Our eyes allow us to see and interpret the world around us, but many different conditions can affect eyesight or eye comfort.
Some eye problems are relatively minor and temporary. Others can cause permanent vision loss if they are not diagnosed and treated promptly.
According to the World Health Organization (WHO), at least 2.2 billion people worldwide have near or distance vision impairment, and at least 1 billion cases could have been prevented or have yet to be addressed.
Globally, important causes of vision impairment include:
- refractive errors,
- cataracts,
- diabetic retinopathy,
- glaucoma,
- and age-related macular degeneration.
Other conditions such as dry eye, conjunctivitis and blepharitis may not usually cause permanent vision impairment but can still produce significant discomfort and require appropriate treatment.
Understanding the differences can help you know when to arrange a routine eye examination and when symptoms require urgent medical attention.
Eye Conditions Are Not All the Same
Eye conditions include many different types of problems and should not all be grouped together as “eye defects.”
Eye problems can instead include several categories:
Refractive errors — problems with how the eye focuses light.
Eye diseases — conditions affecting structures such as the lens, retina, cornea or optic nerve.
Eye infections and inflammation — including conjunctivitis and uveitis.
Congenital conditions — conditions present from birth.
Symptoms — such as photophobia, redness, burning or watery eyes, which may result from many different underlying conditions.
Here are some important eye and vision conditions to understand.
1. Myopia — Nearsightedness
Myopia makes objects in the distance appear blurry while closer objects may remain clear.
It occurs when the shape or length of the eye causes light to focus in front of the retina rather than directly on it.
Myopia often begins during childhood.
Symptoms can include:
- blurred distance vision,
- squinting,
- headaches,
- and eye strain.
Myopia can usually be corrected with:
- eyeglasses,
- contact lenses,
- or refractive surgery in appropriately selected adults.
High myopia is important because it can increase the lifetime risk of conditions including retinal detachment.
2. Hyperopia — Farsightedness
Hyperopia, or farsightedness, is another refractive error.
People with hyperopia may have difficulty seeing nearby objects clearly, although the effect varies depending on the degree of hyperopia and the person's age.
It occurs when the eye's shape prevents light from focusing correctly on the retina.
Possible symptoms include:
- blurry near vision,
- headaches,
- squinting,
- and eye strain.
Eyeglasses or contact lenses can correct hyperopia, and refractive surgery may be an option for selected adults.
3. Astigmatism
Astigmatism can cause blurry or distorted vision at both near and far distances.
It usually results from differences in the curvature of the cornea or lens that prevent light from focusing evenly on the retina.
Symptoms can include:
- blurred vision,
- distorted vision,
- squinting,
- headaches,
- and eye strain.
Astigmatism can usually be corrected with glasses or contact lenses, and surgery may be appropriate for some people.
4. Presbyopia
Presbyopia is the gradual loss of the eye's ability to focus clearly on nearby objects as we get older.
It usually becomes noticeable after approximately age 40 and eventually affects almost everyone.
You may notice that:
- reading small print becomes difficult,
- you hold reading material farther away,
- your eyes become tired during close work,
- or you need brighter lighting for reading.
Presbyopia is a normal age-related change rather than an infection or eye injury.
Options include:
- reading glasses,
- prescription eyeglasses,
- contact lenses,
- and selected surgical procedures.
5. Cataracts
A cataract occurs when the normally clear lens inside the eye becomes cloudy.
Cataracts are a major global cause of vision impairment.
Symptoms can include:
- cloudy or blurry vision,
- colours appearing faded,
- glare,
- halos around lights,
- difficulty seeing at night,
- and frequent changes in eyeglass prescription.
Ageing is an important risk factor, although cataracts can also be associated with:
- diabetes,
- eye injuries,
- certain medicines such as prolonged corticosteroid use,
- and other factors.
Some babies are also born with cataracts.
When cataracts significantly interfere with everyday activities, surgery can remove the cloudy lens and replace it with an artificial lens.
6. Glaucoma
Glaucoma is a group of eye diseases that damage the optic nerve, which carries visual information from the eye to the brain.
Some types of glaucoma are associated with increased pressure inside the eye, but glaucoma can also occur when measured eye pressure is within a statistically normal range.
One of the most common forms, open-angle glaucoma, can develop gradually without obvious early symptoms.
Peripheral vision can slowly deteriorate.
Once vision has been lost from glaucoma, it generally cannot be restored, which is why appropriate eye examinations are important.
Treatment may include:
- prescription eye drops,
- laser treatment,
- or surgery.
7. Age-Related Macular Degeneration
Age-related macular degeneration, commonly called AMD, affects the macula—the central part of the retina responsible for detailed central vision.
