A concussion is a mild traumatic brain injury resulting from a blow, bump, or jolt to the head, or a hit to the body that causes the head and brain to move rapidly. This sudden movement can temporarily disrupt normal brain function, which may manifest through various physical and cognitive symptoms. Since the brain controls every bodily function, including the eyes, checking the pupillary response to light is a simple and immediate assessment that can be performed safely on the sideline or at home. Observing how the eyes react provides a quick, non-invasive window into the state of the central nervous system following an injury.
How Head Trauma Affects Eye Function
The brain controls pupil size through a reflex arc involving pathways in the central nervous system. When light hits the eye, the signal is carried by the optic nerve (Cranial Nerve II), the sensory pathway, traveling to the midbrain. The midbrain processes involuntary functions and sends a response signal via the oculomotor nerve (Cranial Nerve III), the motor pathway. Cranial Nerve III instructs the iris muscle to constrict the pupil, adjusting the amount of light reaching the retina. Head trauma can cause swelling or pressure that stresses the brainstem or Cranial Nerve III, leading to a delayed, weak, or absent pupillary response.
Performing the Pupillary Light Reflex Check
To perform this check, you need a small, bright light source, such as a penlight or a smartphone flashlight, and a semi-dark environment. The room must be dark enough for the pupils to dilate naturally, but light enough for clear observation. Ask the person to focus their gaze on a distant object over your shoulder to prevent constriction due to near focus.
Begin by gently shielding one eye with your hand. Shine the light directly into the center of the first eye from the side, holding it steady for one to two seconds while observing the pupil’s immediate constriction. This is called the direct response.
While keeping the light aimed at the first eye, quickly check the pupil of the other eye to see if it also constricted simultaneously. This simultaneous constriction in the unstimulated eye is the consensual response, confirming the integrity of the bilateral reflex pathway. Repeat the entire process on the second eye, checking for both the direct and consensual responses.
Understanding the Results: Normal Versus Abnormal
A normal pupillary light reflex is characterized by a rapid, brisk constriction in both the illuminated eye and the opposite eye when light is introduced. The pupils should also be equal in size before the light is shined and should constrict to a similar size. A healthy response indicates that the afferent and efferent neurological pathways are functioning correctly.
An abnormal finding is any deviation from this brisk, equal reaction, and it serves as a sign of potential neurological compromise. A sluggish response means the pupil constricts slowly or weakly, rather than quickly, which is a common finding following a concussion or mild TBI. A more serious sign is a fixed or non-reactive pupil, where the pupil remains wide and shows no change in size when exposed to light, suggesting significant pressure on the oculomotor nerve.
Another serious finding is anisocoria, which is a noticeable difference in size between the two pupils. If one pupil is significantly larger or smaller than the other, it can signal increasing intracranial pressure from internal bleeding or swelling. Any of these abnormal findings suggest that the force of the injury may have impacted the brain’s control centers.
When to Seek Emergency Medical Care
While performing the flashlight check is a valuable on-site assessment, it is a screening tool, not a diagnostic one. Any person with a suspected concussion, regardless of the eye check results, requires a professional medical evaluation.
However, the presence of specific findings warrants immediate emergency medical care. You must call emergency services if the individual exhibits unequal pupil sizes (anisocoria) or if one or both pupils are fixed and non-reactive to light. Other red-flag symptoms include a loss of consciousness, repeated or forceful vomiting, or the onset of a seizure. A severe or worsening headache, increasing confusion, or difficulty waking the person also require an immediate trip to the emergency room.