Eye Diseases are disorders affecting the eye and associated visual structures, including the cornea, conjunctiva, sclera, iris, lens, retina, optic nerve and supporting tissues. Vision requires light to pass through transparent ocular structures, be focused onto the retina, converted into neural signals by photoreceptors and processed through retinal circuits and the visual pathways leading to the brain. Eye disease can therefore impair vision by disrupting optical clarity, retinal function, intraocular pressure regulation, ocular blood supply, eye movement or neural transmission. Common disorders include refractive errors, cataract, glaucoma, age-related macular degeneration, diabetic retinopathy, retinal detachment, ocular inflammation and infections. Ophthalmology combines medical, surgical and optical approaches and often overlaps with neurology, endocrinology, rheumatology, infectious disease and general medicine.
Refractive errors occur when the optical system of the eye does not focus incoming light precisely onto the retina without accommodation or correction. Myopia causes distant objects to focus in front of the retina, whereas hypermetropia generally involves focus behind the retina when accommodation is relaxed. Astigmatism results from unequal refractive power in different meridians, often because the cornea or lens has an irregular curvature.
Presbyopia is an age-related reduction in the eye's ability to accommodate for near vision, largely resulting from changes in lens elasticity and associated accommodative mechanisms. Refractive errors are usually corrected with spectacles or contact lenses, while refractive surgery can alter corneal shape in appropriately selected patients. These conditions differ from retinal or optic-nerve diseases because the principal problem is optical focusing rather than damage to the neural structures responsible for vision.
Cataract is an opacity of the crystalline lens that causes progressive reduction in visual quality. Ageing is the most common context, but cataracts can also arise after ocular trauma, prolonged corticosteroid exposure, metabolic disease, congenital abnormalities or other causes. Symptoms include blurred vision, glare, reduced contrast and difficulty seeing in particular lighting conditions. A cataract is not simply a film on the surface of the eye; it is a structural change within the lens.
Diagnosis is generally established through eye examination and visual assessment. When symptoms significantly interfere with daily activities, cataract surgery removes the cloudy lens and replaces it with an intraocular lens designed to provide appropriate optical correction. Modern cataract surgery is highly standardised but remains a procedure with potential complications, including infection, inflammation, retinal problems and refractive outcomes that differ from expectations.
Glaucoma comprises a group of Eye Diseases involving characteristic damage to the optic nerve, often associated with elevated intraocular pressure but not necessarily caused by high pressure alone. Open-angle glaucoma develops through impaired drainage of aqueous humour despite an anatomically open drainage angle, whereas angle-closure mechanisms involve narrowing or obstruction of the drainage angle and can produce acute emergencies. Progressive optic-nerve damage causes characteristic visual-field loss that may initially be unnoticed.
Diagnosis requires assessment of the optic nerve, intraocular pressure, visual fields and the anterior chamber angle, often supplemented by optical coherence tomography. Treatment aims to reduce the risk of further optic-nerve damage and can involve topical medicines, laser procedures or surgery. A major misconception is that normal eye pressure excludes glaucoma; some patients develop glaucomatous damage despite pressure measurements within conventional population reference ranges.
Retinal diseases affect the specialised neural tissue lining the back of the eye. The retina contains rods and cones that detect light and multiple layers of neurons that process visual information before signals pass through the optic nerve. Age-related macular degeneration primarily affects the macula and can impair central vision, with “dry” and “wet” forms having different mechanisms and treatments.
Diabetic retinopathy results from chronic diabetes-related microvascular injury and can progress from non-proliferative abnormalities to proliferative disease with abnormal new blood vessels and risk of vitreous haemorrhage or retinal detachment. Retinal vascular occlusions can abruptly impair vision by reducing blood flow or obstructing venous drainage. Optical coherence tomography, fundus photography, fluorescein angiography and other imaging techniques help characterise retinal pathology.
Corneal diseases can impair vision because the cornea provides a major portion of the eye's refractive power and must remain transparent and structurally stable. Keratitis involves corneal inflammation and can be infectious or non-infectious, while corneal ulcers represent potentially serious defects that may threaten vision. Keratoconus causes progressive thinning and irregular steepening of the cornea, producing distorted vision and astigmatism. Corneal scarring can follow infection, trauma or inflammation and may permanently reduce transparency.
Contact-lens use can increase the risk of particular corneal infections when hygiene or wear practices are inappropriate. Treatment depends on cause and may involve antimicrobial therapy, protective measures, corneal cross-linking for selected progressive keratoconus, specialised contact lenses or corneal transplantation for severe structural or optical impairment.
Eye Diseases can also reflect systemic disease. Diabetes can damage retinal blood vessels, hypertension can produce retinal vascular changes, autoimmune disorders can cause uveitis or scleritis, thyroid-associated disease can affect orbital tissues, and neurological disorders can damage the optic nerve or visual pathways. Sudden visual loss may arise from retinal vascular occlusion, retinal detachment, optic-nerve disease, acute glaucoma or neurological causes and requires urgent assessment.
Diplopia can result from abnormalities of the ocular muscles, cranial nerves, neuromuscular junction or brain rather than the eyeball itself. Because visual symptoms can originate anywhere from the cornea to the visual cortex, the distinction between ocular and neurological disease is fundamental. Ophthalmic examination therefore often provides information relevant to broader systemic diagnosis.
Eye assessment includes visual-acuity testing, refraction, pupil examination, ocular-motility assessment, slit-lamp examination, intraocular-pressure measurement and dilated examination of the retina when indicated. Optical coherence tomography provides cross-sectional imaging of retinal and optic-nerve structures, while visual-field testing measures functional areas of vision that may be lost through glaucoma or neurological disease. Ultrasound can examine structures when the optical media are opaque, and angiographic techniques can evaluate retinal blood vessels. Treatment may include spectacles, topical or systemic medicines, laser procedures and microsurgery.
Early detection is particularly important for conditions that can progress silently, including glaucoma and diabetic retinopathy. A useful distinction is between visual acuity and overall visual function: a person may have relatively preserved central acuity while experiencing significant peripheral-field loss, contrast impairment or other visual deficits.
