- Home
- Knowledge centre
- Eye glossary A–Z
- Sclera: The White of the Eye, Its Function, Colour Changes and Problems
Sclera: The White of the Eye, Its Function, Colour Changes and Problems
Medically reviewed by {{surgeon}}, {{surgeon's title and specialty}}, on {{review date}}. Last updated {{date}}.
Sclera (noun, plural sclerae or scleras): the sclera is the tough, white outer wall of the eye, the part everyone calls "the white of the eye". It protects the eye's delicate inner structures and holds the eyeball in shape, and it is where the eye muscles attach. At Laser Istanbul Clinic, inflammation and surface problems of the eye are assessed by our surgeons, including Doç. Dr. Muhammet Derda Özer (Doç. means Associate Professor), who leads cornea and uveitis care.
Sclera of the eye: key facts
- What it is
- The opaque, fibrous white outer layer of the eye, made mainly of type I collagen
- Where it is
- Covers most of the eyeball (about five-sixths of its outer wall); joins the clear cornea at a ring called the limbus; the optic nerve leaves through its back
- Thickness
- About 1 mm at the back of the eye and as thin as about 0.3 mm just behind the points where the eye muscles attach
- Layers
- Episclera (outer), stroma (the thick middle), and a thin inner suprachoroid lamina
- Blood supply
- Very little: the sclera itself is nearly avascular; most of the red vessels you see on the "white" are in the thin tissues covering it
- Covered by
- The conjunctiva, a clear membrane that lines the eyelids and covers the front of the sclera
- Main jobs
- Protection, shape, and an anchor for the eye muscles
- Specialist
- Ophthalmologist (eye doctor); inflammation is often managed together with a rheumatologist
What is the sclera of the eye?
The sclera of the eye is the outer coat that wraps the eyeball like a firm shell. Most of it is hidden behind the eyelids and the bony socket; the part you see in the mirror is only the front, on either side of the coloured iris. In front, it blends into the cornea, the clear dome over the iris and pupil. Behind, it forms a sleeve around the optic nerve.
Why is it white and opaque while the cornea is clear? Both are built from collagen, but in the sclera the fibres are interlaced and irregular, which scatters light, while in the cornea they are arranged in an exact, regular pattern that lets light through. That one difference in arrangement is the difference between a wall and a window.
The layers of the sclera
| Layer | What it is |
|---|---|
| Episclera | A thin, loose layer of collagen tissue on the outside, just under the conjunctiva |
| Stroma | The thick main layer: dense type I collagen fibres interwoven with networks of elastic fibres; this is what makes the sclera strong |
| Suprachoroid lamina (lamina fusca) | A thin inner layer containing fibroblasts and pigment cells, lying against the choroid |
The sclera is nearly avascular, meaning it has very few blood vessels of its own. This matters for diagnosis: a doctor judges a red eye partly by which layer the visible redness sits in.
Sclera function: what does the sclera do?
The sclera has three main functions.
1. Protection. It forms a strong barrier around the inner eye. Together with the eyelids, the bony socket and the tear reflex, it helps protect the delicate retina, choroid and other tissues from injury.
2. Keeping the eye's shape. The eye works as an optical instrument only if its shape stays stable. The sclera gives the eyeball its architectural support, so that light from the cornea and lens reaches the retina in focus.
3. Anchoring the eye muscles. The muscles that move the eye attach to the sclera and pull on it to turn the eye. This is why the sclera matters in squint (strabismus) surgery, where the muscle attachments on the sclera are adjusted.
A fourth role is social: the visible white of the eye makes it easy to read where someone is looking, and changes in its colour are one of the first things others, and doctors, notice about health.
Sclera vs conjunctiva: what is the difference?
People often mix these up, because both are on the "white" of the eye.
| Sclera | Conjunctiva | |
|---|---|---|
| What it is | The tough white wall of the eye | A thin, clear membrane |
| Where | The eyeball's outer coat | Covers the front of the sclera and lines the inside of the eyelids |
| Blood vessels | Almost none | Many small, visible vessels, which is why a "red eye" usually looks red |
| What goes wrong | Deep inflammation (scleritis), thinning, colour change | Conjunctivitis, subconjunctival bleeds, pinguecula, pterygium |
In short: the sclera is the wall, the conjunctiva is the thin clear covering over it. The conjunctiva and sclera sit so close together that doctors use the slit lamp to tell which one is affected. For example, a subconjunctival haemorrhage, the bright-red patch that appears after a cough or an eye rub, is blood leaking between the conjunctiva and the sclera. It looks alarming but usually heals by itself.
