Nerve terminal degeneration in the rat iris observed following chronic topical 2% epinephrine and 0.1% dipivalyl epinephrine: a quantitative comparison of electron microscopic observations and tissue norepinephrine levels.

This is the first report of nerve terminal degeneration within the iris following topical dipivalyl epinephrine 0.1% treatment. A quantitative comparison of the effects of topically applied epinephrine 2%, dipivalyl epinephrine 0.1%, and placebo vehicle on the nerve terminals within the iris of a rat was made using electron microscopy and a catecholamine radioenzyme assay. Thirty-nine rats were divided into these 3 treatment groups and treated for 10 weeks, after which they were killed and studied. Both the epinephrine 2%- and dipivalyl epinephrine 0.1%-treated groups showed greater nerve terminal degeneration compared with the placebo-treated group (P < 0.05). The epinephrine-treated group showed greater nerve terminal degeneration than the dipivalyl epinephrine-treated group. The clinical significance of these observations is unknown. It is impossible to decide whether these degenerative changes are related unavoidably to a desired therapeutic effect or to an undesirable, potentially avoidable, toxic side effect.