| Feature | Pupil Size | Light Reflex | Near Reflex (Accommodation) | Pathophysiology | Clinical Pearls | Mnemonic |
|---|---|---|---|---|---|---|
| Argyll-Robertson Pupil | Small (miotic) and irregular, often bilateral but can be asymmetric. | Absent. Pupils do not constrict to direct or consensual light. | Intact and brisk. Pupils constrict normally when focusing on a near object. | Believed to be a lesion in the dorsal midbrain, near the Edinger-Westphal nucleus, that selectively damages the light reflex pathway while sparing the near reflex pathway. | Neurosyphilis (classically). Other causes include diabetes, chronic alcoholism, and Lyme disease. | ARP - PRA |
| Accommodation Reflex Present (ARP) but Pupillary Reflex Absent (PRA) |
"Prostitute's Pupil": Accommodates but doesn't react (to light). Classically associated with late-stage syphilis. | | Marcus Gunn Pupil | Normal pupil size at rest. Anisocoria (unequal pupils) is absent. | Impaired (afferent defect). The affected pupil paradoxically dilates when a light is swung from the unaffected eye to the affected one during the "swinging flashlight test." | Normal | A lesion in the afferent pathway (optic nerve or retina) that reduces the signal sent to the brain. This can be caused by optic neuritis, severe glaucoma, or retinal detachment. | Optic neuritis (often a sign of multiple sclerosis), severe glaucoma, or retinal detachment. | "Swinging Flashlight Test": The key diagnostic test showing the paradoxical dilation. | | Holmes-Adie Pupil | Unilaterally or bilaterally large (dilated). | Sluggish or absent. Pupils constrict very slowly and poorly to light. | Slow and "tonic". Pupils constrict slowly and tonically (sustained) when focusing on a near object. They are also slow to redilate. | Damage to the ciliary ganglion or post-ganglionic parasympathetic nerves to the eye. This is often idiopathic but can be caused by viral infections. | Holmes-Adie Syndrome, which includes absent or reduced deep tendon reflexes, most commonly in the Achilles tendon. | "Tonic Pupil": Slow, tonic constriction to near objects and slow redilation. | | Horner’s Syndrome | Small (miotic) pupil on the affected side. | Normal. The pupillary light reflex is intact because the parasympathetic pathway is unaffected. The problem is with dilation. | Normal | A lesion anywhere along the three-neuron sympathetic pathway, which originates in the brain, travels down the spinal cord, and up to the head and eye. | Causes can range from benign to life-threatening (e.g., Pancoast tumor in the lung apex, carotid artery dissection, stroke, or neck trauma).Ptosis (drooping eyelid) and anhidrosis (decreased sweating) on the same side of the face. Anhidrosis may not be present with post-ganglionic lesions. Anisocoria (unequal pupil size) is most obvious in the dark, as the affected pupil cannot dilate properly. | "SPAM": Sympathetic nerve, Ptosis, Anhidrosis, Miosis. | | Hutchinson's Pupil | Initially normal or slightly constricted, then becomes dilated and fixed. | Absent. The pupil is "fixed" and does not react to light. | Absent | Compression of the oculomotor nerve (cranial nerve III) due to a mass effect, most commonly from a brain hemorrhage, tumor, or swelling pushing the brain against the tentorium cerebelli. The pupillary fibers are on the outside of the nerve, making them vulnerable to early compression. | Altered level of consciousness, head injury, and signs of increased intracranial pressure.
This is a grave sign and a medical emergency. The progression to a fixed, dilated pupil signals a critical increase in pressure and potential brain herniation. | "Herniation": A sign of impending brain herniation. |