Other factors to take into account when measuring and comparing pupils are:

Repeated pupil size measurements by the same or different observers

Research has found high levels of agreement between observers as long as formal measurement of papillary size was undertaken with a Haab scale. Simple estimation was associated with poor correlation [14].

Difference between pupil sizes

Table 1: Pathological causes of anisocoria
Previous surgery
Traumatic mydriasis from blunt eye trauma
Topical medication
Oculomotor nerve palsy (dilated pupil)
Holmes-Adie syndrome (dilated pupil)
Horner’s syndrome (constricted pupil)
Argyll Robertson pupil (constricted pupil)

Anisocoria (unequal pupil sizes) is found in 19% of normal subjects at any one time [15] and is more apparent in the dark than the light[16]. Unilateral miosis may be found in the acute red eye and is more prevalent with more serious problems (e.g. keratitis, uveitis) [17]. The mechanism is thought to be due to release of prostaglandins into the eye which act on the sphincter pupillae. Other pathological causes of unequal pupils are shown in Table 1.

Learning bite

Approximately 20% of the normal population has unequal pupils. A smaller pupil is often found in acute red eye.