What Is Keratitis? — Causes and Symptoms
The term keratitis literally means “inflammation of the cornea.” It comes from the prefix kerat, which is Greek for “horn.” How does that translate to cornea? Well, the “horn” on the head of an animal is actually made of keratin, a hard substance from which human hair and nails are made. There are several different types of keratin, and the cornea is made up of one of these types.
Keratitis actually occurs more frequently than most people know. In fact, most people have it several times during their life. It comes in several different forms, the most common of which is dry eye syndrome. Other forms include:
- Superficial Punctate Keratitis
- Stromal Keratitis
- Interstitial Keratitis
- Amoebic Keratitis
- Bacterial, Viral or Fungal Keratitis
- Ulcerative Keratitis
- Exposure Keratitis
- Thygeson's Superficial Punctate Keratitis
These are just a few of the many types of keratitis. Many forms of the disorder can be caused by underlying medical problems including allergies, and autoimmune diseases among other things.
Thygeson's Superficial Punctate Keratitis
Facts About the Cornea
- The cornea gives the eye about 70 percent of its ability to focus; the other 30 percent comes from the crystalline lens.
- The tear film, which consists of three layers, keeps the cornea hydrated and clean, which keeps the vision crystal clear (like looking through a clean window).
- The tear film is controlled by the nerve endings in the cornea. When the cornea needs more hydration, the eye begins to feel irritated and produces tears in response to that irritation. When the irritation subsides, the tear film returns to normal levels.
Keratitis can affect all five layers of the cornea. The names of these five layers are:
- Bowman’s Membrane
- Descemet’s Membrane
The first layer, the epithelium, is the layer that is protected by the tear film and the most likely to be injured by foreign bodies or abrasions. The epithelium also has all the nerve endings. The foreign body sensation, or feeling of something in your eye, comes from these nerve endings. This layer also has the amazing regenerative properties of the cornea. Abrasions tend to heal within 24 hours.
The second layer of the cornea is known as Bowman’s membrane. It is made of randomly placed collagen fibrils, and it has no regenerative properties. It typically scars if injured, and its function is currently unknown in ophthalmology. Interestingly, this layer is only found in the corneas of primates, which suggests primates need this layer for some reason.
The Five Corneal Layers
Location of the Cornea
The third layer is the thickest layer, known as the stroma, which gives the cornea substance. Interestingly, this layer is 78% water; yet, it is the layer that provides stability for the cornea. It does not regenerate and typically scars if injured.
The fourth layer is Descemet’s membrane. This is a very elastic layer. If it is cut, it snaps back, similar to a rubber band. This layer is the basement membrane of the endothelial cells. If it is injured, the eye will usually require a corneal transplant since the endothelial cells depend on this layer for support and it doesn’t regenerate.
The fifth and final layer of the cornea is the endothelium. This layer maintains the corneal deturgescence. In English, this means it maintains the relative dehydration that is necessary to keep the cornea transparent. This layer does not regenerate, and tends to actually degenerate with age.
View of the Cornea from a Slit Lamp
Causes of Keratitis
There are a number of different reasons for the cornea to become inflamed. Some of these include:
- Injury (especially lacerations or penetration of the cornea)
- Chemical exposure
- Viruses (especially the herpes virus)
- Contaminated water (swimming pools that contain chlorine, for example)
- Fungal infections
- Bacterial infections
- Dry eye syndrome
- Contaminated contact lenses
- Corneal foreign bodies
- Exposure to ultraviolet light
- Systemic diseases (such as AIDS)
The most common of these is dry eye syndrome, a condition in which the tear film fails to hydrate the cornea properly either because the tear film is evaporating too quickly or there isn’t enough being produced. This can happen for several reasons including:
- Medications – many medications cause overall dehydration (such as opiate pain killers) and therefore also dry out the eyes.
- Surgery – surgery desensitizes the cornea allowing it to dry out more easily.
- Contact Lenses – people who wear contact lenses on a regular basis, and for long periods of time, reduce their corneal sensitization drastically, which affects tear film production.
Corneal Foreign Body
Injuries to the cornea are another common cause of keratitis. This can be anything from a small corneal abrasion or penetrating laceration to a burn caused by chemical exposure or ultraviolet light.
Corneal foreign bodies are also a common injury, and almost always result in keratitis. Mechanics are the most common patients who present with a foreign body, which is usually metallic in nature and associated with a rust ring. That’s exactly what it sounds like; a ring of rust develops around the foreign body.
Symptoms of Keratitis
Injuries or other afflictions of the cornea are extremely painful. Even dry eyes can be excruciating if left untreated. The most common symptoms of keratitis include:
- Photophobia (sensitivity to light)
- Sensation of something in the eye (foreign body sensation)
- Epiphora (excessive and continuous tearing)
- Injection of the conjunctiva (redness of the white part of the eye)
- Blurred, cloudy or hazy vision
There is one symptom common to any injury affecting the cornea, whether it is keratitis, a foreign body, or simply a case of dryness. Everyone will have the sensation that there is something in the eye. Unfortunately, this can be a very misleading symptom, as many people will actually think there is something in their eye.
One way to tell if the sensation is coming from something actually in the eye is to notice if the sensation moves. Unless it is embedded in the cornea, anything in the eye will move around the eyeball, and the sensation will move with it.
If you feel like there is something in your eye, flush it with tap water or saline solution for at least 15 minutes. If you still feel like there is something in the eye, then there is another problem and you need to see your eye doctor.
Using Fluorescein Eye Drops
Diagnosing and Treating Keratitis
Keratitis is easily diagnosed by your optometrist or ophthalmologist. As with any exam, you will be asked about the history of the symptoms, and then your vision will be checked. Your doctor will then instill a yellow drop known as fluorescein, which will allow them to easily see if there is something wrong with the cornea.
The fluorescein causes any injury to the cornea to fluoresce when exposed to light from a cobalt blue, or UV filter. If there is a foreign body, rust ring, or any type of abrasion, it will be seen immediately.
If the keratitis is ulcerative (has formed an ulcer), the doctor may take a culture of it and send it to a lab for testing, especially if it isn’t healing quickly. The majority of corneal ulcers are sterile, meaning there is no infection; however, there are some bacterial ulcers that are difficult to treat without knowing exactly which bacterium you are treating.
Depending on the cause of the keratitis, there can be a number of different treatments. The most common keratitis, dry eyes, is typically treated with a regimen of artificial tears and ointments. Some patients have to take the drops four times a day; others have to take them every hour. Ointments are used at night to prevent the cornea from drying out while the patient sleeps.
Sometimes the only course of action is to allow the cornea to heal itself, which normally only takes about 24 hours in minor injuries. However, even in these cases, keeping the cornea as moist as possible will facilitate the healing process. Artificial tears every one to two hours will actually reduce the amount of time the cornea takes to heal.
If you have to take antibiotic or other eye drops as well, wait approximately five minutes after using the tears to instill the prescription drops. Using artificial tears prior to prescription drops will also help with the absorption of the drop.
© 2012 Melissa Flagg OSC