Topography of cerebral affection

Although there are many specific causes of unconsciousness, the sites of cerebral affection are either the bilateral cerebral cortex or the brainstem reticular activating system. The most common causes of bilateral cortical disease are deficiencies of oxygen, metabolic disorders, physical injury, toxins, postconvulsive coma, and infections (Table 1).

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table 1 Unconsciousness due to bilateral cortical disease

The reticular activating system maintains the state of wakefulness through continuous stimulation of the cortex. Any interruption may lead to unconsciousness. The reticular activating system can be affected in three principal ways: by supratentorial pressure, by infratentorial pressure, and by intrinsic brainstem lesions ( Table...?).

Table 2 Unconsciousness due to lesions of the reticular activating system

Supratentorial lesions produce impaired consciousness by enlarging and displacing tissue. The skull is a limited area and when additional volume accumulates, pressure is directed to the point of least resistance. Thus the temporal lobes may be forced through the tentorial notch, compressing brainstem structures and cranial nerves. By pressing on the brainstem through this remote mechanism, the reticular activating system is also compressed. The brainstem lies infratentorial in the posterior fossa with the ventricular aqueduct of Sylvius, the fourth ventricle, and the cerebellum. An increase of infratentorial pressure may force the content of the posterior fossa upward through the tentorial notch or downward through the foramen magnum. This again causes compression of the reticular activating system and unconsciousness. Lesions which affect the brainstem itself damage the reticular activating system directly ( PJlum.laQld.PoslQer.l.1982l; IjntinaNi §L§L 1992).

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