The Mechanism of Fixed Dilatation of the Pupil

For many years it has been recognized that when a patient with evidence of increased intracranial pressure becomes suddenly comatose and shows one dilated and fixed pupil, a presumptive localization of an expanding lesion in the ipsilateral cerebral hemisphere is permissible. Macewen1suggested the possible diagnostic value of the sign, and the more recent works of Rand,2Holman and Scott,3Kaplan4and Hoessly5have proved its practical localizing value. The mechanism producing this phenomenon has not been clear. Various explanations have been suggested, the most common being that it is due to direct pressure on the third nerve. A series of patients admitted to the neurosurgical service with unilateral expanding cerebral lesions at some distance from the midbrain have shown ipsilateral pupillary dilatation as a sudden near terminal or terminal sign. Of these, nine patients have been selected for this report because of their conditions