The effect of reduced eyedrop size and eyelid closure on the therapeutic index of phenylephrine

In this study we examined the relative effects of reducing eyedrop size (from 30 μl to 10 μl) and eyelid closure on the ocular efficacy and systemic absorption of 10% phenylephrine. Thirteen subjects participated in a quadruple crossover study that involved dilation with a 10-μl and a 30-μl drop of phenylephrine with and without eyelid closure. The 10-μl drop was just as effective for pupillary dilation as the 30-μl drop. Eyelid closure improved dilation for both drop sizes. Both eyelid closure and reducing the drug volume decreased systemic absorption of phenylephrine as measured by plasma concentration. When used together, eyelid closure and the smaller drop size reduced plasma concentration by 45%. The therapeutic index for 10% phenylephrine appears to be improved by using a 10-μl drop followed by eyelid closure.