Electrodiagnostic Studies as a Diagnostic and Prognostic Tool in Acute Flaccid Myelitis

INTRODUCTION/AIMS: Acute flaccid myelitis (AFM) is a poliomyelitis-like syndrome, characterized by severe flaccid weakness. Nerve conduction studies (NCS) and electromyography (EMG) may have a role in the diagnosis and prognostication of AFM, but have not been well studied. We aim to describe in detail electrodiagnostic studies in AFM and relate findings to outcome. METHODS: In this retrospective multi-center case series, we analyze electrodiagnostic data from patients with a probable or definite diagnosis of AFM. Patients were enrolled at 11 sites from four countries. A detailed description of NCS and EMG results is provided and findings are correlated to muscle strength and functional outcome. RESULTS: There were 85 patients included with definite or probable AFM based on AFM Working Group criteria. Patients were 54.1% male, median age 5.15 years (IQR 3.15-8.37), and median follow-up 833 days (range 40-3228). Electrodiagnostic abnormalities included decreased compound muscle action potential (CMAP) amplitude, reduced recruitment of motor unit action potentials (MUAPs), and fibrillation potentials in both affected and unaffected muscles, appearing as early as 6 days postonset and persisting beyond 7 years. CMAP amplitude and MUAP recruitment predicted muscle strength and functional status at last follow-up. Phrenic nerve studies correlated with respiratory outcomes in the small subset tested. Ten patients showed marked clinical improvement, accompanied by unexpectedly transient electrodiagnostic abnormalities. DISCUSSION: Electrodiagnostic studies in AFM reveal a distinctive pattern resembling poliomyelitis, with early-onset and persistent findings of denervation even in clinically normal muscles. The findings inform their use in diagnosis and prognostication.