Pathophysiology of brainstem lesions due to overdrainage.

Overdrainage in hydrocephalus therapy is a common shunt complication responsible for many different side effects. Especially an association with an impairment of upper brainstem structures causing symptoms of a dorsal midbrain syndrome (DMS) has already been described. Yet apart from these known mesencephalic lesions, we found several more brainstem signs and symptoms resulting from overdrainage. Parinaud's syndrome was diagnosed in all six patients examined; moreover, parkinsonism, memory disturbances, fluctuations in the level of consciousness, and hypothalamic dysfunctions could be detected in five of six patients. In addition hypersalivation combined with peripheral facial nerve palsy and blepharospasm occurred in two patients each, respectively. We postulate an upward herniation of the midbrain into the tentorial notch causing a secondary aqueductal stenosis as causal. An obstructed Sylvian aqueduct and the occurrence of shunt failure can lead to a bulging or enlargement of the third ventricle resulting in diencephalic lesions. If combined with fourth ventricle outlet occlusion, secondary aqueductal stenosis aggravates the situation with a fourth ventricle entrapment. Symptomatology and proposed pathophysiology are presented.