Both multifocal motor neuropathy (MMN) and amyotrophic lateral sclerosis (ALS) affect the peripheral nervous system and present with palsies. Differentiating between these two diseases is important from a prognostic perspective, as MMN should be treated as early as possible with immunomodulatory therapy, while ALS is not currently treatable. Magnetic resonance (MR) neurography has been proposed as a way of differentiating between MMN and ALS. In this cohort study, investigators used MR neurography in 22 participants with ALS, 8 participants with MMN, and 15 matched healthy control participants. The images produced were then given to two blinded readers to examine whether any features of MR neurography could be used to differentiate ALS from MMN and controls. MR neurography images were acquired at mid-thigh, proximal calf, and mid-upper arm of the more clinically affected side. The readers independently rated fascicular nerve lesions and signs of possible denervation using 5-point scales. Investigators found that the mean nerve lesion score was higher in MMN than in ALS (reader 1, score of 2.6 vs. 0.9, p=0.04; reader 2, 3.5 vs. 1.5, p=0.008). Muscle edema and atrophy were found in nearly all participants with ALS and the mean muscle denervation score was significantly higher in ALS than in MMN (reader 1, 1.82 vs. 0.25, p=0.002; reader 2, 2.59 vs. 1.13, p=0.007). Mean muscle denervation score was also higher in ALS than healthy controls (p<0.001 for both readers). There were no differences in signs of muscle denervation between participants with MMN and healthy controls (p=0.99 and p=0.73 for the two readers). The inter-rater agreement between the two readers was kw =0.73 (95% CI 0.59 to 0.87) for nerve lesions and kw=0.61 (95% CI 0.44 to 0.77) for muscle denervation signs. The sensitivity for detection of ALS compared to non-ALS (MMN and control) was 86% (95% CI 67% to 95%), and the specificity was 100% 95% CI 86% to 100%). The sensitivity of detecting MMN versus non-MMN (ALS and control) was 88% (95% CI 53% to 99%), and the specificity was 100% (95% CI 91% to 100%). The results from this study suggest that MR neurography can be used as a way of differentiating MMN with ALS, with signs of nerve denervation playing an important role. This study was limited by its small size and the heterogeneity of treatment and disease duration amongst participants.
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