|Year : 2015 | Volume
| Issue : 1 | Page : 11-14
Assessment of biometry and keratometry in low and high degrees of myopia
Afra I Alsamani1, Atif B Mohamed Ali2
1 Department of LVA, Makkah Eye Complex, Sudan Eye Center, Khartoum, Sudan
2 Faculty of Optometry, Al.Neelain University, Khartoum, Sudan
|Date of Web Publication||10-Nov-2015|
Atif B Mohamed Ali
Faculty of Optometry, Al.Neelain University, Khartoum
Purpose: To investigate the possible ocular biometric and keratometric changes in low and high degrees of simple myopia.
Subjects of Methods: A sample of 60 myopic subjects their ages between 10 and 30 years recruited from the Makkah Eye Hospital in Khartoum. In this sample, 30 subjects with myopia < 3.00 D and other 30 subjects with myopia > 6.00 D. Auto keratometry and A-scan biometry were measured from two eyes of each subject.
Results: Subjects with high myopia have longer axial length (AL) (P < 0.0001) compared to low myopic subjects. No significant difference found in front surface corneal power or type of corneal astigmatism between the two groups.
Conclusion: Like previous studies myopia mainly develops with increases in AL of the eye rather than optical changes in cornea or lens.
Keywords: Biometry, high myopia, keratometry, low myopia
|How to cite this article:|
Alsamani AI, Mohamed Ali AB. Assessment of biometry and keratometry in low and high degrees of myopia. Albasar Int J Ophthalmol 2015;3:11-4
| Introduction|| |
Myopia (short-sightedness) is often regarded as a benign disorder because vision can be corrected with glasses, contact lenses, and refractive surgery. However, myopia has become a major public health concern for different reasons. First, in developed countries in the East and Southeast Asia, the prevalence of myopia has rapidly increased in the past 50–60 years., The highest prevalence estimates for myopia are for young adults in the East Asia, with estimates reaching 90% in some urbanized and highly educated populations. These changes are not restricted to urbanized East Asia, since the prevalence of myopia is also increasing in the North America. Data on children of the Middle Eastern origin are less comprehensive. In general, the prevalence of myopia is low, but urban-rural differences have been noted. In children of sub-Saharan African, the prevalence of myopia is generally low for those growing up in Africa. Second, the WHO recognizes that myopia, if not corrected it will be a major cause of visual impairment. Third, people with high myopia are at a substantially increased risk of potentially blinding myopic pathologies, which are not prevented by optical correction.
Refractive status is a complex variable, determined by the balance of the optical power of the cornea and the lens, and the axial length (AL) of the eye (with its component parts anterior chamber depth, lens thickness, and vitreal chamber depth). Myopia usually results from an eye that has become too long, particularly through elongation of the vitreal chamber. Most children are born hypermetropic, during the 2 years after birth the hypermetropia decreases gradually. This change indicates that there is an active process shaping the distribution of refraction known as emmetropization. After that period the cornea stabilizes. but refraction can become more myopic as AL can continue to increase for another two decades. By contrast, lens power decreases substantially up to the age of about 12 years  with slower decreases for most of adult life. Myopia generally develops during the early to middle childhood years, but significant myopia can also develop in the late teenage years or early adulthood. AL is the most variable factor during development, with the strongest correlation with refractive status, with longer eyes more likely to be myopic than shorter eyes. On other hand, 50 years ago, myopia was believed to be genetic, with only minor environmental influences. However, results from experimental studies support the evidence of environmental factors.
Pathological myopia was originally described as high myopia accompanied by characteristic degenerative changes in the sclera, choroid, and retinal pigment epithelium, with compromised visual function. Not all highly myopic eyes develop pathological myopia, and attempts have been made to define highly myopic eyes at high risk as those with an AL of more than 3 (standard deviation [SD]) from the mean of emmetropic eyes. Although issues associated with differing definitions of high myopia and signs of pathological myopia exist, results from several studies , have shown that few pathological signs are noted in eyes with refractions in the mild to moderate range of myopia, but the prevalence of pathological signs increases steeply with myopia more severe than –5 D to –6 D. The incidence and severity of pathological signs also increase with age, but clinically significant pathological changes can be noted in patients who are middle-aged or younger.
