Pupillographic Investigation of the Relative Afferent Pupillary Defect Associated with a Midbrain Lesion

ObjectiveTo identify clinical and pupillographic features of patients with a relative afferent pupillary defect (RAPD) without visual acuity or visual field loss caused by a lesion in the dorsal midbrain.DesignExperimental study.Participants and ControlsFour patients with a dorsal midbrain lesion who had normal visual fields and a clinically detectable RAPD.MethodsThe pupil response from full-field and hemifield light stimulation over a range of light intensities was measured by computerized binocular pupillography.Main Outcome MeasuresThe mean of the direct and consensual pupil response to full-field and hemifield light stimulation was plotted as a function of stimulus light intensity.ResultsAll 4 subjects showed decreased pupillographic responses at all intensities to full-field light stimulation in the eye with the clinical RAPD. The pupillographic responses to hemifield stimulation showed a homonymous pattern of deficit on the side ipsilateral to the RAPD, similar to that observed in a previously reported patient with an optic tract lesion.ConclusionsThe basis of a midbrain RAPD is the nasal-temporal asymmetry of pupillomotor input that becomes manifest when a unilateral postchiasmal lesion interrupts homonymously paired fibers traveling in the contralateral optic tract or midbrain pathway to the pupillomotor center, respectively. The pupillographic characteristics of an RAPD resulting from a dorsal midbrain lesion thus resemble those of an RAPD resulting from a unilateral optic tract lesion, but without the homonymous visual field defect.Financial Disclosure(s)The author(s) have no proprietary or commercial interest in any materials discussed in this article.