AMD is an important cause of vision loss among older adults.
Symptoms can include:
- blurred central vision,
- straight lines appearing wavy,
- difficulty recognizing faces,
- and a blank or distorted area near the centre of vision.
There are two major forms:
- dry AMD,
- and wet AMD.
Wet AMD can progress rapidly and requires prompt specialist treatment.
New distortion or sudden changes in central vision should be professionally assessed.
8. Diabetic Retinopathy
Diabetic retinopathy is a complication of diabetes that damages blood vessels in the retina.
Early disease may cause no noticeable symptoms.
As it progresses, symptoms can include:
- blurred vision,
- floaters,
- dark areas in vision,
- and vision loss.
People with diabetes should receive eye examinations according to the recommendations of their healthcare and eye-care professionals.
Managing:
- blood glucose,
- blood pressure,
- and other cardiovascular risk factors
can help reduce the risk of diabetic eye complications.
Treatment for advanced disease may include injections into the eye, laser treatment or surgery.
9. Dry Eye Disease
Dry eye occurs when the eyes do not produce enough tears or when tears do not work effectively enough to keep the eye surface adequately lubricated.
Symptoms can include:
- burning,
- stinging,
- gritty sensation,
- redness,
- light sensitivity,
- fluctuating blurred vision,
- and, perhaps surprisingly, excessive watering.
Dry eye can be associated with:
- ageing,
- environmental conditions,
- prolonged screen use,
- certain medicines,
- contact lenses,
- and some medical conditions.
Treatment depends on the underlying cause and severity.
10. Conjunctivitis — Pink Eye
Conjunctivitis is inflammation of the conjunctiva, the thin transparent tissue covering the white part of the eye and lining the inner eyelids.
It can cause:
- redness,
- swelling,
- watering,
- itching,
- burning,
- irritation,
- and discharge.
Conjunctivitis has several possible causes.
Viral conjunctivitis
Viral conjunctivitis is often highly contagious and may occur alongside a respiratory infection.
Bacterial conjunctivitis
Bacterial conjunctivitis can produce thicker discharge and may also spread from person to person.
Allergic conjunctivitis
Allergic conjunctivitis is caused by a reaction to allergens such as pollen, dust mites or animal dander.
It is not contagious.
This distinction is important because not every red or “pink” eye is an infection and not every case requires antibiotics.
11. Blepharitis
Blepharitis is inflammation affecting the edges of the eyelids.
Symptoms may include:
- irritated eyelids,
- crusting around the eyelashes,
- itching,
- burning,
- redness,
- gritty sensation,
- and dry or watery eyes.
Blepharitis can be associated with bacteria on the skin, dysfunction of oil-producing glands in the eyelids and skin conditions such as dandruff or rosacea.
Treatment may involve regular eyelid hygiene and other therapies recommended by an eye-care professional.
12. Stye
A stye, medically known as a hordeolum, is a painful red lump that develops near the edge of an eyelid.
It usually develops when an eyelid gland or follicle becomes infected or inflamed.
Many styes improve on their own.
A warm compress may help.
Do not squeeze or attempt to pop a stye.
Seek professional advice if it:
- does not improve,
- becomes very painful,
- significantly swells the eyelid,
- or affects vision.
13. Eye Floaters
Floaters can appear as:
- spots,
- dots,
- threads,
- cobweb-like shapes,
- or small shadows
moving through your field of vision.
They often become more common with age as the vitreous gel inside the eye changes.
Many longstanding floaters are harmless.
However, a sudden increase in floaters can be an emergency warning sign, particularly if accompanied by flashes of light or a shadow or curtain across your vision.
These symptoms may indicate a retinal tear or retinal detachment.
14. Retinal Detachment
A retinal detachment occurs when the retina pulls away from its normal position at the back of the eye.
This is a medical emergency.
Symptoms can include:
- sudden new floaters,
- flashes of light,
- a dark curtain or shadow across vision,
- or sudden changes in vision.
Retinal detachment can cause permanent vision loss if it is not treated promptly.
Anyone experiencing these symptoms should obtain urgent eye care.
15. Retinal Vein Occlusion
A retinal vein occlusion occurs when a vein carrying blood away from the retina becomes blocked.
The problem is primarily interruption of normal retinal blood circulation.
Symptoms can include sudden or gradual painless visual blurring or vision loss in one eye.
Risk factors may include:
- increasing age,
- high blood pressure,
- diabetes,
- glaucoma,
- and cardiovascular risk factors.
Sudden unexplained vision loss should always receive urgent medical assessment.
