Purpose: With "standard" stimuli (white, Goldmann size III, 200 ms), the Nidek Microperimeter MP1 underestimates retinal light increment sensitivity (LIS). We thoroughly analyze this problem, suggest alternative settings to improve sensitivity to detect dysfunction, and provide true normal values.
Methods: LIS was tested at 55 positions in the macular region using a 4-2-1 staircase strategy with 200 ms white or red stimuli on a 1.3 cd m⁻² background. Stimulus size was Goldmann III and I, and additionally II in the healthy subjects. All participants underwent a complete ophthalmologic examination, spectral domain optical coherence tomography (OCT), and fundus autofluorescence (FAF).
Results: In normals, distributions of LIS for white Goldmann sizes II and III within the central 6° to 10° were clipped off at 20 dB--the MP1 cannot attenuate them any further. When the stimulus size was reduced to Goldmann I or the color changed to red, median LIS in the fovea (∼15 dB) was approximately 5 dB higher than at 10° eccentricity. Estimated from these results, central LIS for white Goldmann sizes II and III stimuli were 21 and 27 dB, respectively. In four patients with either focal or diffuse macular pathology, as confirmed by funduscopy, OCT, or FAF, reduced LIS was detected clearly with Goldmann size I stimuli, but not III.
Conclusions: In all subjects reported here, standard central LIS was above the technical limit of the MP1. To measure true thresholds in healthy subjects, either smaller (Goldmann size I) or dimmer stimuli (red) must be used.
Keywords: macula; microperimetry; perimetry; retinal light increment sensitivity; spatial summation.