If it is a 6-month-old baby with purpura on the lower extremities, the first thing is to find out the child’s underlying diseases and blood clotting function, first of all, exclude serious infections and blood diseases and other diseases, mostly local manifestations of acute and critical illnesses, however, the possibility of it being allergic purpura is very small and almost non-existent. You as soon as possible to the child’s family, let them take the child to a convenient regular hospital, ask an experienced pediatrician to identify the cause, do not let the child accident! Allergic purpura, also known as heng-shu syndrome, is mostly found in children aged 3-8 years old, which means it is more common in preschool and school-age children, and is rare in small infants and toddlers under 3 years old, more in boys than girls. The child mentioned by my colleague was only 6 months old, so of course I had reason to doubt his diagnosis! For the sake of the future and the health of our children, let’s review and review this disease again. 1, What is allergic purpura? Allergic purpura, also known as heng-schu syndrome, is an allergic disease that mainly invades capillaries and is the most common type of vasculitis syndrome. The clinical features are non-thrombocytopenic purpura, mainly skin purpura, arthralgias, abdominal pain, blood in stool, hematuria and proteinuria. It occurs mostly in children aged 2 to 8 years old, more in boys than in girls; it occurs all year round, mostly in spring and autumn. 2.How does allergic purpura occur? The cause of allergic purpura is unknown, the basic pathological changes are more widespread acute aseptic capillary and small artery inflammatory reaction, the affected blood vessel wall can be seen as fibrous swelling and necrosis, surrounded by neutrophils, eosinophils, lymphocytes, monocytes infiltration and plasma exudation. Possible predisposing factors are infections (bacterial, viral or parasitic, etc.), drugs (antibiotics, sulfonamides, isoniazid, salicylates, sodium phenobarbital, etc.), food (fish, shrimp, crab, eggs, milk, etc.) and other factors (pollen inhalation, insect bites, vaccinations, etc.). Individuals with genetic background produce abnormal immune response to these factors to stimulate the proliferation of B-cell clones, forming IgA immune complexes, causing widespread capillaritis, and in severe cases, necrotizing small arteritis, increased permeability of the vessel wall leading to hemorrhage and edema of the skin, mucous membranes and internal organs. What are the clinical manifestations of allergic purpura? The first symptoms are mainly skin purpura, mostly found in lower limbs and arms, symmetrical distribution, appearing in batches, more on the extension side and less on the trunk of the face. The purpura varies in size, is purplish red, higher than the skin, may be accompanied by urticaria, erythema multiforme and angioedema, a few severe purpura can fuse into large blisters or even hemorrhagic necrosis. Some cases first appear abdominal pain, arthralgias or kidney symptoms, and often have a history of upper respiratory tract infection 1 to 3 weeks before the onset of the disease. 4.Is there any abnormality in laboratory test of allergic purpura? Blood routine: white blood cell count is normal or mildly increased, neutrophils and eosinophils may be increased, unless severe anemia, platelet count is usually not anemic; normal or even elevated, bleeding and clotting time, clot retreat test are normal. Urine routine: there may be red blood cells, protein, tubular, and in severe cases, carnivorous hematuria. Urine routine: fecal occult blood test may be positive. Special examination: serum IgA concentration is often increased, bone marrow examination is normal. 5.How is allergic purpura diagnosed? Allergic purpura is a clinical syndrome diagnosis based on characteristic skin purpura, mostly seen on lower limbs and arms, symmetrical distribution, appearing in batches, more on the extensor side and less on the trunk of the face. It can be combined with gastrointestinal, joint or renal symptoms and a history of recurrent episodes. However, attention should be paid to exclude idiopathic thrombocytopenic purpura, rheumatoid arthritis, surgical acute abdomen and other diseases, as well as the characteristics of the occurrence of purpura and the age of the child. 6.How to treat allergic purpura? There is no specific treatment for allergic purpura, it is mainly symptomatic support treatment. During the acute attack period, bed rest should be given, allergens should be sought and avoided as much as possible, infection should be controlled and vitamins should be supplemented. Adrenocorticosteroids can improve abdominal pain and joint symptoms, but cannot reduce purpura or decrease the incidence of kidney damage, nor prevent recurrence. Prednisone: 1-2 mg/(kg-d) orally in divided doses, or dexamethasone or methylprednisolone intravenously can be discontinued after the symptoms are relieved for <10 d. Cyclophosphamide can be tried in severe or complicated nephritis where hormone therapy is ineffective. The antispasmodics should be applied in case of abdominal pain, and in case of gastrointestinal bleeding, the diet should be restricted or fasted, cimetidine 20-40 mg/(kg-d) can be given intravenously, and blood transfusion should be given if necessary. Antihistamine or intravenous calcium drip can help desensitization. 7.What is the prognosis of allergic purpura? Mild cases are cured in 7-10 days, while severe cases can last for weeks to months, or recurrent episodes can last more than 1 year. The majority of patients have a good prognosis and can heal on their own. Some cases have a tendency to relapse, and those with renal failure or intracranial hemorrhage have a poor prognosis. Please refer to the doctor's instructions for the specific medication to be used.