# Anisometropia

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Anisometropia
Other namespronounce = ann-EYE-sə-mi-TROH-pee-ə
Specialty Ophthalmology, optometry

Anisometropia is the condition in which the two eyes have unequal refractive power. Each eye can be nearsighted (myopia), farsighted (hyperopia) or a combination of both, which is called antimetropia. Generally a difference in power of two diopters or more is the accepted threshold to label the condition anisometropia.

## Contents

In certain types of anisometropia, the visual cortex of the brain will not use both eyes together (binocular vision), and will instead suppress the central vision of one of the eyes. If this occurs often enough during the first 10 years of life while the visual cortex is developing, it can result in amblyopia, a condition where even when correcting the refractive error properly, the person's vision in the affected eye is still not correctable to 20/20.

The name is from four Greek components: an- "not," iso- "same," metr- "measure," ops "eye."

An estimated 6% of subjects aged 6 to 18 have anisometropia.

## Treatment

### Spectacle correction

For those with large degrees of anisometropia, spectacle correction may cause the person to experience a difference in image magnification between the two eyes (aniseikonia) which could also prevent the development of good binocular vision. This can make it very difficult to wear glasses without symptoms such as headaches and eyestrain. However, the earlier the condition is treated, the easier it is to adjust to glasses.

It is possible for spectacle lenses to be made which can adjust the image sizes presented to the eye to be approximately equal. These are called iseikonic lenses. In practice though, this is rarely ever done.

The formula for iseikonic lenses (without cylinder) is:

${\displaystyle {\textrm {Magnification}}={\frac {1}{(1-({\frac {t}{n}})P)}}\cdot {\frac {1}{(1-hF)}}}$

where: t = center thickness (in meters) n = refractive index P = front base curve h = vertex distance (in meters) F = back vertex power (essentially, the prescription for the lens)

If the difference between the eyes is up to 3 diopters, iseikonic lenses can compensate. At a difference of 3 diopters the lenses would however be very visibly different - one lens would need to be at least 3mm thicker and have a base curve increased by 7.5 spheres.

### Contact lenses

The usual recommendation for those needing iseikonic correction is to wear contact lenses. The effect of vertex distance is removed and the effect of center thickness is also almost removed, meaning there is minimal and likely unnoticeable image size difference. This is a good solution for those who can tolerate contact lenses.

### Refractive surgery

Refractive surgery causes only minimal size differences, similar to contact lenses. In a study performed on 53 children who had amblyopia due to anisometropia, surgical correction of the anisometropia followed by strabismus surgery if required led to improved visual acuity and even to stereopsis in many of the children [1] (see: Refractive surgery#Children).

## Epidemiology

A determination of the prevalence of anisometropia has several difficulties. First of all, the measurement of refractive error may vary from one measurement to the next. Secondly, different criteria have been employed to define anisometropia, and the boundary between anisometropia and isometropia depend on their definition. [2]

Several studies have found that anisometropia occurs more frequently and tends to be more severe for persons with high ametropia, and that this is particularly true for myopes. Anisometropia follows a U-shape distribution according to age: it is frequent in infants aged only a few weeks, is more rare in young children, comparatively more frequent in teenagers and young adults, and more prevalent after presbyopia sets in, progressively increasing into old age. [2]

One study estimated that 6% of those between the ages of 6 and 18 have anisometropia. [3]

Notwithstanding research performed on the biomechanical, structural and optical characteristics of anisometropic eyes, the underlying reasons for anisometropia are still poorly understood. [4]

Anisometropic persons who have strabismus are mostly far-sighted, and almost all of these have (or have had) esotropia. [5] However, there are indications that anisometropia influences the long-term outcome of a surgical correction of an inward squint, and vice versa. More specifically, for patients with esotropia who undergo strabismus surgery, anisometropia may be one of the risk factors for developing consecutive exotropia [6] and poor binocular function may be a risk factor for anisometropia to develop or increase. [7]

## Related Research Articles

Esotropia is a form of strabismus in which one or both eyes turns inward. The condition can be constantly present, or occur intermittently, and can give the affected individual a "cross-eyed" appearance. It is the opposite of exotropia and usually involves more severe axis deviation than esophoria. Esotropia is sometimes erroneously called "lazy eye", which describes the condition of amblyopia—a reduction in vision of one or both eyes that is not the result of any pathology of the eye and cannot be resolved by the use of corrective lenses. Amblyopia can, however, arise as a result of esotropia occurring in childhood: In order to relieve symptoms of diplopia or double vision, the child's brain will ignore or "suppress" the image from the esotropic eye, which when allowed to continue untreated will lead to the development of amblyopia. Treatment options for esotropia include glasses to correct refractive errors, the use of prisms and/or orthoptic exercises and/or eye muscle surgery. The term is from Greek eso meaning "inward" and trope meaning "a turning".

