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Dark Adaptation of Rod Photoreceptors in Normal Subjects, and in Patients with Stargardt Disease and an ABCA4 Mutation

¹From the Departments of Ophthalmology and Visual Sciences and ³Bioengineering, University of Illinois at Chicago, Chicago, Illinois; the ²Pritzker Institute of Biomedical Science and Engineering, Illinois Institute of Technology, Chicago, Illinois; the ⁴Retina Foundation of the Southwest, Dallas, Texas; the ⁵Department of Ophthalmology and Visual Science, The Carver College of Medicine and the Howard Hughes Medical Institute, University of Iowa, Iowa City, Iowa.

PURPOSE. Psychophysical and electroretinographic (ERG) studies indicate that patients with Stargardt disease exhibit abnormally slow rod dark adaptation after illumination that bleaches a substantial fraction of rhodopsin. However, relatively little information is available concerning rod recovery in this disease after weaker adapting (i.e., conditioning) light. With the use of a paired-flash ERG method, properties of the derived rod response to a low-bleach (<1%) but rod-saturating conditioning flash were investigated in seven normal subjects and in five Stargardt patients with identified sequence variations in the ABCA4 gene.

METHODS. In the first of two experiments, the interval between a fixed conditioning flash (67 or 670 scotopic cd s m^–2) and a bright probe flash of fixed strength was varied to determine the falling-phase kinetics of the derived rod response to the conditioning flash. In the second, the instantaneous amplitude-intensity function for the rod response at an intermediate stage of recovery from the conditioning flash was determined by presenting a test flash of various strengths at a fixed time after the conditioning flash, and a probe flash at 200 ms after the test flash.

RESULTS. The maximum peak amplitude of the dark-adapted, rod-mediated a-wave determined in Stargardt patients (211 ± 87 µV) was on average lower than that determined in normal subjects (325 ± 91 µV; P = 0.06). The derived rod response to the 670 scotopic cd s m^–2 conditioning flash determined in normal subjects and Stargardt patients exhibited a biphasic recovery, and the kinetics of the early stage of this recovery were similar in the two subject groups. For both normal subjects and patients, normalized amplitude-intensity functions describing the dark-adapted derived rod response exhibited half-saturation at approximately 1.5 log scotopic troland second. In both groups, the normalized amplitude-intensity function determined at approximately 2 seconds after the 67 scotopic cd s m^–2 conditioning flash and at approximately 9 seconds after the 670 scotopic cd s m^–2 conditioning flash exhibited an average desensitization (i.e., an increase of test flash strength at half-saturation) of approximately 0.5 to 0.6 log unit relative to that determined under dark-adapted conditions.

CONCLUSIONS. The results indicate that, despite a reduction in the average dark-adapted maximum a-wave amplitude in the Stargardt/ABCA4 patients, the early-stage recovery kinetics of the derived rod response to a low-bleaching conditioning flash as well as the lingering rod desensitization produced by such a flash are similar to those determined in normal subjects.

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