I have previously discussed the important role of saccadic eye movement in visual function. Saccades are rapid eye movements that allow a person to swiftly look from one place to another in the real world. Humans can quickly shift attention to an object in the field of vision that is off to the side, above, or below.
I have described how traumatic brain injury (TBI) can cause measurable abnormalities in rapid eye movements. But imagine watching an airplane in the sky slowly moving across the horizon. The eyes slowly track the object, there are no rapid eye movements, and this is called smooth pursuit and is also very important in visual function. One can also imagine the eyes smoothly moving across a line of writing in a book or computer screen.
These types of eye movements are critical to everyday function and can also become disrupted in a concussion. The ability of the brain to keep a fixed target in sharp focus as it slowly moves is actually a complex brain task. There needs to be constant feedback to the brain from the visual system to keep the eyes and head in harmony. Humans don’t think about this process, it just happens, but it’s really like a well-trained dancer moving across the stage.
What was once effortless becomes difficult when the brain is rapidly shaken, as in a concussion. These patients often complain of fatigue and losing track of simple movements.
In a careful ophthalmologic exam smooth pursuit can be measured by watching a patient stare at the tip of a pencil as the doctor slowly moves the pencil from side to side. In normal patients, both eyes remain riveted on the pencil tip and carefully track the target. One can imagine a predator slowly tracking its prey waiting for that moment to strike. Evolution has carefully designed this system over millions of years.
In an individual with a recent auto accident and head injury the wires get scrambled. The smooth pursuit becomes jerky, with the eyes constantly over- and under-correcting each pursuit.
Most ophthalmologists never check a patient’s smooth pursuit- it takes time and a careful exam. I specialize in treating patients with acute and chronic TBI, and the disrupted elements become observable and are objective evidence of TBI. The small, halting and jerky adjustments become obvious.
The impact on everyday function can be enormous. Suddenly, what was once an unconscious process like slowly walking becomes stumbling. And if a person needs to frequently and consciously correct the “eye walk” it is no longer a dance but an exhausting hike. The brain needs rest and the results are frustrating.
The key again is to measure smooth pursuit and not just straight ahead vision. It is not difficult, but takes that extra effort by the doctor. I have personally examined many traumatic brain injury victims and can describe abnormalities in specific terms. Unfortunately, it is difficult to treat, but often improves with time. It can be an important marker of head concussion recovery- as the brain heals, the eyes can once again follow the plane across the sky. But to the patient with a traumatic injury to the brain and its intricate tracking system, the eyes will hesitate and wobble. The gentle hike becomes a bumpy road.