Cornea !
The cornea is the clear dome at the front of the eye; it does about two thirds of the eye's focusing and is the part reshaped by laser eye surgery.
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What it is
The clear dome at the front of the eye.
What it does
Does about two thirds of the eye's focusing; LASIK reshapes it.
Key facts
- Where
- Front sixth of the outer coat of the eye
- Blood supply
- Avascular: aqueous humour, tears, limbal vessels
- Nerve
- V1 (long ciliary nerves): corneal reflex
- Function
- Two thirds of the eye's focusing power (~43 D)
Relations
- In front
- Tear film, and the eyelids when the eye is closed
- Behind
- Anterior chamber and its aqueous humour, which feeds the endothelium; the iris lies behind, across the chamber
- Contents
- Fine branches of the long ciliary nerves (V1), which lose their myelin as they enter so the stroma stays clear; no blood or lymph vessels
How to find it
The clear dome in front of the iris; its edge is the limbus, where it meets the sclera and the conjunctiva and where corneal stem cells live. Under the slit lamp each layer can be told apart in a thin optical section.
Examination
- Corneal reflex with a wisp of cotton: no blink in either eye points to a V1 lesion on the touched side; a blink only in the other eye points to a facial nerve lesion on the touched side
- Fluorescein staining under blue light: abrasions and ulcers glow green; a branching (dendritic) pattern suggests herpes simplex keratitis
- Seidel test: leaking aqueous washes out concentrated fluorescein, revealing a full-thickness wound
On imaging
Corneal topography and tomography map curvature and thickness (keratoconus shows focal, often inferior, steepening and thinning); pachymetry measures thickness; specular microscopy counts endothelial cells.
Common problems
- Refractive errors (short- or long-sightedness, astigmatism) come partly from its curve.
- Scratches and ulcers (contact-lens wearers are at risk): pain, redness, light sensitivity.
- Keratoconus: the cornea thins and bulges into a cone.
How it is diagnosed
Slit-lamp exam, a dye that shows scratches, and corneal maps (topography) before laser surgery.
Treatment
- Glasses or contact lenses.
- Laser reshaping of the cornea (LASIK and similar).
- Antibiotic drops for ulcers; corneal cross-linking for keratoconus; a corneal transplant for scarring.
Clinical notes
- Thin corneas make tonometry read falsely low and thick ones falsely high, so thickness is measured when assessing glaucoma
- Kayser–Fleischer ring: a golden-brown ring of copper in Descemet membrane at the periphery, a sign of Wilson disease
- Arcus: a grey-white lipid ring just inside the limbus; common in older people, it may point to hyperlipidaemia in the young
- Fuchs endothelial dystrophy: gradual endothelial loss with blurred vision worse on waking; it can be treated by transplanting only the endothelial layer
- Laser refractive surgery reshapes the stroma, either under a cut flap or after removing the epithelium; a thin or irregular cornea rules it out
Development
The epithelium comes from surface ectoderm; the stroma and endothelium come from neural crest cells.
Memory aid
Layers from front to back: Every Boy Should Do Exercise (Epithelium, Bowman, Stroma, Descemet, Endothelium).
Related conditions
- Hurler syndrome (mucopolysaccharidosis type I)
- Morquio syndrome (mucopolysaccharidosis type IV)
- Xeroderma pigmentosum
- Vitamin A deficiency (night blindness and xerophthalmia)
- Wilson disease
- Free-living amoeba infections (Naegleria meningoencephalitis, Acanthamoeba)
- Onchocerciasis (river blindness)
- Chemical eye injury
- Corneal abrasion and ocular foreign body
- Keratitis and corneal ulcer
- Herpes zoster ophthalmicus
- Ocular trauma: hyphema and open globe injury
- Refractive errors and presbyopia
- Trachoma
- Pterygium
- Dry eye disease
- Graves disease (hyperthyroidism)
- Open-angle glaucoma
- Acute angle-closure glaucoma
- Conjunctivitis
Test yourself
Think of the answer, then tap the question to see it.
What are the five classic layers of the cornea from front to back?
Epithelium, Bowman layer, stroma, Descemet membrane and endothelium; the stroma makes up most of the thickness.
What keeps the cornea clear, and why does endothelial loss cause corneal oedema?
The endothelial pump keeps the stroma dehydrated; endothelial cells do not regenerate, so their loss lets the stroma swell.
What are the afferent and efferent limbs of the corneal reflex?
Afferent: V1 (nasociliary nerve); efferent: facial nerve (VII) to orbicularis oculi. Touching one cornea makes both eyes blink.
What is a Kayser–Fleischer ring and what does it indicate?
A golden-brown ring of copper in Descemet membrane at the corneal periphery: a sign of Wilson disease.
Why is corneal thickness measured when assessing glaucoma?
Thin corneas make tonometry read falsely low and thick corneas falsely high.
Educational information, not medical advice. For any symptom, see a health professional.
Its names
- Official Latin (TA2)
- cornea TA2 6744
- Official Spanish (DeCS)
- Córnea DeCS D003315
- English (TA2)
- Cornea
Related
Sources: Terminologia Anatomica (TA2); DeCS; Anatomy, Head and Neck, Eye Cornea; Keratoconus.
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Representative model, for educational purposes only.