Why does my sclera look blue? Is blue sclera dangerous?
Blue sclera is a bluish or grey tint to the white of the eye. It appears when the sclera is thin enough for the dark layers underneath (the uvea) to show through.
It is often harmless. A slightly blue sclera is normal in infants and young children, whose sclera is thinner. It usually fades as the sclera thickens with age.
It can be a clue to a connective-tissue condition. In older children and adults, a persistent blue tint can go with conditions in which collagen is faulty:
- Osteogenesis imperfecta (brittle bone disease). MedlinePlus says many people with milder forms have a blue or grey tint to the sclera, especially the mildest form, type I. The condition affects roughly 1 in 10,000 to 20,000 people, and mutations in the COL1A1 and COL1A2 collagen genes cause about 90 percent of cases.
- Ehlers-Danlos syndrome, another connective-tissue disorder, can also give the sclera a bluish tint.
So, is blue sclera dangerous? The tint itself does not hurt the eye. What matters is whether it comes with other signs, such as bones that break easily or very flexible joints, or a family history. If you or your child have a blue sclera together with any of these, ask your GP or an eye doctor to look into it. It is also worth mentioning a blue sclera at any pre-operative eye assessment.
Other colour changes of the sclera
| What you see | What it can mean | What to do |
|---|---|---|
| Yellow | Jaundice: bilirubin building up in the body. The NHS lists alcohol-related liver disease, gallstones, hepatitis, pancreatitis and sickle cell disease among the causes | See a doctor urgently; the NHS advises an urgent GP appointment or NHS 111 |
| Red, in a patch | Subconjunctival haemorrhage: a small burst vessel under the conjunctiva, often after coughing, sneezing, straining or eye rubbing | Usually heals in a few days to a few weeks without treatment; see a doctor if it keeps coming back or follows an injury |
| Red, all over, with gritty or sticky eyes | Surface irritation or conjunctivitis | Depends on the cause; see a doctor if it lasts or vision is affected |
| Red or violet, deep and painful | Scleritis (see below) | Same-day eye care |
| Yellowish raised spot | Pinguecula (see below) | Routine check |
| Blue or grey | Thin sclera, see the section above | Mention it to your doctor |
Bump on the sclera of the eye: what can it be?
A bump on the white of the eye can come from several different conditions. Most are harmless, but a few need prompt attention, so do not guess.
- Pinguecula. The American Academy of Ophthalmology describes this as a yellowish bump on the white of the eye, a growth linked to ultraviolet light, wind and dust. Many people have no symptoms beyond occasional irritation. A pterygium is a related growth that can extend across the cornea. Both are usually watched, and treated if they cause trouble.
- Nodular episcleritis. A discrete, raised area of inflammation in the superficial layers, with a gradual onset; it is benign and settles by itself.
- Nodular scleritis. A raised, tender nodule in the sclera itself, usually with pain. This one needs urgent assessment.
- Other lumps. Any lump that is getting bigger, changing colour or bleeding should be examined. Doç. Dr. Özer's areas of work include ocular oncology, which is the diagnosis and treatment of eye tumours.
A simple rule: a painless, yellowish, flat-to-slightly-raised patch that has been there for years is usually a pinguecula. A tender, red or violet lump, especially with pain, is not, and needs a doctor the same day.
Sclera swelling and inflammation: episcleritis and scleritis
Sclera swelling has many causes, and some of them involve the conjunctiva above the sclera rather than the sclera itself. The two inflammatory conditions that doctors want to tell apart are episcleritis and scleritis.
| Episcleritis | Scleritis | |
|---|---|---|
| Which tissue | Superficial layers (episclera) | The sclera itself |
| How common and how serious | Relatively common, benign, self-limited | Less common; can threaten vision |
| Pain | Mild or none; mainly redness and irritation | Severe, boring or piercing pain that is worse with eye movement |
| Look | Red eye from congestion of the superficial vessels | Often a violet-bluish hue with swelling of the sclera |
| Usual course | The simple form peaks within about 12 hours and clears in 2–3 days; the nodular form lasts longer; usually gone within a few weeks | Needs specialist treatment |
| Link to body-wide disease | Associated in about 26–36% of cases; most are idiopathic | Up to about 50%, most often rheumatoid arthritis |
| Vision | Not usually affected | May be reduced; can involve the cornea and the inner eye |
What causes scleritis?
