Until better evidence settles the localization of the AR pupil, it is appropriate to screen patients with bilateral tonic pupils for syphilis. Marcus Gunn pupil (MGP) is the term given to an abnormal pupil showing aberrant pupillary response in certain ocular disorders. If segmental iris sphincter palsy is found and the light-near dissociation has tonic features, one must conclude that the mechanism of the pupil disorder is a ciliary (peripheral) rather than a midbrain (central) denervation. To describe the differential diagnosis of. The Argyll Robertson (AR) pupil has been defined as a pupil that is small and constricts poorly to direct light but briskly when a target within reading distance is viewed ('light-near dissociation'). Resolving the issue about the location of the syphilitic lesion that produces the AR pupil will depend on careful examination of patients with techniques designed to disclose segmental palsy of the iris. To list the causes for light-near dissociation (e.g., Argyll-Robertson pupils, diabetic neuropathy, tonic pupil). However, lesions in this region have not been reliably demonstrated in syphilis. Because the AR pupil lacks these features, it has been attributed to a dorsal midbrain lesion that interrupts the pupillary light reflex pathway but spares the more ventral pupillary near reflex pathway. Such features are considered typical of the light-near dissociation of Adie syndrome and of neuropathic tonic pupils, where damage to the ciliary ganglion or ciliary nerves is believed to be the mechanism. Most descriptions of the AR pupil do not mention segmental iris sphincter constriction, or slow, sustained constriction with a near vision effort. The Argyll Robertson (AR) pupil has been defined as a pupil that is small and constricts poorly to direct light but briskly when a target within reading distance is viewed ("light-near dissociation").
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