Brain-Stem Syndromes

Abstract

No other region of the CNS compares with the brain stem in being as densely packed with vital structures. Crowded into the small space of the brain stem are the nuclear groups and nerve fibers of the cranial nerves, sensory tracts ascending from the spinal cord of cochlea to the thalamus and cortex, and the motor pathways descending from the cortex and the subcortical nuclei to the brain stem and spinal cord. In addition, the brain stem contains the reticular formation, with autonomic centers that control respiration, blood pressure, and gastrointestinal functions, as well as centers that mediate arousal and wakefulness. Finally, the brain stem is surrounded by a narrow passage for the circulation of cerebrospinal fluid. For both clinical and therapeutic purposes it is critical to determine whether a lesion lies within or outside the brain stem. Thus, depending on the acuteness of the disease process, brain-stem syndromes require immediate neurological eval-uation. Acute lesions within the brain stem frequently cause coma, either by directly invading and destroying the central core of the brain stem or by impairing its blood supply with subsequent ischemia, necrosis, or hemorrhage. The most common pathologic process that causes primary destruction of the brain stem is cerebrovascular occlusive disease; however, hemorrhage, demyelinating disorders, and encephalitis may produce similar effects. Acute posterior fossa lesions lying adjacent to but outside the brain stem likewise cause brain-stem disturbance in three different ways: (a) direct exertion of pressure on the tegmentum of the pons and midbrain, (b) upward herniation of the superior vermis of the cerebellum through the tentorial notch and, (c) downward herniation of the cerebellar tonsils through the foramen magnum.