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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).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 
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

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 
Paramyotonia congenita (PC) is an inherited muscular Na⁺ channelopathy. In PC, exposure to cold in combination with muscle exercise leads to muscle stiffness and myotonia followed by muscle weakness due to intracellular Na⁺ accumulation. These symptoms will disappear spontaneously within a few hours if the patient is no longer exposed to the stimulus (1). PC is caused by autosomal-dominant point mutations, such as T1313M and R1448C, in the voltage-gated muscle membrane Na⁺ channel encoded by the SCN4A gene (1). Muscle stiffness and weakness are caused by a long-lasting depolarization of muscle fiber membranes. The underlying pathogenetic mechanism is a gating defect of the Na⁺ channel, thereby destabilizing the inactivated state—that is, the channel inactivation may be slow or incomplete, causing a long-lasting depolarizing Na⁺ inward current (1). The resulting elevated intracellular Na⁺ levels lead to membrane depolarization and weakness (1). Muscle stiffness and weakness can be effectively prevented by local anesthetics and class IB antiarrhythmics (1). The agent of choice is mexiletine, which selectively blocks the pathologically inactivating Na⁺ channels (2). Treatment is necessary only in a minority of patients to prevent cold- and exercise-induced myotonia and weakness in cold surroundings. Because the elevated myoplasmic Na⁺ level can be controlled in vivo—that is, triggered by cooling and exercise and blocked by mexiletine—PC is the ideal disorder for which to establish a clinically feasible sodium 23 (²³Na) magnetic resonance (MR) imaging protocol.

²³Na MR imaging is an MR technique that allows for noninvasive measurement of the Na⁺ concentration within tissues. Elevated signal intensity on ²³Na MR images has been shown to correspond to high tissue Na⁺ concentration due to intracellular Na⁺ accumulation and loss of myocardial viability (3). The method is able to discriminate between viable and nonviable myocardial tissue (4,5). The ²³Na signal in vivo decays biexponentially, with a fast (0.5–3.0 msec) and a slow (15–30 msec) component. To measure the total ²³Na signal, sequences with short echo times are necessary. Authors of previous studies have used ²³Na MR imaging to quantify the Na⁺ content in skeletal muscles of patients with progressive hereditary degenerative diseases, such as myotonic dystrophy (6,7). However, in vivo data on skeletal muscle Na⁺ concentration in muscular channelopathies are lacking. Na⁺ channelopathies are especially interesting because they could serve as a paradigm to evaluate different ²³Na MR imaging techniques. As a clinical model, we chose PC because its Na⁺ channels conduct a higher amount of Na⁺ than do physiologic channels, and this results in a myoplasmic Na⁺ accumulation associated with muscle stiffness and weakness.

Thus, the aim of our study was to prospectively examine whether ²³Na MR imaging can be used to visualize acute intracellular Na⁺ accumulation and the effects of specific therapy in patients with PC.

	MATERIALS AND METHODS

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Patients and Volunteers
Sixteen patients (four women, 12 men; mean age, 46.7 years ± 16.7 [standard deviation]) with clinically proved PC were included in the study from November 2003 to January 2005. All patients had typical symptoms of PC, with stiffening and weakness of muscles after exercise and exposure to cold. No patient had any sensory impairment at the time of examination. To confirm mutations typical of PC, whole blood (20 mL from each patient) anticoagulated in ethylenediaminetetraacetic acid was taken for SCN4 mutation analysis. Prior to patient enrollment, from July 2003 to November 2003, 10 healthy volunteers (three women, seven men; mean age, 26.6 years ± 3) with no evidence or history of muscular or cardiovascular disease and no family history of PC (all with normal muscle strength and normal hydrogen 1 [¹H] MR imaging findings) were examined for comparison. The study was approved by the ethics committee of Heidelberg University and Ulm University and was conducted according to the Declaration of Helsinki. Informed consent was obtained from all volunteers and patients after the nature of the examination had been fully explained.

