Examine / Pupil Lab
Light right. Watch both. Swing left. Watch again.
The pupils test the optic nerve on one side and the third nerve and the sympathetic chain on the other, in thirty seconds, with a torch. Learn the two limbs of the reflex, then examine eight patients whose pupils obey the physiology.
Dim room
A model, not a recording: each eye has an afferent gain, a sphincter gain, a resting size in the dark and the light, and a speed; both pupils are driven by what both eyes see. The drawing is a diagram of pupil size, not a photograph.
The patient
A 24-year-old with no visual complaint.
Set the room light, watch the resting sizes, then shine and swing the light. What is wrong, and which side?
Read the pupils
- The swinging flashlight test. Shine into one eye for two or three seconds, then swing quickly to the other and watch that pupil as the light arrives. Normal: it stays small or constricts further. Afferent defect: it dilates, because the drive from the worse eye is less than the consensual drive it had a moment ago. Grade the RAPD by how much it dilates.
- Anisocoria: which pupil is wrong?. Measure both in the light and in the dark. A difference greater in the dark is the small pupil failing to dilate: a sympathetic problem (Horner). A difference greater in the light is the large pupil failing to constrict: a parasympathetic problem (third nerve, tonic pupil, a drug). The same in both: physiological.
The physiology
- The afferent limb
- Retina, optic nerve, chiasm and tract, then the fibres leave before the lateral geniculate for the pretectal nucleus, which projects to BOTH Edinger-Westphal nuclei. That double projection is why light in one eye constricts both pupils.
- The efferent limb
- Parasympathetic fibres travel with the third nerve, on its surface, to the ciliary ganglion and the sphincter pupillae. Compression reaches them first, which is why an aneurysm gives a dilated pupil and a diabetic microvascular palsy usually spares it.
- The sympathetic path
- Three neurons: hypothalamus to the ciliospinal centre at C8 to T2; out over the lung apex to the superior cervical ganglion; up along the carotid to the dilator pupillae and Müller's muscle. A lesion anywhere along it gives a Horner syndrome.
- The swinging flashlight test
- Shine into one eye for two or three seconds, then swing quickly to the other and watch that pupil as the light arrives. Normal: it stays small or constricts further. Afferent defect: it dilates, because the drive from the worse eye is less than the consensual drive it had a moment ago. Grade the RAPD by how much it dilates.
- Anisocoria: which pupil is wrong?
- Measure both in the light and in the dark. A difference greater in the dark is the small pupil failing to dilate: a sympathetic problem (Horner). A difference greater in the light is the large pupil failing to constrict: a parasympathetic problem (third nerve, tonic pupil, a drug). The same in both: physiological.
- What the pupils cannot tell you
- An RAPD is relative. Two equally damaged nerves give no RAPD, and dense cataract or a macular problem rarely gives one. The pupils test the nerve, and a symmetrical disease hides from them.
The pupil with a droopy lid and a big eye: proptosis and the pupil. The nerve behind the RAPD: neuro-ophthalmology.