Clinically AOSD patients are relatively rare, with an incidence between approximately 1 and 34 per 1 million population, equal incidence in both genders, and a "bimodal" age of onset, 15-25 and 36-46 years, respectively . AOSD often has four major clinical features: transient rash in the proximal limbs or trunk at the peak of fever, high fever of 39°C or more, elevated peripheral white blood cell count and neutrophil proportion greater than 80%, generalized polyarticular pain or arthritis. Other clinical manifestations include: pharyngeal pain, myalgia, myositis, lymphadenopathy, splenomegaly, pericarditis, myocarditis, pleuritis, lung disease, hepatitis, increased erythrocyte sedimentation rate and CRP levels, increased ferritin, decreased glycosylated ferritin and coagulopathy . According to the course of the disease, AOSD patients can be divided into three different clinical patterns: a monocyclic pattern, a multicyclic pattern, and a chronic pattern . AOSD can be easily misdiagnosed clinically as infectious lesions or other diseases, because clinical symptoms and laboratory tests are not specific, such as fever, arthralgia, elevated white blood cells. Clinicians are not aware of the possibility of AOSD when conservative treatment fails. After futile antibiotic treatment like the case presented here, AOSD was finally diagnosed by pathological examination of biopsied lymph nodes. Studies have shown that the general time from the appearance of symptoms or signs to the final diagnosis of AOSD ranges from 1.5 years to 4 years .
Adult onset Still's disease (AOSD) usually presents with high fever, arthralgia, rash, and is often accompanied by multiple lymphadenopathy and hepatosplenomegaly .When malignant lymphoma is easily suspected clinically after conservative medical treatment is ineffective, pathological biopsy of lymph nodes is the inevitable choice, combined with pathomorphological features, immunohistochemistry and molecular biological examination to confirm the diagnosis.Y K Jeon et al  summarized the pathohistomorphological changes in 12 AOSD enlarged lymph nodes and classified the lymphadenopathy into four morphological types. The first atypical paracortical hyperplasia pattern ,characterized by hyperplasia in the paracortical areas of the lymph nodes with abundant high endothelial vessels, was composed mainly of reactive proliferating T lymphocytes, scattered large activated B / T immunoblasts and few plasma cells and eosinophils, with a ratio of CD4 to CD8 positive T lymphocytes of approximately 3:2,And mildly hyperplastic histiocytes and focally hyperplastic monocytoid B cells were seen. And the second burnt out histiocytic pattern, characterized by hyperplasia of the paracortical areas, high endothelial vascularity, and sinus histiocyte proliferation with no remnants of lymphoid follicles. Histiocytes expressed CD68 and S-100 and often clustered in a mottled pattern in the cortical areas. And the third exuberant immunoblastic reaction pattern, characterized by patchy or diffuse proliferation of numerous immunoblasts in the paracortical area, predominantly t immunoblasts with numerous mitotic figures and a Ki-67 proliferation index of up to 90%, which is most easily confused with malignant lymphoma. The fourth follicular hyperplasia pattern, is characterized by numerous lymphoid follicles of various sizes distributed throughout the lymph nodes, some with enlarged germinal centers, some with atrophy of germinal centers, and vascular hyalinization with widening of mantle or marginal zones. The histomorphological features of AOSD lymphadenopathy are complex and diverse, and change dynamically with the course of the disease.
Hyoun ah Kim et al  performed histological observation of lymphadenopathy in 48 AOSD patients and summarized 6 morphological patterns of lymph nodes. These include: follicular pattern, dominated by extensive hyperplasia of lymphoid follicles; Paracortical areas pattern, with proliferation and expansion of the paracortical areas and only a few small remnants of lymphoid follicles; Diffuse pattern, diffuse hyperplasia of the paracortical areas, no lymphoid follicular structures seen; Necrotic pattern, proliferative expansion of the paracortical areas, focal scattered necrosis and nuclear fragmentation; Mixed patterns of lymphoid follicles and paracortical areas; And a mixed pattern of diffuse and paracortical areas. It is also mentioned in the text that in almost all morphologic patterns, moderate to severe hyperplasia of histiocytes is seen, and there are more CD8 positive T cells than CD4 positive T cells. The morphologic features of lymph nodes in our case should belong to the mixed pattern of lymphoid follicles and paracortical areas, which were expanded with proliferative and atrophic lymphoid follicular structures, and a slight predominance of CD8 positive T lymphocytes.
