A 68-year-old retired man, formerly employed in the banking sector, had been receiving intramuscular cyanocobalamin 1 mg monthly for vitamin B12 deficiency of unknown aetiology for the last year. His medical history included dyslipidemia, hypertension, type 2 diabetes mellitus, and atrial fibrillation, for which he was being treated with atorvastatin, enalapril, metformin/dapagliflozin, propranolol, and apixaban, respectively. He presented with a one-year history of monthly pruritic eczematous eruptions affecting the trunk, extremities, and genital area, coinciding with cyanocobalamin administration and responsive to corticosteroids. Physical examination revealed multiple symmetrical nummular erythematous-desquamative plaques on the trunk, upper arms, hands, genital region, and lower limbs (Figure 1). The clinical differential diagnosis included generalized eczema versus cutaneous drug eruption. Patch tests were performed using the extended Spanish baseline series. Patch tests were read on day (D)2 and D4, showing a positive reaction to cobalt chloride 1% petrolatum (++/+++) (Figure 1) with probable current relevance due to cyanocobalamin administration. Alternative sources of systemic cobalt exposure (dietary supplements, occupational exposure and metallic implants) were ruled out. Given the suspicion of systemic allergic dermatitis (SAD) due to cyanocobalamin and normal vitamin B12 levels, the treatment was discontinued, with close follow-up by the primary care physician. Complete resolution was achieved, with no flares after 1 year of follow-up. No other drugs were modified, and cyanocobalamin discontinuation was the only intervention before complete remission. Based on these findings, the patient was diagnosed with SAD secondary to intramuscular cyanocobalamin administration. Patch testing with vitamin B12 was considered after improvement, but the patient declined. SAD is a condition that occurs when an individual sensitized to an allergen is re-exposed through a systemic route. The main groups of allergens involved in SAD are metals, plant products, and pharmaceutical drugs 1. Cyanocobalamin contains a central cobalt atom, representing a potential source of systemic exposure in cobalt-sensitized individuals 2. SAD induced by cyanocobalamin has been described only in isolated cases and may elicit various type IV hypersensitivity reactions, as summarized in Table 1 2-6. Most reported cases have been associated with oral supplementation. However, this case is noteworthy for the development of SAD following intramuscular administration. Prevalence of sensitization to cobalt chloride in the general population is high (2.7%) 7, and cyanocobalamin is widely used as a treatment and, particularly, as a dietary supplement. However, few cases of SAD caused by cyanocobalamin in cobalt-sensitized individuals have been reported, suggesting that this condition may be underdiagnosed. Cobalt-sensitized individuals should be warned of potential adverse reactions to vitamin B12. It is advisable to ask about metal sensitization before starting cyanocobalamin therapy. Joan García-Vilar: conceptualization, methodology, writing – original draft, writing – review and editing. Juan Francisco Silvestre-Salvador: conceptualization, methodology, writing – review and editing, writing – original draft. M.ª. Belén Contreras-García: conceptualization, methodology, writing – original draft, writing – review and editing. Written informed consent for publication was obtained from the patient. The authors declare no conflicts of interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
García‐Vilar et al. (Sun,) studied this question.