Anisometropia - Causes, Symptoms, and Treatment

What Is Anisometropia of the Eyes?

Anisometropia of the eyes is a refractive disorder. In this condition, the refractive power of both eyes is different. If the difference is more than three diopters between the eyes, there is unequal focus between the two eyes.

This condition can occur in different scenarios like myopia or hypermetropia with three diopters difference between both eyes, or myopia in one eye and hypermetropia in the other. Also, there is a condition called amblyopia in which the brain favors the eye with a clear picture leading to the development of amblyopia (lazy eye) in the weaker eye.

How Prevalent Is Anisometropia in the Eyes?

Anisometropia is a common ocular condition. More than 20 % of the patients show an inter-ocular power difference of 0.5D or more. In some patients, the difference in eye power between the two eyes may reach 3D. It is prevalent in both males and females equally.

What Causes Anisometropia of the Eyes?

Anisometropia can be congenital or acquired. Congenital anisometropia is linked to genetic factors. Acquired anisometropia of the eyes can be caused due to a variety of factors; majorly, it might be caused due to the difference in shape or size of the eyes. Other reasons for anisometropia are:

• Astigmatism (imperfect curvature of the eyes resulting in blurred vision).

• Asymmetric hyperopia (far-sightedness).

• Asymmetric myopia (near-sightedness).

• Cataracts.

• Eye injury.

• Keratoplasty of one eye.

• Aphakia.

• Pseudoaphakia.

How Does Anisometropia Develop?

Anisometropia develops mainly due to a defect in the connection between the brain cells and the eyes. The difference in the eyes' refractive power affects the eyes' binocular vision. As a result, the brain favors the eye with a clear picture leading to the development of amblyopia (lazy eyes).

Who Is at Risk of Getting Anisometropia?

Anisometropia is mainly seen within the first ten years of life. Children within ten years of age are at a greater risk of getting the disease. In adults, cataracts can worsen the condition. Anisometropia shows a close association with the age of a person. Its risk increases from childhood to adulthood and when a person becomes older.

How Much Refractive Error Difference Can Humans Tolerate?

The human brain cells can tolerate a refractive error difference of 3D or a mismatch of 6 % between the right and the left eye. However, if the difference is greater, it becomes difficult for the brain to form a single image based on the image formed by both eyes.

What Is Binocular Vision?

Binocular vision in humans is the ability to use the information or images used by both eyes at once. It helps in better perception of depth and better visualization of the pictures.

What Are the Different Types of Anisometropia?

Anisometropia is primarily of two types:

1. Absolute Anisometropia: It is a condition where the refractive error between the two eyes is unequal. It is subdivided into four types, such as:

• Simple Anisometropia: In this, only one eye has a refractive error (either myopia or hypermetropia). The other eye is fine and can see the images.

• Compound Anisometropia: In this type of anisometropia, both eyes have refractive error; however, the power of each eye will be different. Therefore, the images seen by one eye will be blurrier.

• Antimetropia: In this, both eyes have refractive errors; however, one eye is myopic while the other is hypermetropic.

• Meridional Anisometropia: It is caused by cylindrical error or high astigmatism. There is a worsening of the refractive error along one axis.

2. Relative Anisometropia: In this, the refractive errors of both eyes are the same, but there is a difference in the axial length, which leads to the formation of retinal images that are different in size.

What Are the Clinical Signs and Symptoms Reported by Patients With Anisometropia?

Many children remain asymptomatic unless the patient presents with wandering eyes.

The signs and symptoms reported in patients suffering from anisometropia are:

• Headache.

• Poor perception of depth.

• Nausea.

• Sensitivity to light (photophobia).

• Tiredness.

• Light-headedness.

What Are the Different Visions Seen in Patients With Anisometropia?

The visions seen in patients with anisometropia are:

• Binocular vision. In patients with mild anisometropia, there is binocular vision.

• Uniocular vision. In patients with amblyopia due to anisometropia, there is uniocular vision.

• Alternating vision. In severe anisometropic patients, one eye sees nearby objects, and the other is used for distant objects.

• Strabismus (squint).

• Double vision.

• Blurred vision.

How Is Anisometropia Diagnosed?

Anisometropia often remains asymptomatic and is diagnosed only during a routine eye examination. Early detection of the condition is possible in children with the help of photo screening (the use of specialized cameras to detect the eye's light reflexes). Binocular vision is tested by

• FRIENDS test.

• Worth’s a four-dot test.

What Are the Complications of Anisometropia?

Anisometropia can lead to amblyopia (reduced vision in the affected eye) in children, as the difference in refractive error will make the brain choose the eye with the least refractive error for better vision. It can also lead to suppression of normal binocular vision in an individual.

Who Is Eligible for Medical Treatment of Anisometropia?

Conservative treatment of anisometropia with spectacles or glasses is conducted in the following patients:

• Patients with amblyopia.

• Children below one year of age with more than 3D refractive error.

• Older children and adolescents who can tolerate full binocular vision correction.

How Is Anisometropia Treated?

Anisometropia can be corrected either by glasses, contacts, or surgery.

Anisometropia of the eyes is corrected primarily by glasses with different refractive errors.

• It is essential to prescribe contact lenses for babies suffering from anisometropia to prevent the progression to amblyopia. Contact lenses are also used to correct high anisometropia.

• In severe cases, refractive surgeries can be performed to improve visual acuity and correct refractive errors. Other surgical procedures that can be performed to treat anisometropia are:

  1. Intraocular lens implantation.

  2. Extraction of clear lens and implantation of an artificial lens.

• If the condition progresses to amblyopia, it has to be treated with conventional approaches used for correcting amblyopia, such as vision therapy, eye patches, eye drops, etc.

What Are the Factors That Determine the Overall Outcome of the Treatment?

The factors that determine the treatment outcome for ametropia are:

• The child's age. The younger the child, the more successful the treatment.

• The initial glass prescription

Is It Possible to Get Rid of Anisometropia of the Eyes Permanently?

Using glass and contact lenses improves the connection between the brain and the eyes; however, the difference in refractive error between the two eyes remains throughout life.

Conclusion

Anisometropia is a common ocular condition concerning the refractive errors of the eyes. It starts at an early age. It remains undiagnosed as most of the patients remain asymptomatic. Early diagnosis of the condition with the help of photo screening will help improve the vision and prevent the development of amblyopia.