Detection of PC Mutation
Mutation screening was performed by two physiologists (K.J. and F.L., with 10 and 20 years of experience, respectively) by using polymerase chain reaction amplification of SCN4A exons 22 and 24 as described previously (8). Polymerase chain reaction products were loaded on 2% agarose gel and stained with ethidium bromide, and the band was cut out under ultraviolet light. Bands were purified and cycle-sequenced with 1 pmol of primer by using the dye terminator kit (Applied Biosystems, Foster City, Calif). Sequencing was performed on 6% denaturating polyacrylamide gels in an ABI 377 HT automated sequencer (Applied Biosystems). All sequences with base exchanges were verified by reverse sequencing of a new polymerase chain reaction product of the same DNA sample. A radiologist (M.A.W.) compared the muscular Na⁺ accumulation after cooling and exercise detected at ²³Na MR imaging with the detected mutations to elucidate possible relationships between Na⁺ channel mutation and Na⁺ influx.

Examination Protocol
In patients and volunteers, ²³Na MR imaging was performed before and after provocation (ie, local cooling of the nondominant left lower leg and exercising). The contralateral lower leg was not cooled and served as the reference leg. Cooling was performed for 10 minutes with ice pads wrapped around the nondominant lower leg while the subject rested on a stretcher. Then, immediately after cooling, subjects had to bend their knees 30 times and stand on their tiptoes 30 times. This standardized exercise procedure induced local muscle weakness of the cooled lower leg in all patients. The exercise procedure was discontinued as soon as the patient was unable to perform the exercise because of severe paresis; this was done in order to avoid a paralysis that might have lasted for several hours. Fourteen of 16 patients and all volunteers were able to perform the whole exercise procedure. Two of the 16 examined patients received mexiletine (Mexitil; Boehringer Ingelheim, Ingelheim, Germany) as permanent therapy, so no MR examination could be performed without Na⁺ channel blockage. These two patients were examined only once.

The muscle strength before and immediately after cooling of the nondominant lower leg and the exercise procedure, as well as muscle strength 45 minutes after the experimentally induced paresis (ie, after the second part of the MR examination), was scored on a six-point scale according to the grading system proposed by the British Medical Research Council (9): score of 0, complete paralysis; 1, minimal contraction; 2, active movement with gravity eliminated; 3, weak contraction against gravity; 4, active movement against gravity and resistance; and 5, normal strength. Examination of the lower limb comprised strength testing of the following: hip flexion, hip extension, hip abduction, hip adduction, knee flexion, knee extension, dorsiflexion, plantar flexion, and eversion of the foot, toe dorsiflexion, and toe plantarflexion. Muscle strength analysis was performed by two neurologists (J.C.W. and H.M.M., with 15 and 30 years of experience in muscular diseases, respectively). The whole experiment, including MR imaging, cooling and exercise, and muscle strength testing, was repeated in 13 patients and in all volunteers after a mean of 3 days. Of the 14 patients who were not undergoing permanent therapy, one was unavailable for repeated testing.

MR Imaging
The study was performed with a 1.5-T clinical MR system (Magnetom Symphony; Siemens Medical Solutions, Erlangen, Germany) that was equipped with hardware for broadband spectroscopy by using a 32 x 39-cm single-resonant (16.84-MHz) ²³Na surface-quadrature coil (Rapid Biomed, Wuerzburg, Germany) placed over the triceps surae muscles for ²³Na measurements and a whole-body coil for ¹H measurements. ²³Na MR imaging was performed before and after local cooling of the nondominant lower leg and exercising.

²³Na MR Imaging Protocol
The ²³Na signal in vivo decays biexponentially, with a fast (0.5–3.0 msec) and slow (15–30 msec) component of the T2 relaxation time. The fast component has been previously related to the intracellular Na⁺ concentration and the slow component to the extracellular Na⁺ concentration (10). In order to measure the total Na⁺ signal, it was proposed that sequences with short echo times of less than a millisecond are needed (7). In this case, a weighted average of intracellular and extracellular Na⁺ concentration was observed. However, as long as the tissue is adequately perfused, the extracellular Na⁺ concentration will remain constant, so changes in ²³Na MR signal intensity will directly relate to changes in the intracellular concentration of Na⁺ (11). Therefore, a two-dimensional (2D) radial gradient-echo ²³Na MR sequence was implemented, which images k-space in a starlike fashion immediately after section selection; the readout gradient and the signal acquisition start simultaneously, achieving an echo time of 0.6 msec. An off-line reconstruction was implemented to regrid the radially acquired data with nearest-neighbor interpolation (taking ramp sampling into account) onto a cartesian grid, followed by a conventional 2D fast Fourier transform by using the IDL software package (version 5.3; Research Systems IDL, Boulder, Colo) (12).

