Oral and Systemic Medications in Eye Care: Part II
BY WILLIAM TOWNSEND, OD
Eyecare providers have been fortunate that with new topical medications, they are rarely forced to use oral medications. Topical medications have less systemic toxicity, and they reach the target site faster than their systemic counterparts. But there are times when, in treating ocular conditions, oral medications are indicated. This is nowhere more obvious than in the case of antibiotics.
Kill the Bug, Spare the Patient
Antibiotics are able to selectively kill or inhibit bacterial growth because of the differences between animal and bacterial cells. Most notably, beta lactams (penicillins and cephalosporins) take advantage of bacteria having a cell wall while animals do not. In prescribing antibiotics, one should understand the basic means by which various classes of antibiotics work. It is also important to understand the mechanisms through which bacteria develop resistance to antibiotics.
In eye care, oral antibiotics are used mainly for lid infections, such as hordeolum and preseptal cellulitis, systemic infection such as chlamydial infection associated with adult inclusion conjunctivitis (AIC) and infections of the lacrimal drainage system, such as canaliculitis and dacryocystitis. Serious conditions such as orbital cellulitis should be referred to a tertiary specialist.
Penicillins and Cephalosporins
These medications inhibit cell wall synthesis by binding to proteins that crosslink the bacterial cell wall. This eventually causes autolysis of the cell. They are primarily excreted by the kidneys, and some need frequent dosing. Concurrent probenicid inhibits tubular secretion of beta lactams and reduces the number of doses needed per day. Penicillins and cephalosporins show cross allergy; when allergy to one of these drugs is confirmed, prescribe another class of medication. Many people erroneously believe they are allergic to penicillin when they have simply experienced some of the common side effects of the medication, including gastric upset, nausea, vomiting and diarrhea. Patients with a history of maculopapular rash, hives, itching or frank anaphylaxis following use of beta lactams should be treated with another class of antibiotic.
Some bacteria, especially staph species, produce beta lactamases, enzymes that inactive many beta lactams. Amoxicillin with clavulanate (Augmentin) is a commonly-prescribed medication that is effective against beta lactamase-producing bacteria. Clavulanate inhibits the action of beta lactamases. Augmentin is highly effective against Hemophilus influenze and is available in 500 mg (125 mg clavulanate), 250 mg (125 mg clavulanate) and 250 mg chewable tablets (62.5 mg clavulanate) or in a pediatric suspension. Augmentin is an excellent medication for treating skin infections; no generic form is available, so it is fairly expensive. The customary adult dosing is one 250 mg tablet tid or one 500 mg tablet bid for seven to 10 days.
Dicloxacillin is a penicillanase-resistant synthetic penicillin. It is effective against Staph aureus and many of the organisms that cause lid infections. Because it is available in generic form, dicloxacillin is a cost-effective antibiotic for your patients. Usual dosing is 250 or 500 mg qid for seven to 10 days. Dicloxacillin may reduce the effectiveness of birth control pills.
Cephalexin is a first-generation cephalosporin that is moderately effective against beta- lactamase producers, but is bactericidal and has a broad spectrum of activity. It is also a good choice for lid infections, and like dicloxacillin, is available in generic form. Cephalexin is typically prescribed for adults at 250 mg qid or 500 mg bid for seven to 10 days.
Dr. Townsend is in private practice in Canyon, Texas, and is a consultant at the Amarillo VA Medical Center. E-mail him at firstname.lastname@example.org.