To describe the clinical, histopathologic, electrophysiologic, imaging and serological features of the patients with biopsy- confirmed inflammatory myositis and normal CK levels.

Creatinine kinase (CK) is a biomarker for muscle injury in inflammatory myositis. Patients can show clinical and histopathologic evidence of active disease with normal levels of CK. This subgroup remains poorly characterized and may be underdiagnosed if diagnosis relies only on CK.

We conducted a retrospective review of adult patients diagnosed with inflammatory myositis from January 2024 to August 2025. The inclusion criteria were 1) muscle biopsy confirming inflammatory myositis and 2) normal serum CK at presentation. Data were collected on demographics, clinical features, MRI, EMG, laboratory values, antibodies tested, treatment and outcomes with emphasis on non-inclusion body myositis(non-IBM) cases.

Fifteen patients met the inclusion criteria (mean age 63.4 years; 67% female). Diagnoses included IBM (n=6), dermatomyositis (n=5), seronegative necrotizing myopathy (n=2), and myositis secondary to graft-versus-host disease (n=1) or adalimumab (n=1). Mean CK was 95 U/L (range 34–212). IBM patients had classic clinical and histopathologic features; four were NT5c1A-positive. Among the nine non-IBM patients, proximal lower limb weakness was most common (67%), followed by upper limb weakness (44%). Aldolase, ALT, and AST were elevated in 67%, 56%, and 44% respectively. Antibodies included anti-SSA 54 kD (n=3) and anti-NXP-2 (n=1). MRI showed myositis in 4 of 7 patients. Biopsies revealed perimysial inflammation (89%), necrotic fibers (67%), and endomysial inflammation (44%), mostly mild and focal. Immunohistochemistry showed universal CD68 positivity; CD3, CD8, and CD20 were positive in five cases. MHC I was expressed in all tested samples (n=6). Most patients received steroids (89%) and/or IVIG (55%), with 67% improving.