To observe the influence of different echo times on the measured muscular ²³Na MR signal intensity, we used three different ²³Na MR sequences, with echo times ranging from 0.2 to 3.53 msec. The ²³Na MR imaging protocol comprised the ²³Na 2D radial gradient-echo sequence (repetition time msec/echo time msec, 13/0.6; resolution, 3.9 x 3.9 x 30 mm; bandwidth, 190 Hz/pixel; number of acquisitions, 400; acquisition time, 5.5 minutes) and a ²³Na 2D fast low-angle shot (FLASH) sequence (13/3.53; resolution, 2.7 x 2.7 x 30 mm; bandwidth, 190 Hz/pixel; number of acquisitions, 300; acquisition time, 8 minutes). In order to evaluate the T2 signal decay, ²³Na free induction decays (FIDs) with a time delay between end of the radiofrequency pulse and start of acquisition (echo time) of 0.2 msec were acquired separately for each lower leg. Further sequence parameters were as follows: repetition time, 1000 msec; bandwidth, 10 kHz; number of points, 1024; and acquisition time, 100 msec.

Analysis of ²³Na MR Data
²³Na MR imaging with 2D radial and 2D FLASH sequences.—In order to quantify the signal intensity enhancement on the ²³Na MR images after exercise and local cooling of the nondominant lower leg, 2D radial and 2D FLASH images were evaluated by means of region-of-interest analysis by using a 0.3% saline solution phantom placed between the lower legs as a reference. Regions of interest had a size of 100 pixels and were placed on both 2D FLASH and 2D radial ²³Na MR images by a neurologist and radiologist in consensus (M.A.W. and H.B.H., with 5 and 3 years of experience in their specialties, respectively). A region of interest was placed on the soleus muscle of each lower leg by using the ¹H MR images for reference, and a third region of interest was placed on the 0.3% saline solution phantom. The signal intensity on 2D radial and 2D FLASH ²³Na MR images was normalized to the 0.3% saline solution phantom for interindividual and intraindividual comparisons—that is, the values of the regions of interest placed on the soleus muscles were divided by the values of the phantom. The signal intensities before and after provocation were analyzed separately for each lower leg. Signal intensity (SI) alterations were considered to reflect changes in muscular Na⁺ concentration. The percentage change between the normalized muscular ²³Na MR imaging signal intensity before (SI_pre) and after (SI_post) provocation (SI%) was calculated, and findings were expressed according to Equation (1):

(1)

²³Na FID.—The acquired FID data (Fig 1) were fitted to the biexponential decay by using a Levenberg-Marquard algorithm to evaluate the fast (T2_fast) and slow (T2_slow) component of the T2 relaxation time, as shown in Equation (2):

(2)

View larger version (17K): [in this window] [in a new window] [Download PPT slide]   Figure 1: Graph shows 23Na FID data representative for all measured FIDs to evaluate the T2 MR signal decay. The FID data clearly show that the last points are already background noise, thus indicating that acquisition time was long enough to fit the envelope. a.u. = arbitrary units.

(3)

Assuming that M_fast mainly relates to intracellular Na⁺ concentration and M_slow mainly relates to extracellular Na⁺ concentration, and since extracellular concentration of Na⁺ remains constant due to perfusion (11), a decrease in M_slow/M_fast should reflect an accumulation of intracellular Na⁺. The percentages of change measured according to Equations (1) and (3) by using the different ²³Na MR sequences were analyzed for a possible correlation.

¹H MR imaging.—In order to exclude other muscular pathologic conditions, ¹H MR imaging was performed in addition to the ²³Na MR imaging protocol before provocation. The ¹H MR imaging protocol comprised a transverse T1-weighted spin-echo sequence (516/15, matrix of 308 x 512, section thickness of 6 mm) and a transverse T2-weighted turbo spin-echo sequence (3000/104, matrix of 308 x 512, section thickness of 6 mm). Image interpretation was performed by two readers (M.A.W. and M.E., with 5 and 11 years of experience in musculoskeletal MR imaging, respectively) in consensus. A muscle edema was defined as an area of localized hyperintensity on T2-weighted MR images. Fatty infiltration, which was defined as areas with signal intensity equivalent to that of subcutaneous fat on T1- and T2-weighted MR images, was interpreted as a sign of chronic myopathy. The MR imaging criterion of muscle atrophy was a reduction of muscle cross-sectional area, which was assessed qualitatively by the two readers in consensus by using the opposite side or other muscle groups for comparison. The readers were asked to assess in a dichotomous fashion whether these criteria were identified.

