D3.372 - Rapid Intramuscular Penicillin Desensitization: A Promising Future

Rapid intravenous penicillin desensitization is a well-established procedure for penicillin-allergic patients; however, it is complex and time-consuming. To date, the mechanism of desensitization has not been fully elucidated, and intramuscular (IM) desensitization has been considered as an alternative approach with potential advantages.Objective:To describe a case of intramuscular penicillin desensitization.Case report:A 31-year-old woman with a history of rheumatic fever had been receiving benzathine penicillin every 21 days for 15 years. Five months prior to consultation, she developed generalized erythematous papules and plaques approximately 30 minutes after penicillin administration. She denied angioedema, dyspnea, or abdominal pain. Physical examination revealed dermographism (2+/4). Specific IgE to penicillin G was negative. An oral provocation test with potassium phenoxymethylpenicillin was performed without immediate reaction, and the patient was discharged after two hours of observation. On her way home, she developed plaques on the abdomen, which resolved spontaneously. Intravenous desensitization was then initiated. During one episode, she developed disseminated pruritic urticaria on the lower limb. At that time, prophylactic antihistamine therapy was started, resulting in complete symptom control. She continued this treatment for eight months, after which rapid IM desensitization was initiated, following guidance from specialists outside Brazil. Fractionated doses of 2,400,000 IU, 360,000 IU, and 600,000 IU were administered at 30-minute intervals, with successful outcomes.The patient maintained this regimen for five months, after which the injection was divided into two doses: 240,000 IU followed by 960,000 IUwithout reactions. She subsequently returned to a single full-dose administration without reactions for seven months. Thereafter, she began to report muscle pain, arthralgia, and cutaneous pruritus after penicillin injections. Laboratory evaluation was unremarkable. Rapid IM desensitization was reintroduced, and the symptoms resolved. Conclusion:Intramuscular desensitization is an effective and safe option and should be considered after appropriate risk stratification, particularly for patients with less severe reactions and low risk of anaphylaxis.