AOSD patients often present with multiple enlarged lymph nodes, and when the diagnosis is still difficult to be established by a combination of clinical manifestations, laboratory tests, and imaging studies, surgical resection with pathological biopsy of lymph nodes is the inevitable choice for the final definite diagnosis. Previous articles have shown that most lymph node lesions histomorphometrically exhibit reactive hyperplasia in the paracortical region, characterized by the proliferation of immunoblasts and high endothelial venules [9-10]. The histomorphological features of AOSD lymphadenopathy are complex and diverse, and change dynamically with the course of the disease, which still needs to be differentiated from the following diseases.(1) Angioimmunoblastic T-cell lymphoma(AITL): AITL is a T-cell lymphoma formed by the proliferation of mature follicular helper T cells with prominent hyperplasia of high endothelial venules and follicular dendritic cells, often showing generalized lymphadenopathy, hepatosplenomegaly, systemic symptoms and polyclonal hypergammaglobulinemia, and often a rash with pruritus. There are many similarities between AITL and AOSD in clinical presentation and histomorphologic features, with the former neoplastic cells often expressing CXCL13, PD1, CD10, BCL6, and ICOS, most cases often showing EBV positive B cells, and an irregular proliferation of CD21 positive follicular dendritic cells surrounding high endothelial venules.（2）Dermatopathic lymphadenopathy(DL)：DL is a special type of proliferative lesion of the paracortical region of lymph nodes that usually presents as lymphadenopathy in the drainage area with chronic skin irritation. Histomorphology often shows a pale nodular appearance in the paracortical region, which is mainly composed of proliferating interdigitated dendritic cells, Langerhans cells, and pigment laden histiocytes. Some studies have shown that in dermatopathic lymphadenopathy lesions , the paracortical areas of lymph nodes contain at least three subsets of dendritic cells with different immunophenotypes: interdigitated dendritic cells (S100 positive, CD1a sparsely positive, langerin negative), Langerhans cells (S100 positive, CD1a positive, langerin positive) and few dendritic cells (S100 positive, CD1a negative, langerin negative).（3）Infectious mononucleosis(IM):IM is an EBV infection induced proliferative lesion of lymph nodes and tonsils, commonly seen in adolescents and young adults, has a short disease course, and histologic features vary with disease duration. Lymphofollicular hyperplasia predominates early in the disease, with monocytoid B-cell and histiocytic hyperplasia. The later stages of the disease show proliferative expansion in the paracortical areas, composed of proliferating immunoblasts, small to medium-sized lymphocytes, and plasma cells with a mottled appearance, dominated by CD8 positive T cells, and the immunoblasts often show EBER positivity.(4) Histiocytic necrotizing lymphadenitis (Kikuchi's disease): also known as Kikuchi Fujimoto lymphadenitis, usually has a self limited, predilection for young adults, especially young Asian women. It is classified into three different subtypes: proliferative, necrotic, and xanthomatous. The early stage was dominated by the proliferation of immunoblasts, crescentic histiocytes, and plasmacytoid dendritic cells in the paracortical region. The necrotic phase showed patchy necrosis without neutrophil infiltration in the paracortical area, with a large number of nuclear debris. The xanthoma stage contains a large number of foamy histiocytes and few immunoblasts. When AOSD lymphadenopathy appears in a necrotic pattern, it needs to be differentiated from Kikuchi's disease, and the simultaneous appearance of both lesions has also been documented .
Depending on the course of the disease, AOSD patients have been clinically classified into three different clinical patterns (monocyclic, multicyclic, and slowly progressive) [13-14]. The chronic progression pattern is most commonly characterized by the occurrence of at least one persistent symptom lasting more than one year, mainly characterized by stable disease progression, persistent inflammation and often erosion of the affected joint. Followed by a multicyclic pattern, manifesting as periodic recurrences with unpredictable deterioration months or years later. A monocyclic pattern, manifesting as a single episode over 2 months but less than 1 year, persisted in remission with no recurrence throughout follow-up. A new approach has divided AOSD patients into two phenotypes: those with systemic features and those with chronic arthritis as the predominant feature .
AOSD often presents as a chronic passage, and patients may develop different complications within the course of the disease, which affect their clinical condition, treatment, and prognosis. Secondary hemophagocytic lymphohistiocytosis (HLH), aka macrophage activation syndrome (MAS), is the most severe complication and is associated with high mortality. Common complications are coagulopathy with multiorgan involvement including heart, lung, liver, spleen and other sites [15-16], and these patients often require more intensive treatment and have a worse prognosis. It has been shown that more than 20% AOSD patients experience recurrence and that patients with severe disease at the initial stage of the disease may be at an increased risk of recurrence, which requires intensive treatment and close follow-up .