Tacklin' Assessment: Function of the Vestibular System

 This is the third part of a series of discussions about the vestibular system. If you remember from the Winter 2020 newsletter, we identified the three main roles of the vestibular system: 1) identify the position and direction of    movement of the head; 2) maintain balance; and 3) keep vision clear while the head is moving (gaze stability).

Many people know that the vestibular system makes a huge contribution to balance, but not many people realize that the vestibular system also plays a large role in vision. While the eye, surrounding eye muscles, and head work   together to help you look around your environment, this system is not capable of keeping your vision clear when the head is moving at very fast speeds. An example of these high-speed movements would be turning your head quickly to look toward a loud, unexpected sound or bouncing up and down while driving on a bumpy road (fast vertical speeds).  Even when you are walking, the impact of your foot hitting the ground causes a subtle high-speed bouncing motion of the head. At these high speeds, the oculomotor system (eye and eye muscles) cannot respond or move quickly enough to keep your vision clear. This is where the vestibular system helps. Through the vestibulo-ocular reflex (VOR), the   vestibular system works with the visual system to move your eyes in the opposite direction of the head and at the same speed as the head so you can keep your visual target in focus. This is an automatic response—you do not think about it at all. As you bounce up, your eyes automatically move down at the same speed so you can still read the street sign clearly. To try to simulate this, hold your arm straight in front of you at eye level. Close your fingers against your palm and keep your thumb straight and pointed toward the ceiling. Keep your hand still while you turn your head side-to-side as fast as you can while looking at your thumb. Your thumb should still appear clear to you because your VOR is helping keep your vision stable. You do not think about moving your eyes to the right while your head turns to the left—the VOR takes care of that. Now hold your head still while you move your arm side-to-side as fast as you can keeping your eyes on your thumb. Your thumb may blur, or you may see two thumbs as you move your hand side-to-side. Your VOR does not help you in the second scenario, which is why you have blurring of your vision. Individuals with low or absent vestibular function will have blurring in both scenarios because their VOR is impaired.

How does this affect deaf or hard of hearing children? Though the numbers vary from one study to another, approximately one-half of children with a severe to profound hearing loss have some degree of vestibular dysfunction.¹  Those with complete vestibular loss will not have a VOR and will experience gaze instability. Those with reduced vestibular function may also have an impaired VOR. This can make it difficult to navigate the environment when walking, running, or driving because of the accompanying visual “blurring” when the head is moving. There is also evidence that individuals with gaze instability have difficulty with reading acuity and require larger print sizes to read effortlessly than children who have a stable gaze.²  While young children’s books use larger print sizes, books for older children and young adults are typically in a smaller print size and may require the student to use extra effort to read. 

 

1.                 Jacot E, Van Den Abbeele T, Debre HR, Wiener-Vacher SR. Vestibular impairments pre- and post-cochlear implant in children. Int J Pediatr Otorhinolaryngol. 2009;73(2):209-217.

2.                 Braswell J, Rine RM. Evidence that vestibular hypofunction affects reading acuity in children. Int J Pediatr Otorhinolaryngol. 2006;70(11):1957-1965.