Diseases of the lids and ocular surface can induce both focal and widespread changes in corneal shape, with primary and secondary effects. A lid lump compressing the central cornea causes central flattening, whereas one compressing the peripheral cornea will cause secondary central steepening.
Disturbances of the tear film can cause focal drying with localised flattening or more widespread irregularity. Epithelial disease often causes multiple small-scale changes that disrupt the image quality. Local elevations or depressions produce corresponding small areas of steepening or flattening.
Local peripheral lumps such as a pterygium or bleb produce flattening due to filling in of the limbus, subepithelial sclerosis and an exaggerated tear film meniscus at its margin.
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