Amblyopia is often defined as a difference in visual acuity of two lines or more on Snellen or equivalent chart in a child with, otherwise, healthy eyes. Amblyopia may be present any time visual acuity is reduced, and the reduction of acuity cannot be explained by findings on clinical examination, even if the difference is even one line only.
The term amblyopia is derived from Greek word (amblys means dull and ops means eye), which literally means dullness of vision. Von Noorden defined amblyopia as a ‘decrease of visual acuity in one eye when caused by abnormal binocular interaction or occurring in one or both eyes as a result of patterned vision deprivation during immaturity, for which no cause can be detected during the physical examination of the eye(s) at which in appropriate cases is reversible by therapeutic measures’.
Amblyopia is a functional reduction in the visual acuity of an eye caused by disuse during critical period of visual development. The mechanism of vision loss is not known, but it is thought to originate in the visual cortex. Amblyopia results in reduced visual acuity, binocularity, depth perception, and contrast sensitivity. Fusion and stereopsis, the central formation of three dimensional images, are dependent upon receiving clear images from each eye simultaneously.
Agarwal Sunita, Agarwal Athiya, Apple David J, Buratto Lucio, Aliό Jorge L, Pandey Suresh K, Agarwal Amar. Textbook of Ophthalmology Vol 1. Jaypee Brothers Medical Publishers (P) Ltd 2002. P 447- 451.
Schwartz M William, Bell Louis M Jr, Bingham Peter M, Chung Esther K, Cohen Mitchell I, Friedman David F, Mulberg Andrew E. The 5 - Minute Pediatric Consult Third Edition. Lippincott Williams & Wilkins 2003. P 110- 111.
Scheiman Mitchell, Wick Bruce. Clinical Management of Binocular Vision – Heterophoric, Accommodative and Eye Movement Disorders Third Edition. Lippincott Williams & Wilkins, a Wolters Kluwer business 2008. P 478- 496.
Von Noorden GK. Amblyopia: a multidisciplinary approach. Proctor lecture. Invest Ophthalmol Vis Sci 1985; 26: 1704- 1716.
The main symptom of amblyopia is decreased foveal visual acuity. Commonly used diagnostic criteria, is the loss of visual acuity of two or more lines on the Snellen vision chart.
A distinction must be made between potentially reversible functional amblyopia and irreversible organic amblyopia. Organic amblyopia is a term used to describe visual impairment due to obvious or non obvious ocular pathology, commonly affecting retina or optic nerve. Examples of organic amblyopia are optic nerve hypoplasia, optic atrophy and foveal hypoplasia. Functional amblyopia may occur along with organic amblyopia. Functional amblyopia normally occurs in an eye that is anatomically normal.
Functional amblyopia is caused by either form of vision deprivation or abnormal binocular interaction. Form- vision deprivation occurs due to conditions that obstruct the visual axis such as cataract, corneal opacity, vitreous haemorrhage, or severe ptosis, but it may also be produced by severe anisometropia. Abnormal binocular interaction refers to the condition in which the image projected onto the fovea of each eye is dissimilar enough to preclude fusion, thus prompting suppression and ultimately amblyopia of the suppressed eye. While strabismus may be the most obvious cause of abnormal binocular interaction, unilateral opacity of the media and anisometropia may participate in this mechanism as well.
Aetiological classification: This is the most commonly used classification and has high prognostic value. It is classified as
- Isoametropic amblyopia: This is related to the age. It is said that high refractive errors either bilateral or unilateral would not affect the development of binocularity and visual acuity before the first year of life. Otherwise, refractive errors not corrected after one year of age are related with a high degree of uni- and bilateral decrease of vision.
In hypermetropia, insufficient accommodative effort which inadequately focuses the images on retina may produce amblyopia. In high myopia, objects close to the patient tend to be in focus and amblyopia is uncommon.
- Anisometropic amblyopia: It is likely to occur because the amount of accommodation necessary to focus the image in the less hypermetropic eye is utilised. Therefore, more hypermetropic eye receives blurred image. When the difference in hypermetropia exceeds two dioptres, amblyopia may be present. Some cases even with less amount of difference may develop amblyopia.
In anisomyopia, more myopic eye may be used for near vision and the less myopic for distance vision. It causes sharp image to be present in both the eyes. Amblyopia is unusual when myopia does not exceed five dioptres.
- Meridional amblyopia: Meridional amblyopia is due to astigmatism producing defocused images with specific orientation. Astigmatism appears to be an important factor for amblyopia, during first year of life.
Diagnosis of amblyopia is based mainly on measurement of visual acuity.
A diagnosis of amblyopia requires complete eye examination to rule out any organic cause as well as correct refractive error under cycloplegia. Clinically an organic cause is suspected in addition to functional amblyopia, if treatment improves vision up to certain level only. Amblyopia may be mimicked by any defect in the afferent visual system. Optic nerve glioma is the most important suspected pathology.
Visual acuity measurement
Subjective and objective methods to test visual acuity have been proposed.
Early management of amblyopia is critical for best visual acuity results. The basic strategy for treating amblyopia is to first provide a clear retinal image, and then correct ocular dominance if ocular dominance is present, as early as possible during the period of visual plasticity (birth to eight years of life).
I. Clear retinal image:
Patients with bilateral hypermetropia (> + 5.00 D) should receive the full hypermetropic correction, as amblyopic eyes do not fully accommodate. Patients who are given partial correction of their high hypermetropia often shows very slow or no improvement in their amblyopia. Prescribe full astigmatic correction to provide a clear retinal image.
II. Correct ocular dominance
Correction of ocular dominance is accomplished by forcing fixation to the amblyopic eye through patching or blurring the vision of the sound eye by
The prognosis of amblyopia depends upon the age of the patient, severity of amblyopia, and type of amblyopia. The earlier the amblyopia occurs and longer it remains untreated, the worse is the prognosis. In general, bilateral amblyopia responds better than unilateral amblyopia, and myopic anisometropic amblyopia responds better than hypermetropic anisometropic amblyopia.