Whole-body muscle MRI improves diagnostic certainty in amyotrophic lateral sclerosis.

Introduction: Early diagnosis of amyotrophic lateral sclerosis (ALS) remains challenging due to the absence of a definitive biomarker and the difficulty of demonstrating widespread lower motor neuron (LMN) involvement. Whole-body muscle MRI (WB-MRI) enables comprehensive assessment of muscle involvement and may improve detection of LMN dysfunction. This study aimed to evaluate whether WB-MRI improves diagnostic certainty in ALS when combined with clinical and electromyography (EMG) assessment. Methods: In this prospective single-center study, 47 patients with ALS underwent clinical examination, EMG, and WB-MRI. Diagnostic classification according to the Awaji criteria was assessed using clinical and EMG data alone and after integration of MRI markers of LMN involvement, including fatty infiltration and muscle edema, or muscle edema alone as a surrogate marker. Results: WB-MRI identified additional LMN-involved regions in 27.7% of patients when both fatty infiltration and muscle edema were considered, and in 42.6% when considering muscle edema alone. This resulted in diagnostic upgrading in 14.9% and 25.5% of patients, respectively. The proportion of definite ALS increased from 8.5% to 17.0% when muscle edema alone was considered. MRI had limited impact on diagnostic classification according to the Gold Coast criteria. Among patients without LMN involvement on clinical and EMG assessment (all with bulbar-onset), 50% were reclassified after MRI. Conclusion: WB-MRI improves detection of LMN involvement and increases diagnostic certainty according to the Awaji criteria, with muscle edema appearing to be the most relevant MRI marker for integration into ALS diagnostic assessment.