If scenes like the picture above bother you, there is a chance you have VVM.
VVM is a common phenomenon we are seeing post-concussion. It is intimately tied to our balance (and its perception), the function of our autonomic nervous system and ultimately our quality of life. It is something almost all of us take for granted as it developed normally in almost all of us from infancy to adolescence.
So, what is it we are discussing here?
Let us back up and talk about dizziness a bit first. Many of our post-concussion patients complain of dizziness and/or imbalance. Ever wonder about the factors that contribute to your balance and/or dizziness? What gives one person the ability to balance on a tight-rope or score an acrobatic goal?
The systems involved are the following:
1. The vestibular system – The vestibular system is discussed in a general way elsewhere on this website. It is the gyroscope of the body that tells your brain how you are oriented relative to gravity. The brain uses this information in the following ways:
2. The somatosensory system: by 3 years of age, somatosensory effectiveness in postural control emerges. The somatosensory system is a broad system of nerves and pathways that relay information about sensations detected all over your body to your brain. There are different types of sensory nerves that have specialized functions. Some of the things your somatosensory system detects are: heat, pain, touch, chemicals, proprioception (a signal to represent the position of a limb relative to an adjacent limb at any given moment), haptic perception (the ability to judge the way your environment is while you are moving, i.e., like a blind person getting around with a probing cane or white cane), and kinesthesia (sense of movement). Anyone who has fumbled around in a dark room before can appreciate how the somatosensory system was helpful.
3. The visual system: although visual system’s effectiveness in postural control is less mature than that of the somatosensory system before age 7.5 years, it remains the the dominant source of information for postural control in standing in children less than 7.5 years old. The visual system is complex and performs many tasks. Some of these tasks include visual acuity, contrast sensitivity, pattern perception, facial perception, being able to see 2 objects side by side (i.e., overlapping in your field of view) as separate, colour perception, depth perception, motion perception and much more. One can see how visual cues can give you information that would be helpful to maintaining posture, whether you are just standing still or in motion.
4. Your belief system: If you have a strong belief about your balance capabilities in different contexts, this will enhance your balance. Having done an acrobatic feat before will give you confidence that you can do it again even if it took you a while to work up to that point. Training in different postures in different contexts is important for the development of postural abilities. The time period between the ages of 4 and 6 years is a transition period in which mature patterns are emerging. These mature patterns involve integration of all of the above systems. Efficiency in doing this evolves between 7 and 15 years of age.
VVM is a type of dizziness that results from your vestibular system telling your brain something that is at odds with what your visual system is telling you.
When you are in a train and the neighbouring train starts to depart, how do you know you are not rolling backwards? The visual signal is identical in both scenarios. The answer is because your vestibular system – and your somatosensory system – tells you that you are not moving. The vestibular system is unique in that it is dependent on gravity (which is fixed, doesn’t change) and it is sensitive to any acceleration. It also, by design, has a physiological setup that is redundant in many aspects. That is, the vestibular system has many structures that do the same task, making it more resilient to injury. In the train example above, your vestibular system does not detect any motion so it tells you that the illusion of self-motion is not real. Thus, the visual system operates in the context of the vestibular system. This makes sense for the above reasons but also when you consider that your eyes are in your head, and an accurate assessment of where your head lies is important for providing context to what you see.
The hierarchy of these systems in our internal map develops sequentially as discussed above. In normally developed individuals, vision plays the least dominant role in postural control when compared to the other systems mentioned above.
This means the brain does not have the same quality of reliable information coming from the vestibular system. This means that patients resort to other systems, i.e., visual system, to compensate for this deficit. Both systems are usually involved in postural control in healthy people, but as previously mentioned, the visual system is usually subservient to the vestibular system. But in VVM patients, this hierarchy is reversed, that is, the visual system becomes the new preferred system and the vestibular system is subservient to it. That is, these patients are said to be visually dominant. This is a problem for several reasons:
1. The information we take in visually is rarely stable/fixed/monotonous like gravity is (you will remember, the vestibular system is like a gyroscope that is dependent solely on motion with respect to gravity);
2. The visual system is less efficient at detecting motion of yourself relative to your environment (remember the train example above);
3. Relying on a weaker strategy (i.e., visual compensation for vestibular shortcomings) prevents patients from training better strategies (i.e., restoring the vestibular system to its throne); that is, it limits a patient’s ability to compensate or perform better.
Moreover, this is particularly problematic and frustrating for many post-concussion patients because 2/3 of these patients also have injury affecting the functioning of the visual system. Any effort to rehabilitate post-concussion (focal, more on this distinction later) visual issues before restoring the vestibular system to its throne may just further empower the minister (i.e., the visual system) in this neurological coup d’ état.
The answer is yes. This shows that neurological damage is not the cause of VVM. It is caused by a faulty compensation to attenuated vestibular signals.