FIGURE 29.5 Congenital Horner syndrome The left iris is lighter in color than the right, the left pupil is miotic and there is a mild ptosis Other local factors include surgical irritation of the iris and pharmacologically induced unilateral miosis Mechanical contact with the iris during any intraocular surgical procedure may result in transient postoperative unilateral miosis Parasympathomimetic or sympatholytic systemic medications and topical drops can also result in transient miosis It is helpful to remember that most topical ophthalmic miotics are supplied in bottles that have green caps Neurologic Factors Congenital Horner syndrome may result from brachial plexus injury and is often associated with ipsilateral iris hypopigmentation (Fig 29.5 ) This sign is not as helpful in the first few months of life when both eyes are normally relatively hypopigmented More than 50% of children with congenital Horner syndrome have a history of difficult extraction at delivery Congenital varicella infection may also be a cause If the presence of Horner syndrome is questioned, one drop of topical 4% cocaine can be instilled into both eyes This testing is best performed by ophthalmology or neurology consultants Because cocaine prevents reuptake of norepinephrine at the terminal myoneural junction of the sphincter muscle, pupillary dilation will occur normally Failure of the miotic pupil to dilate is diagnostic of Horner syndrome Knowledge of the sympathetic system anatomy can be exploited through the use of other topically applied diagnostic agents (e.g.,