The impact of non-adherence on clinical outcomes has gained increasing attention in recent years.
The Collaborative Initial Glaucoma Treatment Study (CIGTS) was a randomized multicenter clinical trial of initial treatment with medications or trabeculectomy in which self-reported medication adherence and visual field (VF) test results were assessed over follow-up in a standardized fashion. Newman-Casey et al. evaluated the longitudinal relationship between medication adherence and visual field progression among participants enrolled in the medication arm of CIGTS.
Adherence to treatment was self-reported and assessed through interviews where specific question was included. Repeated interviews followed the study visits schedule while VF-tests were performed in each visit. Mean deviation over time was the main outcome measure.
Among 306 participants, 46% reported never missing a dose of medication over all available follow-up, 112 patients 37% reported missing medication at up to one third of visits, 10% reported missing medication at one third to two thirds of visits, and 7% reported missing medication at more than two thirds of visits.
An increase in the number of visits during which a patient reported a missed dose of medication was associated significantly with a decrease (worsening) in mean deviation (estimate, -0.14 dB per visit with missed dose, P = 0.0054). For subjects who reported never missing a dose of medication, the average predicted MD loss over 8 years was 0.62 dB, subjects who reported missing medication doses at one third of visits had a loss of 1.42 dB (95% CI, 1.19-3.26; P < 0.0001 and subjects who reported missing mediation doses at two thirds of visits showed a loss of 2.23 dB (95% CI, 1.19-3.26, P < 0.0001).
The CIGTS trial is one of few studies to have captured longitudinal measures of both medication adherence and visual field assessment. This longitudinal analysis demonstrated a statistically and clinically significant association between medication adherence and glaucomatous loss of visual function over eight years of follow-up. In addition, these data display a dose-response relationship between the extent of medication adherence and glaucomatous visual field loss.
Because adherence was self-reported the magnitude of the association between medication adherence and glaucomatous visual field progression found in this study is likely an underestimate because CIGTS participants were more likely to have overestimated their true medication adherence.
Longitudinal information on adherence and key clinical variables collected on a large number (n = 306) of people with newly diagnosed glaucoma followed up for up to eight years and uniform protocol as part of a randomized controlled clinical trial are among the strengths of this study.
On the other hand this was not a randomized controlled trial of the impact of medication adherence on visual field progression. It is possible that the non-adherent participants showed worsening visual field loss because of factors other than medication adherence.
Finally, like most clinical trial participants, CIGTS participants were quite adherent both to their medications and to follow-up and exhibited a fairly small amount of visual field progression. The current study likely underestimates the real world magnitude of the association between medication adherence and visual field progression.