Definition

Anisometropia is a type of refractive error in the eye characterized by a difference in visual sharpness between the two eyes. Refractive errors cause vision to become blurry or double due to the eye's inability to refract light so that it falls precisely on the retina. The retina is the layer inside the eyeball that captures incoming light and transmits it through the optic nerve to the brain.

Causes

Anisometropia can be caused by a problem in one eye during childhood, leading to one eye being more active in development. At birth, a baby's vision is about ten times blurrier than an adult's but develops until the age of 3-5 years. In addition to eye problems, anisometropia can be caused by differences in the eyeball's shape, which can be shorter or longer. This difference can cause the incoming light to not fall precisely on the retina, resulting in blurry vision.

Risk factor

Risk factors for anisometropia include:

• Outward strabismus (eye turning outward)
• History of intensive care treatment at birth
• Family history of myopia (nearsightedness)
• Premature birth
• Developmental delays

Symptoms

Anisometropia may or may not have symptoms. If symptoms are present, they can include mild vision disturbances such as slight blurriness, which is not perceived as very severe. However, some people may experience more severe symptoms, such as significant blurriness in one eye, shifting vision, double vision, or an excessive need to squint.

Diagnosis

Anisometropia can be diagnosed by a general practitioner or an eye specialist. The doctor will check visual sharpness using images or letters. If visual sharpness is below normal, the doctor will determine the eyeglass prescription needed to achieve normal vision. The unit of lens strength for glasses is diopters, with negative values for myopia (nearsightedness) and positive values for hyperopia (farsightedness). Anisometropia is diagnosed if the difference in eyeglass prescription between the two eyes is 1 diopter or more.

Anisometropia is categorized into absolute and relative anisometropia. Absolute anisometropia involves a difference in lens strength of 1 diopter or more. In relative anisometropia, the lens strength required is not much different, but the eye lengths differ, causing each eye to perceive objects in different sizes. Absolute anisometropia is further subdivided into:

• Simple anisometropia: one eye is normal while the other is myopic or hyperopic.
• Compound anisometropia: both eyes are either myopic or hyperopic, but require different lens strengths.
• Mixed anisometropia: one eye is myopic and the other is hyperopic.
• Compound astigmatic anisometropia: both eyes have astigmatism but at different levels.
• Simple astigmatic anisometropia: one eye is myopic or hyperopic, while the other has astigmatism.

In addition to visual sharpness tests, the doctor may perform other vision tests, such as examining eye position, eye movement, and visual field. Eye position tests check for strabismus, while eye movement tests help determine if there is amblyopia (lazy eye), characterized by uncoordinated eye movement. Visual field tests assess the normal range of vision.

The doctor may also measure eye length using optical measurements (laser) or ultrasound (ultrasonography) to determine relative anisometropia risk factors and whether absolute anisometropia is due to a significant difference in visual sharpness.

Management

The treatment of anisometropia aims to equalize visual sharpness in both eyes using glasses or contact lenses tailored to each eye's needs. This encourages the brain to use both eyes together, balancing their development. Glasses or contact lenses must be worn continuously to achieve this.

However, if the difference in lens strength is too large, glasses can cause differences in object shape and size seen by each eye. In such cases, surgery may be recommended. Surgery to correct anisometropia can involve the cornea or lens, both of which help focus light on the retina.

Corneal surgery aims to improve light transmission through the cornea, while lens surgery involves removing or replacing the natural lens with an artificial one. Surgery can provide a permanent correction for anisometropia but carries risks such as infection and incomplete vision correction.

Complications

The most common complication of anisometropia is amblyopia (lazy eye). In anisometropia, the brain tends to rely more on the eye with better vision, leading to reduced use of the other eye. This can cause the lesser-used eye's nerve to become "lazy." Amblyopia is often indicated by uncoordinated eye movement or the child frequently closing the "lazy" eye for clearer vision. Early treatment is crucial to prevent permanent damage or blindness, with younger patients responding better to therapy.

Prevention

Anisometropia cannot be prevented but can be detected and treated early. Vision screening can be done at infancy, ages 3-4, and annually from elementary school age. Schools can also conduct routine health checks, including eye exams. Early detection allows for prompt correction of vision problems.

When to see a doctor?

If screening results from a healthcare facility or school indicate your child has anisometropia, immediately take them to an eye specialist. Screening typically shows a significant difference in eyeglass prescription between the two eyes (over 1 diopter). Although anisometropia symptoms may not be evident in children, untreated anisometropia can severely impact visual development.

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