Na⁺ Channel Blockage
In seven patients with PC and no history of cardiac disease (one woman, six men; mean age, 39 years ± 14), a third experiment that included ²³Na MR imaging, cooling, and muscle strength testing was performed after 4 days of oral medication with a selective Na⁺ channel blocker (200 mg of mexiletine [Mexitil; Boehringer Ingelheim] three times a day).

Before medication was administered, informed consent was again obtained from the patients. Because of the fact that, in rare cases, cardiac arrhythmias can be induced by mexiletine, electrocardiography was performed before medication was administered. The electrocardiogram was normal in all of these seven patients, and there were no adverse events related to this third experiment. Among the other six patients available for repeated testing, two reported that they had developed side effects after previous mexiletine administration, two had cardiac arrhythmias, and two did not give informed consent, so no mexiletine could be administered. The Na⁺ channel blocker was not given to the volunteers, in order to prevent unnecessarily exposing them to the risk of adverse events.

Statistical Analysis
Data entry procedures and statistical analysis were performed with a statistical analysis software system (SPSS for Windows, version 11.5.1, 2002; SPSS, Chicago, Ill). Data were analyzed by using a two-sided parametric t test for testing no difference versus difference and by using one-sided t tests for testing no difference against larger or smaller (level of significance, P = .05). Results were expressed as mean ± standard deviation. For each difference found in our study, the corresponding statistical power was calculated by using the effect size approach (nQuery Advisor version 5.0, 2002; Statistical Solutions, Saugus, Mass). For each statistically significant difference, the corresponding statistical power was at least 80%.

	RESULTS

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Mutation Screening
In all 16 patients, a typical PC mutation was identified: amino acid substitution R1448C in eight patients, T1313M in five patients, and R1448H in three patients.

Muscle Strength
In patients with PC, the muscle strength prior to cooling and exercise was normal in both legs, and the muscle strength after exercise was normal in the noncooled leg (Table 1). Hip muscle function was not influenced by cooling of the lower leg and exercise. All 16 patients with PC presented with muscle weakness in the cooled lower leg, particularly for foot and toe dorsiflexion. Muscle strength was partially recovered 45 minutes after cooling and exercise (British Medical Research Council score: before cooling, 4.9 ± 0.1; immediately after cooling, 4.2 ± 0.5; 45 minutes after cooling, 4.5 ± 0.3). In 13 of 13 patients, there was no significant intraindividual variability between the muscle strength scored during the first experiment and the second experiment 3 days later (P = .07). In all 10 volunteers, muscle strength was normal for all conditions.

View larger version (105K): [in this window] [in a new window] [Download PPT slide]   Figure 2: A, C, E, Transverse T1-weighted spin-echo (516/15) and, B, D, F, transverse T2-weighted turbo spin-echo (3000/104) MR images of both lower legs in a family with PC (R1448C mutation). Images of the 54-year-old father (A, B) show a symmetrically fatty infiltration of both gastrocnemius muscles (arrows) and edema in both soleus muscles (*). The 0.3% saline reference phantom is also visible. Images of the 25-year-old son (C, D) show muscle edema, a precursor and possible causative mechanism of muscle degeneration, bilaterally symmetrical in the medial head of both gastrocnemius muscles, whereas the youngest family member, a 17-year-old daughter (E, F), has not yet developed morphologic changes.

View larger version (19K): [in this window] [in a new window] [Download PPT slide]   Figure 3a: Box plots of percentage change of muscular 23Na MR signal intensity after cooling in patients (gray boxes) and volunteers (white boxes) measured at (a) FID and (b) 2D radial MR imaging. Boxes show the 25–75 percentile, with a middle line that indicates median; error bars = 10 and 90 percentile; = extreme values. FID data (a) show a decrease in the ratio of extra-to-intracellular Na+ concentration after cooling in patients with PC, which reflects intracellular Na+ accumulation. No change is seen in volunteers or in the reference leg in patients. Two-dimensional radial data (b) show Na+ accumulation in patients with PC by means of an increase in 23Na MR signal intensity in the cooled leg. No change is seen in volunteers.

