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Table of Content - Volume 4 Issue 3 - December 2017
Axial length of the eyeball in ametropia in comparison to emmetropia by A-Scan: A cross sectional study
R Havilah Twinkle1, G Parvathi2*
1Assistant Professor, 2Professor, Department of Physiology, Great Eastern Medical School and Hospital, Srikakulam, Andhra Pradesh, INDIA. Email: rhtwinkle@yahoo.co.in
Abstract Background: Axial length is regarded as one of the primary determinants of refractive error. The correlation with refractive error is larger for axial length than for any other component. Objective: To study the axial length of the eyeball in ametropia in comparison to emmetropia. Material and Methodology: The present cross sectional study was carried out from October 2012 to September 2013 at Ophthalmology outpatient department, Regional eye hospital, Vishakapatnam. A study group of 150 cases divided into three groups by simple random sampling as emmetropes, myopes and hypermetropes in the age group of 10-30 years were included in the study. After taking informed consent the patient was enrolled into the study. A scan biometry using Appascan 2000 was done in all cases to know axial length. Results: The majority of patients in all groups were from age group 16-20 years with no gender difference. The axial length in emmetropes, myopes and hypermetropes was 23.60 ± 0.70 mm, 25.98 ± 1.00 mm and 21.58 ± 0.76 mm respectively. The mean axial length in the three groups was analyzed using ANOVA test and was found to be statistically significant (p< 0.05). Conclusion: Axial length is greater in myopic eyes as compared to that of hypermetropic eyes. The axial length is directly proportional to the degree of refractive error in myopic eyes and inversely proportional in hypermetropic eyes.
The Axial Length (AL) is the distance from the corneal surface to an interference peak corresponding to the retinal pigment epithelium/Bruch’s membrane.1,2 Majority of axial length elongation takes place in the first 3 to 6 months of life and a gradual reduction of growth over the next two years, 3 and by three years the adult size is attained. It is found that the depth and volume of the anterior chamber diminish with age and are related to the degree of ametropia. The large scale studies on the growth of the ocular components suggest that the eye usually reaches its adult emmetropic axial length by the age of 13 years.4,5 In the adult, axial length remains practically unaltered. A slight but steady change towards hyperopia is the rule, especially after the age of 40. The human eye grows extensively after birth. The full term newborn eye has a mean axial length of 16-18 mm and mean anterior chamber depth 1.5-2.9 mm. The mean adult values for axial length are 22-25 mm and mean refractive power -25.0 -+1.0 D.6,7 Ocular biometrics is among the most important factors affecting refractive errors.8,9 Compared with other ocular components such as the cornea and crystalline lens, axial length is typically regarded as the primary determinant of refractive error. The correlation with refractive error is larger for axial length than for any other component. Most studies suggest axial length (AL) is the most important determinant of refractive errors.9,10 Hence, the present study was done to study the axial length of the eyeball in ametropia in comparison to emmetropia.
MATERIAL AND METHODOLOGY The present cross sectional study was undertaken to study the axial length of the eyeball in ametropia in comparison to emmetropia. The present study was carried out from October 2012 to September 2013 at Ophthalmology outpatient department, Regional eye hospital, Vishakapatnam. A study group of 150 cases divided into three groups by simple random sampling as Group E (50 emmetropes), Group M (50 myopes) and Group H (50 hypermetropes) in the age group of 10-30 years were included in the study. After taking informed consent the patient was enrolled into the study. A detailed history regarding complaints was taken along with demographic details. Ophthalmic examination was done comprising of Visual Acuity (VA) – with and without pinhole, retinoscopy (under cycloplegia, slit lamp examination to exclude cases of index ametropia, Keratometry (Bausch and Lomb Keratometer) to rule out curvature ametropia. The cases with a keratometric reading between 42.00 – 43.00 dioptre (D) were included in the study. A scan biometry using Appascan 2000 was done in all cases to know axial length. Refractive error ≤ -0.50 D was considered as myopia and refractive error ≥ + 0.50 D was considered as hypermetropia. Data Analysis was done with the help of SPSS software version 20 with appropriate statistical test and P value less than 0.05 was taken as significant level. OBSERVATIONS AND RESULTS Table 1: Age distribution among various groups
Table 2: Sex-wise distribution among various groups
Table 3: Axial length among various groups
Table 4: Distribution according to change in dioptric power per one millimetre in axial length:
DISCUSSION Ultrasonic biometry is a simple technique which gives accurate measurements of axial lengths as well as the lens thickness and anterior chamber depth. The purpose of this study was to make a comparative study of A – Scan biometry among emmetropes and ametropes. Patients in the age group of 10-30 years were specifically selected as by the age of 10 years, the eyeball attains its maximum size. Patients above the age of 30 years were excluded to eliminate index ametropia. In the present study axial length in emmetropes ranged from 22.10 to 24.50 mm. The mean axial length in emmetropes was 23.60 ± 0.70 mm. These findings were comparable well with other studies like Francois J et al11, Robert A. Gordon et al12, were mean axial length in emmetropes was 24.00 ± 0.80 and 23.60 ± 0.70 mm respectively. Similar findings related to mean axial length in emmetropes were observed by David A. Atchison et al13 and Vijay S. Lodha et al. 14 The axial length in myopes ranged from 23.95 to 28.00 mm with mean axial length of 25.98 ± 1.00 mm. This compares well with other studies like David A. Atchison et al13 and Vijay S. Lodha et al. 14 were mean axial length among myopes was 25.40 ± 1.10 and 25.60 ± 1.20mm respectively. The refractive error in myopes ranged from -1.00 to -13.50 D and showed that in myopic eyes a proportional increase in axial length of eye with an increase in the degree of refractive error was observed with a mean value of 2.85 ± 0.18 D/mm increase in axial length. These findings were compares well with the findings of David A. Atchison et al13, Lourdes Llorente et al15 and Vijay S. Lodha et al. 14 The findings related to hypermetropes, axial length ranged from 20.00 to 22.80 mm with mean axial length of 21.58 ± 0.76 mm. Similar findings were also seen by Lourdes Llorente et al15 and Dr. Nial C. Strang et al16 in whom mean axial length among hypermetropes was 22.62 ± 0.66 and 21.80 ± 0.90 respectively. The refractive error in hypermetropes ranged from +2.5 to +9.00 D and observed that in hypermetropic eyes a proportional decrease in axial length of eye with an increase in the degree of refractive error was observed with a mean value of 2.74 ± 0.18 D/mm decrease in axial length. This was similar to the findings of Dr Nial C. Strang et al. 16 The mean axial length in the three groups was analyzed using ANOVA test and was found to be statistically significant (p< 0.05). The change in refractive error for one millimetre change in axial length was found to be statistically significant (p< 0.05). This showed that, the axial length among myopic eyes was greater than hypermetropic eyes with statistical significance. (P<0.05)
CONCLUSION The study concludes that, axial length is greater in myopic eyes as compared to that of hypermetropic eyes. The axial length is directly proportional to the degree of refractive error in myopic eyes and inversely proportional in hypermetropic eyes.
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