Question:

After an unprotected sexual exposure, a 44-year old man came to us with symptoms of anterior dysuria and scanty mucoid urethral discharge. Diagnosed non-gonococcal urethritis with pus cells found on urethral smear and with a negative history of drug allergy we prescribed a course of tetracycline 500mg QID for seven days. On the third day after prescription, he came back with a swollen plaque on his right forearm. On more careful questioning he admitted a history of skin eruption on the same site twenty years before following taking an unknown drug for a similar problem. That rash subsided in two weeks with a residual grayish patch which took 5 years to fade.

Physical examination showed an oedematous erythematous patch on the right forearm with intense itchiness. Tetracycline was stopped and the lesion was treated with KMnO4 compresses and anti-histamine. It subsided in 10 days with grayish pigmentation. We completed the treatment of non-gonococcal urethritis with a single dose of azithromycin 1gm without any side effects.

Answer: B. Fixed drug eruption

The patient suffered from fixed drug eruption from tetracycline. Fixed drug eruption is an adverse cutaneous drug eruption characterised by itchy, sharply marginated, round or oval localised dermatitis in reaction to administration of a drug and tends to recur in the same sites when the drug is administered again. The lesions may be solitary or multiple. At the beginning of a reaction the lesions are erythematous and dusky, sometimes become bullous and then desquamate and evolve into hyperpigmentation which will remain for months or longer before fading. A variant is non-pigmenting fixed drug eruption and is speculated to be a dermal reaction rather than the usual epidermal predominant reaction.

A long list of drugs may cause fixed drug eruption: phenolphthalein, barbiturates, penicillin, tetracycline, erythromycin, trimethoprim, sulfaonamides, salicylates, acetaminophen are a few of them. Pseudoephedrine is one of the causes of the non-pigmented variants. Food additives such as artificial flavours, colours and preservatives are also possible causes.

Diagnosis is usually made with a careful history, taking all the medications and diet into consideration. Confirmation with oral or topical provocation is practised in some centers but most of the time the diagnosis is clinical. Management includes withdrawal of the drug and avoidance of subsequent re-exposure. Lesions will subside with discontinuation of the drug and symptomatic treatments with anti-histamines and cold compresses may help to relieve the itchiness.