**Background:** Blau syndrome (BS) is a rare autosomal dominant autoinflammatory condition caused by mutations in the NOD2 gene, characterized by granulomatous dermatitis, arthritis, and uveitis. Juvenile dermatomyositis (JDM) is an autoimmune inflammatory myopathy primarily affecting muscles and skin. Both conditions can present with skin rashes, leading to diagnostic confusion. This case report highlights a pediatric patient initially diagnosed with JDM who was later confirmed to have BS through genetic testing.
**Methods:** This is a case report of a 4-year-old Arab male who presented at age 1 with a widespread brownish scaly eczematous rash. Initial workup for suspected JDM included electromyography (EMG), muscle biopsy, and skin biopsy, all of which were normal and showed no granulomas. Based on the rash and absence of alternative diagnoses, the patient was treated with oral prednisolone, with improvement after 2 weeks and complete resolution after 3 weeks. At age 3, the patient developed joint pain and swelling in wrists, PIP joints, MCP joints, and ankles, with morning stiffness and difficulty walking. The rash reappeared. Symptoms initially responded to steroids but persisted upon reintroduction. At age 4, the patient returned with eye twitching and visual problems. Ophthalmologic evaluation revealed total posterior synechiae and a complex anterior polar cataract in the right eye, and minimal posterior synechiae in the left eye. Physical examination showed bilateral camptodactyly, joint effusion, and limited range of motion in PIPs, MCPs, wrists, knees, and ankles. Laboratory findings included elevated CRP (27.6 mg/l, reference 0–6 mg/l) and ESR (40 mm/h, reference 0–20 mm/h). Based on the triad of dermatitis, polyarthritis, and uveitis, BS/early-onset sarcoidosis (EOS) was suspected. Genetic testing for NOD2 mutations revealed a heterozygous p.Arg334Trp (R334W) mutation in exon 4, confirming BS/EOS. The mutation was absent in both parents. Treatment was initiated with subcutaneous adalimumab 20 mg every 2 weeks, subcutaneous methotrexate 10 mg once weekly, oral folic acid 5 mg once weekly, tapering steroids, and vitamin D supplementation.
**Key Results:** The patient showed improvement in skin rashes and joint swelling at follow-up. Ophthalmologic review revealed no deterioration in eye manifestations at 3 weeks, with significant improvement. Skin rash improved noticeably after 4 weeks, and joint manifestations improved at 6 weeks. This case represents the second reported case of BS in the region.
**Clinical Implications:** This case underscores the diagnostic challenges of BS, which can be misdiagnosed as JDM, Kawasaki disease, or juvenile idiopathic arthritis due to overlapping clinical features. The absence of granulomas on biopsy does not rule out BS. Genetic testing for NOD2 mutations is essential for definitive diagnosis. A multidisciplinary approach involving rheumatologists, ophthalmologists, and dermatologists is crucial for timely recognition and management. Early diagnosis and appropriate treatment (e.g., anti-TNF therapy) can prevent morbidity from arthritis and uveitis. Clinicians should maintain a high index of suspicion for BS in children presenting with dermatitis, arthritis, and uveitis, even when initial histology is negative.