Scleritis is often a sign of an underlying autoimmune condition. The American Academy of Ophthalmology links it with rheumatoid arthritis, lupus, inflammatory bowel disease and Sjögren's syndrome. Eye injuries and infections, including rare fungal or parasitic ones, can also trigger it. Anterior scleritis (the front of the eye) is the more common type; posterior scleritis, affecting the back of the eye, is rarer but can cause serious complications such as retinal detachment.
Symptoms of scleritis
- Severe, dull, boring pain, which can spread to the jaw or face and is worse on eye movement
- Redness and swelling of the white of the eye, sometimes with a violet-blue look
- Blurred vision, watering and sensitivity to light
- Vision loss, if untreated
How scleritis is treated
Treatment depends on severity and is chosen by a specialist. EyeWiki describes oral anti-inflammatory tablets (NSAIDs) as first-line; systemic corticosteroids for moderate to severe disease; and immunomodulating drugs such as methotrexate for stubborn cases. Because up to half of patients have an underlying body-wide disease, a rheumatologist is often involved. Episcleritis is usually treated with simple supportive care and sometimes oral anti-inflammatories.
Treatment with us: inflammatory eye disease falls within the uveitis and cornea expertise of Doç. Dr. Muhammet Derda Özer. If you have been diagnosed with scleritis and want a second opinion, send us your report and scans.
How is the sclera examined?
Most checks are quick and painless:
- History and visual acuity test. Your doctor asks about pain, onset, injuries, contact lenses, joint problems and medical history.
- Slit-lamp examination. A bright, thin beam through a microscope shows the conjunctiva, episclera and sclera, so the depth and extent of redness, swelling or a nodule can be judged. Doctors look for the characteristic violet-bluish hue with swelling that points to scleritis.
- Phenylephrine test. Phenylephrine 2.5% drops blanch the conjunctival and episcleral vessels but leave the scleral vessels undisturbed. If the redness disappears, it is superficial (episcleritis); if it stays, scleritis is more likely.
- Blood tests. For scleritis, tests screen for autoimmune disease, for example rheumatoid factor, antinuclear antibodies and ANCA.
Eye tests like these are explained in our guide to eye examinations and diagnostic tests.
Sclera lenses: scleral lenses, and black or red "sclera lenses"
"Sclera lenses" can mean two very different things.
Scleral lenses (medical)
A scleral lens is a large contact lens that rests on the sclera, not on the cornea, and creates a tear-filled vault over the cornea. That fluid layer conforms to an irregular cornea so vision can be restored comfortably. Scleral lenses are used for keratoconus, corneal ectasia, severe dry eye and Stevens-Johnson syndrome. The American Academy of Ophthalmology notes that scleral contact lens correction may significantly reduce the rate of transplant surgery in patients with severe keratoconus. They are individually fitted by an eye-care professional.
Black, red and other "sclera lenses" (decorative)
Searches for black or red sclera lenses are usually about costume lenses that cover the whole visible eye, including the white. These are decorative lenses. The FDA classes decorative lenses as medical devices and says they need a prescription and a fitting from an eye doctor, because unfitted, shared or novelty-shop lenses can cause corneal abrasions, allergic reactions, serious infections and vision loss, potentially blindness. If a decorative lens causes redness, pain or discharge, take it out and see an eye doctor the same day.
If you have keratoconus and are weighing lenses, cross-linking and other options, send us your corneal scans so Doç. Dr. Özer can review them.
When to see a doctor about your sclera
Seek same-day care from an eye doctor or eye emergency service if you have
- a red eye with deep, boring pain, especially pain that spreads to the jaw or face or gets worse when you move the eye
- a violet or dark red, tender area or lump on the white of the eye
- reduced vision or sensitivity to light with a red eye
- a red eye after an eye injury, a chemical splash, or contact lens wear (take lenses out)
- pain, redness or discharge after wearing decorative lenses
See a doctor promptly if
- the white of your eyes, or your skin, turns yellow (jaundice; the NHS advises urgent advice)
Book a routine appointment if you notice
If you already have a diagnosis, such as scleritis or keratoconus, and want a second opinion, send your scans before you travel.
