A biometric investigation of ocular components in amblyopia

Abstract

The aim of this study was to investigate the ocular parameters and their contribution to total axial length, and any inter-ocular differences when compared to the fellow non-amblyopic eye, in a population of paediatric amblyopes. Inter-ocular differences in visual acuity, refractive error, corneal curvature, anterior chamber depth and their contribution to total axial length were analysed in four populations: adult controls (n=26), paediatric controls (n=24), paediatric strabismic amblyopes (n=18) and paediatric anisometropic amblyopes (n=27). In the two control groups, there were small inter-ocular differences between the components, none of which were statistically or clinically significant. Anisometropic amblyopic eyes were found to have statistically and clinically significant differences in refractive error, crystalline lens power, vitreous chamber depth and total axial length when compared to the fellow eye. Anterior and vitreous chamber depths, when expressed as a percentage of the anterior and vitreous chambers in the non-amblyopic fellow eye, were approximately 95% of the size. When parameters were normalised by expressing as a percentage of total axial length, the contribution of the anterior chamber, crystalline lens and vitreous chamber to total axial length in both eyes were comparable, suggesting that all components of the anisometropic amblyopic eye are proportionately reduced in size. The strabismic amblyopic eyes were in the main isometropic. There were however statistically significant differences in anterior chamber depth, crystalline lens power, vitreous chamber depth and total axial length, but not in crystalline lens thickness. When the components were expressed as a percentage of the component in the non-amblyopic fellow eye, the anterior and vitreous chambers were approximately 90% of the size of the fellow non-amblyopic eye and made a significantly smaller contribution to total axial length when normalised. Crystalline lens thickness contributed significantly more to the total axial length in a strabismic eye. This suggests that, unlike control eyes or anisometropic amblyopic eyes, the strabismic eye is physically, though not proportionately, reduced in size.

Biometric relationships of ocular components in esotropic amblyopia

“…We found similar individual contribution from anterior cham ber depth to total axial length in amblyopic and non-amblyopic eyes. This finding is consistent with the previous cited study (21) . Even though interocular differences for the individual contribution from lens thickness and vitreous chamber depth to total axial length in amblyopic patients were not clinically significant in our stu dy, these represented statistically significant differences.…”

“…One previous study found similar re sults and hypothesized that amblyopia may cause a stalling in the emmetropization process, leaving the lens disproportionately thicker (21) . The role played by the ocular components in emmetropization in normal eyes is not completely understood.…”

“…During emmetropization, both the anterior chamber and the vitreous chamber deepen, the cornea flattens and the lens flattens and become thinner due to equatorial stretch during the ocular sagittal ex pansion (25,26) . Some investigators have suggested that in amblyopia the lens is disproportionately thicker because the eye appears to be under-developed, as a result of impaired emmetropization in the infantile phase, before the lens thinning due to sagittal expansion of the globe (21) . Theories explaining the emmetropization process involve feedback mechanisms in which retinal image quality may affect axial (20,27,28) .…”

“…They found that the contribution of anterior chamber depth and vitreous chamber depth to axial length were similar between amblyopic and fellow eyes. However, lens thickness contributed more to the total axial length in the amblyopic eye when compared to the fellow eye (21) . Nevertheless, these investigators included only 18 strabismic patients and used through-the-lid A-scan ultra sound biometry.…”

The relationship of axial length to cycloplegic refraction and keratometry in amblyopic eyes of hyperopic children

“…In the same paper, 8 the authors described 27 patients with anisometropic amblyopia (greater than 11.00 D interocular difference) without strabismus and concluded that the affected more hyperopic eyes were proportionally smaller than their fellow eyes-in other words, that the relations among biometric components of affected eyes were the same as in fellow eyes. Again, these conclusions were limited by large overlapping 95% confidence intervals, multiple paired t test comparisons, and the performance of A-scan ultrasound through closed lids.…”