In biology, binocular vision is a type of vision in which an animal having two eyes is able to perceive a single three-dimensional image of its surroundings. Neurological researcher Manfred Fahle has stated six specific advantages of having two eyes rather than just one:

1. It gives a creature a spare eye in case one is damaged.
2. It gives a wider field of view. For example, humans have a maximum horizontal field of view of approximately 190 degrees with two eyes, approximately 120 degrees of which makes up the binocular field of view flanked by two uniocular fields of approximately 40 degrees.
3. It can give stereopsis in which binocular disparity provided by the two eyes' different positions on the head gives precise depth perception. This also allows a creature to break the camouflage of another creature.
4. It allows the angles of the eyes' lines of sight, relative to each other (vergence), and those lines relative to a particular object to be determined from the images in the two eyes. These properties are necessary for the third advantage.
5. It allows a creature to see more of, or all of, an object behind an obstacle. This advantage was pointed out by Leonardo da Vinci, who noted that a vertical column closer to the eyes than an object at which a creature is looking might block some of the object from the left eye but that part of the object might be visible to the right eye.
6. It gives binocular summation in which the ability to detect faint objects is enhanced.

Far-sightedness, also known hypermetropia, is a condition of the eye in which light is focused behind, instead of on, the retina. This results in close objects appearing blurry, while far objects may appear normal. As the condition worsens, objects at all distances may be blurry. Other symptoms may include headaches and eye strain. People may also experience accommodative dysfunction, binocular dysfunction, amblyopia, and strabismus.

Strabismus is a condition in which the eyes do not properly align with each other when looking at an object. The eye that is focused on an object can alternate. The condition may be present occasionally or constantly. If present during a large part of childhood, it may result in amblyopia or loss of depth perception. If onset is during adulthood, it is more likely to result in double vision.

Amblyopia, also called lazy eye, is a disorder of sight in which the brain fails to process inputs from one eye, and over time favors the other eye. It results in decreased vision in an eye that otherwise typically appears normal. It is the most common cause of decreased vision in a single eye among children and younger adults.

Refractive eye surgery is non essential eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea (keratomileusis), lens implantation or lens replacement. The most common methods today use excimer lasers to reshape the curvature of the cornea. Successful refractive eye surgery can reduce or cure common vision disorders such as myopia, hyperopia and astigmatism.

Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, diagonally, or rotationally in relation to each other. It is usually the result of impaired function of the extraocular muscles, where both eyes are still functional, but they cannot turn to target the desired object. Problems with these muscles may be due to mechanical problems, disorders of the neuromuscular junction, disorders of the cranial nerves that innervate the muscles, and occasionally disorders involving the supranuclear oculomotor pathways or ingestion of toxins.

Ocular dominance, sometimes called eye preference or eyedness, is the tendency to prefer visual input from one eye to the other. It is somewhat analogous to the laterality of right- or left-handedness; however, the side of the dominant eye and the dominant hand do not always match. This is because both hemispheres control both eyes, but each one takes charge of a different half of the field of vision, and therefore a different half of both retinas. There is thus no direct analogy between "handedness" and "eyedness" as lateral phenomena, which can be a considerable factor to unusual outstanding sports performance, such as cricket or tennis, where athletes may take some physical advantages from the space-time relation and the significant quickness of the moving object as well as the usual brief remaining space of time, or short windows lots, for tagging the projectile with the oponent's movement. Aside from physical advantages, it might be some functional or visuospatial processing advantages when the left eye is preponderant on the scene's information intake.

Duane syndrome is a congenital rare type of strabismus most commonly characterized by the inability of the eye to move outwards. The syndrome was first described by ophthalmologists Jakob Stilling (1887) and Siegmund Türk (1896), and subsequently named after Alexander Duane, who discussed the disorder in more detail in 1905.

Exotropia is a form of strabismus where the eyes are deviated outward. It is the opposite of esotropia and usually involves more severe axis deviation than exophoria. People with exotropia often experience crossed diplopia. Intermittent exotropia is a fairly common condition. "Sensory exotropia" occurs in the presence of poor vision. Infantile exotropia is seen during the first year of life, and is less common than "essential exotropia" which usually becomes apparent several years later.

Suppression of an eye is a subconscious adaptation by a person's brain to eliminate the symptoms of disorders of binocular vision such as strabismus, convergence insufficiency and aniseikonia. The brain can eliminate double vision by ignoring all or part of the image of one of the eyes. The area of a person's visual field that is suppressed is called the suppression scotoma. Suppression can lead to amblyopia.

