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

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

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.

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.

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.

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