Purpose
Visual field testing uses high contrast stimuli in areas of severe visual field loss. However, retinal ganglion cells saturate with high contrast stimuli, suggesting that the probability of detecting perimetric stimuli may not increase indefinitely as contrast increases. Driven by this concept, this study examines the lower limit of perimetric sensitivity for reliable testing by standard automated perimetry.
Design
Evaluation of diagnostic test.
Participants
34 participants with moderate to severe glaucoma (Mean Deviation (MD) on their last clinic visit averaged −10.90dB, range −20.94dB to −3.38dB). 75 of the 136 locations tested had perimetric sensitivity ≤19dB.
Methods
Frequency of seeing curves were constructed at four non-adjacent visual field locations by the method of constant stimuli (MOCS), using 35 stimulus presentations at each of 7 contrasts. Locations were chosen a priori, and included at least two with glaucomatous damage but sensitivity ≥6dB. Cumulative Gaussian curves were fit to the data, first assuming a 5% false negative rate, and subsequently allowing the asymptotic maximum response probability to be a free parameter.
Main Outcome Measures
The strength of the relation (R2) between perimetric sensitivity (mean of last two clinic visits) and MOCS sensitivity (from the experiment), for all locations with perimetric sensitivity within ±4dB of each selected value, at 0.5dB intervals.
Results
Bins centered at sensitivities ≥19dB always had R2>0.1. All bins centered at sensitivities ≤15dB had R2<0.1, an indication that sensitivities are unreliable. No consistent conclusions could be drawn between 15–19dB. At 57 of the 81 locations with perimetric sensitivity <19dB, including 49 of the 63 locations ≤15dB, the fitted asymptotic maximum response probability was <80%, consistent with the hypothesis of response saturation. At 29 of these locations the asymptotic maximum was below 50%, and so contrast sensitivity (50% response rate) is undefined.
Conclusions
Clinical visual field testing may be unreliable when visual field locations have sensitivity below approximately 15–19dB, due to a reduction in the asymptotic maximum response probability. Researchers and clinicians may have difficulty detecting worsening sensitivity in these visual field locations and this difficulty may occur commonly in glaucoma patients with moderate to severe glaucomatous visual field loss.