Pupil

Pupilla

Also known as: pupillary aperture, miosis, mydriasis, anisocoria

The central round aperture of the iris (≈2–8 mm) through which light enters the posterior eye; its size is set by the balance of parasympathetic (constrictor) and sympathetic (dilator) tone.

Show on body map Eye — Horizontal Section

Overview

Diameter in bright light≈2–4 mm

Diameter in darkness≈4–8 mm

Physiological anisocoria<1 mm in ≈20% of people

The pupil is slightly nasal to the centre of the iris. Normal diameter is ≈2–4 mm in bright light and ≈4–8 mm in darkness; pupil size decreases with age (senile miosis). Pupils are normally equal (round, central, equal), though ≈20% of people have physiological anisocoria of <1 mm that is equal in light and dark.

The pupil constricts (miosis) in bright light (pupillary light reflex), during near vision (part of the near triad), in sleep and with opioids or cholinergic drugs; it dilates (mydriasis) in darkness, with sympathetic arousal (fear, pain), and with anticholinergic or sympathomimetic drugs. Smaller pupils increase depth of field and reduce aberrations; larger pupils admit more light at the expense of image quality.

Pupil examination — size, shape, direct and consensual light responses, the swinging-flashlight test and the near response — is one of the most informative parts of the neurological examination, because the afferent (optic nerve) and efferent (oculomotor, sympathetic) limbs can be tested separately.

Location: Centre of the iris, slightly inferonasal; aligned with the lens.

Function

Control retinal illumination
Improve depth of focus and image sharpness (pinhole effect)
Provide a window to brainstem and autonomic function

Physiology: Pupil area can vary ≈16-fold (2–8 mm diameter), only a fraction of the ≈10¹⁰ range of light intensities the visual system handles — most adaptation is retinal. Constriction latency to light ≈200–300 ms.

Clinical significance

Relative afferent pupillary defect (Marcus Gunn pupil)

In the swinging-flashlight test, both pupils dilate when the light swings to the affected eye, because that eye's optic nerve sends a weaker signal. Indicates asymmetric optic nerve disease (optic neuritis, ischaemic optic neuropathy, severe glaucoma) or extensive retinal disease; cataract does not cause an RAPD.

Pharmacological pupils

Pinpoint pupils: opioids, organophosphates, pontine haemorrhage. Dilated pupils: anticholinergics (atropine, scopolamine patch, tricyclics), sympathomimetics (cocaine, amphetamines), oculomotor nerve compression (e.g. uncal herniation — a unilateral fixed dilated pupil in a drowsy patient is a neurosurgical emergency).

Light–near dissociation

Pupils that constrict better to near effort than to light: Argyll Robertson pupils (small, irregular — neurosyphilis), Adie tonic pupil (large, ciliary ganglion damage), dorsal midbrain (Parinaud) syndrome and severe bilateral afferent disease.

Test yourself

A 65-year-old hyperopic woman has a sudden painful red eye, haloes, a hazy cornea, vomiting and a fixed mid-dilated pupil after watching a film in a dark cinema. Which drug would be contraindicated?

Pilocarpine
Acetazolamide
Atropine
Timolol

Answer: Atropine

This is acute angle-closure glaucoma. Mydriasis (darkness, anticholinergics) crowds the peripheral iris into the angle. Atropine dilates the pupil and would worsen closure; pilocarpine, acetazolamide and timolol lower IOP, followed by laser peripheral iridotomy.

In the swinging-flashlight test, both pupils dilate when the light moves from the right to the left eye. Where is the lesion?

Right oculomotor nerve
Left optic nerve
Left oculomotor nerve
Right optic tract
Left ciliary ganglion

Answer: Left optic nerve

This is a left relative afferent pupillary defect (Marcus Gunn pupil): the left optic nerve conducts a weaker signal, so the consensual constriction driven by the right eye is lost when the light swings to the left eye. Efferent (CN III) lesions cause anisocoria instead.

Which statement about the pupillary light reflex is correct?

It requires an intact primary visual cortex
Its efferent limb is sympathetic via the superior cervical ganglion
Pretectal neurons project bilaterally to the Edinger–Westphal nuclei, producing a consensual response
Its postganglionic fibres travel in the long ciliary nerves

Answer: Pretectal neurons project bilaterally to the Edinger–Westphal nuclei, producing a consensual response

Optic tract fibres reach the olivary pretectal nucleus, which projects to both Edinger–Westphal nuclei (via the posterior commissure). Efferents: CN III → ciliary ganglion → short ciliary nerves → sphincter pupillae. Cortical blindness spares the light reflex.