Abstract: 【Abstract】 Objective To investigate clinical and histopathological features of adult erythema nodosum （EN）. Methods Clinical data were collected from 54 adult inpatients with histopathologically confirmed EN in Department of Dermatology and Venereology, the First Affiliated Hospital of Wannan Medical College from November 2019 to July 2022, and analyzed retrospectively. Results Among the 54 EN patients, there were 6 males and 48 females, their ages were 42.50 ± 11.68 years （range, 18 - 73 years）, and their disease course ranged from 1 day to 10 years; 30 patients （55.56%） were diagnosed with idiopathic EN, and 24 （44.44%） with secondary EN. The most common etiological factor in secondary EN was infection （17 cases）, including respiratory tract infection （9 cases）, tuberculosis infection （6 cases）, upper respiratory tract infection comorbid with active hepatitis B virus infection（2 cases）; the following common etiological factor was connective tissue disease （7 cases）, including Behcet′s syndrome （4 cases）, Sj?gren′s syndrome （1 case）, and undifferentiated connective tissue diseases （2 cases）. The patients′ ages were significantly younger in the secondary EN group （38.33 ± 12.15 years） than in the idiopathic EN group （46.17 ± 10.20 years, t = 2.58, P = 0.013）. All patients had skin lesions on their lower limbs, lesions were limited to both lower limbs in 24 patients with idiopathic EN and 12 with secondary EN, and the proportion of patients with lesions limited to both lower limbs was significantly lower in the secondary EN group than in the idiopathic EN group （χ2 = 5.44，P = 0.020）. Compared with the idiopathic EN group, the secondary EN group showed significantly increased white blood cell counts （［7.56 ± 2.46］ × 109/L vs. ［6.04 ± 1.60］ × 109/L, t = 2.62, P < 0.05） and C-reaction protein levels （34.34 ± 46.48 mg/L vs. 11.45 ± 18.13 mg/L, t = 2.28, P < 0.05）. In the idiopathic EN group, 23 patients mainly showed histopathological features of septal panniculitis, while 17 patients in the secondary EN group mainly showed histopathological features of mixed panniculitis or lobular panniculitis, and the proportion of patients with histopathological features of mixed panniculitis or lobular panniculitis was significantly higher in the secondary EN group than in the idiopathic EN group （χ2 = 12.18, P < 0.001）. Conclusion EN was more common in female adults; idiopathic EN was the most common type, and secondary EN may be a cutaneous sign of systemic diseases; for EN patients at a relatively young age, with lesions involving both lower limbs or more sites, higher white blood cell counts and C-reaction protein levels, and histopathological manifestations of lobular panniculitis, systemic examinations were required to rule out underlying causes.

Key words: Erythema nodosum, Etiology, Infection, Connective tissue diseases, Histopathological characteristics