Vestibular-ocular reflex

The vestibular-ocular reflex (VOR)  is a special reflex in the brain which enables head motion to be yoked to eye movements. You may have never wondered this but when your head moves to the left, your eyes must move to the right in order to remain fixated on a stationary target.  This does not happen voluntarily, that is, you do not instruct your eyes to go to the right.  Your brain does this “automatically” or reflexively, via the VOR.

The vestibular sensors in the inner ear are designed to sense motion of the head, and these sensors are connected via a series of nerves, to the muscles in the eyes, so that the eyes can receive the message about which way the head is moving, at lightning speed!  As a matter of fact, this nerve pathway in the brain is the fastest pathway in the entire human body.   The VOR enables gaze stability.  Gaze stability is required to enable the clear focus of an image whilst the head is in motion, such as when walking.  If the VOR is not functioning well, the eyes will not move accurately in response to the head motions, and therefore will not remain focused on the target. This may cause a “slip” of the target from focus, which is often referred to as oscillopsia.

To assess the VOR, the head is moved in certain directions, and the movement of the eye is measured.  If the movement of the eye is delayed or inaccurate, based on the movements of the head, the VOR is deemed to be dysfunctional.  Because we know the neural pathways that make up the VOR, it is possible to determine if these structures specifically are part of the presenting complaint, and then target the structures along these pathways as part of the rehabilitation process towards improving gaze stability and symptoms of dizziness.

For those who are interested in the nerve pathways that connect the semicircular canals to the muscle of the eyes, see the diagram below.  The VOR has been called the “stretch reflex of the brain” by vestibular expert Dr. Halmagyi, and interestingly the nerves in the VOR reflex are the fastest conducting nerves in the human body!

Dynamic visual acuity 

The movement of the eyes in response to movements of the head is not always easy to see from the naked-eye of the observer.  Specialized cameras are designed to capture microsecond delays, as well as measure speed of the eyes in response to angular head motion. But this is not the ONLY way to measure the function of the VOR. Since the function of the VOR is to enable the clear focus of any image during head motions, the subject may simply attempt to read text whilst the head is in motion to determine if the VOR is intact.

The dynamic visual acuity assessment requires the subject to move the head left to right, whilst trying to read the text of a certain size, at a given distance. The visual acuity of the subject does not matter, however, the change in visual acuity once the head motion is applied, does matter.  That is, if the movements of the head are enough to cause the text to become particularly blurry, it is likely that the reason is because the VOR is not functioning well enough to ensure the eyes are remaining focused on the given target.

To test the DVA, the assessor can use simple wall charts, or special devices which capture eye movements.