Relative risk of progressive glaucomatous visual field loss in patients enrolled and not enrolled in a prospective longitudinal study.

OBJECTIVE To establish the relative risk of progressive visual field loss in a sample of glaucomatous eyes enrolled in a prospective longitudinal study vs a matched sample of eyes not enrolled in a study. METHODS The first visual field records of 66 glaucomatous eyes enrolled in a prospective longitudinal study (mean follow-up time, 3.4 years; mean number of visual field tests, 8.3) were matched to 66 eyes from patients not enrolled in a study (mean follow-up time, 3 years; mean number of visual field tests, 3.7). Eyes were matched on the basis of (1) time of enrollment, (2) length of follow-up, and (3) the extent and spatial pattern of visual field loss. Linear regression of global visual field indexes was used to measure change and the relative risk of progression was calculated for a series of progression criteria sample. RESULTS The relative risk of progressive visual field loss was on average 368% (range, 209%-673%) higher in the eyes not enrolled in a prospective longitudinal study. CONCLUSION Selection bias may reduce the risk of progressive visual field loss in patients enrolled in longitudinal studies.