Anisometropia

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Anisometropia
Pronunciation
Specialty Ophthalmology, optometry   OOjs UI icon edit-ltr-progressive.svg
Symptoms Eyes' refractive power differs significantly
Complications Amblyopia
Antimetropia
Specialty Ophthalmology, optometry   OOjs UI icon edit-ltr-progressive.svg
Symptoms One eye has myopia and other has hyperopia
Complications Amblyopia

Anisometropia is a condition in which a person's eyes have substantially differing refractive power. [1] Generally, a difference in power of one diopter (1D) is the threshold for diagnosis of the condition. [2] [3] Patients may have up to 3D of anisometropia before the condition becomes clinically significant due to headache, eye strain, double vision or photophobia. [4]

Contents

In certain types of anisometropia, the visual cortex of the brain cannot process images from both eyes simultaneously (binocular summation), but will instead suppress the central vision of one of the eyes. If this occurs too often 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 may still not be fully correctable to 20/20.

The name of the condition comes from its four Greek components: an- "not", iso- "same", metr- "measure", ops "eye".

Antimetropia is a rare sub-type of anisometropia in which one eye is myopic (nearsighted) and the other eye is hyperopic (farsighted). This condition occurs in about 0.1% of the population. [5]

Causes

Anisometropia is caused by common refractive errors, such as astigmatism, far-sightedness, and myopia, in one eye. [6]

Anisometropia is likely the result of both genetic and environmental influences. [7]

Some studies suggest, in older adults, developing asymmetric cataracts may cause worsen anisometropia. However, anisometropia is associated with age regardless of cataract development: a rapid decrease in anisometropia during the first years of life, an increase during the transition to adulthood, relatively unchanging levels during adulthood but significant increases in older age. [7]

Diagnosis

Anisometropia causes some people to have mild vision problems, or occasionally more serious symptoms like alternating vision or frequent squinting. However, since most people do not show any clear symptoms, the condition usually is found during a routine eye exam. [8]

For early detection in preverbal children, photoscreening can be used. In this brief vision test specialized cameras detect each eye's light reflexes, which the equipment's software or a test administrator then interprets. If photoscreening indicates the presence of risk factors, an ophthalmologist can then diagnose the condition after a complete eye exam, including dilating the pupils and measuring the focusing power of each eye. [6]

Treatment

Spectacle correction

For those with large degrees of anisometropia, the wearing of standard spectacles 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:

where:
t = center thickness (in metres);
n = refractive index;
P = front base curve (in 1/metres);
h = vertex distance (in metres);
F = back vertex power (in 1/metres), (essentially, the prescription for the lens, quoted in diopters).

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 3 mm thicker and have a base curve increased by 7.5 spheres.

Example

Consider a pair of spectacles to correct for myopia with a prescription of −1.00 m−1 in one eye and −4.00 m−1 in the other. Suppose that for both eyes the other parameters are identical, namely t = 1 mm = 0.001 m, n = 1.6, P = 5 m−1, and h = 15 mm = 0.015 m.
Then for the first eye ,
while for the second eye .

Thus, in the first eye the size of the image formed on the retina will be 1.17% smaller than without spectacles (although it will be sharp, rather than blurry), whilst in the second eye the image formed on the retina will be 5.36% smaller.

As alluded to above, one method of producing more iseikonic lenses would be to adjust the thickness and base curve of the second lens. For instance, theoretically it could be set to t = 5 mm = 0.005 m and P = 14.5 m−1, with all other parameters unchanged. Then for the second eye the magnification would become ,
which is much closer to that of the first eye.

In this example the first eye, with a −1.00 diopter prescription, is the stronger eye, needing only slight correction to sharpen the image formed, and hence a thin spectacle lens. The second eye, with a −4.00 diopter prescription, is the weaker eye, needing moderate correction to sharpen the image formed, and hence a moderately thick spectacle lens—if the aniseikonia is ignored. In order to avoid the aniseikonia (so that both magnifications will be practically the same, while retaining image sharpness in both eyes), the spectacle lens used for the second eye will have to be made even thicker.

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 [9] (see: Refractive surgery).

