A concussion is medically defined as a mild traumatic brain injury that temporarily disrupts normal brain function. This injury, often from a jolt to the head or body, causes a cascade of neurophysiological changes within the skull. Because more than half of the brain’s circuits are dedicated to processing vision and controlling eye movements, visual symptoms are one of the most common and immediate indicators of injury. Although the eyes may appear physically normal, the way the brain interprets and manages visual information is significantly altered.
Subjective Changes in Visual Perception
The initial signs of a concussion often involve a noticeable decline in the quality of sight as perceived by the patient. Blurred vision is a frequently reported symptom, where objects at a distance or while reading become indistinct and difficult to resolve clearly. This is often due to the brain’s struggle to maintain proper focus, a function known as accommodation. This difficulty in focusing can lead to significant eye strain and fatigue, especially during tasks that require sustained visual effort.
Another common complaint is diplopia, or seeing double, which arises when the eyes cannot align precisely to form a single, unified image. This symptom can be particularly disorienting and may be intermittent, worsening with fatigue or movement. Some individuals also report changes to their peripheral vision, describing a sensation of “tunnel vision” or reduced awareness of their surroundings.
Observable Impairments in Eye Movement
A concussion can cause objectively observable deficits in the motor control of the eyes. One such impairment is difficulty with saccades, which are the rapid, voluntary movements the eyes make when shifting fixation from one stationary target to another, like reading across a line of text. A person with saccadic dysfunction will exhibit movements that are slowed, inaccurate, or require multiple attempts to land on the target. This inefficiency directly impacts the ability to read and concentrate.
Tracking a moving object, known as smooth pursuit, also becomes noticeably impaired after a concussion. An observer may see the patient’s eyes follow a moving finger or pen with jerky, uneven motions instead of a fluid sweep. This deficit is a direct sign of disruption in the brain pathways that coordinate eye position and velocity.
A particularly common finding is convergence insufficiency, where the eyes struggle to turn inward to maintain focus on a near object, such as a phone or a book. When testing this, a healthcare provider observes the point at which one eye drifts outward as the target is moved closer to the nose. In more severe cases, an involuntary, rhythmic, and rapid eye movement called nystagmus may be present, often indicating a profound disruption to the brainstem’s control centers.
Reflexive Changes: Pupils and Light Sensitivity
Concussions frequently affect the autonomic nervous system, leading to changes in the reflexive functions of the eyes. Photophobia, or an extreme sensitivity to light, is one of the most common post-concussion symptoms. This symptom is an uncomfortable or painful reaction to normal levels of illumination, forcing individuals to seek dim environments. The brain’s regulatory mechanisms for light intake are overstimulated, making fluorescent lighting or sunlight intolerable.
The pupils, which are controlled by cranial nerves and brainstem nuclei, may also exhibit abnormal responses to light. In a healthy eye, both pupils constrict equally and quickly when exposed to light, but after a concussion, their reaction can become sluggish or delayed. In some instances, the pupils may appear unequal in size, a condition called anisocoria, which can be an important indicator of pressure or damage affecting the midbrain. These reflexive changes suggest a direct disruption to the pathways that govern the body’s automated responses.
How Concussion Affects Visual Pathways
The various visual disturbances following a concussion stem from a widespread, functional injury to the brain rather than structural damage to the eyes themselves. The violent forces of the injury cause the brain tissue to rapidly accelerate and decelerate, which stretches and injures the delicate white matter tracts. These tracts are the brain’s communication cables, and their disruption impairs the rapid signaling needed for complex visual tasks.
The concussive impact triggers a neurochemical and metabolic cascade that temporarily slows the brain’s processing speed and efficiency. Areas like the visual cortex, which interprets images, and the brainstem nuclei, which command eye movement and pupillary reflexes, become functionally impaired.