Quantitative MRI Differentiates Electromyography Severity Grades of Denervated Muscle in Neuropathy of the Brachial Plexus

Abstract

Background: Quantitative MRI (qMRI) metrics reflect microstructural skeletal muscle changes secondary to denervation and may correspond to conventional electromyography (EMG) assessments of motor unit recruitment (MUR) and denervation.

Hypothesis: Differences in quantitative T₂ , diffusion-based apparent fiber diameter (AFD), and fat fraction (FF) exist between EMG grades, in patients with clinically suspected neuropathy of the brachial plexus.

Study type: Prospective.

Population: A total of 30 subjects (age = 37.5 ± 17.5, 21M/9F) with suspected brachial plexopathy.

Field strength/sequence: 3-Tesla; qMRI using fast spin echo (T₂ -mapping), multi-b-valued diffusion-weighted echo planar imaging (for AFD), and dual-echo Dixon gradient echo (FF-mapping) sequences.

Assessment: qMRI values were compared against EMG grades (MUR and denervation). qMRI values (T₂ , AFD, and FF) were obtained for five regional shoulder muscles. A 4-point scale was used for MUR/denervation severity.

Statistical tests: Linear mixed models and least-squares pairwise comparisons were used to evaluate qMRI differences between EMG grades. Predictive accuracy of EMG grades from qMRI was quantified by 10-fold cross-validated logistic models. A P value < 0.05 was considered statistically significant.

Results: Mean (95% confidence interval) qMRI for "full" MUR were T₂ = 39.40 msec (35.72-43.08 msec), AFD = 78.35 μm (72.52-84.19 μm), and FF = 4.54% (2.11-6.97%). Significant T₂ increases (+8.36 to +14.67 msec) and significant AFD decreases (-11.04 to -21.58 μm) were observed with all abnormal MUR grades as compared to "full" MUR. Significant changes in both T₂ and AFD were observed with increased denervation (+9.59 to +15.04 msec, -16.25 to -18.66 μm). There were significant differences in FF between some MUR grades (-1.45 to +2.96%), but no significant changes were observed with denervation (P = 0.089-0.662). qMRI prediction of abnormal MUR or denervation was strong (mean accuracy = 0.841 and 0.810, respectively) but moderate at predicting individual grades (accuracy = 0.492 and 0.508, respectively).

Data conclusion: Quantitative T₂ and AFD differences were observed between EMG grades in assessing muscle denervation.

Level of evidence: 2 TECHNICAL EFFICACY: Stage 1.

Keywords: muscle denervation; neuropathy; quantitative MRI.