Seronegative Immune-Mediated Necrotizing Myopathy Presenting as Acute Onset Quadriparesis

Immune-mediated necrotizing myopathy (IMNM) is a rare, rapidly progressive inflammatory myopathy that closely resemble Guillain-Barré syndrome (GBS), making early diagnosis particularly challenging. We report the case of a 32-year-old man who initially presented with a five-day history of fever and diffuse body aches, followed by rapidly progressive-,ascending-,symmetrical limb weakness in the lower limbs with progression to the upper limbs. A neurological examination revealed hypotonia with preserved deep tendon reflexes. Sensory, cranial, and cerebellar functions were intact. Given the acute, flaccid nature of the paralysis, GBS was initially suspected; however, cerebrospinal fluid analysis showed no albuminocytologic dissociation. Significantly elevated serum creatine phosphokinase levels, along with persistent myalgia, shifted the clinical suspicion toward a myopathic process. Magnetic resonance imaging (MRI) revealed diffuse-, symmetric hyperintensities in multiple muscle groups, and muscle biopsy confirmed the diagnosis of necrotizing myopathy. Despite extensive autoimmune workup, including myositis specific antibodies, all results were negative, classifying the condition as seronegative IMNM. The patient responded promptly to high-dose intravenous corticosteroids, followed by oral methotrexate and maintenance oral steroids with recovery at the one-month follow-up. This case highlights the importance of considering IMNM in patients presenting with acute flaccid paralysis and preserved reflexes, as timely diagnosis and early initiation of immunosuppressive therapy are crucial for achieving favorable outcomes.