What is Anisometropia?
Anisometropia is a condition in which the two eyes have a difference in their refractive powers. This means that one eye is more nearsighted or farsighted than the other. A large amount of anisometropia can cause problems with depth perception and stereopsis (3D vision). Correcting this condition with eyeglasses or contact lenses is important, especially in childhood, so that both eyes can work together properly and see clearly.
- Anisometropia is a condition where each eye has a different refractive power.
- Anisometropia can be caused by several factors, including congenital abnormalities, disease, injury, surgery,or medication.
- Anisometropia can lead to lazy eye (strabismus) and permanently decreased vision in both eyes (amblyopia) if not corrected with glasses or contact lenses in childhood.
Factors Causing Anisometropia
Several different conditions can cause anisometropia. These include:
- Congenital abnormalities in the development of the eye, such as in-utero infections
- Diseases that affect the eye, such as diabetes
- Injury to the eye
- Surgery of the eye
- Medications that cause changes in the refractive power of the eye, such as Topamax
If not corrected, anisometropia can lead to lazy eye (strabismus) and permanently blurry vision that cannot be corrected with glasses (amblyopia). Anisometropia is treated with eyeglasses, contact lenses, or refractive surgery. Treatment for children with sight-threatening amounts of anisometropia may include eye patching or atropine drops.
Anisometropia is the difference in the refractive power of the two eyes. For example, if someone has anisometropia, one of their eyes may be more nearsighted than the other. The degree of anisometropia is quantified by the difference in dioptric power between the two eyes. For example, an individual with mild anisometropia may have a difference of 1.00 diopter between their two eyes. In contrast, someone with a more severe case may have a difference of 2.50 dioptres or more. Typically for anisometropia, the average difference in power between the eyes is approximately 1.00 diopter. Approximately 25% of the human population has a 0.50 to 1.00 diopters of anisometropia, or power difference between the eyes.
Pathophysiology of Anisometropia
The majority of humans have a mild amount of anisometropia due to natural variations of growth factors in each eye. It is rare for someone to have the exact same prescription in both eyes. However, a large amount of anisometropia (a large prescription difference between the eyes) causes unequal input to the visual cortex. Over time, the brain may favor the input from the clear eye and ignore the input from the blurry eye. This may result in one eye having permanently blurred vision because the brain suppresses the “weak” eye and ignores its blurry visual image (amblyopia). The person may eventually lose binocular depth perception since the two eyes aren’t working together.
Signs and Symptoms of Anisometropia
Symptoms of anisometropia depend on the amount of difference in the eyes. A large amount of anisometropia, with a large difference in prescription between the two eyes, will be more likely to cause symptoms. Blurred vision is the most common symptom of anisometropia.
Other symptoms may include:
- Reduced depth perception, such as trouble walking on stairs or seeing a 3D movie
- Eye fatigue
- Difficulty reading
- Difficulty seeing at night
- Double vision
Diagnostic Methods for Anisometropia
Anisometropia is diagnosed with a comprehensive eye exam. During the exam, the doctor will test each eye separately and then together to determine the refractive power of each eye. Detecting anisometropia in the exam consists of five main tests:
- Visual acuity
This measures the clarity of vision in each eye at different distances.
This test uses light to shine into the eye and determine the prescription objectively, as the doctor observes the reflection of the retina; used primarily in infants and young children.
This classic ‘1 or 2’ test uses a phoropter to test the power of different lenses using your feedback of the lenses you prefer.
- Binocular vision testing
This set of tests measures your eyes’ ability to work together as a team.
This test measures depth perception using polarized glasses to see which figure pops out of the page.
The best treatment for anisometropia is to correct the refractive error in both eyes with glasses or contact lenses. This helps to improve vision and prevent amblyopia, strabismus, and decreased vision in both eyes. It is important to screen children at a young age. An eye doctor will determine if the amount of anisometropia is sight-threatening to the child and may prescribe a pair of glasses. It is critical for children with large amounts of anisometropia to wear glasses.
The most important time to correct this problem in children with glasses is between 5 to 8 years old. After 8 years old, children are outside of the developmental period and eye conditions caused by anisometropia are unlikely to resolve. It is recommended that children have eye exams at 6 months old, 3 years old, 5 years old, and then every year.
In some cases, treatment may include patching or atropine drops if there is a risk of permanent vision loss (amblyopia). Patching involves covering one eye with an eye patch or sticker. Patching can stimulate the weak eye and force the brain to receive input from the weak eye. Typically, the patch will cover the normal or lower-prescription eye. This gives the weak or higher-prescription eye the opportunity to become stronger and not be suppressed by the brain.
Atropine dilating drops may also be put into the stronger eye, to blur out the image from the stronger eye and allow the weaker eye to develop.
Refractive surgery may be performed to correct the refractive error in both eyes. This is a permanent solution, but not a fit for everyone. It is crucial to discuss treatment options with an eye doctor before making a decision.
If anisometropia is not treated in children, it can lead to lazy eye (strabismus), permanent vision loss (amblyopia), and symptoms such as reduced depth perception. However, if it is treated with glasses or contact lenses at a young age, the prognosis is good, and most people can see clearly in both eyes.