Purpose: To study the correlation between macular thickness and different grades of myopia in their axial length and spherical equivalent refraction. Methods: A total of 207 young myopes between the ages of 11 and 34 were selected. A comprehensive eye examination was done, including cycloplegic refraction, and measurement of axial length by A-Scan biometry. Subjects were categorized into low and medium (134 eyes), high (51 eyes) and super high (15 eyes) degrees of myopia. Foveal thickness assessment and macular evaluation was done by SD-OCT. Results: The macular thickness in all the four quadrants of inner and outer circles significantly decreases as the degree of myopia increases. With an increase in the grade of myopia, the central foveal thickness has also increased. The central fovea and the temporal outer macula were thinnest of all areas in all the groups. Conclusion: This study highlights the foveal and macular thickness changes as influenced by myopia progression and increase in the axial length.
Key words: Myopia, refractive error, optical coherence tomography, macular thickness, axial length.
OCT – Optical coherence tomography, SD – OCT Spectral domain optical coherence tomography, LMMG – Low to moderate myopia group, HMG – High myopia group, SHMG – Super high myopia group, TAMT – Total average macular thickness, CST – Central subfield thickness, BCVA – Best corrected visual acuity, SIM – Superior inner macula, IIM – Inferior inner macula, NIM – Nasal inner macula, TIM – Temporal inner macula, SOM – Superior outer macula, IOM – Inferior outer macula, NOM – Nasal outer macula, TOM – Temporal outer macula, IOP – Intraocular pressure
Myopia is an important cause of visual disability in both the developed and the developing worlds. Its prevalence varies in different geographic regions, age groups and ethnic groups. 1 Myopia usually commences in the school age, 5-15 years, and tends to stabilize by second decade of life. There may be an association between high myopia and the occurrence of vitreous degeneration, posterior pole degeneration, peripheral retinal degenerations, retinal breaks and retinal detachment. Reports from Singapore-China study show that the prevalence of myopia in 7-9 year old children to be 36.7% in Singapore, 18.5% in Xiamen City southern China and 6.6% in rural Xiamen. 2 The prevalence of myopia among high school students in Taiwan is reported as 84%. 3 The prevalence is much lower in some other countries in East Asia. The prevalence of myopia in India is reported as 4.9%; higher in urban population (6.9%) in comparison to rural population (2.77%). From the surveys conducted in 1970s, the prevalence of myopia in India is reported as more than 4% among school children, (4) Murthy etal reported a prevalence of 7.45% of myopia in the urban population of New Delhi. 5 Report from a rural location in Andhra Pradesh puts the prevalence at 4.1%. 6 Andhra Pradesh Eye Disease Study has shown refractive error to be the main cause of moderate visual impairment in younger population. 7 A correlation between macular thickness and different grades of myopia is reported by a number of studies. Studies from South East Asian counties and some of the European countries have reported on macular thickness using OCT. There are very few reports of macular thickness measurement on OCT in myopia cases from Indian centres.
The purpose of our study was to evaluate the variations in macular thickness in myopic patients in this south Indian population, at a Government Medical College tertiary eye hospital, with different diopter (D) degrees using spectral domain-OCT (SD-OCT).
2. Material and Methods:
2.1 Study Design:
It is an observational study. Two hundred eyes of young myopic patients presenting to Sarojini Devi Eye Hospital for a period of 17 months from December 2014 to April 2016 were included in the study. An informed consent was taken before subjecting each patient to required investigations. Basic demographic details, presenting complaints, duration of myopia, past history, and family history were documented. These patients were divided into three groups according to their refractive error/axial length as Low and medium myopia group(LMMG)(Refractive Error up to -6D/axial length 24-26mm), High myopia group(HMG)(RE- 6 to -10D/axial length 26- 28mm) and Super high myopia group(SHMG)(RE >-10D/axial length >28mm).
SD-OCT was used to evaluate macula (TAMT, CST, inner/outer macular thickness and MV). The patients were subjected to detailed ophthalmological examination including BCVA using Snellens chart, refraction, slit lamp biomicroscopy, indirect ophthalmoscopy, gonioscopy and applanation tonometry. A-scan biometry was performed in each patient to determine the axial length. Fundus photographs were taken when needed.
2.2 Inclusion criteria:
All myopes with Best Corrected Visual Acuity of at least 6/24.
2.3 Exclusion criteria:
1. Macular edema due to Diabetes, Uveitis, Retinal vein occlusions, Post operative cases and trauma
2. All diagnosed cases of open angle glaucoma.
3. Any other macular pathologies like Choroidal Neovascular Membrane, Macular scars, Pigment Epithelial Detachments and Dystrophies.
OCT was performed with the Cirrus HD-OCT system (Carl Zeiss Meditec, Cirrus HD-OCT 5000-2408, and Soft version 220.127.116.112) and macular cube 512×128 protocol. This comprises of a cube of data in a 6 mm square grid recording 128 horizontal scans having 512 A-scans. This protocol has a scan with signal strength more than 6 that correctly portrays the retinal layers without any image artifacts that may be caused by eye movements. All the scans were performed by the same examiner under similar conditions. The chart has nine sectoral thickness measurements zones, bounded by three concentric circles with diameters of 6, 3 and 1 mm. (figure 1) The area bounded by the outer(6mm)and middle(3mm) circles forms the outer ring (OR) while the area bounded by the middle(3mm) and inner circles(1mm) forms the inner ring(IR). Each ring is divided into temporal, nasal, inferior and superior zones. CST stands for central subfield thickness, SIM, IIM, NIM, TIM stand for superior inner macula, inferior inner macula, nasal inner macula, and temporal inner macula respectively. The nine zones in the chart are hence labeled as; CST for central subfield thickness, TIM, NIM, IIM, and SIM represent temporal inner macula, nasal inner macula, inferior inner macula and superior inner macula respectively. Superior outer macula, inferior outer macula, nasal outer macula and temporal outer macula are indicated as SOM, IOM, NOM, TOM. Total macular thickness and macular volume over the 6mm diameter circle, centered at the true fovea location were also obtained from the computational software output.