ORIGINAL ARTICLE Year : 2006  Volume : 54  Issue : 2  Page : 99103 Evaluation of relationship of ocular parameters and depth of anisometropic amblyopia with the degree of anisometropia Simi ZakaurRab Institute of Ophthalmology, J.N. Medical College, Aligarh Muslim University, Aligarh, India Correspondence Address: Purpose: (1) To find out the relationship of the depth of amblyopia with the degree of anisometropia, in untreated cases of anisometropic amblyopia without strabismus, for both myopic and hypermetropic individuals. (2) To find out the relationship between various ocular parameters, such as axial length and corneal curvature, with the degree of anisometropia between the two eyes. Materials and Methods: This prospective study was conducted between January 2001 and March 2003, in 85 cases of untreated anisometropic amblyopia, who attended the author«SQ»s out patient department. All these patients were subjected to a meticulous ocular examination, with special emphasis on (1) refraction under cycloplegia (2) best corrected visual acuity (3) measurement of axial length by A Scan (4) keratometry . The depth of amblyopia was calculated in two ways: (1) By finding out decimal visual acuity for each eye and subsequently calculating their difference. (2) By converting the Snellen acuity into Log MAR units, which was calculated by finding the Logarithm of the reciprocal of the decimal visual acuity for the two eyes, followed by calculating the difference between the two. The difference in refraction between the two eyes as a measure of anisometropia, was determined by the difference in spherical equivalent between the refraction for each eye. Results: On comparing hypermetropic and myopic cases, a significant correlation was found between depth of amblyopia and the degree of anisometropia, in both myopic and hypermetropic patients. The correlation coefficients were however, found to be greater for hypermetropic than myopic individuals. It was observed that the difference between the axial length of the two eyes contributed to a major part of anisometropia, more so in myopic cases. Conclusions: The depth of amblyopia correlates with the degree of anisometropia in previously untreated anisometropic amblyopia patients.
Materials and Methods The study was conducted between January 2001 and March 2003, in 85 cases of untreated anisometropic amblyopia, who attended the author's out patient department. Patients with strabismus, eccentric fixation, microtropia, cataracts, history of trauma or previous eye surgery or use of spectacles before the age of 7 years, observable pathology of media or fundi and age more than 50 years were excluded from the study. In the present study, anisometropia is reckoned as the difference in refraction of one diopter or more, in either sphere or cylinder, between the two eyes and amblyopia was taken to be the difference in visual acuity of one or more lines on the standard Snellen chart between the eyes (with no known disease to account for the same). All these patients were subjected to a detailed history taking and meticulous ocular examination with special emphasis on (1) refraction under cycloplegia (2) best corrected visual acuity (3) measurement of axial length by A Scan (4) keratometry (5) slit lamp examination and (7) fundus examination. In all the cases, a senior refractionist did the cycloplegic refraction and postmydriatic test, manually. Incidentally, there was no case that could read only a few letters and not the entire line, so line acuity was measured in all the cases by using Snellen test types by the same refractionist, which was counterchecked by the author. The biometry and keratometry was done in all the cases by a different observer. The patients were given code numbers and the observer who had done biometry and keratometry had no information about the refractive status or the visual acuity of the cases. To increase the accuracy, an average of three biometry and keratometry readings were taken by the same person, in all cases. For the keratometry, a mean of three keratometric readings were taken for K1 and K2 and then a mean K value was calculated for both the eyes. All the cases were referred to squint clinic for a detailed orthoptic checkup and 4 prism base out test, to rule out microtropia. The orthoptic findings were counterchecked by the author and only those cases in which no disparity was found, were included in the study. All the patients were subjected to a meticulous slit lamp and fundus examination by direct and indirect ophthalmoscopy and slit lamp biomicroscopy by a 90 D lens in the retina clinic by the author herself, to rule out any other ocular pathology. The depth of amblyopia is equal to the difference in the best corrected visual acuity between the two eyes, at postmydriatic test. This was calculated in two ways: (1) By converting the Snellen fraction into decimals, i.e, finding out decimal visual acuity for each eye and then calculating their difference. (2) By converting the Snellen acuity into Log MAR units (Logarithm of the minimal angle of resolution). This was calculated by finding the Logarithm of the reciprocal of decimal visual acuity for both eyes and then calculating the difference between the two. The difference in refraction between the two eyes as a measure of anisometropia, was determined by finding out the difference in spherical equivalent between the cycloplegic refraction for each eye. The study groups were divided into hypermetropic and myopic individuals for statistical analysis. The amount of anisometropia was correlated to each of the two measures of depth of amblyopia by Pearson Rank Correlation Coefficients. These coefficients were also used to find out the relationship between various ocular parameters and amount of anisometropia. Statistical significance was defined as P P P P =0.285). A strong relationship was, however, observed between the amount of anisohypermetropia and the difference in axial length between the two eyes, with a Pearson Correlation Coefficient of 0.609, which was significant at 0.01 level. In myopic cases, a positive correlation was found between the amount of anisometropia and the depth of amblyopia, with Pearson Correlation Coefficient being 0.468 ( P =0.009) for difference in Snellen fraction and 0.463 ( P =0.010) for difference in Log MAR units. There was a negative correlation between the amount of anisomyopia and the difference in keratometry