A person riding in a car down a bumpy road can look out the window and read street signs, recognize faces, and keep everything in focus, even when rotating the head from one side of the car to the other.
If you have an intact vestibular system and you look at a spot on the wall, then rotate your head from side to side, your VOR (as described below) keeps the eyes focused on that spot.
When you move your head 10 degrees to the right, the movement is sensed by the vestibular system, and a signal is sent to the eye muscles to rotate the eyes 10 degrees to the left. Likewise, when you move your head up 10 degrees, the eyes automatically compensate via the VOR and move 10 degrees down. Within limits, the head can freely move about, while the eyes can keep focused on a distant object.
Oscillopsia (bouncing vision):
A person with the condition known as oscillopsia sees only a jerky blur, even when the roadway is fairly smooth. Walking causes motion of the head which results in a jerky, bouncing visual field. Below are three video clips created to show the sensation of oscillopsia:
Passersby are often difficult to visualize.
Maintaining a stable gaze on objects moving by is difficult.
This video was produced by Dr. Timothy Hain, a neurologist who specializes in balance disorders.
Keeping focused while moving:
Airplanes rely on a gyroscope to keep pilots oriented in three dimensional spaces when outside visual references are obscured. Thanks to the spinning gyroscope, pilots know which way is up, which way is down, and where the horizon is, even when they are flying through a thick layer of clouds, and the airplane is changing heading or altitude. The internal portion of the gyroscope always remains pointed in the same direction, level with the horizon, even when the airplane and the attached outer portion of the gyroscope changes direction, or points upward or downward.
Humans don't have gyroscopes. Instead, we rely on the vestibulo-ocular reflex, or VOR, which is a complex nerve pathway between the inner ear and eye muscles, to keep us balanced and oriented in space, especially when there is no outside visual reference.
The process of maintaining a clear visual field while moving the head is known as gaze stabilization. There is a computer aided test (GST) for gaze stabilization. Dr. Timothy Hain has a webpage devoted to gaze stabilization exercises.
When the inner ear is damaged by gentamicin, there is decreased or absent information available to the vestibulo-ocular reflex
If you have been the victim of gentamicin poisoning, your vestibular system may be damaged to a degree that there is no input to the VOR to keep the eyes steady. Any time the head moves at all, even while walking or riding in a car, the eyes move also. The result is bouncing vision, or oscillopsia (also called spontaneous oscillopsia).
Gentamicin attacks and kills “hair cells” in the inner ear that sense even very small movements of the head. If enough of these cells are destroyed, there is no vestibular input to the brain, so the brain has to rely only on vision and proprioception to determine what is up or down, or where the body is positioned in three-dimensional space. This is why patients who have gentamicin poisoning experience balance problems and bouncing vision.
How can a person be tested for oscillopsia?:
A person describing jerky or bouncing vision that develops after taking gentamicin most likely has oscillopsia. The degree of oscillopsia can be semi-quantified with a dynamic visual acuity test. The examiner holds a miniature version of an eye chart about 18 inches in front of the subject to be tested. The person reads the smallest line possible, and this represents the person's best normal vision. The examiner then rotates the head forward and backward about once per second and asks the person to read the smallest line possible. A person with an intact vestibular system and VOR should be able to read the same line. A person with oscillopsia, however, can only read the lines with much bigger print. The number of lines "lost" is indicative of the degree of oscillopsia. For example, a person with severe oscillopsia might go from 20/20 vision to 20/200 vision when their head is in motion. The test is repeated by moving the head from side to side.
The presence of oscillopsia impacts nearly every facet of a person's life:
For a victim of gentamicin poisoning, the presence of oscillopsia is usually as devastating as the sense of loss of balance. Every time the head is in motion, even slightly, vision is affected. The degree of oscillopsia experienced is somewhat related to the degree of vestibular injury: a person with a complete, bilateral loss (both inner ears have been completely damaged) usually has more severe oscillopsia than persons with incomplete bilateral losses (both inner ears partially destroyed) or persons with unilateral losses (only one side completely or partially destroyed).
A person with moderate to severe oscillopsia usually cannot safely drive:
When they walk down the street, faces are unrecognizable, and street signs are a bouncing blur. To recognize a face, or read a sign, one must stop, look at the object, and hold one's head perfectly still until the bouncing stops. The constant bouncing vision is very distracting, and seems to cause mental fatigue. This is especially true in low light situations, when the presence of oscillopsia makes it difficult to see the ground ahead, and to perceive bumps or cracks in the pavement.
We recently became aware of a middle aged person who had moderate vestibular damage due to gentamicin who continued to drive, because she just couldn’t bear the thought of loosing her independence. She was driving on the freeway, rear-ended a car in front of her, and died when her car rolled over. While every circumstance is different, we strongly urge persons with moderate to severe oscillopsia to realistically assess their ability to react quickly to moving objects, such as a darting child.
Reading is usually much more difficult for persons with oscillopsia. The ability to track along a line of text and going from line to line is partially a function of the VOR. Persons with moderate to severe oscillopsia usually report having to re-read passages to get their meaning. Reading speed and comprehension are drastically impaired.
Watching television, or using a computer is often reported as being more difficult, although high refresh rate flat screen monitors seem to cause less distraction. Some clients report an increased difficulty when they are under fluorescent lighting.
There is no cure for oscillopsia, but treatment by a physical therapist or other medical professional who specializes in vestibular rehabilitation can help you learn to compensate for the problem.
Links to other sites describing oscillopsia: