Anisometropia

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.