Amblyopia?

Retina and Optic Disc Characteristics in Amblyopic and Non-amblyopic Eyes of Patients with Myopic or Hyperopic Anisometropia

Objectives:

To compare retinal and optic disc characteristics between amblyopic and non-amblyopic eyes in patients with myopic and hyperopic anisometropia measured with optical coherence tomography (OCT).

Materials and Methods:

Patients with myopic (25 patients: 17 female, 8 male; median age 27 years, range 16-40 years) and hyperopic (31 patients: 19 female, 12 male; median age 20 years, range 13-41 years) anisometropic amblyopia were included. Eye examination included determination of best-corrected visual acuity (BCVA) with a Snellen chart, measurement of manifest and cycloplegic refraction after pupillary dilation, alternate cover testing, globe movement evaluation, A-scan biometry for axial length, biomicroscopy, fundus examination, and OCT scanning. Main outcome measures were spherical equivalence, BCVA, axial length, retinal nerve fiber layer (RNFL) thickness, macular thickness, macular volume, and optic disc area.

Results:

In both myopic and hyperopic patients, the absolute value of the mean spherical equivalence was significantly greater in the amblyopic than non-amblyopic eyes, and the mean BCVA was significantly less in the amblyopic than the non-amblyopic eyes. In both myopic and hyperopic patients, there were no significant differences in mean RNFL thickness, macular thickness, macular volume, axial length, or optic disc area between amblyopic and non-amblyopic eyes.

Conclusion:

The amblyopic process may have no significant effect on the RNFL, macula, or optic disc. Further studies with more patients, including postmortem studies, may clarify the retinal, histopathologic, and anatomic differences between amblyopic and non-amblyopic eyes.

The relationship between anisometropia and amblyopia

This review aims to disentangle cause and effect in the relationship between anisometropia and amblyopia. Specifically, we examine the literature for evidence to support different possible developmental sequences that could ultimately lead to the presentation of both conditions. The prevalence of anisometropia is around 20% for an inter-ocular difference of 0.5D or greater in spherical equivalent refraction, falling to 2-3%, for an inter-ocular difference of 3D or above. Anisometropia prevalence is relatively high in the weeks following birth, in the teenage years coinciding with the onset of myopia and, most notably, in older adults starting after the onset of presbyopia. It has about one-third the prevalence of bilateral refractive errors of the same magnitude. Importantly, the prevalence of anisometropia is higher in highly ametropic groups, suggesting that emmetropization failures underlying ametropia and anisometropia may be similar. Amblyopia is present in 1-3% of humans and around one-half to two-thirds of amblyopes have anisometropia either alone or in combination with strabismus. The frequent co-existence of amblyopia and anisometropia at a child's first clinical examination promotes the belief that the anisometropia has caused the amblyopia, as has been demonstrated in animal models of the condition. In reviewing the human and monkey literature however it is clear that there are additional paths beyond this classic hypothesis to the co-occurrence of anisometropia and amblyopia. For example, after the emergence of amblyopia secondary to either deprivation or strabismus, anisometropia often follows. In cases of anisometropia with no apparent deprivation or strabismus, questions remain about the failure of the emmetropization mechanism that routinely eliminates infantile anisometropia. Also, the chronology of amblyopia development is poorly documented in cases of 'pure' anisometropic amblyopia. Although indirect, the therapeutic impact of refractive correction on anisometropic amblyopia provides strong support for the hypothesis that the anisometropia caused the amblyopia. Direct evidence for the aetiology of anisometropic amblyopia will require longitudinal tracking of at-risk infants, which poses numerous methodological and ethical challenges. However, if we are to prevent this condition, we must understand the factors that cause it to develop.

Defining Amblyopia: The Need for a Joint Classification

Recent years have shown an impressive rise in high quality research on amblyopia. Unfortunately, the condition is not sufficiently defined, and consequently different studies have been used different definitions of amblyopia. Aspects in need of consideration include the degree of visual acuity reduction, magnitude of interocular difference in acuity, method of visual acuity testing used, the effect of refractive adaption, the presence of amblyogenic factors, the absence of organic cause and the treat ability of the deficit. Vision scientists worldwide are encouraged to jointly decide on what is, and what is not, amblyopia.

Optic nerve axons: life and death before birth

In the 2004 Bowman Lecture, I give a panegyric for Sir William Bowman, an estimate of the importance and the epidemiology of anterior visual pathway developmental disorders, followed by a history of the anterior visual system. I review the normal development of the optic nerve and chiasm and the main developmental disorders: Optic Nerve Aplasia, Optic Nerve Hypoplasia and Achiasmia.

The axial length/disc area ratio in anisometropic hyperopic amblyopia: a hypothesis for decreased unilateral vision associated with hyperopic anisometropia

OBJECTIVE

Hyperopic anisometropia is a risk factor for amblyopia. Disc areas and axial lengths are reduced in amblyopic eyes. This study's aim was to investigate the differences in the relative size of the optic disc in 5 groups of eyes: (1) normal right eyes; (2) smaller eyes of nonamblyopic, anisometropic, bilateral hyperopes; (3) larger eyes of nonamblyopic, anisometropic, bilateral hyperopes; (4) fellow (nonamblyopic) eyes of eyes of amblyopic, anisometropic, bilateral hyperopes; and (5) amblyopic eyes of bilateral hyperopic anisometropes. To determine the relative size of the discs, the relationships of axial length to disc area in the above groups were evaluated.

DESIGN

Case-control study.

PARTICIPANTS

Nine hundred twenty-seven records from the author's private practice, which included axial length measurements and optic disc photographs or digital images, were analyzed to form the 5 groups above.

METHODS

Disc areas were measured using objective techniques developed by Bengtsson and Krakau. Axial lengths were determined by ultrasound biometry or with a Zeiss IOL Master.

MAIN OUTCOME MEASURES

Optic disc areas, corrected for magnification, axial lengths, and a derived ratio, axial length/disc area (AXL/DA).

RESULTS

The AXL/DA for the amblyopic eyes (group 5) was significantly greater than that of all of the other groups, indicating that amblyopic eyes have discs that are both absolutely smaller and, when adjusted for eyes' size, relatively smaller than those of nonamblyopic eyes.

CONCLUSION

The relatively small size of the discs of amblyopic eyes points to an organic as opposed to a functional amblyopia in these patients.