Examination of the Pupil

1. Neuroanatomical Basis

• Light stimulates retinal rods and cones → optic nerve → pregeniculate optic tracts → pretectal nucleus (midbrain-diencephalic junction) → bilateral Edinger-Westphal nuclei → travel with CN III → ciliary ganglion → ciliary nerves → pupillary sphincter muscle.

• 1st-order neuron: Ventral lateral hypothalamus → descends ipsilaterally through brain stem → synapses at C8-T2 (intermediolateral column)
• 2nd-order (preganglionic) neuron: Exits with ventral roots of C8, T1, T2 → ascends the neck → superior cervical ganglion
• 3rd-order (postganglionic) neuron: Follows the internal carotid artery → ophthalmic branch of CN V → pupillodilator muscle

2. Method of Examination

Step 1 - Assess Size and Shape

• Examine in a dimly lit room (pupils naturally dilated).
• Shine a dim light from below so both pupils are seen simultaneously in indirect illumination.
• Record size in millimeters (normal: 2-6 mm) and note whether pupils are equal, round, and regular.
• Anisocoria (unequal pupils): measure in both darkness and bright light to distinguish cause.

Step 2 - Direct and Consensual Light Reflex

• Ask the patient to fixate on a distant target (to minimize accommodative miosis).
• Shine a bright light directly into one eye - note direct constriction in that eye.
• Note consensual constriction in the opposite eye simultaneously.
• Repeat for the other eye.

Step 3 - Swinging Flashlight Test (for RAPD)

1. Patient fixates on an immobile distant target.
2. Use the brightest light available for maximum constriction.
3. Grade each pupil's direct light response on a 1-4 scale.
4. Swing the light between eyes at ~1 second intervals (swinging flashlight test).
5. Observe each pupil for initial constriction or dilation.
6. Limit to 3-4 swings to avoid retinal bleaching.

Note: RAPD is absent when both optic nerves are equally damaged. Severe inner retinal pathology can also produce RAPD.

Step 4 - Near (Accommodation) Reflex

• Ask the patient to look at a distant object, then quickly at their thumb held 15-30 cm away.
• Normal: pupil constricts (as part of the accommodation-convergence-miosis triad).
• A near reaction can be elicited even in a blind patient via proprioceptive influences.

Recording Format

3. Pupillary Responses with Lesions

4. Light-Near Dissociation

• Adie tonic pupil
• Dorsal midbrain syndrome (Parinaud syndrome)
• Severe bilateral optic neuropathies
• Argyll Robertson pupils

5. Common Pupillary Abnormalities

Physiologic Anisocoria

• Long-standing unequal pupils with normal reflex reactions in both eyes.
• Disparity remains constant in both light and dark.
• Benign; no localizing significance.

Adie Tonic Pupil

• Medium-to-large pupil (3-6 mm) that constricts little/not at all to light, very slowly to accommodation.
• Constricts with dilute pilocarpine 0.125% (denervation hypersensitivity).
• Usually unilateral, more common in women aged 25-45.
• Likely post-viral denervation of ciliary ganglion.
• Benign; may have associated areflexia (Adie syndrome).

Horner Syndrome

• Miosis (small pupil)
• Ptosis (partial, from Müller muscle paresis)
• Anhidrosis (of forehead/face, variable with postganglionic lesions)
• Also: enophthalmos, lower eyelid elevation ("upside-down ptosis")

Argyll Robertson Pupils

• Small (1-2 mm), unequal, irregular pupils.
• Fixed to light (no direct or consensual reaction).
• Constrict briskly to near (light-near dissociation).
• Classical cause: tertiary neurosyphilis.

Pharmacologic Mydriasis

• Dilated, unresponsive pupils from mydriatic drugs (atropine, scopolamine patches).
• Fails to constrict with 1% pilocarpine - distinguishes from CN III palsy (which does constrict with pilocarpine).

Third Nerve Compression

• Dilated pupil (6-7 mm), unresponsive to light or near.
• Always associated with other CN III signs.
• Causes: posterior communicating artery aneurysm, brain herniation, neoplasia.

• Goldman-Cecil Medicine - Examination of the Pupil, Pupillary Pathways
• Bradley & Daroff's Neurology in Clinical Practice - Examination of the Pupils, Box 16.1
• S Das: A Manual on Clinical Surgery, 13th Edition - Oculomotor Nerve Examination