STRABISMUS AND AMBLYOPIA IN ADULTS
Stephen P. Kraft, MD, FRCSC
Professor, Department of Ophthalmology and Vision Sciences, University of
Toronto
Staff Ophthalmologist, The Hospital for Sick Children and Toronto Western
Hospital (University Health Network), Toronto
1. Normal Vision with Two Eyes
When the eyes are directed parallel to one another the images seen by the two eyes are unified into one clear image by the brain. This process also leads to vision in 3 dimensions, known as stereovision. As long as the brain receives equal stimulation from the two eyes it will influence the eye muscles to remain in balance.
Any problems that upset the balance in the eye muscles or compromise the vision in one or both eyes can lead to loss of stereovision and to disruption of the normal alignment of the eyes. In contrast to a child, an adult who develops a turned eye will not develop amblyopia, or “lazy eye”, since the visual system has already fully developed.
2. Amblyopia in Adults – It is treatable in some cases !
In children who have strabismus, the eye that is usually turned may lose vision as the brain stops focusing with that eye. The eye may also lost vision if there is a significant difference in the focusing ability of the two eyes, due to the blurry vision in the more defocused . This condition is termed “amblyopia”, or “lazy eye”. In children this condition responds to glasses to restore more normal and symmetric focussing in the two eyes, and also to patching or blurring of the good eye to make the poorer eye stronger.
It has been thought for many years that amblyopia was not treatable if a patient was older than 8 or 9 years as the visual system and the parts of the brain dealing with vision were “set in their ways” by this age. This notion has been proven false in several studies in recent years. In fact, up to one-third of adults and older children can gain some reversal of amblyopia by patching or blurring of the good eye to force the poorer eye to work harder, in addition to glasses if they are indicated.
It is no longer proper to discourage any adult or older child from trying therapy for amblyopia, especially if he or she is willing to undergo such a treatment trial. A trial should be planned for at least 8 to 12 weeks. Studies have shown that if therapy does not improve the vision at all within 3 months, then it is unlikely that there will be any reversal.
3. Strabismus
Strabismus is the medical term for eyes that are not straight. An eye can turn in either the horizontal or vertical direction. Thus, there are different possible misdirections:
a) If the eye turns inward it is often called a “crossed eye” and the medical term for this is “esotropia”.
b) If the eye turns outward it is sometimes called a “wall-eye” and the medical term for this is “exotropia”.
c) If one eye is higher than the other eye the medical term is “hypertropia”.
d) An eye can drift at an oblique angle such that it is misaligned in both the horizontal and vertical directions at the same time.
4. Causes of Strabismus in Adults
An adult may have strabismus that has persisted from the time he or she was a child, and it either was not successfully treated in childhood or else has recurred after being in a satisfactory state for a period of time. However, strabismus in adults can arise from many other conditions, including:
a) Loss of vision in one eye, as a result of diseases in the eyes such as cataracts, glaucoma, or retina abnormalities
b) Trauma to the eye or the eye socket, causing damage to the muscles of the eye or the nerves that send messages to move the eye muscles
c) Neurologic disorders, such as strokes and head trauma that affect the signals from the brain to the eye muscles
d) Other medical conditions such as diabetes or thyroid disease in which the muscles or nerves that move the eyes do not function properly
Whereas a child may learn to ignore the image of a misaligned eye, an adult who develops strabismus for the first time will likely have difficulties such as double vision, if the angle of the misalignment is large, or blurring of vision and eyestrain if the misalignment is intermittent or of a smaller degree. This is especially true if the misalignment is in the vertical direction, as the brain has more difficulty coping with images displaced in the vertical direction than in the horizontal plane.
5. Misconceptions about Adult Strabismus – Treatment is more than cosmetic !
If an adult with strabismus complains of double vision or eyestrain, then restoring good alignment of the eyes has a good chance of eliminating these complaints.
Another problem that can be caused by strabismus is torticollis, or an abnormal head posture. If the eyes are not aligned when the head is held straight then the patient may adopt a head posture as a compensation to keep the eyes aligned and working together. Treatment to realign the eyes in the straight-ahead position can eliminate the head posture. Torticollis left untreated for many years can lead to permanent neck muscle changes later in life, and even possibly arthritis of the neck bones.
For adults with longstanding strabismus (since childhood) or who do not suffer from such complaints it is often stated that treatment of the condition is “only cosmetic”. We now know that this perception is not true, for several reasons:
a) Treating an adult can improve depth perception as long as the angle of misalignment is reduced to a small degree.
b) The adult can regain fusion, or synthesis of images seen by the two eyes into one percept. This is possible in a majority of cases even if the strabismus has been present for many years and even if amblyopia (‘lazy eye”) has been present since childhood.
c) The panorama of the field of vision (the visual field or “peripheral vision field”) can be normalized. In patients with esotropia (crossed eyes) the field of vision is reduced by 20 to 30 degrees from normal in some cases. Realigning the eyes can expand the peripheral vision field back to normal. In patients with exotropia (wall-eye) the excessive field of vision can be restored to its normal expanse.
d) Many patients report enhanced self-esteem, communication skills, job opportunities, reading and driving. These factors have been confirmed by several well-designed studies.
Depending on the type of strabismus and its cause, several options are available:
a) No intervention at all ! There are some conditions that either improve on their own or do not cause enough difficulty to justify therapy.
b) Glasses: Correcting vision with glasses can lead to improved alignment and reduced symptoms in some cases, just as seen in children.
c) Prisms: These are optical devices that can be pasted onto glasses or ground into glasses to help move the images seen by the two eyes closer to each other, allowing single vision.
d) Eye exercises: Some forms of adult strabismus can be helped by exercises for the focussing of the eye muscles.
e) Botulinum toxin (marketed as Botox or Dysport): Injections of eye muscles with botulinum toxin, which relaxes overacting muscles, can realign the eyes in some cases where the angle of misalignment is not excessive.
f) Surgery: Eye muscle surgery can be performed on any of the eye muscles to restore normal alignment
g) Patching or eye occlusion: In cases of double vision or eyestrain where surgery or other interventions are not possible, patching or some other occlusive device (such as a frosted lens) may be the only means to provide visual comfort.
7. Helpful Websites: