Epinephrine auto-injectors are the standard first aid treatment for anaphylaxis. Intramuscular delivery into the anterolateral aspect of the thigh is recommended for optimal onset of action of epinephrine. The most frequently prescribed auto-injector in North America and Canada is the EpiPen®, which has a needle length of 15.2 mm. Currently, it is unknown whether this needle length is adequate for intramuscular delivery of epinephrine in adult patients at risk of anaphylaxis.
One hundred consecutive adult patients with confirmed food allergy requiring an epinephrine auto-injector were recruited. Skin to muscle depth (STMD) at the right mid-anterolateral thigh was measured using ultrasound under minimal (min) and maximum (max) pressure. The EpiPen® needle length was considered adequate if STMDmax was ≤15.2 mm. Baseline characteristics including age, gender, ethnicity, and body mass index (BMI) were compared in patients with STMDmax ≤15.2 mm vs. >15.2 mm.
The EpiPen® needle length of 15.2 mm was inadequate for intramuscular delivery in 19 of the 100 enrolled patients (19%), all of whom were female; 28% of women had a STMDmax >15.2 mm. The mean STMDmax in the ≤15.2-mm and >15.2-mm groups were 9 ± 4 mm and 20 ± 4 mm, respectively (p = 0.0001). Linear regression analysis found BMI to be significantly associated with STMDmax after adjusting for age (p < 0.001).
The needle length of the epinephrine auto-injectors may not be adequate for intramuscular delivery of epinephrine in a large proportion of women with food allergy; this may impact morbidity and mortality from anaphylaxis in this patient population.