The classic midbrain syndrome associated with depressed consciousness involves midsized pupils, often unilateral cranial nerve III paresis with compensatory forced adduction (wall-eyes) and retractory nystagmoid movements. Decerebrate or decorticate posturing can occur, and a severe respiratory alkalosis can arise from central neurogenic hyperventilation. Rarely, patients with midbrain dysfunction can actually remain awake but have an altered state termed peduncular hallucinosis of L'hermitte. In this situation, the patient experiences intense visual hallucinations in a clear sensorium. These phenomena have been likened to dreams in an awake state, and disrupted brain stem pathways involved in sleep-wake physiology may be implicated in this remarkable syndrome.
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