Stimulus deprivation amblyopia (SDA) develops due to an obstruction to the passage of light, preventing clear formation of an image on the retina (e.g. cataract, ptosis). It is particularly severe and can be resistant to treatment, leading to poor visual prognosis. Precise estimates of SDA prevalence are difficult to come by but it probably constitutes less than 3% of all amblyopia cases. In developed countries, most patients present under the age of one; in less developed parts of the world, presentation is likely to be significantly later than this. The mainstay of treatment is occlusion of the better-seeing eye, but regimens vary, can be difficult to execute and are traditionally believed to lead to disappointing results.