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. [10]

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. [10]

One study estimated that 6% of those between the ages of 6 and 18 have anisometropia. [11]

Notwithstanding research performed on the biomechanical, structural and optical characteristics of anisometropic eyes, the underlying reasons for anisometropia are still poorly understood. [12]

Anisometropic persons who have strabismus are mostly far-sighted, and almost all of these have (or have had) esotropia. [13] 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 [14] and poor binocular function may be a risk factor for anisometropia to develop or increase. [15]

Related Research Articles

<span class="mw-page-title-main">Esotropia</span> Form of strabismus in which the eyes turn inward

Esotropia is a form of strabismus in which one or both eyes turn 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, orthoptic exercises, or eye muscle surgery.

<span class="mw-page-title-main">Farsightedness</span> Eye condition in which light is focused behind instead of on the retina

Far-sightedness, also known as long-sightedness, hypermetropia, and hyperopia, is a condition of the eye where distant objects are seen clearly but near objects appear blurred. This blur is due to incoming light being focused behind, instead of on, the retina due to insufficient accommodation by the lens. Minor hypermetropia in young patients is usually corrected by their accommodation, without any defects in vision. But, due to this accommodative effort for distant vision, people may complain of eye strain during prolonged reading. If the hypermetropia is high, there will be defective vision for both distance and near. People may also experience accommodative dysfunction, binocular dysfunction, amblyopia, and strabismus. Newborns are almost invariably hypermetropic, but it gradually decreases as the newborn gets older.

<span class="mw-page-title-main">Presbyopia</span> Medical condition associated with aging of the eye

Presbyopia is physiological insufficiency of accommodation associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Also known as age-related farsightedness, it affects many adults over the age of 40. A common sign of presbyopia is difficulty reading small print which results in having to hold reading material farther away. Other symptoms associated can be headaches and eyestrain. Different people will have different degrees of problems. Other types of refractive errors may exist at the same time as presbyopia. This condition is similar to hypermetropia or far-sightedness which starts in childhood and exhibits similar symptoms of blur in the vision for close objects.

<span class="mw-page-title-main">Strabismus</span> Eyes not aligning when looking at something

Strabismus is a vision disorder in which the eyes do not properly align with each other when looking at an object. The eye that is pointed at 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 lazy eyes, and loss of depth perception. If onset is during adulthood, it is more likely to result in double vision.

<span class="mw-page-title-main">Amblyopia</span> Failure of the brain to process input from one eye

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

<span class="mw-page-title-main">Refractive surgery</span> Surgery to treat common vision disorders

Refractive surgery is an optional 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. Refractive eye surgeries are used to treat common vision disorders such as myopia, hyperopia, presbyopia and astigmatism.

<span class="mw-page-title-main">Refractive error</span> Problem with focusing light accurately on the retina due to the shape of the eye

Refractive error is a problem with focusing light accurately on the retina due to the shape of the eye and/or cornea. The most common types of refractive error are near-sightedness, far-sightedness, astigmatism, and presbyopia. Near-sightedness results in far away objects being blurry, far-sightedness and presbyopia result in close objects being blurry, and astigmatism causes objects to appear stretched out or blurry. Other symptoms may include double vision, headaches, and eye strain.

<span class="mw-page-title-main">Diplopia</span> Double vision

Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally or vertically in relation to each other. Also called double vision, it is a loss of visual focus under regular conditions, and is often voluntary. However, when occurring involuntarily, it results from 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.

Pediatric ophthalmology is a sub-speciality of ophthalmology concerned with eye diseases, visual development, and vision care in children.

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.

<span class="mw-page-title-main">Exotropia</span> Visual disorder where eyes work independently

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 in one eye. Infantile exotropia is seen during the first year of life, and is less common than "essential exotropia" which usually becomes apparent several years later.

<span class="mw-page-title-main">Strabismus surgery</span> Surgery to correct strabismus

Strabismus surgery is surgery on the extraocular muscles to correct strabismus, the misalignment of the eyes. Strabismus surgery is a one-day procedure that is usually performed under general anesthesia most commonly by either a neuro- or pediatric ophthalmologist. The patient spends only a few hours in the hospital with minimal preoperative preparation. After surgery, the patient should expect soreness and redness but is generally free to return home.

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. If the ocular image size in both eyes are equal, the condition is known as iseikonia.

In visual perception, the near point is the closest point at which an object can be placed and still form a focused image on the retina, within the eye's accommodation range. The other limit to the eye's accommodation range is the far point.

