The incidence of BD is regional, with a high incidence in Turkey, Iran, Iraq and other places along the ancient Silk Road. China is also a country with a high incidence of BD, but research on BD in China is lacking. Professor Guan began to pay attention to BD and conducted clinical research more than 20 years ago. They summarized its epidemiological characteristics, developed a series of concepts from diagnostic processes and treatment strategies to clinical prognosis, and redefined BD. BD is a form of vasculitis of unknown etiology is characterized by a variety of clinical manifestations and usually affects young adults. Oral ulcers is the most common first symptom, and gradually accompanied by vulvar ulcers, nodular erythema and other skin mucosal diseases, selective ophthalmitis, intestinal ulcers, aortic valve regurgitation, venous thrombosis, aneurysms, arterial stenosis, arthritis and hemocytopenia.

BD was first classified into 8 clinical phenotypes: skin mucosal BD, eye BD, intestinal BD, heart BD, blood vessel BD, nerve BD, blood BD and joint BD [3]. The patients have repeated oral ulcers as the first symptom, followed by pale red nodules on the skin of both lower limbs, accompanied by pain in both the knee joints and ankles, followed by vulvar ulcers. In this case, the left calf skin biopsy showed that the dilation and congestion of blood vessels in the superficial to middle layers of the dermis, leading to red blood cell exudation. Some blood vessel walls become thickened and damaged, lymphocytes and tissue cells also produce an infiltrate around blood vessels. The basic characteristic of BD is vasculitis. So, the diagnosis of BD of this case is accurate.

According to the diagnostic/classification criteria of ICBD for BD [4], a score ≥ 4 indicates BD. The ICBD for this patient was 5, so the clinical diagnosis was BD. During this period, the patient developed severe proteinuria and hematuria, and IgAN was diagnosed after percutaneous renal biopsy. Combined with the above medical history and pathogenesis, the patient was finally diagnosed as the co-occurrence of IgAN with BD. Altay et al. [5] reported 2 patients with BD combined with IgAN. Zheng [6] et al. reported that there are 16 kidney-related diseases among 618 patients with BD, accounting for 2.6%. Two of them had pathological features of IgAN after renal biopsy.

At present, although the pathogenesis of BD and its associated kidney disease is not clear, some studies [7, 8] believe that genetic factors and immune abnormalities play important roles in the occurrence and development of BD. Various immune cells and cytokines, especially autoimmune regulatory T cells (Tregs) and auxiliary T cells type 22 (Th22), play a role in the immune pathogenesis of BD. The dose at which BD causes nephropathy is currently unclear. Some studies have shown that [9] the amount of circulating immune complex produced by abnormal T cells is related to the occurrence and development of micropathogenic glomerulopathy (MCD) in patients with BD. Overexpression causes the disappearance of MCD-related foot cells and proteinuria [10, 11]. Based on the above research, we believe that the abnormal function of T cells and their overexpression by IL-13 may also be related to the occurrence of IgAN in patients with BD, but further research is needed.

The prognosis of the co-occurrence of IgAN with BD is relatively optimistic, while only a few patients experience renal failure. The patient we reported was treated with hormonal anti-inflammatory, immunomodulatory, kidney protection, and protein-lowering urine. After 3 years of follow-up, the patient reappears oral ulcers, reddish nodules on the skin of both lower limbs and renal dysfunction. It has been reported [5, 12] that BD secondary to nephropathy can be relieved in some patients, but some studies have shown that the disease can further progress. Some patients developed from IgAN II to stage IV 2 years later.

Here we present a case of co-occurrence of BD and IgAN. Immunofluorescence staining was negative for IgA in the left calf skin biopsy. But Granular deposits of IgA (2+) was found in mesangial areas of renal biopsy. So, attributing this case to secondary IgAN is inappropriate. Based on this, we believe that it is a co-existence relationship between BD and IgAN. At present, it cannot be ruled out that BD patients may develop primary IgAN, including this case. IgAN is one of the most common types of glomerulonephritis, which can occasionally occur in patients with BD. BD is a chronic, recurrent autoimmune or inflammatory disease based on vasculitis, which can lead to various types of glomerulonephritis. Comparing the immunological characteristics of these two diseases, there may be a relationship between IgAN and BD, but we cannot rule out simple coincidences. After renal biopsy, IgAN was found in this patient, but we cannot explain the sequence and causal relationship between BD and IgAN. Therefore, further research is needed to investigate the pathogenic correlation between BD and IgAN.

Due to the hidden symptoms or intermittent attacks of BD with renal injury, it is easy to misdiagnose, which may be one of the reasons for the long-term neglect of BD syndrome with renal injury. Therefore, for such patients, attention should be given to routine urinary parameters, blood creatinine levels and clearance rates to detect renal lesions as early as possible and prevent the occurrence and development of disease.