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A Short Course in Evaluation of Pupil Sizes

In the case of brain injury (concussion, for example) the pupils sometimes give us information about where and how severely the brain might be effected.

Normal Responses to Light and dark

There are two main pupillary responses to light, the Direct and Indirect.

When a light is shined into the eye, its strongest reaction is from the Direct Response, and is just what you expect: the pupil should get smaller when a light is shined directly into it. The Indirect Response is when fellow eye sends messages to the tested eye, telling it to dilate again, because the fellow eye is still in the dark.

The Direct Response of the pupil to its own environment should always be the stronger, over the Indirect Response.


Look at both pupils in normal room light, to get an idea of two things: How large or small they are under usual conditions, and if they are roughly equal in size when compared.

Darken the room and wait one or two minutes for the pupils to adapt to the darkness. The expected response here is that both pupils will dilate (expand) to allow more light into the eye under low light conditions.

  1. Using a narrow-beam bright flashlight such as a penlight (NOT a laser pointer!), shine the light into either eye, but not into the other one just yet. It is not necessary that the patient cover the other eye, just that light isn’t allowed to cross the bridge of the nose into the other eye.
  2. Watch the pupil as you shine the light into the eye; it should start out somewhat larger to begin with than it was in normal room light. (Exactly how large depends on a lot of other factors, like how dim the room is, and how bright the light is, and any meds the person may be using, so don’t worry about exactly HOW large, just that it is larger than “normal” for that eye.)
  3. As the pupil reacts to the bright light, its normal response will be to get smaller. This first Direct Response is the pupil reacting to the dimness of the room and then to the bright light. There should be a rapid decrease in pupil size, and then, as the tester watches without removing the light just yet, it will get slightly larger and “bounce” its size up and down a little bit. This is normal and expected
  4. As the light is kept on the first eye for several seconds, the pupil should maintain its smaller size (with small variations as it “bounces” slightly), and should not get significantly larger as you watch.
  5. Remove the light and wait a few seconds for the eyes to equalize in the dim light again, and test the second eye as you did the first. Again, expected response is for both pupils to get larger in the dimness, and then to react quickly and strongly to the bright light by getting smaller and then bouncing a little as you watch.
  6. Watch the pupils with the bright light on each eye for several seconds. Expected results are that each pupil should maintain its contraction from the bright light, with slight variations and size “bounce.”
  7. Take the time needed to allow the pupils to react, both to the light and to the dimness in the room. Pupils do not react instantaneously, and require at least a little time to expand and contract.

Expected Result: Pupils are equal in size, right to left, and react equally to bright light shined directly into the eye by contracting quickly and then re-dilating slightly, bouncing in size.

The Indirect Response comes from the fellow eye, still in dim light, trying to get the tested eye to expand again, because the fellow eye is still in dim light conditions. The small amount of pupil enlargement after the initial contraction from the bright light, and the “bouncing” pupil size are both due to the indirect response. This is normal and expected.

Unusual Findings

The pupils, as already stated, should be equal in size, shape (round) and reaction to bright and dim light. Both Direct Response and Indirect Response should be present and equal. The reaction to bright light – the Direct Response – should always override that coming in from the fellow eye – the Indirect Response, but both Direct and Indirect should be present and equal.

Any variation in this should be noted. Unequal pupil size and/or reaction to light and dark conditions are possible cause for concern.

A small percentage of the population has unequal pupils, known as essential anisocoria, but the reactions to light and dark, direct and indirect, should still be present and equal, allowing for the difference in starting pupil size, (which is usually less than 2 mm or so). Some people may already be aware that their pupils are always slightly different, but for some, the first notation of unequal pupils only comes after a head injury and can make it difficult to differentiate from essential anisocoria. In general, pupils that are two or more millimeters different in size should be evaluated in more detail by an eyecare practitioner.