View larger version (19K): [in this window] [in a new window] [Download PPT slide]   Figure 3b: Box plots of percentage change of muscular 23Na MR signal intensity after cooling in patients (gray boxes) and volunteers (white boxes) measured at (a) FID and (b) 2D radial MR imaging. Boxes show the 25–75 percentile, with a middle line that indicates median; error bars = 10 and 90 percentile; = extreme values. FID data (a) show a decrease in the ratio of extra-to-intracellular Na+ concentration after cooling in patients with PC, which reflects intracellular Na+ accumulation. No change is seen in volunteers or in the reference leg in patients. Two-dimensional radial data (b) show Na+ accumulation in patients with PC by means of an increase in 23Na MR signal intensity in the cooled leg. No change is seen in volunteers.

View larger version (56K): [in this window] [in a new window] [Download PPT slide]   Figure 4a: (a, b) Transverse 2D FLASH (13/3.53) and (c, d) transverse 2D radial (13/0.6) 23Na MR images of both lower legs, before and after provocation, show Na+ accumulation in a 39-year old patient (T1313M mutation). Images after cooling (b, d) show signal intensity increase in the cooled left leg (arrow). B1 field inhomogeneity caused by the surface coil, with a penetration depth of 7.8 cm, leads to highest intensity near the coil windings. The pathologic signal intensity increase can be evaluated because the right (reference) leg is the same distance from the coil and thus submitted to a similar B1 field. The radial images have blurring due to decay of the short T2 component during data acquisition. The 0.3% saline solution phantom is seen between the lower legs. Blood vessels and bones appear as areas of high and low signal intensity, respectively (7).

View larger version (57K): [in this window] [in a new window] [Download PPT slide]   Figure 4b: (a, b) Transverse 2D FLASH (13/3.53) and (c, d) transverse 2D radial (13/0.6) 23Na MR images of both lower legs, before and after provocation, show Na+ accumulation in a 39-year old patient (T1313M mutation). Images after cooling (b, d) show signal intensity increase in the cooled left leg (arrow). B1 field inhomogeneity caused by the surface coil, with a penetration depth of 7.8 cm, leads to highest intensity near the coil windings. The pathologic signal intensity increase can be evaluated because the right (reference) leg is the same distance from the coil and thus submitted to a similar B1 field. The radial images have blurring due to decay of the short T2 component during data acquisition. The 0.3% saline solution phantom is seen between the lower legs. Blood vessels and bones appear as areas of high and low signal intensity, respectively (7).

View larger version (41K): [in this window] [in a new window] [Download PPT slide]   Figure 4c: (a, b) Transverse 2D FLASH (13/3.53) and (c, d) transverse 2D radial (13/0.6) 23Na MR images of both lower legs, before and after provocation, show Na+ accumulation in a 39-year old patient (T1313M mutation). Images after cooling (b, d) show signal intensity increase in the cooled left leg (arrow). B1 field inhomogeneity caused by the surface coil, with a penetration depth of 7.8 cm, leads to highest intensity near the coil windings. The pathologic signal intensity increase can be evaluated because the right (reference) leg is the same distance from the coil and thus submitted to a similar B1 field. The radial images have blurring due to decay of the short T2 component during data acquisition. The 0.3% saline solution phantom is seen between the lower legs. Blood vessels and bones appear as areas of high and low signal intensity, respectively (7).

View larger version (49K): [in this window] [in a new window] [Download PPT slide]   Figure 4d: (a, b) Transverse 2D FLASH (13/3.53) and (c, d) transverse 2D radial (13/0.6) 23Na MR images of both lower legs, before and after provocation, show Na+ accumulation in a 39-year old patient (T1313M mutation). Images after cooling (b, d) show signal intensity increase in the cooled left leg (arrow). B1 field inhomogeneity caused by the surface coil, with a penetration depth of 7.8 cm, leads to highest intensity near the coil windings. The pathologic signal intensity increase can be evaluated because the right (reference) leg is the same distance from the coil and thus submitted to a similar B1 field. The radial images have blurring due to decay of the short T2 component during data acquisition. The 0.3% saline solution phantom is seen between the lower legs. Blood vessels and bones appear as areas of high and low signal intensity, respectively (7).

