Tolerance to topical preparations: cold or warm?

Ocular irritation from topical preparations is related to their viscosity, osmolarity, and pH value. We investigated the effect of temperature on ocular discomfort in 40 volunteers without a history of neuropathy and keratopathy. The dilating drops, tropicamide and phenylephrine were available at two different temperatures, 4 degrees C ("cold") versus 20 degrees C ("warm"). Each patient received cold drops in one eye and warm drops in the fellow eye. Ocular discomfort was similar in both groups. Eyes that received cold drops had a delayed irritation versus an instantaneous irritation with the warm drops. Cold drops may have a transient anesthetic effect on pain nerve endings that fades once the drops warm up to body temperature.