Hypertropia is a condition of misalignment of the eyes (strabismus), whereby the visual axis of one eye is higher than the fellow fixating eye. Hypotropia is the similar condition, focus being on the eye with the visual axis lower than the fellow fixating eye. Dissociated vertical deviation is a special type of hypertropia leading to slow upward drift of one or rarely both eyes, usually when the patient is inattentive.

Strabismus surgery is surgery on the extraocular muscles to correct strabismus, the misalignment of the eyes. With approximately 1.2 million procedures each year, extraocular muscle surgery is the third most common eye surgery in the United States. The earliest successful strabismus surgery intervention is known to have been performed on 26 October 1839 by Johann Friedrich Dieffenbach on a 7-year-old esotropic child; a few earlier attempts had been performed in 1818 by William Gibson of Baltimore, a general surgeon and professor at the University of Maryland.

Aniseikonia is an ocular condition where there is a significant difference in the perceived size of images. It can occur as an overall difference between the two eyes, or as a difference in a particular meridian.

Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the eye lens in a neutral or relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and the axial length of the eye balance out, which focuses rays exactly on the retina, resulting in perfect vision. A human eye in a state of emmetropia requires no corrective lenses; the vision scores well on a visual acuity test.

Gene Folk was an American ophthalmologist who specialized in the diagnosis and treatment of strabismus. A charter member of the American Association for Pediatric Ophthalmology and Strabismus, he later served as President of this organization. With Martin Urist, Folk helped found the "Chicago" school of strabismus, whose ideas competed with and stimulated those of Marshall M. Parks, Arthur Jampolsky, and other prominent strabismologists. During the 1950s and 1960s, Urist and Knapp's contributions led to a much improved understanding of so-called A and V "pattern" strabismus, where the amplitude of deviation varies in up- and downgaze.

Infantile esotropia is an ocular condition of early onset in which one or either eye turns inward. It is a specific sub-type of esotropia and has been a subject of much debate amongst ophthalmologists with regard to its naming, diagnostic features, and treatment.

Stereopsis recovery, also recovery from stereoblindness, is the phenomenon of a stereoblind person gaining partial or full ability of stereo vision (stereopsis).

Botulinum toxin therapy of strabismus is a medical technique used sometimes in the management of strabismus, in which botulinum toxin is injected into selected extraocular muscles in order to reduce the misalignment of the eyes. The injection of the toxin to treat strabismus, reported upon in 1981, is considered to be the first ever use of botulinum toxin for therapeutic purposes. Today, the injection of botulinum toxin into the muscles that surround the eyes is one of the available options in the management of strabismus. Other options for strabismus management are vision therapy and occlusion therapy, corrective glasses and prism glasses, and strabismus surgery.

The management of strabismus may include the use of drugs or surgery to correct the strabismus. Strabismus is a misalignment of the eyes and may also result in amblyopia or impairments of binocular vision.

## References

1. William F. Astle; Jamalia Rahmat; April D. Ingram; Peter T. Huang (December 2007). "Laser-assisted subepithelial keratectomy for anisometropic amblyopia in children: Outcomes at 1 year". Journal of Cataract & Refractive Surgery. 33 (12): 2028–2034. doi:10.1016/j.jcrs.2007.07.024. PMID   18053899.
2. Barrett BT, Bradley A, Candy TR (September 2013). "The relationship between anisometropia and amblyopia". Progress in Retinal and Eye Research. 36: 120–58. doi:10.1016/j.preteyeres.2013.05.001. PMC  . PMID   23773832.
3. Czepita D, Goslawski W, Mojsa A. "Occurrence of anisometropia among students ranging from 6 to 18 years of age." Klin Oczna. 2005;107(4-6):297-9. Polish. PMID   16118943.
4. Vincent SJ, Collins MJ, Read SA, Carney LG (2014). "Myopic anisometropia: ocular characteristics and aetiological considerations" (PDF). Clinical & Experimental Optometry (Review). 97 (4): 291–307. doi:10.1111/cxo.12171. PMID   24939167.
5. "When strabismus is present in an anisometropic individual, it is almost always of the convergent type and is generally found in anisohyperopes but not anisomyopes." Barrett BT, Bradley A, Candy TR (September 2013). "The relationship between anisometropia and amblyopia". Progress in Retinal and Eye Research. 36: 120–58. doi:10.1016/j.preteyeres.2013.05.001. PMC  . PMID   23773832.
6. Yurdakul NS, Ugurlu S (2013). "Analysis of risk factors for consecutive exotropia and review of the literature". Journal of Pediatric Ophthalmology and Strabismus. 50 (5): 268–73. doi:10.3928/01913913-20130430-01. PMID   23641958.
7. Fujikado T, Morimoto T, Shimojyo H (November 2010). "Development of anisometropia in patients after surgery for esotropia". Japanese Journal of Ophthalmology. 54 (6): 589–93. doi:10.1007/s10384-010-0868-z. PMID   21191721.