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Videonystagmography for diagnosing brain dysfunction
When both of your eyes are not looking at the same object this can cause you to experience a myriad of different issues. Many patients report problems with eye movements, reading, visual attention, accommodation, visual motion sensitivity, neck pain, headaches, and other visually related aspects. These are all common complains that can occur after a traumatic brain injury but let’s discuss what may cause an individual to experience this in the first place.
During a concussive event, of course the patient can experience a whiplash injury which can cause damage to the structural integrity of the cervical spine and can lead to pain in the neck and even into the head or jaw but let’s take an even closer look at the mechanism behind a traumatic brain injury and how that can lead to deviation of the eyes.
The brain is encased in a boney vault that we call a skull and when we experience these jarring forces of the head during a concussion your brain can become injured in a multitude of ways. Depending on the force of the injury you can experience anything from a skull fracture, vertebral fracture, contusion, cerebral bleeding, subdural bleeding, etc but the brain also sits on top of a brain stem. Think of it like a basketball sitting on top of a post and since the brain is round, when it hits, it rotates, and your brainstem takes the brunt of the rotational forces which leads to sheering of the neuronal pathways throughout the brainstem. Many times this can lead to patients experiencing vestibular complaints, visual complaints, autonomic complaints, etc.
Think of each one of your eyes as an individual camera and in order for you to be able to experience clear vision both of these cameras need to be looking at the exact same location. When these cameras are off by a mere 2 degrees an individual can experience double vision and even blurry vision. There is a lot of complex machinery involved that allows you to be able to lock both of your eyes on the same target. These pathways are very susceptible to injury during head trauma because of the long winding route that they take as they traverse their way from the cortex down into the brainstem and eventually out to the individual eye muscles. When this happens the visual and oculomotor centers in the brainstem can become physiologically altered and lead you to experience all the symptoms mentioned above.
The most obvious mechanism that could cause an individual to experience one of these symptoms would be if there was actual damage to the eye muscle(s). The eyes were designed to be situated in the center of the orbit and there are 6 different muscles in each eye that contribute to the movement of each eye. These eye muscles are like rubber bands that are always fighting to remove the tension placed on them. For example, if an individual were to experience damage to the lateral rectus muscle which is involved in excursion of the eye outward the patient could display an eye that has a tendency to come in due to the lack of the elastic pull of the eye out.
During a traumatic brain injury an individual could also experience damage to the abducens nerve and or abducens nucleus. These structures are last stop along the route before relaying the signal to the lateral rectus muscle to contract when placed in the situation where you need/want to look laterally.
Lastly, an individual could experience these symptoms due to damage to the vestibular system. The vestibular system plays an intimate roll with the oculomotor system mainly through the vestibulo-ocular reflex (VOR). The VOR is the mechanism that allows you to keep your gaze steady during movement of your head because the visual system alone is not fast enough to compensate for the various perturbations of the head that you experience on a daily basis.
For example, take your thumb and place it out in front of you. Now move your thumb back and forth and take note of how fast you have to move your thumb before it begins to get blurry. Now keep your thumb steady and shake your head at that same speed. What you should see if that your thumb remains clear even though your eyes are moving just as fast as they were when you were moving only your thumb. When you move your head the vestibular system relays that information to the brainstem and the oculomotor nuclei to coordinate your eyes moving in an equal and opposite direction, so your gaze stays steady and the object of interest doesn’t become blurry. The vestibular system, just like the eyes utilize a push pull mechanism meaning if the vestibular system is activated on one side, the opposing area on the other side is reflexively inhibited. Therefore, during a head injury, the patient can damage various structures within the vestibular system and experience a reflexive upregulation of the vestibular system on one side compared to the other.
For example, if I damaged my right vestibular system, I will have a higher level of resting activation on my left side. These signals are sent to the brainstem and tell your brain that your head is turned to the left which is then compared against what the input from your neck is saying about where it is, and finally what your visual system is saying about where the environment is relative to you. Your brain then has to try and figure out where the poor or incorrect information is coming from. What this can lead to is if you damage the right vestibular system and the left side is overactive the eyes will be pushed to the right because your brain thinks your head is rotated to the left which can then cause you to experience these various visual complaints.
There are multiple reasons as to why an individual could experience these symptoms and with each different area of dysfunction, they require their own unique treatment applications. This is why it is important to find a practitioner that is well versed in the pathophysiology of concussion to be able to confidently and accurately localize the region of dysfunction so that they can apply the appropriate treatment applications in order to get you back to the you that you once were.
Dr. Matthew Shem DC