Amblyopia

Definition

Amblyopia is a decrease in vision in a healthy eye that is caused by problems with the eye and the brain failing to work together in correctly processing information from the "bad" or amblyopic eye. Lazy eye is a common non medical term used to describe amblyopia because the eye with poorer vision does not seem to be doing its job of seeing.


Demographics

Amblyopia is the most common cause of impaired vision in children. It affects about three out of every 100 people or two to four percent of the population.



Description

Vision is a combination of the clarity of the images received from the eyes (visual acuity) and the processing of those images by the brain. If the images produced by the two eyes are substantially different, the brain may not be able to fuse the images. Instead of seeing two different images or double vision (diplopia), the brain suppresses the blurrier image. This suppression can lead to amblyopia.

The critical stage for binocular vision development occurs between the ages of five and seven months with continued development through about age eight years. Ambylopia is most likely to develop early in childhood and leads to poor visual development in the blurrier eye. Amblyopia can also occur in adults if one eye is damaged or vision is reduced by the development of a cataract.

Risk factors

Children who were premature, who are developmentally delayed, have other eye problems, or who have a family history of amblyopia are at higher risk for developing this disorder.

Causes and symptoms

Some of the major causes of amblyopia are as follows:

  • Strabismus. A misalignment of the eyes (strabismus) is the most common cause of functional amblyopia. The two eyes are looking in two different directions at the same time. The brain is sent two different images and this causes confusion. The brain turns off images from the misaligned or "crossed" eye in order to avoid double vision.
  • Anisometropia. This is another type of functional amblyopia. In this case, there is a difference of refractive states between the two eyes (in other words, a difference of prescriptions between the two eyes). For example, one eye may be more nearsighted than the other eye, or one eye may be farsighted and the other eye nearsighted. Because the brain cannot fuse the two dissimilar images, it will suppress the blurrier image, causing the eye to become amblyopic.
  • Cataract. Clouding of the lens of the abnormal eye will cause the image to be blurrier than the image from the normal eye. The brain "prefers" the clearer image. The eye with the cataract may become amblyopic.
  • Ptosis. This is the drooping of the upper eyelid. If light cannot enter the eye because of the drooping lid, the eye is essentially not being used. This condition can lead to amblyopia.
  • Nutrition. Nutritional deficiencies or chemical toxicity may result in amblyopia. Alcohol, tobacco, or a deficiency in the B vitaminsmay result in toxic amblyopia.
  • Heredity. Amblyopia can run in families.

Barring the presence of strabismus or ptosis, children may or may not show signs of amblyopia. Children may hold their heads at an angle while trying to favor the eye with normal vision. They may have trouble seeing or reaching for things when approached from the side of the amblyopic eye. Parents may notice that one side of approach is preferred by the child or infant. If an infant's good eye is covered, the child may cry.

Examination

Because children with outwardly normal eyes may have amblyopia, regular vision screenings are recommended beginning at a young age. There is some controversy regarding the age at which children should have their first vision examination. Some authorities recommend that children have their vision checked by their pediatrician, family physician, ophthalmologist, or optometrist at or before six months of age. Others recommend testing by at least the child's fourth birthday. In actuality, children's eyes can be examined at any age, even at one day of life. The earlier amblyopia is found, the better the possible outcome. Most physicians test vision as part of a child's medical examination. If there is any sign of an eye problem, the child may be referred to an eye specialist.

Generally, a difference of two lines or more (on an eye-chart test of visual acuity) between the two eyes would be defined as amblyopia.For example, if someone has 20/20 vision with the right eye and only 20/40 with the left, and the left eye cannot achieve better vision with corrective lenses, the left eye is said to be amblyopic. Objective methods such as retinoscopy can measure the refractive status of the eyes. This can help determine anisometropia. In retinoscopy, a hand-held instrument is used to shine a light in the child's (or infant's) eyes. Using hand-held lenses, a rough prescription can be obtained. Visual acuity can be determined using a variety of methods. Many different eye charts are available (e.g., tumbling E, pictures, or letters).

In amblyopia, single letters are easier to recognize than when a whole line is shown. This is called the "crowding effect" and helps in diagnosing amblyopia. Neutral density filters also may be held over the eye to aid in the diagnosis. Sometimes visual fields to determine defects in the area of vision will be performed. Color vision testing also may be performed. Amblyopia is a diagnosis of exclusion, so many tests may be performed to rule out visual or health problems that also can cause a decrease in vision.

Treatment

The treatment plan should be discussed with the doctor to fully understand the purpose of the treatment, its length, and expected results.

Traditional

Treatment should be begun as early as possible in a child's life. The primary treatment is occlusion therapy, which is performed by a child's wearing a patch over the good eye. This forces the amblyopic eye to work and the brain to process information from this eye. Clinical trials sponsored by the United States National Institutes of Health (NIH) have shown that for many children, wearing a patch over the good eye for two hours daily effectively treats mild to moderate amblyopia; patching for six hours daily generally improves severe amblyopia. Limited patching time allows the child to wear the patch at home, avoiding the stigma of looking different in public. Initially it was thought that if patching were not done by age seven, it would be ineffective, but research as of 2010 has shown that the effective period for reversing or reducing amblyopia can extend through age 17 years in some individuals; information on reversing amblyopia in older adults is limited. When patched, eye exercises may be prescribed to force the amblyopic eye to focus and work. This is called vision therapy or vision training. Even after vision has been improved in the weak eye, part-time patching may be recommended to maintain the improvement.

Drugs

An alternative to patching is treatment with the drug atropine. Atropine eyedrops are applied to the "good" (stronger) eye. Atropine dilates the pupil and causes vision in the good eye to become blurry. This forces the brain to process information from the "bad" or ambylopic eye. While patching or atropine treatment is necessary to get the amblyopic eye/brain processing to work, it is just as important to correct any underlying reason for the amblyopia. Glasses may also be worn if there are errors in refraction. Surgery or vision training may be necessary in the case of strabismus. Better nutrition is indicated in some toxic amblyopias.

Prognosis

It is important to diagnose and treat amblyopia early because significant vision loss can occur if left untreated. The best outcomes result from early diagnosis and treatment.However, treatment may be successful in older children. Success in the treatment of amblyopia also depends upon how severe the amblyopia is, the specific type of amblyopia, and patient compliance.

Prevention

To protect their child's vision, parents must be aware of amblyopia as a potential problem. Parents should be encouraged to take young children for vision exams early on in life and certainly before they begin school. Proper nutrition is important in the avoidance of toxic amblyopia.