The Study of Progression of Adult Nearsightedness (SPAN): design and baseline characteristics

The Study of Progression of Adult Nearsightedness (SPAN) is a 5-year observational study to determine the risk factors associated with adult myopia progression. Candidate risk factors include: a high proportion of time spent performing near tasks, performing near tasks at a close distance, high accommodative convergence/accommodation (AC/A) ratio, and high accommodative lag.

Subjects between 25 and 35 years of age, with at least -0.50 D spherical equivalent of myopia (cycloplegic autorefraction), were recruited from the faculty and staff of The Ohio State University. Progression is defined as an increase in myopia of at least -0.75 D spherical equivalent as determined by cycloplegic autorefraction. Annual testing includes visual acuity, noncycloplegic autorefraction and autokeratometry, phoria, accommodative lag, response AC/A ratio, cycloplegic autorefraction, videophakometry, ultrasound, and partial coherence interferometry (IOLMaster). Participants' near activities were assessed using the experience sampling method (ESM). Subjects carried a pager for two 1-week periods and were paged randomly throughout the day. Each time they were paged, they dialed into an automated telephone survey and reported their visual activity at that time. From these responses, the proportion of time spent performing near work was estimated.

Three-hundred ninety-six subjects were enrolled in SPAN. The mean (+/- standard deviation) age at baseline was 30.7 +/- 3.5 years, 66% were female, 80% were white, 11% were black, and 8% were Asian/Pacific Islander. The mean level of myopia (spherical equivalent) was -3.54 +/- 1.77 D, the mean axial length by IOLMaster was 24.6 +/- 1.1 mm, and subjects were 1.7 +/- 4.0 Delta exophoric. Refractive error was associated with the number of myopic parents (F = 3.83, p = 0.023), and the number of myopic parents was associated with the age of myopia onset (chi2 = 13.78, p = 0.001). In a multivariate analysis, onset of myopia (early vs. late) still had a significant effect on degree of myopia (F = 115.1, p < 0.001), but the number of myopic parents was no longer significant (F = 0.65, p = 0.52). For the ESM, the most frequently reported visual task was computer use (mean, 18.9%; range, 0-60.0%) and, overall, subjects reported near work activity 34.1% of the time (range, 0-67.3%).

The design of SPAN and the baseline characteristics of the cohort have been described. Parental history of myopia is related to the degree of myopia at baseline, but this effect is mediated by the age of onset of myopia.