16. Uveitis
Uveitis refers to inflammation inside the eye, involving the uvea and sometimes nearby structures.
Symptoms can include:
- eye pain,
- redness,
- blurred vision,
- floaters,
- and sensitivity to light.
Possible causes include:
- autoimmune or inflammatory diseases,
- infections,
- eye injury,
- and other conditions.
In many cases, no specific cause is identified.
Uveitis can damage vision if untreated and requires professional eye assessment.
17. Corneal Ulcer
A corneal ulcer is an open sore affecting the cornea, the clear front surface of the eye.
It can result from infection or injury.
Contact-lens-related infection is an important risk.
Symptoms can include:
- severe eye pain,
- redness,
- discharge,
- blurred vision,
- light sensitivity,
- and a feeling that something is in the eye.
A suspected corneal ulcer requires urgent eye care because infection can damage the cornea and threaten vision.
Contact-lens wearers with a painful red eye should be particularly cautious.
18. Colour Vision Deficiency
Colour vision deficiency—often called colour blindness—means a person has difficulty distinguishing certain colours.
The common inherited forms are usually present from birth and are more common in males.
The most common type affects the ability to distinguish some shades of red and green.
Other forms affect blue and yellow discrimination.
Complete inability to perceive colour is rare.
Colour vision problems can also sometimes develop later because of:
- eye disease,
- neurological disease,
- injury,
- or certain medicines.
19. Coloboma
Coloboma is a condition in which part of the tissue forming the eye is missing from birth.
It develops when parts of the eye do not form completely during pregnancy.
A coloboma can affect structures including:
- the iris,
- retina,
- choroid,
- or optic nerve.
Its effect on vision varies widely depending on the location and extent of the missing tissue.
Some people have little visual impairment, while others have significant vision problems.
20. Adie Pupil
Adie pupil, also called tonic pupil, is a condition in which one pupil typically reacts abnormally or slowly to light.
The affected pupil is often larger than the other pupil.
Some people may also have difficulty focusing at near distances.
Different conditions can cause unequal pupil sizes, so a new or unexplained change in pupil size should be professionally assessed.
If unequal pupils develop suddenly with:
- severe headache,
- eye pain,
- weakness,
- confusion,
- drooping eyelid,
- or other neurological symptoms,
seek urgent medical care.
21. Night Blindness
Night blindness, or nyctalopia, means having unusual difficulty seeing in dim light or darkness.
It is a symptom rather than one single disease.
Possible causes include:
- uncorrected refractive error,
- cataracts,
- retinal disorders,
- and vitamin A deficiency.
Vitamin A deficiency is an important cause in some parts of the world but should not be assumed without assessment.
Taking excessive vitamin A supplements can be harmful.
Treatment depends on the underlying cause.
22. Photophobia — Light Sensitivity
Photophobia means abnormal sensitivity or discomfort in response to light.
It is a symptom, not a disease caused simply by exposure to bright lights.
Possible causes include:
- migraine,
- corneal problems,
- uveitis,
- eye infections,
- eye injury,
- and certain neurological conditions.
New or severe photophobia—particularly when accompanied by eye pain, redness or reduced vision—requires medical assessment.
23. Watery Eyes
Excessive tearing can occur when the eyes produce too many tears or when tears do not drain normally.
Possible causes include:
- dry eye,
- allergies,
- infection,
- blocked tear ducts,
- eyelid problems,
- wind,
- smoke,
- and other irritants.
The underlying cause determines the appropriate treatment.
Persistent watering, especially if accompanied by pain, swelling or changes in vision, should be assessed.
What About Eye Redness and Burning?
Eye redness and burning are symptoms, not single diseases.
Possible causes include:
- dry eye,
- allergies,
- conjunctivitis,
- blepharitis,
- contact-lens problems,
- foreign material,
- chemical irritation,
- corneal disease,
- uveitis,
- and glaucoma.
A mild red eye caused by irritation may resolve quickly.
However, a painful red eye with reduced vision or severe light sensitivity needs urgent assessment.
What About Double Vision?
Double vision can have many causes.
Some are related to the eye itself, while others involve:
- nerves,
- muscles controlling eye movement,
- or the brain.
Myasthenia gravis is an autoimmune neuromuscular disease that can affect the muscles controlling the eyelids and eyes.
Eye-related symptoms can include:
- drooping eyelids,
- and double vision.
New or sudden double vision should be medically assessed, particularly when accompanied by neurological symptoms.
Why Regular Eye Examinations Matter
Some serious eye diseases develop before a person notices obvious symptoms.
Glaucoma and diabetic retinopathy are important examples.
