are blocked preventing translocation and absorption. SSAS, spinal subarachnoid space; FM, foramen magnum; Cp, choroid plexus; AV, arachnoid villi.
An expanding mass distorts the adjacent brain tissue and can generate pressure gradients in the cranium. The local deformation is reflected more distally in mechanical shifts and herniations resulting in compression and ischemia.
Four patterns of herniation can occur: subfalcine (cingulate gyrus), transtentorial (uncal or temporal), tonsillar (cerebellar or through the foramen magnum), and transcalvarial (traumatic) (Fig, 2). Subfalcine herniation may entrap the branches of the anterior cerebral artery. In transtentorial herniation the hippocampus and uncus of the temporal lobe herniate through the tentorial notch compressing the midbrain, and entrapping the occulomotor and abducens nerves and branches of the posterior cerebral artery. When the tonsils of the cerebellum herniate through the foramen magnum (tonsillar herniation), the medulla is compressed.
Fig. 2 Schematic representation of four different types of herniation of brain: (1) subfalcine (cingulate gyrus); (2) transtentorial (uncal or temporal); (3) tonsillar (cerebellar or through the foramen magnum); (4) transcalvarial (traumatic).
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