Tear Film Alterations in Type 2 Diabetes Mellitus: A Systematic Review and Meta-Analysis

<b>Background</b>: Type 2 diabetes mellitus (T2DM) is increasingly recognized as affecting not only the retina but also the ocular surface. Chronic hyperglycemia can disrupt meibomian gland function, reduce tear secretion, and impair corneal sensitivity, leading to tear film instability and symptoms of dry eye disease. However, previous studies have reported variable findings, and the extent of these alterations remains uncertain. <b>Methods</b>: Following PRISMA guidelines, this systematic review and meta-analysis evaluated observational studies that compared tear film parameters between adults with T2DM and non-diabetic controls. Eligible studies assessed one or more of the following: invasive or non-invasive tear break-up time, Schirmer test, tear meniscus height, or Ocular Surface Disease Index (OSDI). <b>Results</b>: Twenty-four studies involving approximately 3500 eyes were included. Most reported significantly reduced tear stability and secretion in diabetic participants compared with controls. Tear break-up times were consistently shorter in T2DM, indicating a less stable tear film. Schirmer test results demonstrated lower tear production correlated with diabetes duration and poor glycemic control. Tear meniscus height was modestly reduced in T2DM, reflecting decreased tear reservoir volume. Subjective symptoms, as measured by OSDI, were generally higher among patients with T2DM, suggesting greater ocular surface discomfort. <b>Conclusions</b>: T2DM is strongly associated with tear film instability, reduced tear secretion, and increased dry eye symptoms. These findings suggest that diabetic care should include routine ocular surface assessment and highlight the need for standardized, longitudinal investigations.