Sweep visual evoked potential evaluation of contrast sensitivity in Alzheimer's dementia

The purpose of this study was to evaluate primary afferent visual pathway function by objectively testing contrast sensitivity in persons with Alzheimer's dementia (AD), using a sweep visual evoked potential technique. Twenty-five patients, 16 with AD and 9 elderly control (EC) subjects, were enrolled from the University of Southern California Rancho Los Amigos Medical Center. The patients with AD had clinical dementia ratings ranging from 0.5 to 3, corresponding to mild to moderate disease. All participants underwent refraction and screening for ophthalmic disease. Subjects were evaluated with the sweep visual evoked potential technique. Each trial consisted of logarithmically increasing contrast over a 10-second period. Subjects were evaluated monocularly at spatial frequencies of 1, 5, and 8 cyc/deg. Patients were not required to integrate and respond to stimuli. Mean contrast sensitivity thresholds were significantly higher in patients with AD than in EC subjects. The mean contrast sensitivities in the AD group were 4.0%, 9.6%, and 18.6%, at 1, 5, and 8 cyc/deg, respectively. The corresponding sensitivities in the EC group were 2.1%, 5.3%, and 11.4%, at 1, 5, and 8 cyc/deg, respectively. These threshold differences were significant at probabilities of 0.01, 0.05, and 0.07. There was no correlation between clinical dementia ratings and reduction of contrast sensitivity thresholds. Confounding factors such as age, gender, nuclear sclerosis, and visual acuity were evaluated. Visual acuity was the only factor significantly different between AD responders and AD nonresponders at 1 and 5 cyc/deg. These results suggest patients with AD have deficits in contrast sensitivity attributable to dysfunction of the primary afferent visual pathway.