Chemical injuries account for only 10 percent of all ocular injuries, but when these cases present, we must manage them promptly and properly.

INITIAL CHARACTERISTICS

Chemical burns vary greatly in severity and appearance, but ocular injection is generally a consistent sign. In mild cases, damage may be restricted to the inferior cornea and conjunctiva, largely a result of Bell's phenomenon. You'll often observe generalized hazing of the corneal surface and isolated areas of epithelial loss. Anterior chamber reaction is usually limited.

More serious cases may present with a relatively white eye (the result of vascular thrombosis), severe corneal edema, loss of most or all corneal epithelium, and partial or total obstruction of iris details.

ALKALIS & ACIDS

Alkalis (pH > 7) are far more damaging to ocular tissue than an acid of equal strength. Unless the pH of an acid is less than 2.5, a natural buffering system afforded by precipitation of proteins often limits the damage. Even moderately alkaline substances are damaging because they saponify fats and destroy mucoproteins and collagen, removing the natural barrier to chemical penetration. The extent of tissue damage depends on pH, concentration and contact time of the substance. (Severe cases often lead to blindness and eventually require penetrating keratoplasty.)

Commonly encountered alkalis include ammonium hydroxide (ammonia), calcium hydroxide (lime) and sodium hydroxide (lye). Ammonia is fat soluble and is generally considered the most damaging to corneal tissue. Sulfuric acid, found in car and marine batteries, is the most common cause of acid burns.

CORNEAL INJURY FROM ANHYDROUS AMMONIA.

PLAN OF ACTION

If a patient calls seeking help for a chemical burn, instruct him to irrigate with water immediately, and to bring the chemical container to your office for identification. Evaluate him immediately and irrigate again until the pH of the tears is between 7.0 and 7.3. Be sure to keep litmus paper on hand to monitor the pH.

A contact lens can act as a barrier that protects an eye from an offending substance. But once the lens absorbs the chemical, it may increase the contact time and concentration of the substance, increasing the deleterious effects. After irrigation, remove and discard the contact lenses. The patient should resume wear only after the cornea clears and the immune response subsides.

Even if you know what the compound is, don't try to neutralize it with another compound of opposite pH. The resultant exothermic reaction could cause far more damage than the burn itself. Evert the upper and lower lids and remove any solid foreign matter with sterile forceps.

Solvents such as alcohol, gasoline and acetone degrade proteins, leading to epithelial desiccation and keratitis. Detergents and surfactants emulsify lipids in the cell membrane and may lead to surface injury. After solvents and detergents have been irrigated out, the greatest risk is for secondary bacterial infection. Topical antibiotics and cycloplegics are the mainstay in treatment. Due to the toxicity of aminoglycosides, we prophylactically treat the eye with a fluoroquinolone such as Ocuflox.

Debride severely damaged epithelium with a sterile platinum spatula. Patients often require pressure patching, and bandage contact lenses may help promote comfort and re-epithelialization. Increased IOP is common and may be treated with a beta blocker or topical anti-hypertensive such as Alphagan.

If you must prescribe topical steroids due to a significant anterior chamber reaction, restrict their use to the first few days of treatment. Steroids inhibit collagen synthesis and enhance collagenase, which is produced by damaged corneal epithelium and is linked to the development of late ulcers in alkali- burned corneas.

Following any chemical burn, the immune system may also contribute to late tissue destruction by releasing collagenase. Ascorbic acid taken orally may promote healing in alkali-burned corneas. Topical EDTA and acetylcysteine (Mucomyst) are also effective. CLS

Dr. Townsend is in private practice in Canyon, Texas, and is a consultant at the Amarillo VA Medical Center.