In 13 of 13 patients, both before (P = .25) and after (P = .52) provocation, the muscular ²³Na signal intensity was not significantly different between the first experiment and the second experiment 3 days later in the reference leg and cooled leg (P = .25 before and P = .58 after provocation, Table 4). No significant differences were found in the percentage change of muscular ²³Na signal intensity after provocation between the first and second experiment (P = .83 for the reference leg and P = .52 for the cooled leg, Table 5). In 10 of 10 volunteers, before (P = .06) and after (P = .06) provocation, the muscular ²³Na signal intensity was not significantly different between the lower legs (Table 3), and no significant increase in ²³Na signal intensity could be observed in the reference leg (P = .86) or in the cooled leg (P = .41, Table 2).

Two-dimensional FLASH MR Data
In 14 of 14 patients, the ²³Na signal intensity increase in the cooled lower leg after provocation was less pronounced on 2D FLASH MR images in comparison with 2D radial MR images (Table 2). Before provocation, ²³Na signal intensity was not significantly different between the two legs (P = .22), but after provocation, ²³Na signal intensity was significantly higher in the cooled leg than in the reference leg (P < .001, Table 3), which showed no increase in muscular ²³Na signal intensity (mean, –2% ± 8). The ²³Na signal intensity was significantly higher in the 14 patients than in the 10 volunteers (reference leg, P < .001; cooled leg, P < .001). There was no significant difference in percentage change of muscular ²³Na signal intensity after provocation between the reference leg in the 14 patients and that in the 10 volunteers (P = .32), but the percentage change was significantly higher in the cooled leg of patients than in that of volunteers (P = .003).

In 13 of 13 patients, both before (P = .69) and after (P = .92) provocation, the muscular ²³Na signal intensity was not significantly different between the first experiment and the second experiment 3 days later in the reference lower leg and the cooled lower leg (Table 4; P = .68 before and P = .91 after provocation). In 10 of 10 volunteers, the ²³Na signal intensity was not significantly different in the lower legs before (P = .93) or after (P = .73, Table 3) provocation and was not influenced by cooling and exercise (reference leg, P = .29; cooled leg, P = .62).

FID Data
In 14 of 14 patients with PC, the ratio of extracellular-to-intracellular Na⁺ concentration prior to provocation was not significantly different between the two lower legs (P = .18). The concentration ratio in the reference leg before and after exercise was also not significantly different (P = .10; Table 3). In comparison with the reference leg, there was a significant decrease in extracellular-to-intracellular concentration of Na⁺ in the cooled leg after provocation (P < .001; Tables 2, 3); this decrease was detected in all patients. There was no difference in percentage change of muscular ²³Na signal intensity after provocation between the reference leg in the 14 patients and that in the 10 volunteers (P = .051), but there was a significant difference between the cooled leg in the 14 patients and that in the 10 volunteers (P < .001; Fig 3a).

In 13 of 13 patients, no significant differences in percentage change of muscular ²³Na MR signal intensity after provocation were found between the first experiment and the second experiment 3 days later (reference leg, P = .94; cooled leg, P = .37; Table 5), and the ratio of extracellular-to-intracellular Na⁺ concentration was not significantly different between the first and second experiments in the reference leg before (P = .09) and after (P = .13; Table 4) provocation and in the cooled leg before (P = .07) and after (P = .07; Table 4) provocation. In 10 of 10 volunteers, the ratio of extracellular-to-intracellular Na⁺ concentration was not significantly different (P = .89) in the two legs before provocation and remained unchanged after exercise in the reference leg (P = .18) and cooled leg (P = .57; Table 3). In 10 of 10 volunteers, no significant increase of percentage change in muscular ²³Na signal intensity could be observed after provocation (P = .72; Table 2). The correlation between the percentage change in muscular ²³Na signal intensity after provocation measured at 2D radial and FID ²³Na MR imaging was higher than the other correlations (linear regression, r² = 0.53; Fig 5), while correlation was low when comparing 2D FLASH with FID (linear regression, r² = 0.12) and 2D radial (linear regression, r² = 0.13) MR imaging.

