• L A Marshman, C E Polkey, and C C Penney.
• Department of Neurosurgery, King's College Hospital, Denmark Hill, London SE5 9RS, England. marshman@btinternet.com
• Neurosurgery. 2001 Nov 1;49(5):1251-5; discussion 1255-6.

Objective And ImportanceAlthough other focal signs may prove "false localizing," it is a neurosurgical axiom that unilateral fixed dilation of the pupil occurs ipsilateral to a supratentorial mass.Clinical PresentationA 25-year-old man collapsed with a dense right hemiplegia and a Glasgow Coma Scale score of 6 (eye opening, 1; motor, 4; verbal, 1) after rupture of a left middle cerebral artery aneurysm associated with an intrasylvian hematoma. Initially, both pupils had remained equal-sized and reactive: however, within hours, the right (contralateral) pupil became fixed and dilated (i.e., false localizing). For some time, the left (ipsilateral) pupil remained small and reactive; at emergency craniotomy, this also became fixed and equally dilated.InterventionAfter evacuation of the clot and wrapping of the aneurysm, both pupils rapidly became equal-sized and reactive. Twenty-four hours later, concurrent with massive left hemispheric swelling and a midline shift, the left (ipsilateral) pupil became unilaterally fixed and dilated (i.e., false localizing). Eventually, the right (contralateral) pupil also became fixed and dilated, concurrent with cardiovascular collapse. Death occurred within 10 hours.ConclusionUnilateral fixed dilation of the pupil in patients with hemispheric mass lesions may be false localizing. Furthermore, disparate "herniating mechanisms" can arise despite mass effect emanating from the same side. Because such mechanisms cannot be witnessed, their nature remains speculative. An extensive review is contained in this article.

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