**6.4 Thickness of photoreceptor outer segment (PROS)**

Advancement in technology has allowed the measurement of the thickness of the photoreceptor layer with SD-OCT in patients with diabetes. Patients with DR, DME, or diabetes but no retinopathy, have reported a thin photoreceptor layer in comparison to healthy individuals [46, 68]. Variation in visual acuity has been correlated to the thickness of the outer segment of the photoreceptor (PROS) in eyes with DME. This thickness of PROS is measured from the inner boundary of IS/OS junction to the inner boundary of the RPE layer [69]. The correlation of thickness of PROS with visual acuity is significant particularly when measuring it at the fovea [49].
