Mechanism of Argyll-Robertson Pupils
The Argyll-Robertson pupil results from damage to the ciliary ganglion or ciliary nerves (peripheral denervation) rather than a dorsal midbrain lesion, causing light-near dissociation where pupils constrict to accommodation but not to light. 1
Pathophysiologic Mechanism
The traditional teaching attributed Argyll-Robertson pupils to a dorsal midbrain lesion that selectively interrupts the pupillary light reflex pathway while sparing the more ventral near reflex pathway. 1 However, this central mechanism has been challenged by evidence showing that lesions in this midbrain region have not been reliably demonstrated in syphilis patients. 1
Evidence for Peripheral Mechanism
- Segmental iris sphincter abnormalities and irregular iris margins are consistently found in patients with Argyll-Robertson pupils, suggesting peripheral nerve pathology rather than central lesions. 2
- Abnormal striations of the iris occur in light-rigid pupils associated with neurosyphilis, further supporting a peripheral mechanism. 2
- Some pupils with light-near dissociation in neurosyphilis demonstrate cholinergic supersensitivity and tonic features characteristic of ciliary ganglion or ciliary nerve damage. 3
- When mydriasis is induced with cocaine or ephedrine in these pupils, they can regain the ability to constrict to light, indicating the lesion is at the peripheral nerve level. 2
Clinical Features
The classic Argyll-Robertson pupil presents with:
- Small, miotic pupils (typically bilateral) that constrict poorly or not at all to direct light 1, 4
- Brisk constriction when viewing near targets (light-near dissociation) 1
- Irregular pupillary margins and iris abnormalities 2
- Absence of segmental iris sphincter constriction or slow, sustained tonic constriction that characterizes Adie's tonic pupil 1
Pathognomonic Association
Argyll-Robertson pupils are regarded as pathognomonic of neurosyphilis, particularly in the context of tabes dorsalis or general paresis. 4 The finding should prompt immediate serologic testing for syphilis. 3
Anatomic Localization
The weight of evidence suggests the lesion occurs at the ciliary ganglion or short ciliary nerves rather than in the dorsal midbrain, though the exact mechanism by which Treponema pallidum causes this selective peripheral denervation remains incompletely understood. 1, 2 Until definitive pathologic studies clarify the precise localization, patients with bilateral tonic pupils should be screened for syphilis. 1, 3
Clinical Caveat
Unilateral Argyll-Robertson pupils can occur, though bilateral presentation is more typical. 2 The presence of any light-near dissociation with miotic pupils and iris irregularities warrants syphilis serologic testing regardless of laterality. 2, 3