Human dynamic closed-loop accommodation augmented by sympathetic inhibition.
OBJECTIVE
A ciliary alpha-adrenoceptor accommodative effect has been proposed, caused by a small population of alpha1-inhibitory receptors in excised human ciliary muscle. This study was intended to investigate the effect on the closed-loop dynamic accommodative process of modulating alpha1-adrenoceptor activity by topical instillation of the alpha1-adrenergic agonist, phenylephrine hydrochloride.


METHODS
A group of 10 visually normal subjects viewed a photopic (30 candela/m2) high-contrast Maltese cross, which was modulated sinusoidally (0.05-0.6Hz) and stepwise over a 2-D range (2-4 D). Monocular temporal accommodation responses were measured using a continuously recording dynamic tracking infrared optometer under two trial conditions: after instillation of saline control solution and 50 minutes subsequent to the instillation of 0.27 microl 0.4% benoxinate hydrochloride and 0.27 microl 2.5% phenylephrine hydrochloride. Pupil size and accommodative amplitude were measured at 90-second intervals for 50 minutes after drug instillation. All accommodative measurements were recorded through a fixed 4-mm pupil.


RESULTS
A significant reduction in accommodative amplitude (11%; P < 0.05) was recorded, whereas pupil size showed a significant increase (33%; P < 0.05). No significant change in step-response dynamics was observed. However, phenylephrine hydrochloride caused a significant increase in accommodative gain in the low and midtemporal frequency ranges compared with the effect of a saline control treatment. No significant variation in phase lag was observed.


CONCLUSIONS
For the first time in humans, this study shows that augmentation of the alpha1-inhibitory sympathetic contribution results in increased accommodative gain at low and midtemporal frequencies, which is consistent with findings in animal studies.
Culhane
H M
HM
Department of Optometry, University of Bradford, West Yorkshire, United Kingdom.
Winn
B
B
Gilmartin
B
B
D000065:Accommodation, Ocular / Q000187:drug effects / Q000502:physiology*; D000287:Administration, Topical; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D009433:Neural Inhibition / Q000502:physiology*; D010656:Phenylephrine / Q000494:pharmacology; D011680:Pupil / Q000187:drug effects / Q000502:physiology; D011942:Receptors, Adrenergic, alpha / Q000502:physiology; D012016:Reference Values; D013564:Sympathetic Nervous System / Q000502:physiology*; D013997:Time Factors
D000316:Adrenergic alpha-Agonists; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine