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

To prospectively examine whether sodium 23 (23Na) 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).

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. 23Na 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 23Na 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 23Na 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.

After exercising, all patients developed considerable weakness exclusively in the cooled lower leg; no weakness was observed in volunteers. In patients, all 23Na MR images showed a significant increase in 23Na 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 23Na MR signal intensity in patients.

23Na MR imaging enables visualization of muscular Na+ accumulation associated with muscle weakness in patients with PC, and effects of specific therapy can be detected.