Correlation of Blood Sugar Level and Refractive Status: Clinical Evidence, Mechanisms, and Implications for Diabetic Retinopathy

Diabetes mellitus is a global health burden with a well-recognized ocular morbidity, most notably early cataract and diabetic retinopathy. Beyond retinal pathology, fluctuating blood glucose levels exert significant effects on the optical components of the eye, particularly the crystalline lens, resulting in transient and long-term refractive changes. These alterations may manifest clinically as myopic or hyperopic shifts and are often observed during periods of acute hyperglycaemia or intensive glycaemic control. Over the past century, numerous clinical and experimental studies have investigated the relationship between blood sugar levels and refractive status, revealing complex biochemical, osmotic, and structural mechanisms within the lens. More recently, epidemiological evidence has suggested that refractive status especially myopia and axial length, may influence both the prevalence and severity of diabetic retinopathy. Several large population-based studies and meta-analyses reports show an inverse association between myopia and diabetic retinopathy, proposing axial elongation and altered retinal microcirculation as potential protective factors. This emerging paradigm has prompted renewed interest in refractive error not merely as a visual condition, but as a possible modifier of diabetic retinal disease. This review synthesizes historical foundations, mechanistic insights, and contemporary clinical research regarding the correlation between blood glucose levels and refractive status. Acute refractive changes related to hyperglycaemia and treatment initiation are examined alongside structural lens alterations mediated by polyol pathway activity. Furthermore, the relationship between refractive error, axial length, and diabetic retinopathy is critically appraised using data from cohort studies, national surveys, and systematic reviews. Finally, the clinical implications for patient counselling, refractive assessment, and diabetic eye care are discussed. Understanding these interrelationships is essential for optimizing visual outcomes and refining risk stratification in individuals with diabetes.