Author(s): Srinivasan Shanmugam1
Myopia is a worldwide common type of refractive error. Axial myopia is responsible for around 75% of the refractive error-related complications with serious social and economic consequences. In Western Europe, the estimated prevalence of myopia is over 25%. Myopia is a common cause of reversible blindness in India with a prevalence of 27%. The axial length will not only identify the determinants of eye elongation, but also provide aetiological evidence for myopia. WHO recent report reveals that in 2002, the number of people who have visual impairment caused by myopia and other ocular disorders reached over 161 million.
Majority of eye growth takes place in the first 18 months of life. The changes in axial length appears to be compensated by the progressive corneal flattening with age in normal eyes; The majority of axial length elongation takes place in the first 3 to 6 months of life and a gradual reduction of growth occurs in 2 years and adult eye size is attained by 3 years of age. The higher prevalence of myopia seen among Asian countries such as China, Singapore, Malaysia and Taiwan.
MATERIALS AND METHODS
Our study was a retrospective, clinical comparative study in axial length and corneal radius between low myopia, moderate myopia and high myopia. Total number of cases studied was 120 cases of different types of myopia. After getting written informed consent from all the individuals included in this study, clinical examination was conducted.
This study was conducted at Ophthalmology Department, Government Kilpauk Medical College and Hospital, Kilpauk, Chennai. The sample size was 120 in numbers. Only myopia of various levels like low myopia up to -3.0 dioptre spherical power, moderate myopia from more than -3.0 to -6.0 dioptre spherical powers and high myopia of more than -6.0 dioptre spherical were studied.
The normal axial length is 22 to 25 mm. In this study, the association of posterior segment degenerative changes in relation with increased axial length >25 mm will also be studied.
Total cases considered for refractive error evaluation were 7203. Out of these, 1800 individuals were suffering from various refractive errors like myopia, hypermetropia and astigmatism. Myopia of various types was noticed in 600 cases. As per age group range from 5 to 40 years, there were 120 cases of different types of myopic individuals were taken into consideration for our study. Both genders of male and female sex were included in this study. Since, in the study, the age-related cataract association also excluded, the upper age limit was taken as 40 years. Total duration of study period was 7 months from January 2017 to July 2017.
Routine eye examination like distant visual acuity by Snellen’s chart and auto refractometer, colour vision by Ishihara chart, Corneal Curvature (CR) by auto kerato-refractometer, intraocular pressure by Schiotz tonometer, slit-lamp examination, fundus examination by direct or indirect ophthalmoscope, axial length of eye by A scan, fundus photograph and in selected cases B-scan, etc. were performed.
In our study, various types of only myopia were 33.33%. Our study of 120 cases of various types of myopia between the age group of 5 to 40 years showed increased female sexual prevalence. The sexual prevalence as male:female = 46:74. All 120 cases were associated with myopic refractive error of both eyes either same type of myopia or in combination. In all types of myopic cases, increased female sexual prevalence was noticed. The age group prevalence has reported around 11 to 20 years in all types of myopic individuals. Axial lengths of normal range of 22 to 24 mm were noticed in 67 cases of mild myopic individuals. The visual acuity status in 76 cases of unilateral low myopic individuals reported 65 of them had normal vision (85.53%). Normal ranges of corneal radius as 7.51 to 7.8 mm range 3 cases and between 7.0 to 7.5 mm range 73 cases were reported. Nine cases of low myopic also have elongated AL of more than 24 mm and two cases with AL of more than 25 mm. Among these individuals, eight individuals were having CR less than 7.50 mm correlating the reduction of CR as elongation of AL of eyeball occurs. In two cases of low myopia, the axial length was shown more than 25.0 mm values with reduction of CR 7.15 mm and 7.35 mm without any degenerative changes in posterior segment of eye. The axial length of more than 25 mm was reported in totally 17 cases and majority of cases were associated with posterior segment degenerative changes in the fundus.
The 14 high myopic cases having laterality as unilateral:bilateral = 6:8. As far as corneal radius is concerned, between 7.0 to 7.5 nine and between 7.51 to 7.8 mm five individuals were found. This reveals that in high myopia the corneal curvature steepening resulting in less corneal radius value is seen. This reveals that 57.1% of cases presented with normal vision among high myopia detected and only 42.9% of cases only presented with 20% visual disability. The increased axial length more than
Jebmh.com Original Research Article
J. Evid. Based Med. Healthc., pISSN- 2349-2562, eISSN- 2349-2570/ Vol. 4/Issue 79/Oct. 02, 2017 Page 4674
25 mm was noticed in six cases, of which four cases were bilateral high myopia and two cases were presented with unilateral high myopia. Three individuals with high myopic power of more than -10.0 dpt with more than 25 mm of AL were having CR 7.5 in one case and 7.6 mm and 7.7 mm one case each. This again depict that high myopia with increased AL, need not present with reduction of CR.
Out of 52 moderate myopic individuals, bilateral moderate myopia 34 cases and unilateral moderate myopia 18 cases were reported. In unilateral moderate myopia, out of 18 cases, 15 individuals were presented with 22 to 25 mm normal range of AL, but three cases were more than 25 mm of AL. The corneal radius between 7 to 7.5 mm in 28 cases in bilateral moderate myopia, while in 17 cases in unilateral myopia between 7 to 7.5 mm of corneal radius was present.
Various ocular findings in pathological myopia leads to visual loss are complicated cataract, rhegmatogenous retinal detachment, macular haemorrhage, Forster-Fuchs spot and macular hole. Forster-Fuchs spot is hyperplasia of retinal pigment epithelium associated with subretinal neovascularisation and choroidal haemorrhage in the macula giving rise to Forster-Fuchs spot, which is the common cause for loss of vision in pathological myopia.
This study indicates the early identification of degenerative changes in high myopia and early management will prevent visual loss in the individuals. The study clears that the yearly follow up of low myopic from second decade will identify the progression of low myopia into moderate myopic level and moderate myopia to the high myopic level earlier. Also, vitreoretinal degeneration, asthenopic symptoms, amblyopia and visual loss causing posterior segment lesions can be identified and early management can be done. In our study, we noticed that the increased axial length with steepening of cornea with low CR values need not reflect in all cases of moderate and high myopic individuals.