reading, between the two eyes. This was statistically insignificant, with a low Pearson Correlation Coefficient of  0.020 ( P =0.917). A strong relationship, nevertheless, was observed between the amount of anisomyopia and the difference in axial length between the two eyes, with a Pearson Correlation Coefficient of 0.674, which was significant at 0.01 level. On comparing hypermetropic [Table 4] and myopic [Table 5] cases, a significant correlation was found between depth of amblyopia and the degree of anisometropia, in both myopic and hypermetropic patients. The correlation coefficients were however, greater for hypermetropic, than myopic individuals. Further, it was observed that the difference between the axial length of the two eyes contributed to a major part of anisometropia, more so in myopic cases. Discussion Disagreement exists among workers with regards to the correlation of depth of amblyopia, with the difference in refraction between the two eyes. Some authors have found no relationship between the degree of anisometropia and the depth of amblyopia,[4],[5],[6] whereas others have noted a strong relationship between them.[7],[8] While it is well known that myopia and hypermetropia influence amblyopia differently, majority of previous studies[4],[5],[7] on the relationship between anisometropia and amblyopia, did not separate hypermetropia from myopia, upon statistical analysis. In the few studies[6],[8] where separation was done, the sample size was quite small to draw any statistical significant correlation. In the present study, the difference in manifest refraction and the depth of amblyopia, was found to be strongly correlated in anisometropic individuals and the difference between the axial length of the two eyes contributed to a major part of anisometropia. Helveston[4] defined anisometropia as a difference of 0.50 diopter or more in sphere or cylindrical spherical equivalent. He studied 57 amblyopic cases with anisometropia and found no relationship between the degree of anisometropia and depth of amblyopia in either strabismic or nonstrabismic individuals. Malik et al[5] examined 212 patients with anisometropia, but could not find any correlation between the degree of anisometropia and the depth of amblyopia, in patients with central fixation. In their study, however, the myopes and hypermetropes were not examined independently. Kutschke et al ,[6] found no relationship between degree of anisometropia and depth of amblyopia in the myopic, hypermetropic or astigmatic groups. The age of patients in their study ranged between 11 months to 9 years. In such an age group, the difference in vision could not truthfully reflect the difference in refraction, because such a population is visually not mature and might still be somewhere in the process of developing amblyopia. Sen[7] defined anisometropia as the difference of one or more diopters in sphere or cylinder. He, in his study, performed cycloplegic refractions on 172 individuals with anisometropic amblyopia. He, however, included only 5 cases of anisometropic amblyopia, who were younger than 7 years and noted that the higher degree of anisometropia was associated with a more severe degree of amblyopia. Townshend et al[8] defined anisometropia as the difference in refraction of 0.75 diopter in either sphere or cylinder between the two eyes and amblyopia as the difference in visual acuity between eyes of one or more lines, on the standard Snellen Chart. They studied 35 cases of untreated anisometropic amblyopia (older than 7 years) and demonstrated a positive relationship between the amount of anisometropia and the depth of amblyopia, for both myopic and hypermetropic individuals. On statistical analysis, they observed an unexpected finding, that this correlation was greater for myopic than hypermetropic individuals. Cobb et al[9] in a retrospective analysis of 112 children with anisometropic amblyopia, observed that the age of presentation of a child with anisometropic amblyopia, did not appear to have significant effect on the final visual acuity, but the amount of refractive error and degree of anisometropia at presentation, did correlate strongly with final visual acuity. In the present study, the correlation between the difference in manifest refraction and depth of amblyopia, was found to be greater for hypermetropic cases. This finding was in accordance with expectation,[2] since retina of the more ametropic eye of a pair of hypermetropic eyes never receives a clear image. In anisometropic myopia, however, one eye can sometimes be used for near work and the less myopic eye, for distance. In myopic anisometropic cases therefore, either eye may have a clear image. It was also observed that the difference between axial length of the two eyes contributed to a major part of anisometropia, more so in myopic cases. Although several studies[10],[11],[12],[13],[14],[15] have been conducted in humans to find out the relationship between various types of refractive errors and ocular parameters such as axial length and corneal curvature of the eyes, no such work has been done on cases of anisometropia. In myopia, the corneal curvature was found to play a minor role in determination of ocular refraction, specially in mild to moderate cases,[12] whereas, the measurements of axial length paralleled the degree of myopia, i.e, higher the myopia, longer is the axial length.[10],[11],[12] Stang[13] has reported that hypermetropia, like myopia, is predominantly axial in nature, although the corneal radius also plays a role in determining refractive error magnitude. Touzeau et al[15] recorded subjective refraction, biometric parameters using orbscan and echography in 190 normal eyes (including eyes with ametropia), out of 95 patients. They observed that biometric characteristics of the eye (excluding cornea characteristics), vary with subjective spherical equivalent. Axial length presents the strongest correlation with the subjective spherical equivalent and correlates with the other ocular biometric parameters, i.e, the axial length plays a major role in ocular biometry and refraction. As the depth of amblyopia correlates with the degree of anisometropia, screening of preschool children for large differences in refractive errors at an early age, may be important in the diagnosis, prevention and treatment of anisometropic amblyopia, which otherwise, may go undetected, being asymptomatic at an early age. References


