A 31-year-old man with insidious onset of limited mobility of joints visited our clinic for evaluation. Six months prior, he found his palms could not make full contact with each other (the “prayer sign”). He had not been able to fully clench his fists for 2 months or squat for the past 10 days. No erythema, edema, or sclerotic skin plaque had been found on his limbs or trunk. No Raynaud phenomenon had occurred. His medical and family histories were unremarkable. On examination, he showed bilateral limited mobility of interphalangeal (hands), metacarpophalangeal, wrist, interphalangeal (feet), metatarsophalangeal, and ankle joints (Video). No muscular atrophy or sclerodactyly was found. The prayer sign (Figure 1A) and the “groove sign” (Figure 1B) were observed. Blood test results indicated eosinophilia (absolute eosinophil count, 1300 μL; reference, <500μL [to convert to ×109/L, multiply by 0.001]), elevated erythrocyte sedimentation rate (ESR, 31 mm/h; reference, <15 mm/h) and C-reactive protein (CRP, 3.01 mg/dL; reference, <0.8 mg/dL [to convert to mg/L, multiply by 10]). Liver and kidney function tests, creatine kinase level, electrolyte level, thyroid function, globulin concentration, and antinuclear antibody (ANA) level were unremarkable. Electrocardiogram, cervical spine magnetic resonance imaging, and plain radiographs of hands, wrists, feet, and ankles were normal. Nerve conduction studies performed in all 4 limbs were normal. Electromyography (EMG) showed spontaneous activities in muscles of limbs, including fibrillation potentials and positive sharp waves (1+ or 2+). Myotonic discharges were seen in bilateral extensor digitorum.