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Evaluation of Patients with Paramyotonia at ²³Na MR Imaging during Cold-induced Weakness¹

¹ From the Department of Radiology (M.A.W., H.U.K., M.E.) and Department of Medical Physics in Radiology (S.N., L.R.S.), German Cancer Research Center, Im Neuenheimer Feld 280, D-69120 Heidelberg, Germany; Department of Neurology, University of Heidelberg, Heidelberg, Germany (H.B.H., H.M.M.); Department of Neurology, University Hospital Mannheim, University of Heidelberg, Mannheim, Germany (J.C.W.); and Department of Applied Physiology, University of Ulm, Ulm, Germany (K.J., F.L.). From the 2004 RSNA Annual Meeting. Received May 1, 2005; revision requested June 30; revision received July 24; final version accepted August 25. Supported by the Medical School Research Council of the University of Heidelberg (196/2002) and by the German Research Foundation (DFG, JU470/1). Address correspondence to M.A.W. (e-mail: m.a.weber{at}dkfz.de).

Purpose: 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).

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. 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.

Results: 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.

Conclusion: 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.

© RSNA, 2006