- a persistent blue or grey tint in an older child or adult, particularly with easy fractures or very flexible joints
- a yellowish or growing bump on the white of the eye
- red patches that keep coming back
- a pinguecula or pterygium that is irritating, getting bigger or affecting vision
Frequently asked questions
The sclera is the white, fibrous outer wall of the eye. It protects the eye, keeps its shape and anchors the muscles that move it. It joins the clear cornea at the limbus.
Mostly dense type I collagen fibres, interwoven with elastic fibres. Its fibres are arranged irregularly, which is why it is opaque and white, whereas the cornea's regular fibres make it transparent.
Protection of the inner eye, maintaining the eyeball's shape and providing the attachment point for the eye muscles.
The sclera is the tough white wall of the eye. The conjunctiva is a thin, clear membrane that covers the front of the sclera and lines the inside of the eyelids. Most of the visible red vessels in a red eye are in the conjunctiva, not the sclera.
Not by itself. It is normal in babies and often harmless. In older children and adults it can be associated with conditions such as osteogenesis imperfecta or Ehlers-Danlos syndrome, so it is worth mentioning to a doctor, particularly if bones break easily or joints are very flexible.
Often a pinguecula, a yellowish, usually harmless bump caused by sun, wind and dust. A painful, red or violet lump can be nodular scleritis and needs same-day care. Any bump that grows or changes should be examined.
Swelling can be caused by inflammation such as scleritis or episcleritis, or by problems in the conjunctiva above it. Swelling with deep pain, a violet colour or reduced vision should be seen the same day.
It can be. Scleritis can lead to vision loss if untreated, and up to half of patients have an underlying body-wide disease such as rheumatoid arthritis. Early treatment by an eye specialist is important. Episcleritis, in contrast, is usually mild and clears on its own.
Only if they are prescribed and fitted by an eye doctor. Decorative lenses bought without a fitting can scratch the cornea or cause serious infections, and the FDA warns of possible vision loss and blindness.
Sources for the facts on this page
- American Academy of Ophthalmology. Sclera (eye anatomy). https://www.aao.org/eye-health/anatomy/sclera-list
- American Academy of Ophthalmology. Conjunctiva (eye anatomy). https://www.aao.org/eye-health/anatomy/conjunctiva-list
- EyeWiki (AAO). Basic Histology of the Eye and Accessory Structures. https://eyewiki.org/Basic_Histology_of_the_Eye_and_Accessory_Structures
- Wikipedia. Sclera (coverage and thickness figures; to be verified, see note on top). https://en.wikipedia.org/wiki/Sclera
- MedlinePlus Genetics. Osteogenesis imperfecta. https://medlineplus.gov/genetics/condition/osteogenesis-imperfecta/
- NHS. Jaundice. https://www.nhs.uk/conditions/jaundice/
- American Academy of Ophthalmology. What Is Subconjunctival Hemorrhage? https://www.aao.org/eye-health/diseases/what-is-subconjunctival-hemorrhage
- American Academy of Ophthalmology. Pinguecula and pterygium. https://www.aao.org/eye-health/diseases/what-are-pinguecula-pterygium
- EyeWiki (AAO). Scleritis. https://eyewiki.org/Scleritis
- EyeWiki (AAO). Episcleritis. https://eyewiki.org/Episcleritis
- American Academy of Ophthalmology. What Is Scleritis? https://www.aao.org/eye-health/diseases/what-is-scleritis
- American Academy of Ophthalmology. Scleral lenses for keratoconus. https://www.aao.org/eye-health/diseases/scleral-lenses-keratoconus
- Wikipedia. Scleral lens (design description). https://en.wikipedia.org/wiki/Scleral_lens
- FDA. Decorative contact lenses: Halloween and more. https://www.fda.gov/medical-devices/contact-lenses/decorative-contact-lenses-halloween-and-more
- Wikipedia. Blue sclera (infants, thin sclera showing the uvea, Ehlers-Danlos). https://en.wikipedia.org/wiki/Blue_sclera
Unsure about a change in the white of your eye?
Send us your eye report or scans. One of our surgeons will review them and tell you what the next step should be, with no obligation. For severe pain, a violet or very red eye, or sudden vision loss, see an eye emergency service the same day.
3 eye surgeons
Your case is reviewed by a named surgeon, not a salesperson. Reply within {{response time}}.
Prof. Dr. Ahmet Satıcı
Doç. Dr. Muhammet Derda Özer
Op. Dr. Aysan Bayram