Differential Diagnosis for Vasculitic Rash in a Patient with Crohn's Disease

Given the patient's history of Crohn's disease and treatment with infliximab, along with the presentation of non-palpable purpura on both lower limbs in a symmetrical distribution, the following differential diagnoses are considered:

• Single Most Likely Diagnosis
  • Henoch-Schönlein Purpura (HSP): Although HSP typically presents with palpable purpura, the condition can sometimes manifest with non-palpable purpura, especially in adults or in those with atypical presentations. The symmetrical distribution and the patient's background of an autoimmune condition (Crohn's disease) treated with immunosuppression (infiximab) could predispose to this condition, though it's more commonly associated with IgA vasculitis and often seen in children.
• Other Likely Diagnoses
  • Leukocytoclastic Vasculitis (LCV): This condition is a form of small vessel vasculitis that can present with non-palpable purpura. It can be associated with various factors, including medications, infections, and autoimmune diseases like Crohn's. The patient's use of infliximab and history of Crohn's disease make this a plausible diagnosis.
  • Infliximab-induced Vasculitis: Although less common, certain medications, including biologics like infliximab, can induce vasculitis. Given the patient's treatment regimen, this is a potential cause of the vasculitic rash.
• Do Not Miss Diagnoses
  • Disseminated Intravascular Coagulation (DIC): This is a serious condition that can present with purpura among other symptoms. It can be triggered by severe infections, malignancies, or other critical conditions. Missing this diagnosis could be fatal.
  • Meningococcemia: This condition, caused by Neisseria meningitidis, can present with a petechial or purpuric rash and is a medical emergency. Although less likely given the description of non-palpable purpura, it's crucial not to miss this diagnosis due to its high mortality rate if untreated.
• Rare Diagnoses
  • Cryoglobulinemic Vasculitis: This condition involves the deposition of cryoglobulins (proteins that precipitate from blood serum at cold temperatures) in blood vessel walls, leading to vasculitis. It's associated with infections (like hepatitis C), autoimmune diseases, and certain types of cancer. The patient's immunosuppressed state and autoimmune background make this a rare but possible consideration.
  • ANCA-associated Vasculitis: Conditions like granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA) are characterized by the presence of ANCA (anti-neutrophil cytoplasmic antibodies) and can present with various symptoms, including vasculitic rashes. These conditions are less common but should be considered in the differential diagnosis due to their potential severity and the patient's underlying autoimmune condition.