Intersession repeatability of humphrey perimetry measurements in patients with retinitis pigmentosa.

PURPOSE: To determine the relationship between intersession test repeatability in static perimetry and the degree of local sensitivity reduction in patients with retinitis pigmentosa (RP).
METHODS: Visual field data were obtained from 27 patients with RP using FASTPAC 30-2 of the Humphrey Field Analyzer and stimulus sizes III and V. Each test was repeated at two subsequent visits after an initial practice session. An intersession sensitivity difference was defined for each patient at each measured point in the visual field by subtracting the sensitivity value obtained during session 2 from the value obtained during session 3 at the corresponding test location. In addition, the mean sensitivity value for sessions 2 and 3 was determined at each location for each patient.
RESULTS: Bland-Altman plots of intersession sensitivity differences as a function of mean sensitivities showed that test-retest variability was greatest at test regions with sensitivities of approximately 10 dB, with lower variability at locations with higher or lower sensitivity. The pattern of findings was similar for sizes III and V stimuli. The mean intersession sensitivity difference was +0.26 dB for stimulus size III and +0.45 dB for stimulus size V, representing a small improvement in sensitivity from session 2 to session 3.
CONCLUSIONS: Intersession repeatability of visual field measurements in patients with RP is nonmonotonically related to the magnitude of the sensitivity loss. Therefore, apparent changes in visual field sensitivity as a result of time or treatment should be evaluated in light of the degree of the expected variability at particular sensitivity levels.