Visual deficits in anisometropia

Abstract

Amblyopia is usually associated with the presence of anisometropia, strabismus or both early in life. We set out to explore quantitative relationships between the degree of anisometropia and the loss of visual function, and to examine how the presence of strabismus affects visual function in observers with anisometropia. We measured optotype acuity, Pelli-Robson contrast sensitivity and stereoacuity in 84 persons with anisometropia and compared their results with those of 27 persons with high bilateral refractive error (isoametropia) and 101 persons with both strabismus and anisometropia. All subjects participated in a large-scale study of amblyopia (McKee et al., 2003). We found no consistent visual abnormalities in the strong eye, and therefore report only on vision in the weaker, defined as the eye with lower acuity. LogMAR acuity falls off markedly with increasing anisometropia in non-strabismic anisometropes, while contrast sensitivity is much less affected. Acuity degrades rapidly with increases in both hyperopic and myopic anisometropia, but the risk of amblyopia is about twice as great in hyperopic than myopic anisometropes of comparable refractive imbalance. For a given degree of refractive imbalance, strabismic anisometropes perform considerably worse than anisometropes without strabismus--visual acuity for strabismics was on average 2.5 times worse than for non-strabismics with similar anisometropia. For observers with equal refractive error in the two eyes there is very little change in acuity or sensitivity with increasing (bilateral) refractive error except for one extreme individual (bilaterally refractive error of -15 D). Most pure anisometropes with interocular differences less than 4D retain some stereopsis, and the degree is correlated with the acuity of the weak eye. We conclude that even modest interocular differences in refractive error can influence visual function.

Figure 1

Distribution of refractive errors in each eye of anisometropes (red), strabismic anisometropes (blue) and refractives (gray). We use this color code in all subsequent figures. The vector blur refractive error of the strong eye (abscissa) is plotted against the vector blur refractive error of the weak eye (ordinate). The solid black lines denote zero refractive error (horizontal – weak eye; vertical – strong eye). The green diagonal band represents equal refractive error in the two eyes, ±1 D. The upper right quadrant above the red lines represents anisohyperopes – emmetropic or hyperopic in the strong eye, and more hyperopic in the weak eye. The upper right quadrant below the red line represents “anomalous” anisohyperopes – emmetropic or hyperopic in the strong eye, and less hyperopic in the weak eye. The lower right quadrant represents myopic antimetropes. The upper left quadrant represents hyperopic antimetropes – emmetropic or hyperopic in the weak eye, and myopic in the strong eye. The lower left quadrant contains anisomyopes. Anisomyopes below the red line – emmetropic or myopic in the strong eye, and more myopic in the weak; “Anomalous” anisomyopes above the red line – emmetropic or myopic in the strong eye, and less myopic in the weak eye. Symbol size is used to coarsely code visual acuity (top) and stereoacuity (bottom)

Figure 2

Optotype acuity vs. degree of anisometropia (left panel) and absolute refractive error (right panel). The meandering lines show the running mean acuity for each of the three groups. Vector blur anisometropia is defined as the absolute value of the difference in vector blur between the two eyes, given the sign of the refractive error of the weak eye.

Figure 3

Stereoacuity vs. degree of anisometropia (left panel) and absolute refractive error (right panel).

Fig. 4

The relationship between stereoacuity and MAR. The arrows show the upper and lower limits of the test. The data for strabmismic anisometropes (in blue) have been displaced for clarity.

Figure 5

A. The cumulative probability of being amblyopic (defined as having acuity of 20/40 or worse in the weak eye) as a function of the absolute value of the amount of anisometropia. B. The cumulative probability of stereo-acuity being 40 arc seconds or worse. Cumulative probabilities for positive and negative values of vector blur anisometropia were computed separately, beginning at 0.

Figure 6

The thick green lines shows the optotype acuity predicted from refractive blur by a dioptric vector addition model (Raasch, 1995 – see text). The squares show the effect of lens induced blur on optotype acuity (from Bedell, Patel & Chung, 1999). The circles replot the optotype acuity of our anisometopes (from Fig. 2). The red lines fit to the data are the defocus tolerance fits described earlier (see text).