Pupil size

Examination of the pupil must include assessment of the size, symmetry and reflexes.

How is pupil size measured?

Although pupil size is often guessed, a ruler will provide a more accurate measure. If a ruler is unavailable, or in an emergency, either use a Haab scaleor, remembering that a normal cornea measures 12 x 12mm, make a rough estimate of the proportion that the pupil takes up and, thereby, its size, e.g. 50% = 6mm.

How do you know if the pupil is dilated or constricted?

In a situation where a comparison cannot be made with the other eye or it is clinically important to judge if the pupils are dilated or constricted, the normal range for the size of pupils in different light conditions is often guessed. A study of pupillary size in bright (penlight or ophthalmoscope) and fluorescent light found that pupil sizes greater than 3.6mm or less than 1.9mm in bright light, or greater than 5mm or less than 2mm in fluorescent light, were likely to be abnormal [13].

Learning bite

A pupil smaller than 2mm or larger than 5mm measured in a room lit by fluorescent light is likely to be pathological.