A concussion is a mild traumatic brain injury (TBI) resulting from a blow, bump, or jolt to the head that causes the brain to move rapidly inside the skull. This sudden movement disrupts normal brain function, leading to various symptoms. Because the eyes are directly connected to the brain, pupil changes are a recognized concern and an important tool in assessing head trauma. Understanding these changes helps distinguish a mild injury from a life-threatening complication.
Pupil Changes Following Head Trauma
The most medically significant pupil change associated with head trauma is a difference in size between the two pupils, known as anisocoria. While generalized dilation can occur due to shock or pain, anisocoria is a key focus during a neurological examination. A mild concussion may not cause a sustained difference in size, but it can cause pupils to react sluggishly to light, indicating a disruption in the neural pathways controlling constriction. Although some people naturally have benign anisocoria, any new or worsening difference in size following an injury is a serious red flag.
The Neurological Basis of Altered Pupil Response
The size of the pupils is controlled by the Autonomic Nervous System (ANS), which balances two opposing forces. The sympathetic nervous system dilates the pupil (mydriasis), while the parasympathetic nervous system constricts it (miosis) when exposed to bright light. The parasympathetic pathway responsible for constriction travels along the Oculomotor Nerve, also known as Cranial Nerve III (CN III). This nerve is vulnerable to pressure or swelling within the skull. When concussive force or subsequent brain swelling occurs, the resulting pressure can impair CN III function, leading to symptoms like slow reaction or unequal pupil size.
When Pupil Changes Indicate a Medical Emergency
The most concerning pupil change is the sudden development of a fixed and dilated pupil, a classic sign of rapidly increasing intracranial pressure (ICP). This pressure is often caused by a severe traumatic brain injury (TBI), such as an epidural or subdural hematoma (bleeding on the brain). As the hematoma expands, it compresses the vulnerable CN III, paralyzing the pupil’s ability to constrict. If one pupil becomes significantly larger than the other and fails to react to light, it requires immediate professional medical attention, such as calling 911. This signifies a life-threatening situation where the brain is being dangerously compressed.
Other Crucial Symptoms of Concussion
The majority of concussions are diagnosed based on other symptoms, and the absence of pupil changes does not rule out the injury. Concussion symptoms are typically grouped into four categories:
- Physical symptoms include headache, dizziness, nausea, vomiting, or balance problems.
- Cognitive symptoms involve mental fogginess, confusion, memory issues, or difficulty concentrating.
- Emotional symptoms can manifest as irritability, sadness, anxiety, or uncharacteristic mood swings.
- Sleep-related symptoms include drowsiness, sleeping more or less than usual, or difficulty falling asleep.
Because symptoms can be delayed, anyone who has experienced a jolt or blow to the head should be monitored for any combination of these changes in the hours and days following the incident.