Blindness and Vision Loss: Causes and Warning Signs

What blindness means

Blindness covers a range. Some people see nothing at all (no light perception), but most people described as blind have some remaining vision. In the US:

• Low vision: vision that limits daily tasks despite glasses, usually worse than 20/40 in the better eye
• Legal blindness: best-corrected vision of 20/200 or worse in the better eye, or a visual field narrowed to 20 degrees or less

Blindness can affect one eye or both, and can be temporary or permanent.

Warning signs of permanent blindness

These signs mean the eye or its blood supply is under threat, and fast treatment can save sight:

1. Sudden vision loss in one eye, painless, like a blind being pulled down. A brief episode (amaurosis fugax) is a warning of stroke.
2. New floaters and flashes, or a dark curtain spreading across vision, which suggest a retinal tear or detachment.
3. A painful red eye with halos and nausea, the signs of acute angle-closure glaucoma, where eye pressure rises fast.
4. Headache, scalp tenderness and jaw pain in someone over 50, the signs of giant cell arteritis, which can blind the second eye within days if untreated.
5. Wavy or missing central vision, which can mean wet macular degeneration.
6. Gradual narrowing of side vision, often unnoticed until late, typical of glaucoma.

Causes of sudden blindness

Causes of gradual blindness

• Cataract: clouding of the lens, causing blurred, dim vision and glare. The leading cause of blindness worldwide, and reversible with surgery.
• Glaucoma: optic nerve damage, usually from raised eye pressure. Side vision is lost first, without pain, so many people do not notice until late. Lost vision cannot be restored.
• Diabetic retinopathy: damage to the retinal blood vessels from long-term high blood sugar. A leading cause of blindness in working-age adults.
• Age-related macular degeneration: loss of central vision in people over 50, making reading and recognizing faces difficult.
• Other causes: retinitis pigmentosa and other inherited retinal diseases, uncorrected refractive error, corneal scarring, vitamin A deficiency, trachoma and pituitary tumors pressing on the optic nerves.

Temporary blindness

Short episodes of vision loss can come from:

• Migraine aura: zigzag lines or a shimmering blind spot lasting 5 to 60 minutes, often followed by headache
• Transient ischemic attack: sudden painless loss in one eye for minutes, a stroke warning that needs same-day assessment
• Standing up quickly with low blood pressure: brief graying-out of vision
• Raised pressure in the skull: brief blackouts of vision lasting seconds, with headaches

Never assume brief vision loss is harmless without a medical check, unless it is a typical migraine aura you have had before.

How doctors diagnose the cause

An eye doctor will check:

• visual acuity with an eye chart
• pupil reactions
• eye pressure
• visual fields
• the front of the eye with a slit lamp, and the retina and optic nerve after dilating drops

Further tests include optical coherence tomography (OCT) scans of the retina, fluorescein angiography, blood tests (ESR and CRP for giant cell arteritis, blood sugar), and brain MRI or carotid ultrasound if a stroke or nerve cause is suspected.

Treatment

Treatment targets the cause:

• Cataract: surgery to replace the cloudy lens, which restores sight in most people
• Glaucoma: eye drops, laser or surgery to lower pressure and protect remaining vision
• Acute glaucoma: emergency pressure-lowering medicine and laser
• Retinal detachment: urgent surgery or laser
• Diabetic retinopathy and wet macular degeneration: injections into the eye, laser, and blood sugar and blood pressure control
• Giant cell arteritis: high-dose steroids started immediately
• Stroke or TIA: emergency stroke care and prevention of further strokes

Living with low vision or blindness

Low-vision specialists provide magnifiers, high-contrast lighting, screen readers and smartphone apps. Orientation and mobility training, a white cane or a guide dog, and state vocational rehabilitation services help people stay independent.

Preventing blindness

• Have a dilated eye exam every year if you have diabetes, and regularly after age 40, earlier if glaucoma runs in your family
• Control blood sugar, blood pressure and cholesterol
• Do not smoke, which raises the risk of macular degeneration and cataracts
• Wear protective eyewear for sports, DIY and chemicals
• Wear UV-blocking sunglasses outdoors

Frequently asked questions

What are the warning signs of permanent blindness?

Sudden vision loss, even briefly; new flashes and floaters or a curtain over vision; a painful red eye with halos; headache with jaw pain or scalp tenderness after 50; and wavy central vision. These need same-day or emergency care. Slowly shrinking side vision can signal glaucoma and needs an eye exam.

What causes sudden blindness in one eye?

The main causes are a blocked retinal artery or vein, retinal detachment, giant cell arteritis, acute glaucoma, bleeding inside the eye and optic neuritis. Many of these can cause permanent loss within hours, so call 911 or your local emergency number or go to an ER immediately.

What is the most common cause of blindness?

Worldwide, cataract is the leading cause of blindness and is curable with surgery. In the US, age-related macular degeneration, glaucoma, diabetic retinopathy and cataract are the main causes in adults.

Can blindness be reversed?

Some can. Blindness from cataract, refractive error and some retinal detachments or bleeds can be reversed with treatment. Vision lost from glaucoma, optic nerve damage or a retinal artery blockage usually cannot be restored, which is why early detection matters.

Is temporary blindness serious?

It can be. Brief loss of vision in one eye can be a mini-stroke and needs same-day assessment. Migraine aura, which causes shimmering zigzags for under an hour, is a common harmless cause in people who have had it before.