An eye examination can assess:
- visual acuity,
- refractive error,
- eye pressure when appropriate,
- the retina,
- optic nerve,
- and other eye structures.
How often you need an examination depends on:
- age,
- symptoms,
- family history,
- diabetes,
- existing eye conditions,
- medications,
- and other risk factors.
Follow the recommendations of a qualified eye-care professional.
Children's Eye Health
Children may not realize that their vision is abnormal because they may assume everyone sees the same way.
Possible signs of a vision problem include:
- frequent squinting,
- sitting unusually close to screens,
- holding books very close,
- headaches,
- eye rubbing,
- difficulty seeing the board at school,
- closing or covering one eye,
- or difficulty with reading and schoolwork.
Vision screening and comprehensive eye examinations when indicated can identify problems that may otherwise be missed.
Protecting Your Eyes
Some eye problems cannot be prevented, but several measures can help protect vision.
These include:
- having appropriate eye examinations,
- wearing suitable eye protection during hazardous work, DIY and sports,
- using sunglasses that provide appropriate ultraviolet protection,
- avoiding smoking,
- using contact lenses safely,
- managing diabetes and high blood pressure,
- eating a balanced diet,
- and seeking prompt care for sudden changes in vision.
For children, spending more time outdoors may help reduce the risk of developing myopia.
Contact Lens Safety
Contact lenses require proper hygiene.
Poor contact-lens practices can increase the risk of serious corneal infection.
Important precautions include:
- washing and drying hands before handling lenses,
- following the prescribed replacement schedule,
- using appropriate contact-lens disinfecting solution,
- avoiding water exposure unless specifically advised otherwise,
- removing lenses before swimming or showering,
- and not sleeping in lenses unless they are specifically approved and prescribed for overnight wear.
Never share contact lenses.
If you wear contact lenses and develop a painful, red eye or sudden blurred vision, remove the lenses and seek prompt professional assessment.
When Is an Eye Problem an Emergency?
Some eye symptoms should not wait for a routine appointment.
Seek urgent medical or specialist eye care for:
- sudden loss of vision,
- sudden major reduction in vision,
- a dark curtain or shadow across your vision,
- sudden flashes of light with new or increasing floaters,
- severe eye pain,
- a painful red eye with reduced vision,
- penetrating eye injury,
- chemical injury,
- sudden double vision with neurological symptoms,
- severe light sensitivity with pain or reduced vision,
- or sudden unexplained changes in pupil size accompanied by neurological symptoms.
If chemicals enter the eye, begin immediate irrigation with plenty of clean water while arranging urgent medical care.
Do not delay rinsing while searching for a particular eye wash.
The Bottom Line
There are many different eye conditions, and they should not all be described as “eye defects.”
Some, such as:
- myopia,
- hyperopia,
- astigmatism,
- and presbyopia,
are refractive errors that can often be corrected with glasses or contact lenses.
Others, including:
- cataracts,
- glaucoma,
- diabetic retinopathy,
- age-related macular degeneration,
- retinal detachment,
- and uveitis,
are diseases that may require medical or surgical treatment.
Symptoms such as:
- redness,
- burning,
- watering,
- photophobia,
- floaters,
- or blurred vision
can have many possible causes.
The most important rule is:
Never ignore sudden vision loss or a sudden major change in your eyesight.
Prompt diagnosis and treatment can protect vision in many eye conditions.
Medical Disclaimer
This article provides general health information for an international audience and is not a substitute for examination, diagnosis or treatment by a qualified eye-care or healthcare professional. Eye symptoms can have many causes, and some vision-threatening conditions initially resemble minor problems. Seek urgent medical care for sudden vision loss, severe eye pain, penetrating or chemical injury, a curtain or shadow across vision, or other sudden significant visual changes.
Sources and Further Reading
World Health Organization — Blindness and Vision Impairment
https://www.who.int/news-room/fact-sheets/detail/blindness-and-visual-impairment
World Health Organization — Eye Care, Vision Impairment and Blindness
https://www.who.int/health-topics/blindness-and-vision-loss
World Health Organization — Refractive Errors
https://www.who.int/news-room/questions-and-answers/item/blindness-and-vision-impairment-refractive-errors
National Eye Institute — Eye Conditions and Diseases
https://www.nei.nih.gov/eye-health-information/eye-conditions-and-diseases
National Eye Institute — Refractive Errors
https://www.nei.nih.gov/eye-health-information/eye-conditions-and-diseases/refractive-errors
Centers for Disease Control and Prevention — Conjunctivitis (Pink Eye)
https://www.cdc.gov/conjunctivitis/
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