Allergic Purpuras

Allergic purpura is a nonthrombocytopenic purpura, an acute or a chronic vascular inflammation that affects the skin, joints, and GI and genitourinary (GU) tracts in association with allergy symptoms. When allergic purpura primarily affects the GI tract with accompanying joint pain, it is called Henoch-Schönlein syndrome or anaphylactoid purpura. However, the term allergic purpura applies to purpura associated with many other conditions such as erythema nodosum. An acute attack of allergic purpura can last for several weeks.

Fully developed allergic purpura is persistent and debilitating. This disorder affects males more commonly than females and is most prevalent in children ages 3 to 7 years. The prognosis is more favorable for children than for adults. The course of Henoch-Schönlein syndrome is usually benign and self-limiting, lasting 1 to 6 weeks if renal involvement is not severe.

Causes

The most common identifiable cause of allergic purpura is probably an autoimmune reaction directed against vascular walls and triggered by a bacterial infection (particularly a streptococcal infection, such as scarlet fever). Typically, upper respiratory tract infection occurs 1 to 3 weeks before the onset of signs and symptoms.

Other possible causes include allergic reactions to some drugs and vaccines; allergic reactions to insect bites; and allergic reactions to some foods (such as wheat, eggs, milk, and chocolate).

Diagnostic tests

No laboratory test clearly identifies allergic purpura (although white blood cell count and erythrocyte sedimentation rate are elevated). Diagnosis necessitates careful clinical observation, often during the or third attack.

These laboratory test results may aid diagnosis allergic purpura:

• guaiac-positive stools
• hematuria identified on urinalysis
• elevated blood urea nitrogen and serum creatinine levels and proteinuria, indicating glomerular involvement
• normal coagulation and platelet function (with the exception of a positive tourniquet test).

Small-bowel X-rays may reveal areas of transient edema. Diagnosis must rule out other forms of non­thrombocytopenic purpura.

Treatment

In allergic purpura, treatment is usually based on symptoms; for example, severe allergic purpura may require corticosteroids to relieve edema and analgesics ro alleviate joint and abdominal pain. Some patients with chronic renal disease may benefit from immunosuppression with azathioprine or corticosteroids, along with identification of the provocative allergen.

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