DuoNeb is an inhalation solution combining ipratropium bromide (an anticholinergic) with albuterol sulfate (β2-agonist) to promote bronchodilation. During nebulization, aerosolized ipratropium can leak to the ipsilateral conjunctiva, causing the pupil to dilate (mydriasis). On exam, one would appreciate unequal pupils (anisocoria) with the blown pupil unreactive to light.
Intracranial pathology has to be considered with anisocoria, but if a patient’s neurological exam has not otherwise changed, how can we determine if DuoNeb caused the blown pupil? The pilocarpine eye drop test!
Pilocarpine is a muscarinic (M3) receptor agonist administered as ophthalmic drops to constrict the iris sphincter muscle (causing pupil constriction – “miosis”) and on the ciliary muscle to promote the drainage of aqueous humor via the trabecular meshwork.
Pharmacologic anisocoria due to ipratropium will be unresponsive to the pilocarpine eye drop test. In contrast, pathology like oculomotor (CN-III) compression will respond to pilocarpine since the sphincter muscle remains responsive to cholinergic stimulation. Therefore, if pilocarpine causes miosis, alternative diagnoses must be considered!