Different Tendencies in Muscle Ultrasound Characteristic in Amyotrophic Lateral Sclerosis and Chronic Inflammatory Demyelinating Polyradiculoneuropathy

Introduction: The difference in muscle ultrasound (MUS) characteristics in primary axonal degeneration and demyelination has not been well established. The authors aimed to investigate the subject based on the correlation between MUS findings (echo intensity and muscle thickness) and compound muscle action potential (CMAP) amplitude in amyotrophic lateral sclerosis (ALS) and chronic inflammatory demyelinating polyradiculoneuropathy. Methods: Fifteen patients with ALS and 16 patients with chronic inflammatory demyelinating polyradiculoneuropathy were examined. For each patient, echo intensity and muscle thickness of the abductor pollicis brevis, abductor digiti minimi, and first dorsal interosseous muscles were investigated. Compound muscle action potential amplitudes were measured by median and ulnar nerve conduction studies. Results: In total, 45 muscles were evaluated in each group. The ALS group showed a linear correlation between the MUS finding and CMAP amplitude (rs = −0.70 and 0.59 for echo intensity and muscle thickness, respectively), whereas the chronic inflammatory demyelinating polyradiculoneuropathy group showed a weaker correlation than the ALS group (rs = −0.32 for echo intensity and rs = 0.34 for muscle thickness). Conclusions: The relationship between MUS abnormalities and CMAP amplitude showed different tendencies in ALS and chronic inflammatory demyelinating polyradiculoneuropathy. The results suggested that MUS abnormalities substantially reflect the muscle function in primary axonal degeneration, whereas a discrepancy between MUS findings and muscle function can be frequently seen in demyelination; specifically, MUS findings tend to be normal even though CMAP showed a reduction. These tendencies originating from underlying pathophysiology should be considered when MUS findings are used as biomarkers of disease severity.