Anisometropia

Anisometropia is an optical state with unequal refraction of the two eyes. The amount of spherical refractive error (myopia or hypermetropia) is usually about the same for both the eyes in most of the people. Generally, anisometropia is considered to exist if the refraction differs by 1.0 dioptres (D) or more for the two eyes. The measuring unit for refractive error is dioptre (D), which is defined as the reciprocal of the focal length in meters. Anisometropia is the most insidious refractive condition because it is often asymptomatic. The term antimetropia is used when one eye is myopic and the other is hypermetropic.

The word anisometropia is derived from the Greek words anisos (unequal), metron (measure), and ops (vision).

Emmetropia is the condition where the eye has no refractive error and requires no correction for distance vision. Refractive power of the eye is determined predominantly by variables like power of the cornea, power of the lens, and axial length of the eyeball. In emmetropia, these three components of refractive power combine to produce normal refraction to the eye. In an emmetropic eye, rays of light parallel to the optical axis focuses on the retina. The far point in emmetropia (point conjugate to retina in non- accommodating state) is optical infinity, which is 6 meters. Ametropia (refractive error) results when cornea and lens inadequately focus the light rays.

The term ametropia (refractive error) describes any condition where light is poorly focused on light sensitive layer of eye, resulting in blurred vision. This is a common eye problem and includes conditions such as myopia (near-sightedness), hypermetropia (far-sightedness), astigmatism, and presbyopia (age-related diminution of vision).

Anisometropia due to refractive myopia or hypermetropia is known as refractive anisometropia and that due to axial ametropia is known as axial anisometropia. Anisometropia due to difference in refractive error along one meridian only is called meridional anisometropia.

In myopic anisometropia, one expects the distance visual acuity in each eye to be lower than normal, the more myopic eye having the poorer visual acuity. However, when the amount of myopia in the less myopic eye is small (minus 0.25 or 0.50 D), the visual acuity in that eye is sufficiently good so that the patient may not be aware of the problem, even if the visual acuity in the more myopic eye is quite poor.

In hypermetropic anisometropia, the visual acuity of both eyes is relatively good as long as the patient has sufficient accommodation.

In myopic anisometropia, hypermetropic anisometropia and antimetropia, the individual may not have complaint of asthenopia (eyestrain) and the anisometropia may be discovered during routine eye examination only. However, some cases of hypermetropic anisometropia may have asthenopia due to their inability to focus simultaneously.

Because both the eyes accommodate equally, an uncorrected anisometrope has the problem of never having sharply focused image on both retinas at the same time. For example, a person with 0.25 D of myopia in one eye and 3 D of myopia in other eye will have a sharp focus for one eye for objects at a distance of 4 meters and a sharp focus for the other eye for objects at a distance of 33 cm. Due to this, the person uses less myopic eye for distance vision and the more myopic eye for near vision. Although stereopsis (binocular depth perception) is poor, such individuals have an advantage in later years because bifocal reading glasses may not be required.

On the other hand, a person having 0.25 D of hypermetropia in one eye and 3 D of hypermetropia in the other eye has more severe difficulty. The less hypermetropic eye is used for distance vision requiring 0.25 D of accommodation only. However, more hypermetropic eye requires 3 D of accommodation for distance and 5.50 D of accommodation for a reading distance of 40 cm. Therefore, less hypermetropic eye never has a sharply focused image. It may lead to anisometropic amblyopia (non correctable visual acuity without an obvious cause) in early life.

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Anisometropia occurs because of uncorrected unequal refractive error between fellow eyes. Uncorrected anisometropia produces

Accommodative asthenopia.
Alternating vision.
Blurring of image in one eye.
Abnormal binocular interaction produced by dissimilar images on retina.
Diplopia (double vision).
Amblyopia (inability to see image from one eye).
Strabismus (squint).

Anisometropia may be congenital or acquired.

Congenital and developmental anisometropia: This is produced due to differential growth of each eyeball. It is hereditary in origin.
Acquired anisometropia: This is produced by

– Post cataract surgery uniocular aphakia.

– Incorrect power of intraocular lens implant in patients of pseudophakia .

– Eye injury.

– Inadvertent surgical treatment of refractive error.

– Keratoplasty in one eye.

Diagnosis depends upon retinoscopic examination in patients with defective visual acuity.

Clinical types of Anisometropia:

Anisometropia may be absolute or relative.

I. Absolute anisometropia: In this condition, refractive power of the two eyes is not equal.

This is further divided into

Simple anisometropia: In this type one eye is normal and the other eye is hypermetropic or myopic.
Compound anisometropia: In this both eyes are ametropic i.e. either myopic or hypermetropic.
Mixed anisometropia: In this both eyes are ametropic with one being myopic and the other being hypermetropic. This is also called antimetropia.
Simple astigmatic anisometropia: In this type one eye is emmetropic and the other has simple astigmatism either myopic or hypermetropic.
Compound astigmatic anisometropia: Both eyes are astigmatic with unequal degree.

