Effect of continuous positive airway pressure treatment for obstructive sleep apnoea on visual processing of degraded words.

Background: In a previous uncontrolled study, continuous positive airway pressure (CPAP) therapy for obstructive sleep apnoea (OSA) improved vision in patients with diabetic macular oedema.

Objectives: We investigated whether the above improvement in vision (or visual processing) might have been due to reduced sleepiness, rather than a true improvement in retinal function.

Methods: Twelve normal control subjects and 20 patients with OSA were tested for their ability to recognise degraded words, by means of a computer programme displaying 5-letter words every 4 s for 10 min, with variable amounts of the bottom half of the word missing; the percentage of the word necessary to achieve correct identification on average half the time was 'hunted' (the test score). All subjects were tested twice, 2-3 weeks apart; the OSA group after the commencement of CPAP. The Epworth Sleepiness Score (ESS) in patients was measured at the same visit.

Results: The test score at visit 1 was 26.7% for normal subjects and 31.6% for patients with OSA. At visit 2, the test score was 25.0% for normal subjects and 29.9% for patients with OSA. The groups showed a small and identical improvement over the trial period in the test score, of 1.7% (p = 0.01 and p = 0.03 for the normal and OSA groups, respectively). The group with OSA experienced a drop in ESS of 7.5 (SD 5.5) points following treatment.

Conclusion: The small and identical improvement in both groups suggests only a similar learning effect rather than any improvement due to reduced sleepiness.