Primary open-angle glaucoma is usually a chronic, slowly progressive optic neuropathy with associated atrophy and cupping of the optic nerve head. The onset is usually insidious with characteristic patterns of painless peripheral visual field loss. Although usually bilateral the disease can be quite asymmetrical. Central vision is relatively unaffected until there is advanced glaucomatous optic atrophy. Glaucoma is the second commonest cause of blindness worldwide behind cataracts.