The Vestibular System and EOMs

Like so many of our body’s systems, the vestibular system is something we take for granted until it malfunctions.  Yet the vestibular system is truly a sixth sense, as our colleague Dr. Curt Baxstrom from Federal Way, WA is fond of reminding us.  I stumbled upon a book that organizes recent research and thinking on this sixth sense, and it dovetails nicely with some of the concepts I began to explore with you last year on visual centers of gravity.  I’m going to highlight a few things about the semicircular canals, which play such a crucial role in orienting us to the environment.  There are three coplanar canals functioning as partners in encoding rotary head movements.  Each of these pairs works in a push-pull fashion similar to the three pairs of EOMs to encode the component of angular motion in its plane.

The figure above shows how the canals and EOMs are linked so that the push/pull excitation/inhibition of the semicircular canals signals eye movements.  The planes of these three coplanar semicircular canals nearly align with the planes of the extraocular muscles to which they are connected by three-neuron reflex arcs. The figure shows how a specific pattern of excitation/inhibition the left anterior and right posterior canals results in upward movement of the eyes.  As indicated by the coloring, excitation (green) of the left anterior canal and inhibition (red) of the right posterior canal causes contraction (green) of the left superior rectus and right inferior oblique, and relaxation of their antagonists – the left inferior rectus and right superior oblique.

Once you start thinking more about these interrelationships, you will no longer view subtle ocular motor deficits in isolation as “an eye problem”.  Consider for example a subtle head tilt with an induced cyclovertical imbalance that can occur secondary to unilateral loss of function in the utricle.  The utricle is the part of the labyrinth serving the semicircular canals that detects head tilts in the horizontal plane.  The diagram below shows an ocular tllt reaction secondary to a left head due to impairment of the left utricle

 

Vestibulopathies and vestibular neuritis are not as uncommon as one would think, and the increasing numbers of patients experiencing ABI due to concussions and other forms brain injury mean that you will be collaborating with ENTs more than ever before.  We recently had a patient who was being managed at Johns Hopkins for superior canal dehiscence, and I have had a number of patients through the years who suffered from BPPV, the most common vestibular disorder.  Any patient with a primary or secondary vestibular disorder may potentially benefit from lenses, prisms, or vision therapy as a complement to pharmacologic, surgical, or VRT considerations.