STRABISMUS AND AMBLYOPIA IN CHILDREN
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
A normal child is born with the capacity for clear focussing by both eyes as well as parallel (straight) positions of the two eyes. In addition, when the child wishes to look in different directions the two eyes move in tandem so that their lines of sight remain parallel to each other at all times. The lines of sight will remain parallel as long as the eye muscles that move the eyes are in balance.
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. A child’s visual system is especially sensitive to such problems, as it is not fully developed until age 8 or 9 years.
2. Strabismus
Strabismus is the medical term for eyes that are not straight. An eye can turn in either the horizontal or vertical direction, or it can be rotated about the straight-ahead axis. Thus, there are different possible misdirections:
a) If the eye turns inward it is often called a “crossed eye” or convergent strabismus, and the medical term for this is “esotropia”.
b) If the eye turns outward it is sometimes called a “wall-eye” or divergent strabismus, 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.
e) The eye can also rotate about the straight ahead axis, creating a tilt of the images, or cyclotorsion.
Children can be born with strabismus, and the parents will notice that the eyes are crossing within the first few weeks of life. More commonly, however, strabismus develops several months or years after birth either as a result of problems with the focusing mechanism of the eyes or as a result of disorders in the eyes themselves.
a) Strabismus due to focusing problems
If a child is very far-sighted, a condition known as “hyperopia” or “hypermetropia”, the eyes must focus more strongly than usual in order to create a clear image. This excessive focussing stimulates the eyes to cross because there is a link between the focussing mechanism and the messages to the eye muscles that turn the eyes inward toward the nose.
Alternatively, if the two eyes do not focus equally then the image from one eye may be blurred. This condition is known as “anisometropia”. As a result, the eye that sees the blurry image may wander because the brain will turn off the awareness of that eye. It may no longer send the signals to the eye muscles to keep the eyes straight.
For either of these conditions the initial treatment is glasses to help the focussing of the eyes. In the case of hyperopia the glasses will be prescribed to correct all or part of the focussing demand, thus making it easier for the child to see images clearly. If the eyes do not focus equally, the glasses will make up the difference between the two eyes so that they can work together more easily.
In some cases patching of the better eye or defocussing the image of the better eye with eye drops is also needed to equalize the vision in the two eyes. (see Amblyopia, below) If these measures do not lead to a straighter eye then eye muscle surgery is performed in order to put the eye muscles back into balance.
b) Strabismus due to disorders within the eyes
A problem such as glaucoma or cataract can reduce the vision in an eye to the point that the brain ignores the image from that eye and concentrates vision only with the good eye. The eye with poor vision may then begin to wander since the brain no longer sends signals to its eye muscles to keep it aligned with the fellow eye.
The treatment of these conditions involves treating the disease affecting the vision. For example, if the eye has a cataract, the lens has to be removed with surgery. If the eye has glaucoma, then either medications or surgery are often needed to control the pressure in the eyes. Contact lenses or glasses, and often patching of the good eye, are needed in order to restore the vision in the abnormal eye. If these measures do not lead to a straighter eye then eye muscle surgery may be required to put the eyes into a more normal alignment.
3. Amblyopia
When one eye is not focussing equally with the other eye (anisometropia) or when one eye is not straight (strabismus) then the brain can turn off its awareness of the image in the eye. In a child, if this process remains untreated for several weeks the vision in that eye can slowly worsen. This condition is known as a “lazy eye”, and the medical term for this is “amblyopia”. By contrast, if unequal focussing or strabismus develops in an adult the vision in the eye rarely deteriorates.
The options for treatment of amblyopia include:
a) Correcting the focussing problem with glasses. This may lead to improved vision in the poorer eye.
b) Patching of the good eye, to force the poorer eye to work harder and regain the inputs form the brain to improve the vision.
c) Defocussing the image of the better eye with eye drops, to stimulate the poorer eye to regain the inputs from the brain.
d) Oral medications, which may help improve the results in some children with amblyopia
When a child’s eye develops cataract or glaucoma the loss of clarity of the image can lead to amblyopia in addition to the loss of vision from the disease itself. Therefore, treating the cataract or glaucoma problem may not, by itself, restore good vision. Treatment for amblyopia, such as glasses and/or patching, may also be needed.
4. Association between Strabismus and Amblyopia
In a child, amblyopia is often associated with strabismus. Both problems involve disruption of the visual signals that normally come into the brain equally from the two eyes.
Strabismus, or a misalignment of an eye, causes the eye to lose focus on a target. As a result, the brain may ignore the signals coming from that eye and this can lead to amblyopia of that eye. Conversely, if an eye develops amblyopia then the brain will receive less visual input from that eye compared to the fellow eye. As a result, it may reduce the intensity of messages it sends to the eye muscles of that eye and this may lead to strabismus.
It should be noted that strabismus (eye turn) can often occur without amblyopia, meaning that the vision remains good in both eyes. Amblyopia treatment is not required in such cases. Conversely, amblyopia can develop in children whose eyes are straight, usually due to difference in focusing between the two eyes. In these patients therapy is only needed to treat the vision deficit, most commonly in the form of glasses and patching, and no eye muscle treatment is required.
Fortunately, if amblyopia and strabismus are diagnosed and treated in childhood then the chance for recovery of vision and alignment are very good. Thus, it is important for family doctors and pediatricians to check the eyes for signs of strabismus, amblyopia, cataract, and glaucoma during all routine visits starting immediately after birth.
5. Treatment of Strabismus in Children
In summary, there are many possible options for therapy for children with strabismus:
a) No intervention: In some cases the problem can improve on its own, or it is of such minimal degree that therapy is not indicated.
b) Treatment of an underlying eye condition: Prompt and successful treatment of such conditions as cataracts or glaucoma can restore vision in an eye and prevent strabismus or lead to recovery of normal alignment.
c) Glasses: In many cases of esotropia (crossed-eye) caused by hyperopia (far-sightedness), use of glasses to ease focussing can straighten the eyes. In some cases of exotropia (wall-eye) the use of near-sighted (myopic) lenses can help control the eye turn.
d) Patching: Treatment of amblyopia can lead to improved vision in the poorer as well as improved alignment. Patching of an eye can also improve the alignment in some cases of exotropia.
e) Exercises: Some forms of strabismus, such as near vision problems, can be helped with daily eye exercises.
f) Eye drops: Patients with crossed eyes can sometimes be helped by temporary administration of eye drops to help their depth of focus.
g) Botulinum Toxin (BotoxTM, licensed by Allergan, Inc.) This is an alternative to surgery for some children with small to moderate degrees of strabismus.
h) Surgery: This is indicated when none of the other interventions is indicated or they have not worked.
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