Keratitis is inflammation of the cornea, the transparent dome at the front of the eye. It can be the result of infection or injury followed by infection. With early treatment, it is usually curable but untreated, keratitis can lead to ulceration of the cornea, scarring and in severe cases, vision loss.
 

What increases your risk of keratitis?
The eyelids and the outer layer of the cornea (the epithelium) form a barrier that protects the eye. This protective function is aided by the production of tears, which also help prevent infection. You may be prone to developing keratitis if you have any damage or abnormality that affects the eyelids, epithelium, or problems with tear production.
 

Specific situations that make keratitis more likely include:

Environmental factors:

  • Injury: an object scratching the surface of the cornea or penetrating it can cause inflammation and can also allow infection to get into the eye.
  • Foreign body in the eye.
  • Contact lens wear increases the risk. Contact lenses can be contaminated with infectious agents and may also damage the corneal epithelium. Over-use of contact lenses or poor lens hygiene are therefore risk factors. Keratitis is more common in people using extended-wear lenses or wearing them overnight.
  • Exposure to ultraviolet light.
  • Corticosteroid eye drops.
  • Swimming in water contaminated with pathogens that can enter the eye if the corneal surface is damaged by injury or contact lens wear.

Another underlying eye condition:

  • Dry eye syndrome: not enough tears are produced or they evaporate too fast.
  • Untreated eye rosacea [Link to condition page on eye rosacea].
  • An underlying health problem (not eye related):
  • Vitamin A deficiency.
  • Allergies.
  • Reduced immunity because of disease or poor nutrition.
  • Infection with bacteria, viruses, fungi and parasites: this usually occurs as an opportunistic infection, after eye damage caused by the above.

Symptoms and complications of keratitis
The early symptoms of keratitis include blurred vision, sensitivity to light and painful eyes. It is also common to feel that you have something in your eye, or it can feel itchy. Your eyes may water or produce a discharge and they will look red and sore. If keratitis is not treated, the damage to the surface of the eye continues and this can cause corneal erosion, corneal ulcers and scarring.

Bacterial keratitis

Most bacteria can only cause keratitis if the corneal epithelium is damaged and the main risk factor here is the use of contact lenses, especially extended-use lenses.
 

The exceptions are Neisseria gonorrhoeae and Haemophilus influenzae, which can cross an intact epithelium.
 

Other bacteria that cause keratitis include:

  • Streptococcus species.
  • Pseudomonas: the increasing use of soft contact lenses is thought to be a factor in the increasing incidence of corneal ulcers caused by Pseudomonas aeruginosa.
  • Enterobacteriaceae species.
  • Staphylococcus species.
  • Sometimes, the primary site for inflammation is the stroma, one of the middle layers of the cornea. This interstitial keratitis can have many causes, infectious and non-infectious, but the most common is infection by Treponema pallidum, the bacteria that causes syphilis.

Viral keratitis
Two types of herpes virus are responsible for most cases of viral keratitis:

  • Herpes simplex virus, the virus responsible for cold sores, can lurk in nerves near the eye, erupting from time to time. Repeated attacks can lead to keratitis as the cornea becomes scarred.
  • Herpes zoster virus, the virus responsible for chicken pox and shingles. Usually keratitis occurs as a complication of shingles.

Fungal and parasitic keratitis

Fungal keratitis is a significant cause of infectious keratitis in tropical areas and it can be very serious and difficult to treat.
The fungi that cause it, most often Aspergillus and Fusarium species, can only gain access to the eye after corneal injury. Agricultural injuries, where sharp leaf edges or twigs from plants scrape the eye, are at the root of many cases.
 

Parasitic keratitis is often caused by acanthamoeba, a common single-celled organism that lives in water, air, soil and sewage. It can cause a serious corneal infection that can involve the stroma. Wearing contact lenses while swimming in contaminated waters is often responsible.
 

Non-infectious keratitis

Other types of keratitis include:

  • Peripheral ulcerative keratitis, which is a serious complication of rheumatoid arthritis.
  • Ultraviolet keratitis or corneal flash burn is caused by sun or sun lamps.
  • Inflammation from eye injuries or chemicals.
  • Inflammation caused by abrasions produced by contact lenses not being clean, well-fitting, worn for too long, or trapping dust or dirt behind them.