Median-ulnar Ratios in Differentiating Multifocal Motor Neuropathy From Amyotrophic Lateral Sclerosis
Jennifer Morganroth1, Sujata Thawani2, Holly Elser3, Kenneth Gorson4, Thomas Brannagan5
1Neurology, MGH, 2NYU Neurology Associates, 3Hospital of the University of Pennsylvania, 4Tufts University School of Medicine, 5Columbia University
Objective:
To determine whether median-to-ulnar compound muscle action potential (M/U) ratios can help differentiate between multifocal motor neuropathy (MMN) and amyotrophic lateral sclerosis (ALS).
Background:
Differentiating MMN from ALS is challenging, particularly in lower motor neuron (LMN)-predominant ALS, due to overlapping clinical and electrodiagnostic features. Prior studies comparing compound muscle action potential (CMAP) normalized ratios between the median and ulnar (M/U) motor nerves have suggested that the ratios are higher in MMN than in ALS, but it is unclear whether this holds across ALS subtypes.
Design/Methods:
We retrospectively analyzed 59 patients referred for EMG with suspected motor neuron disease. M/U ratios were calculated from median and ulnar CMAP amplitudes and compared across groups using t-tests, linear regression (with and without adjustment for age and sex), and density plots. ALS cases were stratified into LMN predominant and upper and lower limb onset.
Results:
There were 16 patients with MMN, 30 with ALS, and 13 with LMN-ALS. Median M/U ratios were higher in MMN (1.85 [IQR 1.15–2.85]) compared with ALS (0.74 [0.42–1.07], p = 0.03). Arm- and leg-onset ALS had significantly lower ratios than MMN (p = 0.03 and p = 0.04). whereas the differences between LMN-ALS and MMN ratios were not significant. In regression models, both ALS and LMN-ALS were associated with lower M/U ratios vs. MMN. Density plots revealed overlap, but elevated ratios (>2) occurred almost exclusively in MMN.
Conclusions:
M/U ratios are higher in MMN than ALS and LMN-ALS and may be used to distinguish these conditions in ambiguous cases. In patients with focal LMN presentations, elevated ratios > 2 may identify those with MMN.
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