Increasing short-term fluctuation by increasing the intensity of the fixation aid during perimetry

An increase in short-term fluctuation is a clinically useful clue in the diagnosis of acquired disorders of the visual pathways. However, short-term fluctuation can also be increased in normal subjects by several factors. We found an increase in short-term fluctuation occurred in normal subjects when the visual field was tested using a bright fixation aid. Eight normal subjects underwent automated perimetry with the Octopus 2000R, in which the dimmest (12.5 candelas/m2) and brightest (435 cd/m2) available fixation aids were used. Mean short-term fluctuation values were 1.63 +/- 0.27 dB with the dimmest aid, and were 2.65 +/- 1.26 dB with the brightest aid. The difference was significant using the paired t-test (P = .037). Moreover, mean sensitivity was reduced from 35.67 +/- 2.26 dB to 33.66 +/- 1.71 dB when the brightest fixation aid was used (P = .004). In six of eight subjects, the relative changes in short-term fluctuation after an increase in brightness of the fixation aid were more pronounced than those in mean sensitivity. An increase in intensity of the fixation aid may cause visual changes in normal subjects that resemble those induced by disorders in the visual pathways. Whenever possible, minimal intensity of the fixation aid should be used to allow for an adequate interpretation of short-term fluctuation values.