II. Relative anisometropia: It is that refractive anomaly in which although the total refraction of the two eyes is nearly equal, the component elements in each show relatively large differences. Therefore, total refraction of the two eyes may be equal, but the axial length may be different. This produces clear retinal image with aniseikonia (different size of retinal images).

Visual status in anisometropia:

There are various visual possibilities

Binocular single vision: Binocular vision is possible in small degrees of anisometropia. A difference of more than 5 D between two eyes causes loss of binocularity. Since accommodation is distributed equally between two eyes and is not dissociated, the image of one eye is blurred always. It produces symptoms of accommodative asthenopia.
Amblyopia: If visual acuity is not good and the hypermetropia is high, then this eye may be excluded from vision and the eye develops amblyopia. Thus these patients have uniocular vision in the unaffected eye.
Alternating vision: When there is high anisometropia with good visual acuity in eyes, then the patient develops habit of using hypermetropic eye for distance and the myopic eye for near vision. These patients are usually comfortable and do not seek any medical help for vision correction.
Strabismus: The eye with the visual defect become convergent and the child may develop concomitant convergent squint.

Tests for binocular vision:

Binocular vision may be assessed by using FRIEND test or Worth’s four-dot test.

I. FRIEND test: In this test, alternate letters are green and red in colour. Letters F, I and N are in green colour and the letters R, E and D are in red colour. This is available in Snellen’s distance vision chart. Patient is seated 6 meters from the Snellen’s chart and wears diplopia goggles with red glass in front of the right eye and green glass in front of the left eye. Patient then reads letters FRIEND on Snellen’s chart. Result is interpreted as

Patient reads all letters as FRIEND: In the presence of binocular single vision, the patient reads all letters as FRIEND at once.
Patient reads either green FIN letters or red RED letters: In amblyopia or uniocular vision, patient reads either green letters or red letters i.e. it is read persistently either as FIN (read with left eye) or RED (read with right eye).
Patient reads sometimes FIN and RED: In alternate vision, patient reads sometimes FIN and RED at other times.

II. Worth’s four-dot test: Patient wears diplopia goggles with red glass in front of the right eye and green glass in front of the left eye. Patient views Snellen’s distance vision chart having four dots of light viz. one red, two green and one white. The results are interpreted as

Patient sees all the four lights: When all the four lights are seen in the absence of manifest squint, patient has normal binocular single vision. Patient sees four lights also in the presence of a manifest squint with abnormal retinal correspondence.
Patient sees two red lights only: This indicates suppression of vision in the left eye.
Patient sees three green lights only: This indicates suppression of vision in the right eye.
Patient sees three green lights and two red lights alternately: This indicates presence of alternating suppression of vision.
Patient sees three green lights and two red lights (five lights): This indicates the presence of diplopia in patient.

Management should be carried out under medical supervision.

Medical therapy

Principal factors which influence optical correction of anisometropia are aniseikonia and amblyopia. Corrective guidelines are

Anisometropia should be fully corrected in the presence of amblyopia.
Anisometropia of 3 D or more at one year of age should be corrected as it may produce amblyopia in 60% of cases.
Degree of anisometropia between 1.50 D to 3 D that do not decline on follow-up should also be corrected.
Older children and adults may not be able to tolerate full correction binocularly. Therefore, reduced optical correction of the more ametropic eye may be required even with compromised visual acuity.
Large differences in refraction may require contact lenses, and are prescribed if

– Aniseikonia is present. Contact lenses reduce the difference in the size of the image e. g. as in aphakia. Binocular single vision may be restored in favourable cases.

– There is no improvement of visual acuity in anisometropic amblyopia in children with a difference in refraction of 4 D or above.

– Trial contact lenses show a better binocular function as compared to spectacles.

Patients with marked anisometropic myopia may show hypophoria in primary position with a head tilt to the side of more myopic eye to maintain a comfortable binocular single vision (the ‘heavy eye’ phenomenon).

Medical therapy includes

Spectacles: The corrective glasses may be tolerated up to a maximum difference of 4 D in two eyes. A difference more than 4 D produces diplopia. In children younger than 12 years of age, where best visual correction is required in both the eyes, contact lenses are preferred. In adults, best correction may be prescribed which does not result in ocular discomfort.
Contact lenses: Contact lenses are advised for higher degrees of anisometropia. These are more useful in young children with high anisometropia, who may become amblyopic in more ametropic eye.

Surgical therapy

This is the preferred mode of treatment.

Surgical therapy includes

Intraocular lens implantation: Intraocular lens may be implanted for uniocular aphakia.
Refractive corneal surgery: Refractive corneal surgery (refractive laser) may be done for unilateral myopia, astigmatism and hypermetropia. Photorefractive surgery, both photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK), is being used increasingly to correct anisometropia.
Removal of crystalline lens: Crystalline lens may be removed for unilateral high myopia.
Phakic IOL: Intraocular lens may be implanted without removing the crystalline lens.
Clear lens extraction with the implantation of an IOL: Clear lens extraction with the implantation of an IOL, preferably foldable IOL or a piggyback IOL, may be done. Piggyback IOL means two IOLs are placed in the eye, one on top of the other. This is done if the biometry is +40 D and one does not have a lens of that power to implant.