Evaluation of patients with paramyotonia at 23Na MR imaging during cold-induced weakness

Purpose: To prospectively examine whether sodium 23 (Na-23) magnetic resonance (MR) imaging can be used to visualize acute intracellular Na+ accumulation and the effects of specific therapy in patients with paramyotonia congenita ( PC). Materials and Methods: Ethics committee approval and informed consent were obtained. Sixteen patients ( four women, 12 men; mean age, 46.7 years +/- 16.7 [standard deviation]) with confirmed PC and 10 healthy volunteers ( three women, seven men; mean age, 26.6 years +/- 3) were examined by using a 1.5-T MR system with a 16.8-MHz surface coil. Na-23 MR imaging was performed before and after local cooling of the nondominant lower leg and exercising, with experimentally induced weakness scored by a neurologist. The Na-23 MR examination was repeated in 13 patients and all volunteers after 3 days and, additionally, in seven patients after 4 days of oral administration of mexiletine, which blocks Na+ channels. The Na-23 MR protocol comprised two-dimensional (2D) fast low-angle shot ( FLASH), 2D radial, and free induction decay (FID) sequences. The FID data were fitted to a biexponential decay curve to evaluate the slow and fast components of the T2 relaxation time. Fast and slow components were assigned to intra- and extracellular Na+ concentrations, respectively. Radial and FLASH MR images were evaluated by means of a region-of-interest analysis by using 0.3% saline solution for reference. T1- and T2-weighted MR imaging were also performed. Data were analyzed by using a parametric t test. Results: After exercising, all patients developed considerable weakness exclusively in the cooled lower leg; no weakness was observed in volunteers. In patients, all Na-23 MR images showed a significant increase in Na-23 signal intensity in the cooled lower leg ( P < .001) in comparison with nonsignificant findings in volunteers. After treatment with mexiletine, cooling and exercise induced almost no muscle weakness and no changes in Na-23 MR signal intensity in patients. Conclusion: Na-23 MR imaging enables visualization of muscular Na+ accumulation associated with muscle weakness in patients with PC, and effects of specific therapy can be detected.