Observing pupil shape, location, and size A normal patient’s pupils should be round, symmetrical, and centered within the iris. The red reflex provided when viewing through the direct ophthalmoscope can be helpful when comparing the two eyes. Non-round pupillary shape can occur as a result of a surgical complication, posterior synechia from intraocular inflammation, or iris atrophy from age, ischemia, inflammation, or trauma. Other gross observations for abnormalities could include evidence of corectopia (displaced pupil), polycoria (multiple pupils), leukocoria (white pupil, which can be an ominous sign of a serious ocular form of cancer known as retinoblastoma), or iris heterochromia (difference in iris colors between the two eyes). Although pupil testing provides gross observations in these areas, the slit lamp can be used to examine the pupil and iris in more detail. Measurement of pupil size should occur under normal lighting conditions to the nearest 0.5 mm using a millimeter ruler or pupillary gauge while the patient fixates on a distant, non-accommodative target (Figure 1). To avoid stimulating the accommodative response and consequential constriction, the ruler should be held away from the visual axis of the patient. It can be particularly challenging to accurately measure the size of a patient’s pupils if his irises are dark. If needed, the clinician can view the patient’s pupils through the direct ophthalmoscope, and measure the size of the red reflex. In addition, the ophthalmoscope can also be used as a dim flashlight to measure pupils while looking from outside the instrument. In either situation, it is imperative that both pupils be illuminated equally and simultaneously. Under normal illumination, the average adult’s pupil size measures around 3.5 mm but can range from 1.0 mm to 10 mm and decreases as one ages due to senile miosis.2 Pupils should be within 1 mm in size of each other. Any difference in pupil size between the two eyes is known as anisocoria
Pupillary reaction to light The pupillary light response consists of both an afferent (optic nerve, CN II) and efferent (oculomotor nerve, CN III) pathway. Under normal conditions, when light is shone into one eye, it will cause a direct response in that eye to constrict, and a consensual response in the opposite eye to also constrict. When observing a pupil’s direct and consensual responses to light, the set-up should be normal to dim room illumination with the patient fixating on a distant non-accommodative target. Standing off to one side, the clinician should direct the transilluminator into the right eye (held approximately one inch away) and hold for two to four seconds. Make sure the light is pointed directly into the pupil—avoid holding the light too low because you do not want it directed at the patient’s cheek and watch for stray light entering the opposite eye. Constriction OD would indicate the right eye’s direct response to light. Constriction OS (with the light shone into the right eye) would indicate the consensual response of the left eye
Swinging flashlight test The purpose of the swinging flashlight test is to compare the strength of the direct pupillary response with that of the consensual response in the same eye. In a dark room, with the patient fixating on a non-accommodative distant target, the light beam is directed into the right eye and held for two to four seconds, then quickly moved to the left eye and held for two to four seconds. This process should be repeated for at least three to four cycles. When moving the light between the eyes, use a slight u-shaped motion, making sure to avoid the transilluminator crossing the patient’s visual axis, which may stimulate accommodation. It is critical that the magnitude and duration of the light be kept the same for each eye. Observe the response of the pupil receiving the light, the degree or rapidity of pupillary escape, and the response and size of the pupil not receiving the light. A normal patient should show equal direct responses between the two eyes. In addition, the rate and amount of constriction should be the same for both pupils. When the consensual response is greater than the direct response in the affected eye, then the patient is classified as having a relative afferent pupillary defect (RAPD, APD, Marcus-Gunn pupil), signifying unilateral or asymmetric damage to the anterior visual pathway.