Blurred vision means that visual information appears indistinct, out of focus or lacking normal sharpness. It is a symptom rather than a diagnosis and can arise from problems in the eye's optical system, the retina, optic nerve or brain. Common causes include refractive errors such as short-sightedness, long-sightedness and astigmatism, as well as presbyopia, dry-eye disease, cataract and changes in blood glucose.
Blurring may affect one eye or both eyes, may be constant or intermittent, and may occur at particular distances. The pattern is diagnostically important: distance blur commonly occurs with myopia, near blur in older adults can reflect presbyopia, and fluctuating blur that improves after blinking can occur with an unstable tear film. Sudden or rapidly progressive blurred vision can have more urgent causes and should not automatically be attributed to needing stronger glasses.
Refractive errors are among the most common causes of blurred vision. In myopia, the eye focuses parallel rays of light in front of the retina when accommodation is relaxed, commonly because the eyeball is too long or because its refractive power is too strong. In hyperopia, the focal point tends to lie behind the retina under relaxed accommodation, often because the eye is too short or its optical power is insufficient.
Astigmatism occurs when the optical system has different refractive powers in different meridians, producing blur or distortion at multiple orientations. Presbyopia is an age-related reduction in the ability of the crystalline lens to change shape sufficiently for near focusing. These conditions are generally corrected with appropriately prescribed spectacles, contact lenses or, in selected cases, refractive surgery.
The tear film is an important optical surface, so dry-eye disease can produce intermittent or fluctuating blur even when the cornea and retina are otherwise healthy. A normal tear film forms a smooth refractive surface over the cornea. When tears evaporate too quickly, are produced inadequately or have an abnormal composition, the optical quality of the tear film can become unstable. Blinking temporarily redistributes tears and can briefly improve clarity.
Long periods of reading or screen use can reduce blink frequency and exacerbate symptoms. Contact-lens wear can also disturb the tear film. Blurred vision associated with burning, grittiness, fluctuating clarity or symptoms that worsen with prolonged visual concentration therefore has a different likely mechanism from persistent optical blur caused by an uncorrected refractive error.
Cataract is another common cause of progressive blurred vision. A cataract is an opacity or alteration in the crystalline lens that reduces the amount and quality of light reaching the retina. It can cause hazy vision, glare, difficulty seeing in low contrast, halos around lights and changes in refractive prescription. Cataracts become more common with ageing but can also be associated with diabetes, corticosteroid exposure, previous eye trauma, radiation or congenital factors. Other media opacities, including corneal scarring or abnormalities and vitreous changes, can also reduce visual clarity. Because different structures can interfere with light transmission, a comprehensive eye examination is needed when blur cannot be adequately explained by a simple refractive error.
Retinal disorders can cause blurred or distorted central vision even when the cornea and lens are clear. Age-related macular degeneration affects the macula and can impair detailed central vision, while diabetic retinopathy can damage retinal blood vessels and produce retinal swelling or haemorrhage. Macular oedema from several causes can distort the retinal architecture and reduce visual acuity. Retinal detachment can cause visual disturbance and is often accompanied by new flashes or a sudden increase in floaters, sometimes followed by a shadow or curtain across the field of vision. These symptoms require urgent ophthalmic assessment because prompt treatment can be important for preserving vision.
Changes in blood glucose can temporarily alter the refractive properties of the lens and cause fluctuating vision, particularly in people with diabetes when glucose levels change substantially. Certain medications can also affect vision directly or indirectly. Eye inflammation, glaucoma, optic-nerve disorders and neurological disease can cause blurred or reduced vision, sometimes with pain, colour-vision changes or visual-field loss. A distinction should be made between blur, which refers to reduced image sharpness, and visual-field loss, in which part of the visual field is missing. A person may describe either as "blurred vision", but they represent different physiological problems and require different investigations.
The timing and onset of blurred vision are particularly important. Gradual blur affecting both eyes is commonly associated with refractive or age-related causes, whereas sudden blur in one eye can indicate retinal vascular occlusion, retinal detachment, optic-nerve disease or other acute pathology. Blur accompanied by severe eye pain, halos, nausea and a red eye can occur with acute angle-closure glaucoma and requires emergency assessment. New neurological symptoms such as facial weakness, limb weakness, speech disturbance or severe imbalance accompanying visual symptoms may indicate an acute neurological event. New flashes, numerous floaters or a curtain-like field defect warrant urgent eye assessment.
Diagnosis generally involves visual-acuity testing, refraction, examination of the anterior eye, assessment of the retina and, when indicated, measurement of intraocular pressure and additional imaging. Treatment depends on the cause rather than the symptom. Refractive blur can usually be corrected optically, dry-eye-related blur may improve when the ocular surface is treated, cataract-related blur can be treated by cataract surgery when it significantly affects vision, and retinal or optic-nerve disorders require cause-specific treatment. Persistent, unexplained or progressive blur should be assessed by an optometrist, ophthalmologist or other appropriate clinician. Sudden visual loss, a new field defect, flashes with many new floaters, severe eye pain or visual symptoms associated with neurological abnormalities requires prompt or emergency assessment.
