How Useful is the Pupillary Light Reflex in Assessing Head Injury?


The term “mild traumatic brain injury,” or mTBI, is one of the most troubling misnomers in medicine. In addition to simply being incorrect (severity is implicit in the context of brain injury), the adjective “mild” implies that, in the absence of a penetrating head wound or a symptom such as loss of consciousness, early intervention may not be necessary to ensure full recovery. In the case of sports-induced injury, the lack of such obvious symptoms following mTBI and the difficulty assessing neurological damage are often the cause of continued exposure, which significantly increases the likelihood of long-term damage as a result of a single head injury. The criteria established using the Glasgow Coma Scale, the standard metric for assessing head injury, are ambiguous: does the feeling of being “dazed and confused” qualify as an alteration of consciousness? With the increased awareness of sports-related TBIs, and the growing knowledge of the dangers of continued exposure, it is critical that we establish a standard protocol for assessing mTBI.

Anecdotally, we know that examination of the pupillary light reflex (PLR), during which we check for pupillary constriction and dilation in response to the presentation and removal of a bright light, is a quick way to assess for a concussion following a head impact. Changes in pupillary dynamics, as well as other vision-related changes are common in head injuries of all severities. Head injuries that qualify as moderate or severe may present deficiencies in the PLR that are obvious to the lay observer: pupillary constriction may be sluggish, or may not occur at all.

However, literature suggests that the effects of mTBI events on pupillary dynamics, though consistent, are not robust enough to be detected by gross examination of the PLR in the event of a suspected head injury. The findings of Truong and Ciuffreda (2015, 2016), among others, suggest that mTBI does result in distinct changes in various phases of the PLR: individuals who have experienced mTBIs display decreased velocity of pupil dilation and constriction, increased latency of pupil dilation and constriction, and decreased amplitude of pupil constriction.

In other words, following mTBI, initiation of the PLR in response to a light stimulus is slower, pupil dilation and constriction is slower, and the amount of constriction is less. Though these may seem like definitive criteria, the differences in velocity and latency in these studies between the mTBI groups and the control groups is on the order of milliseconds, and would certainly not be noticeable without the use of a tool such as a pupillometer. In addition, the magnitude of the effects across all conditions is dependent upon the ambient light conditions and the brightness of the stimulus.

The bottom line—simply shining a flashlight to check for a concussion does not allow for optimal conditions to determine such subtle changes as are present in mTBI.

How effective, then, is the PLR as a diagnostic metric for assessing head injury by a concerned parent or coach on the sidelines of a sporting event? In the case of mTBI, it would be nearly impossible to notice the millisecond-scale differences in pupillary dynamics that would indicate the occurrence of a head injury. In the event of moderate or severe head injury, there will likely be other symptoms, such as loss of consciousness, blurred or double vision, or difficulty with speech, that indicate the severity of injury.

Therefore, when assessing head injury immediately after impact, we are more likely to identify mTBI if we are aware of other subtle symptoms. The presence of headache and/or nausea is a reliable indicator of mTBI, but the onset of these symptoms is often delayed, and can occur as long as 24-48 hours post injury. Instantaneous metrics that can offer some diagnostic insight require monitoring of behavior and speech immediately after and for a period of time following injury. Confusion, slow movement, tiredness, and verbalization of any symptoms are indicators that an mTBI may have occurred.

It should be noted, however, that because the presentation of symptoms of mTBI are often not robust, many experts are suggesting that we take a “better safe than sorry” approach in the context of sports-related head injuries, and remove players from the game. While the ability to identify the occurrence of a mild brain injury will typically not result in a treatment recommendation other than to rest, adherence to this recommendation is critical to preventing a mild traumatic brain injury from causing major long-term consequences.

For more information on your legal options after a brain injury, please contact Scarlett Law Group to speak with a San Francisco brain injury lawyer.

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