Quantitative MRI of Muscle Denervation in Subacute Parsonage–Turner Syndrome: A Prospective, Longitudinal Study

Parsonage-Turner syndrome (PTS) is a spontaneous neuropathy characterized by severe upper extremity pain and muscle denervation and is considered to be a rare disease that is under-recognized. Quantitative MRI (qMRI) characterizes muscle denervation but has not been previously assessed in a longitudinal PTS cohort. The aims of this study are to prospectively and longitudinally characterize qMRI changes in PTS patients at baseline (< 6 months' symptom onset) and at follow-up timepoints (3, 6, and 12 months), to measure associations against electromyography (EMG) and muscle strength, and to predict muscle strength at follow-up. A total of 49 subjects (age = 47.2 ± 14.0 years, 31 M/18 F) underwent 3-Tesla qMRI with T2-mapping, diffusion-based muscle fiber diameter, volumetry, and fat fraction (FF) mapping. Image segmentation of involved muscles was performed by two raters. Linear regression between qMRI metrics and days from symptom onset (DSO) was performed. Pearson's correlation quantified correlations between qMRI metrics, and Kendall's tau assessed correlations between qMRI and EMG and muscle strength. For predictive modeling of muscle strength, a generalized linear model was used, and the coefficient of determination (r²) was compared for combinations of baseline inputs. Regression detected a mean T2 increase of 0.66 ms/week and a mean muscle fiber diameter decrease of 0.96 μm/week within DSO of 100. Muscle fiber diameter correlated with muscle volume (r = 0.850). T2 correlated with EMG (|τ| = 0.34-0.78) and muscle strength (|τ| = 0.40-0.83) in most muscles that could be analyzed. Muscle fiber diameter was correlated to EMG (|τ| = 0.43-0.72) and muscle strength in some muscles (|τ| = 0.39-0.56). The addition of baseline T2 values improved the prediction of muscle strength at 3-month (from r² = 0.57 to 0.67, with -0.057 to -0.068 muscle grade per ms T2), at 6-month (r² = 0.40-0.59, -0.057 to -0.071 grade per ms), and at 12-month follow-up (r² = 0.40-0.62, -0.053 to -0.080 grade per ms). Muscle qMRI measurements in PTS depict muscle denervation and provide complementary characterization of muscle quality for diagnosis and follow-up assessment.