| Subjects and Methods|| |
This was a comparative cross-sectional study conducted at Makkah Eye Hospital in Khartoum. Sixty myopic subjects were selected and invited to participate in this study (32 male and 28 female). Auto ref-keratometry and ocular biometry were measured in each eye to all subjects. This sample was further divided into two groups. Thirty subjects (14 male and 16 female) having low myopia (− 0.5 D to <− 3.0 D) and the other 30 subjects (18 male and 12 female) having high myopia (more than − 6.0 D). The inclusion criteria included that each subject should be free from ocular diseases, the visual acuity with correction (0.00 Log MAR equivalent to 6/6 by Snellen) and the age of subjects between 10 and 30 years. Myopia in this study was defined as spherical equivalent refraction obtained as the mean of three measures of ocular refraction in each eye without cycloplegia using (Nidek 510 A) auto ref-keratometer. The accuracy and vertex distance of the instrument were set to the default settings of 0.12 D and 12 mm, respectively. As well a mean of three measures of auto-keratometry (differing by <0.1 mm) was recorded. AL was measured with (Nidek) biometry. AL was the mean of a minimum of five measures (differing by <0.2 mm and standard error <0.05 mm) were obtained per eye. These measurements were carried out in one session by the same examiner for each subject. A verbal consent was obtained from all subjects agreed the protocol before the examination. Results of measurements were compared in each group, and differences in parameter were defined as significant at P < 0.05.
| Results|| |
In high myopia [Table 1], [Table 2], [Table 3], [Table 4], [Table 5] the range of refraction (− 6.5 D to − 14.0 D) and (− 6.5 D to − 15.0 D) diopter with mean − 7.68 D ± 1.57 SD and − 7.58 D ± 1.69 SD in right and left eyes, respectively. In low myopia [Table 2], [Table 3], [Table 4], [Table 5] the range of refraction (− 0.5 D to − 2.0 D) diopters both eyes with mean − 1.18 D ± 0.5 SD and − 1.23D ± 0.47 SD in right and left eyes, respectively [Table 5].
|Table 3: K-reading right and left eyes in subjects with high myopia (in diopters)|
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|Table 4: K-reading right and left eyes in subjects with low myopia (in diopters)|
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The AL [Table 3] and [Table 5]and the keratometric readings (K-readings) [Table 3] and [Table 4] show a significant correlation (P < 0.0001) between right and left eyes among each group [Table 1], [Table 2], [Table 3], [Table 4].
| Discussion|| |
Most of the previous studies indicated that the axial myopia occurs when the AL of the eye is greater than 22.22 mm. In this study, all high myopic subjects had AL more than 22.22 mm and in low myopic subjects more than 95% had AL longer than 22.22 mm. The result agrees with Llorente et al. the AL was longer in myopia than in hypermetropia. Chen et al. also reported that eyes with more myopic refractive error tended to have greater AL. It has been reported that high myopia could possibly develop retinal changes causing deterioration in visual acuity. Therefore, an understanding of structural and dimensional changes in the refractive system and retina is important for myopic subjects.
Keratometry, as well as topography, is one of the fundamental methods of measuring corneal front surface. Their proportions are essential for different parameters selection; especially when intended to correct refractive errors. In older studies, the average power of corneal curvature was about 43.00 diopter  and more than 70%, 50% in high and low myopia, respectively, had corneal curvature steeper than 43.00 diopter. However, this study agrees and confirms the previous studies, although there was no significant difference in corneal curvature found between low and high myopia in the current study.
In this study, the absolute astigmatism increases when the eyes become more myopic, [Table 6] suggesting that corneal surface irregularity or a symmetry of the optical system increases with myopic progression but similar among the two myopic groups. The results of this study show more than half patient with myopia associated with rule astigmatism this agree with the study of Heidary et al. who found high prevalence with rule astigmatism in myopic patient.
| Conclusion|| |
There was a significant difference in AL between low and high degree of myopia. Moreover, the AL increases with the degree of myopia. The corneal curvature is steeper than average in emmetropia. However, there was no significant difference in corneal curvature found between low and high myopia.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]