The proper procedure for testing pupils

Because of its potential to reveal serious retinal, neurologic or other disease, pupil testing is a crucial part of the ophthalmic examination and requires astute observation.

This procedure should be included as a component of every comprehensive examination or any time a patient needs to be dilated-in addition to any problem-focused visit involving eye health, such as a red eye visit, ocular emergency, or intraocular pressure (IOP) check.

This article will focus on the proper procedure for testing pupils as well as point out some of the more commonly encountered pupil abnormalities.

Before testing pupils, the patient should be instructed to remove her spectacle correction.

A distant, non-accommodative target two to three lines larger than the patient’s uncorrected visual acuity should be utilized. If the patient is unable to see the 20/400 E, the red/green filter can be utilized over the optotype, and the patient should be instructed to fixate on the colors.

A target that is too small for the patient might result in accommodation, with associated with pupillary constriction, which you will want to avoid when testing pupils.¹

The equipment required to perform pupil testing is minimal: all you need is a millimeter ruler or pupillary gauge and a transilluminator (which is preferred over a disposable penlight due to the intensity of the light). 

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.

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 and can be physiologic (which occurs in approximately 20 percent of normal patients), pharmacologic, or pathologic in nature.³

How do you know if the anisocoria is a problem?

And, if it is problematic, is it the bigger pupil or the smaller pupil that you need to be worried about? In order to differentiate physiologic anisocoria from a pathologic or pharmacologic cause, pupil sizes should be re-measured in bright and dark lighting conditions, isolating the parasympathetic and sympathetic pathways.

If the amount of anisocoria is the same in both the bright light and the dark, then the anisocoria is physiologic. If there is a greater amount of anisocoria in the bright light, and the larger pupil is not constricting like it should, you are likely dealing with a parasympathetic pupil problem.

Some of the more common big pupil problems include Adie’s tonic pupil, cranial nerve III palsy, and pharmacologic dilation.

If there is a greater amount of anisocoria in the dark, and the smaller pupil is not dilating like it should, you are likely dealing with a sympathetic pupil problem. Potential causes of small pupil problems include Horner’s syndrome, Argyll-Robertson pupils, and pharmacologic constriction.

Whenever anisocoria is suspected, it’s always a good idea to ask the patient if he has taken any new medications recently or if he may have gotten something in his eye. Also, be on the lookout for other clues such as ptosis or EOM involvement which might help you further narrow down your differentials.

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. Both of these responses indicate the integrity of the afferent pathway on the right side.

The procedure is repeated several times while observing and grading the magnitude (quantity) of change and rapidity (quality) of change. The magnitude is graded on a scale of 1 (small change) to 3 (large change). In terms of quality, a fast response is indicated with “+” and slow response is indicated with “-.” The procedure is then repeated with the transilluminator directed into the left eye, looking for a direct response OS and a consensual response OD.  

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, equal consensual responses between the two eyes, and equal direct and consensual responses of the same eye. 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.

When the light beam is directed into the eye with the RAPD, you will notice a reduced direct response in that eye, along with a reduced consensual response in the opposite (unaffected) eye. When the light beam is directed into the unaffected eye, both eyes will constrict normally.

When classifying a RAPD, it is important to specify which eye. It is important to carefully look for very subtle RAPDs because they can vary in magnitude.

RAPD causes include any unilateral or asymmetric damage to the anterior visual pathway, such as severe retinal disease, optic nerve disease or compromise, or a mass/lesion behind the eye. Severe but bilaterally equal disease will not cause an RAPD. An RAPD cannot be caused from disorders of ocular media or refraction, even if extreme.⁴

Important practical considerations to remember:

• A patient’s visual acuity does not necessarily correlate with the magnitude of the RAPD, although it is always a good idea to look carefully for one in cases of significantly reduced visual acuity in one eye

• RAPDs do not cause anisocoria; the pupils look the same size because of the normal consensual response

• You need to have only one functioning pupil to be able to test for a RAPD in either eye

Pupillary reaction to a near stimulus

When a patient fixates on a near target, three things should happen automatically: convergence, accommodation, and pupillary constriction.¹ This response can be tested by instructing the patient to slowly alternate fixation between a distant target and a near target.

This is rarely added to routine pupil testing because the near reflex is always present when the direct light reflex is intact. In other words, if a normal direct reaction to light is observed, then there is no need to check the near response.

If a pupil does fail to respond to light but the near response is intact, then this signifies that the afferent pathway is interrupted and the efferent pathway is intact, also known as light-near dissociation. This can be evident in certain midbrain lesions, amaurotic pupils, or aberrant regenerations.²

Putting it all together

If all of the above results are normal, then your patient has normal pupillary function. The acronym PERRLA is often used when recording pupils.

PE:  pupils equal

R:  round

RL:  reactive to light (direct and consensual)

A: responsive to accommodation (near target)

The A at the end can be left off if the near response was not tested. It is also necessary to document the absence (-) or presence (+) of a relative afferent pupillary defect and in which eye if positive.   

Pupillary testing is an important component of the eye exam. Careful observation can reveal important information about the autonomic nervous system and anterior visual pathways.

References:

1. Grosvenor T. Primary Care Optometry 5th ed.  St. Louis: Butterworth Heinemann Elsevier, 2007. Print.

2. Pensyl CD, Benjamin WJ. “Ocular Motility” Borish’s Clinical Refraction 2nd ed. St. Louis: Butterworth Heinemann Elsevier, 2006. 356-65. Print.

3. Lam BL, Thompson HS, Corbett JJ. The prevalence of simple anisocoria.  Am J Ophthalmol. 1987 Jul 15; 104(1): 69-73.

4. Wilhelm H. Neuro-ophthalmology of pupillary function-practical guidelines. J Neurol. 1998 Sept; 245(9):573-83.