After a head injury, one of the first things a healthcare provider will often do is check the patient’s eyes by shining a light in each eye and observing the response.
But why this eye test? What information does it reveal, and what does recent research say about its accuracy in detecting concussions?
In this article, we’ll discuss the latest findings on the pupillary light reflex (PLR) and explore how this simple test is becoming a valuable tool in concussion detection.
When a healthcare provider checks your eyes after a head injury, they’re looking for the “pupillary reflex” – how your pupils respond to light. The pupil, the dark center of your eye, controls how much light reaches the retina, where visual information is sent to the brain.
In bright light, your pupils constrict to limit the light; in dim conditions, they dilate to let in more light. Pupils also change size based on focus—a process called accommodation. For example, when focusing on a close object like a pencil held 6 inches (15 cm) from your eyes, your pupils constrict to sharpen the image. When shifting to a distant object, they dilate. This can be likened to a camera lens that zooms in for a narrow, detailed view or zooms out for a broader perspective.
Interestingly, pupil size can also be influenced by emotional states. Nervous before a big presentation? Your sympathetic nervous system might trigger pupil dilation.
The pupillary light reflex involves a complex interplay between the optic nerve (CN II), the midbrain, and the oculomotor nerve (CN III). Here’s a brief overview of this pathway:
Because of this 4-step connection, a quick examination of how your pupils respond to light can give healthcare providers valuable insights into brain function. Additionally, beyond the scope of this blog, research has shown that cognitive load, emotions, and the autonomic nervous system also influence the pupillary light response, meaning that stress, attention, and mood can affect how your pupils react. This highlights that the PLR is more than a simple reflex—it’s a nuanced indicator of brain activity and neurological health. Changes in this reflex may reveal underlying issues, making it a potentially helpful tool in assessing concussions and other brain injuries.
Measurable differences in the PLR between healthy and concussed individuals provide essential insights into brain function post-injury, aiding in both detection and recovery monitoring.
Aderman et al. established the first normative PLR values in healthy U.S. Military Academy cadets, showing that sex, age, race, and sleep can influence PLR measurements. These baseline values are essential for future research and can help determine how concussions impact PLR, potentially paving the way for an objective diagnostic tool for mild traumatic brain injury (mTBI) (9).
Carrick et al. observed significant differences in pupil size and reaction speed between concussed and non-concussed individuals using a specialized app, demonstrating progress in using PLR for concussion detection (1).
Maxin et al. showcased the potential of smartphone technology in mTBI diagnosis through PLR analysis. Their study demonstrated that PLR data could differentiate between concussed and healthy individuals, with a computer model achieving an impressive 93.5% accuracy (8).
Master et al. provided additional support for the use of PLR as an objective marker, observing changes in adolescent athletes with concussions (2).
On the other hand, Murphy et al. (2024) found good-to-excellent reliability for the NeurOptics PLR-3000 pupilometer in assessing female field hockey athletes but did not note significant differences between those with and without a concussion history. They did observe trends toward slower metrics in recent concussions, suggesting the need for larger studies to validate PLR as a responsive biomarker (10).
Having standard, non-concussed pupil response norms for different age groups and genders is essential for comparison when assessing potential concussions. Research has shown that PLR responses vary with age and gender, reinforcing the value of demographic-specific data for informed concussion assessment (1).
One significant finding is that while most concussion patients (approximately 60%) experience symptom resolution within 5-7 days (3,4,5,6), PLR abnormalities can persist beyond symptom resolution. This indicates that PLR could be a valuable tool for determining when full recovery has occurred.
As Carrick et al. (2021) noted, “The metrics obtained from the PLR can assist in determining whether a patient has suffered a concussion regardless of symptomatology.” While this statement may have been ahead of its time, it highlights the potential of PLR data to enhance the safety and confidence of return-to-play protocols. Using objective PLR metrics can strengthen healthcare providers’ ability to confirm an athlete’s full recovery.
Carrick et al. showed that PLR testing can be effectively conducted outside of traditional lab settings, making it feasible for use on the sidelines of sports fields, in gyms, clinics, and hospitals. This practicality is crucial as healthcare providers often express concerns about the accessibility and cost-effectiveness of concussion detection tools (1).
A recent study by McGrath et al. highlighted the PupilScreen smartphone app’s success in distinguishing between normal and abnormal PLR data. This development provides a practical and cost-effective tool for improving concussion detection. However, it’s worth noting that McGrath’s study focused on severe TBI, which limits its direct application to concussions, typically categorized as “mild TBI” (7).
The pupillary light reflex (PLR) is proving to be a valuable tool in understanding brain function and detecting concussions. Advances such as the PupilScreen app are making PLR assessments more accessible and cost-effective, even outside traditional clinical environments. As research continues to expand, these technologies and objective biomarkers could play a significant role in concussion evaluation and recovery monitoring.
While more research is needed to confirm the PLR’s utility across broader populations and use cases, the current findings represent a meaningful step forward in concussion care.
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