Pupillary responses to dilute pilocarpine in preganglionic 3rd nerve disorders

Supersensitivity of the iris sphincter to dilute parasympathomimetic agents is considered a diagnostic hallmark of a postganglionic oculomotor nerve disorder. Nine of 13 patients with preganglionic 3rd nerve palsies showed supersensitive pupillary responses using pilocarpine 0.1%. The presence of supersensitivity was not related to the cause of 3rd nerve dysfunction or interval time from onset to testing, but was related to the extent of associated iris sphincter paresis. Some patients with long-standing preganglionic 3rd nerve palsies had features of postganglionic damage, including light-near dissociation and segmental paresis of the iris sphincter. These observations suggest that 1 mechanism of cholinergic supersensitivity in some chronic cases of preganglionic 3rd nerve disorders may be transsynaptic degeneration of postganglionic fibers. In another set of experiments, pharmacologically dilated pupils in normal subjects constricted more to dilute pilocarpine than their normal-sized fellow pupils. Cholinergic supersensitivity in pupil-involving 3rd nerve palsies might also occur simply because the affected pupil is larger than the unaffected pupil.