The effects of thymoxamine, phenylephrine and cyclopentolate on the accommodative process in man

Abstract

Accommodation of the eye was measured in a cross-over study in 11 healthy volunteers (20-35 years). In 5 subjects the near point was determined before and after topical instillation of 5 .mu.l of 0.1% and 0.5%, and 5 .times. 5 .mu.l 0.5% thymoxamine, 5 .mu.l of 2% and 10%, and 5 .times. 5 .mu.l 10% phenylephrine and 5 .mu.l of 0.04%, 0.2%, and 1% cyclopentolate. All concentrations of thymoxamine increased the accomodative amplitude by about 1.5 dioptres. Accommodation decreased by about 0.5 dioptre after instillation of 5 .times. 5 .mu.l 10% phenylephrine. The cycloplegic effects of 0.2% and 1% cyclopentolate were similar. Accommodation was also determined after application of 5 .mu.l 1% cyclopentolate followed by either 5 .times. 5 .mu.l 0.5% thymoxamine or 10% phenylephrine. Addition of thymoxamine did not alter the cycloplegic response of cyclopentolate alone. Addition of phenylephrine caused a more prolonged but similar maximum response compared to that of cyclopentolate alone. In the 6 other test subjects, the accommodation was compared before and after topical instillation of 5 .mu.l of 0.2% and 1% and 40 .mu.l (one standard eye-drop) of 1% cyclopentolate and followed during 6h. There was no difference between the maximum value of 5 .mu.l and 40 .mu.l 1% cyclopentolate. We conclude from these data that alpha-stimulation by phenylephrine decreases and alpha-inhibition by thymoxamine increases the accommodative amplitude in man. Furthermore, the results indicate that a smaller drop volume or weaker solution of cyclopentolate than routinely used in clinical practice produces a similar cycloplegic effect.