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New research has shown that a longer axial length may prove protective of incident diabetic retinopathy.
New research has shown that a longer axial length (AL) may prove protective of incident diabetic retinopathy (DR).1
In this population-based cohort study-1,562 eyes of 840 subjects-Man et al. specifically focused on the association of refractive error, axial length, anterior chamber depth, and corneal curvature on DR development and progression.
The risk and protective factors associated with DR pathogenesis are well known, and refractive error is but one factor. Further, several studies have shown “the presence of a more myopic refraction is associated with a lower likelihood of DR in both Asian and white populations.”1
Man et al. analyzed a subset of patients who enrolled in the Singapore Malay Eye Study (SiMES-1 and 2) and Singapore Indian Eye Study (SiNDI-1 and 2) cohort trials.2-5
To be included in this analysis, patients had to have diabetes; gradable fundus images at baseline and follow-up; available refraction, ocular biometric (axial length, anterior chamber depth, and corneal curvature); and covariate data; and no history of cataract or refractive surgery.
For the purposes of this study, refractive error was calculated as sphere plus half negative cylinder, while AL, anterior chamber depth, and corneal curvature were assessed using optical biometry.
Of the 840 patients analyzed, the average age was 57 years and 48% were female. DR was graded from 2-field retinal photographs (macular and optic disc) using the modified Airlie House Classification system for the Early Treatment Diabetic Retinopathy Study (ETDRS).
Of the 1,562 eyes included, 18.5% had DR at baseline-7% of which were classified as vision-threatening. A total of 164 of the remaining 1273 (12.9%) eyes without DR at baseline had incident DR at follow-up; and 17 of 1542 (1.1%) eyes without VTDR at baseline developed it at follow-up. Finally, 75 of 269 (27.9%) of eyes with at least minimal DR and no VTDR at baseline progressed >1 step in DR severity at follow-up.
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The researchers found that patients with DR tended to be younger, have a higher body mass index and hemoglobin A1c level, and were more likely to be myopic. Interestingly, they were also more likely to have a shorter axial length compared with patients without DR.
After adjusting the cohort for age and gender, patients were at a lower risk of developing DR if they had a longer axial length (risk ratio 0.81, 95% CI [0.69–0.95]; p = 0.008) and lower spherical equivalent (risk ratio 0.92, 95% CI [0.85–1.00]). This decreased risk was independent of anterior chamber depth, corneal curvature, or refractive error.
Researchers found no longitudinal associations between axial length with DR progression or the development of VTDR. These findings suggest that axial elongation “is a major pathophysiological driver of the earlier observed protective myopia-DR relationship.”1
The authors added the small number of cases may explain why no significant associations between AL and incident VTDR or DR progression was found.
“Moreover, the non-association of ACD with any of the DR outcomes suggests that it is the elongation of the posterior ocular chamber-and not the anterior ocular chamber-that underlies the protective impact of AL on DR,” they wrote. 1 They advocate for larger studies to confirm their findings, and that other ethnicities beyond ethnic Malays and Indians be included.
Eye-care professionals have long sought to accurately predict and identify the diabetic patients at highest risk for DR incidence and progression in order to tailor treatment and improve care. Several studies have found that more myopic refraction is associated with a lower likelihood of DR,6 but this paper is one of the first to specifically examine the role AL plays.
Finally, axial elongation may result in retinal neurodysfunction, particularly in the outer retina, leading to a reduced metabolic demand and a consequent decrease in hypoxic burden in the diabetic retina, the authors hypothesize.
1. Man RE, Gan AT, Gupta P, et al. Is Myopia associated with the Incidence and Progression of Diabetic Retinopathy? Am J Ophthalmol 2019.
2. Foong AW, Saw SM, Loo JL, et al. Rationale and methodology for a population-based study of eye diseases in Malay people: The Singapore Malay eye study (SiMES). Ophthalmic Epidemiol 2007;14(1):25-35.
3. Rosman M, Zheng Y, Wong W, et al. Singapore Malay Eye Study: rationale and methodology of 6-year follow-up study (SiMES-2). Clin Exp Ophthalmol 2012;40(6):557-68.
4. Lavanya R, Jeganathan VS, Zheng Y, et al. Methodology of the Singapore Indian Chinese Cohort (SICC) eye study: quantifying ethnic variations in the epidemiology of eye diseases in Asians. Ophthalmic Epidemiol 2009;16(6):325-36.
5. Sabanayagam C, Yip W, Gupta P, et al. Singapore Indian Eye Study-2: methodology and impact of migration on systemic and eye outcomes. Clin Exp Ophthalmol 2017;45(8):779-89.
6. Lim LS, Lamoureux E, Saw SM, et al. Are myopic eyes less likely to have diabetic retinopathy? Ophthalmology 2010;117(3):524-30.