The pupil is the round opening in the centre of the iris through which light reaches the lens and retina. Its size normally ranges from about 2 to 8 mm and is set by the balance between the parasympathetic sphincter pupillae and the sympathetic dilator pupillae. It constricts in bright light and for near vision, and widens in dim light or with arousal.
| Location | Centre of the iris, in front of the lens |
|---|---|
| Parts |
|
| Nerve Supply | Oculomotor nerve (CN III) parasympathetic fibres via the ciliary ganglion and short ciliary nerves; Sympathetic fibres from the superior cervical ganglion via the long ciliary nerves |
| Blood Supply | Major arterial circle of the iris |
Acts as an adjustable aperture that controls retinal light exposure and increases depth of field when small.
The pupillary light reflex tests the optic nerve (afferent) and oculomotor nerve (efferent). A relative afferent pupillary defect, shown by the swinging flashlight test, indicates optic nerve or severe retinal disease on one side. A new dilated, unreactive pupil with ptosis suggests oculomotor nerve compression, for example by an aneurysm, while a small pupil with ptosis suggests Horner syndrome.
Both pupils dilate when light swings to the affected eye, showing reduced input from that eye's optic nerve or retina. Common causes are optic neuritis and ischaemic optic neuropathy.
A small pupil with mild ptosis and sometimes reduced facial sweating from interrupted sympathetic supply. Causes include carotid dissection and apical lung tumours.
A dilated pupil that reacts poorly to light but slowly to near effort, from damage to the ciliary ganglion. It is usually benign.
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