<span class="mw-page-title-main">Emmetropia</span> State of vision

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

Burt Kushner is an American pediatric ophthalmologist specializing in the diagnosis and treatment of strabismus. Kushner's contributions include demonstration of improved visual fields of patients following strabismus surgery, elucidation of torsional contribution to patients with diplopia, corticosteroid treatment of periocular capillary hemangioma, and novel hypotheses on the mechanism of "overacting" extraocular muscles.

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.

<span class="mw-page-title-main">Stereopsis recovery</span> Medical phenomenon

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

The management of strabismus may include the use of drugs or surgery to correct the strabismus. Agents used include paralytic agents such as botox used on extraocular muscles, topical autonomic nervous system agents to alter the refractive index in the eyes, and agents that act in the central nervous system to correct amblyopia.

Retinal birefringence scanning (RBS) is a method for detecting the central fixation of the eye. The method can be used in pediatric ophthalmology for screening purposes. By simultaneously measuring the central fixation of both eyes, small- and large-angle strabismus can be detected. The method is not invasive and requires little cooperation by the patient, so it can be used for detecting strabismus in young children. The method provides a reliable detection of strabismus and has also been used for detecting certain kinds of amblyopia. RBS uses the human eye's birefringent properties to identify the position of the fovea and the direction of gaze, and thereby to measure any binocular misalignment.

References

  1. ↑ "Anisometropia - American Association for Pediatric Ophthalmology and Strabismus". aapos.org. Retrieved 10 February 2020.
  2. ↑ Hammond, Christopher J (2013-01-01), Hoyt, Creig S; Taylor, David (eds.), "Chapter 5 - Emmetropization, refraction and refractive errors: control of postnatal eye growth, current and developing treatments", Pediatric Ophthalmology and Strabismus (Fourth Edition), London: W.B. Saunders, pp. 31–35, doi:10.1016/b978-0-7020-4691-9.00005-4, ISBN   978-0-7020-4691-9 , retrieved 2022-05-02
  3. ↑ Nunes, Amélia F; Batista, Maria; Monteiro, Pedro (2022-01-10). "Prevalence of anisometropia in children and adolescents". F1000Research. 10: 1101. doi: 10.12688/f1000research.73657.3 (inactive 2024-09-12). ISSN   2046-1402. PMC   8729023 . PMID   35035896.{{cite journal}}: CS1 maint: DOI inactive as of September 2024 (link)
  4. ↑ Krarup, Therese Grønhøj; Nisted, Ivan; Christensen, Ulrik; Kiilgaard, Jens Folke; la Cour, Morten (June 2020). "The tolerance of anisometropia". Acta Ophthalmologica. 98 (4): 418–426. doi: 10.1111/aos.14310 . ISSN   1755-375X. PMID   31773911. S2CID   208321443.
  5. ↑ Vincent, Stephen J.; Read, Scott A. (1 July 2014). "Progressive adult antimetropia". Clinical and Experimental Optometry. 97 (4): 375–378. doi: 10.1111/cxo.12129 . PMID   24438511.
  6. 1 2 "Anisometropia - American Association for Pediatric Ophthalmology and Strabismus". www.aapos.org. Retrieved 2022-02-08.
  7. 1 2 Vincent, Stephen J; Collins, Michael J; Read, Scott A; Carney, Leo G (2014-07-01). "Myopic anisometropia: ocular characteristics and aetiological considerations". Clinical and Experimental Optometry. 97 (4): 291–307. doi:10.1111/cxo.12171. ISSN   0816-4622. PMID   24939167. S2CID   205048169.
  8. ↑ "Guide to Anisometropia (& Treatment Options)". NVISION Eye Centers. Retrieved 2022-02-08.
  9. ↑ 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. S2CID   1886316.
  10. 1 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   3773531 . PMID   23773832.
  11. ↑ Czepita D, Goslawski W, Mojsa A (2005). "Occurrence of anisometropia among students ranging from 6 to 18 years of age". Klin Oczna. 107 (4–6): 297–9. PMID   16118943.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. ↑ Vincent SJ, Collins MJ, Read SA, Carney LG (2014). "Myopic anisometropia: ocular characteristics and aetiological considerations". Clinical & Experimental Optometry (Review). 97 (4): 291–307. doi: 10.1111/cxo.12171 . PMID   24939167. S2CID   205048169.
  13. ↑ "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   3773531 . PMID   23773832.
  14. ↑ 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.
  15. ↑ 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. S2CID   9298405.
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