View larger version (23K): [in this window] [in a new window] [Download PPT slide]   Figure 5: Graph shows correlation of 23Na FID and 2D radial MR data. The percentage change of muscular 23Na signal intensity after cooling in the nondominant lower leg is correlated between FID and 2D radial MR imaging. Linear regression, r2 = 0.533.

View larger version (17K): [in this window] [in a new window] [Download PPT slide]   Figure 6: Graph shows results in seven patients with PC before (first and second examination) and after (third examination) blockage of pathologic Na+ channels. Percentage change of muscular 23Na signal intensity after cooling (measured by means of FID) is shown for each patient. There is a decrease in the extracellular-to-intracellular Na+ concentration ratio in the cooled leg at the first and second examination (without mexiletine). At the third examination, concomitant with an improvement in muscle strength, there is an increase in the concentration ratio to values comparable to those in the reference leg.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

The observed tissue Na⁺ concentration is composed of the weighted average of extracellular and intracellular Na⁺ concentrations in the examined tissue. Extracellular Na⁺ concentration at 140 mmol/L is about 10-fold higher than intracellular concentration, which is about 10–15 mmol/L (11). Arguably, the more physiologically relevant information is intracellular Na⁺ concentration, which reflects the function of Na⁺ channels to conduct Na⁺ along the electrochemical gradient at the membrane and the cell's ability to pump out Na⁺. Extracellular concentration of Na⁺ will remain virtually constant as long as there is adequate perfusion of tissue, so that under these circumstances, despite the inability to resolve intra- and extracellular components of the ²³Na signal, the use of short echo times, such as 0.6 msec, provides a measurement of intracellular Na⁺ concentration (11). The FID sequence with an ultrashort time delay between the end of the radiofrequency pulse and the start of data acquisition showed a muscular Na⁺ accumulation after cooling in PC that takes place in the intracellular compartment because of an Na⁺ influx from the extracellular compartment; this is because the ratio of extracellular-to-intracellular Na⁺ measured by means of FID decreased in the cooled lower leg that developed muscle weakness. The increase in intracellular Na⁺ concentration strengthens the hypothesis (10) that the fast component of the T2 relaxation time is associated with intracellular and the slow component with extracellular Na⁺ concentration.

A primary concern when using single-quantum ²³Na MR imaging methods is the need to limit signal loss from fast T2 decay by keeping the echo time as short as possible. By using FID, we were able to use a time delay as short as 0.2 msec between the end of the radiofrequency pulse and the start of acquisition. The FID measurements, however, were nonselective, so that ²³Na signal from the whole lower leg was acquired. The use of the surface coil minimized this problem because of its small region of sensitivity. For direct visualization of the muscular ²³Na signal intensity, we established a 2D radial MR sequence that enables the use of region-of-interest analysis. We demonstrated in our study that although the echo time of the 2D radial sequence is 0.4-msec longer than that of the FID sequence, the results were comparable. Hence, for further analysis of muscular Na⁺ content, we recommend the use of radial MR imaging techniques. A limitation of ²³Na MR imaging is the low signal-to-noise ratio at 1.5 T and, consequently, the relatively long measurement time. However, our MR imaging protocol was well tolerated by all patients. ²³Na MR imaging in muscle is more challenging than that in the brain, because the total Na⁺ concentration in muscle is about 32% lower than that in the brain, with 43–45 mmol per kilogram wet weight (7). This translates directly into a reduced signal-to-noise ratio that leads to decreased spatial resolution and prolonged image acquisition times.

One of the advantages of the 2D radial MR technique is that Na⁺ accumulation was visualized not only in the cooled muscles of patients but also in the noncooled muscles, which was partially provoked by exercise. In a warm environment, most patients with paramyotonia do not present with myotonic stiffness during rest but become stiff with continued strong activity. This phenomenon is called paradoxical myotonia because it is contrary to the relief of stiffness during repeated contractions in patients with myotonia congenita, a chloride channel disease (warm-up phenomenon). The paradoxical myotonia is caused by reopening of Na⁺ channels, which can generate bursts of action potentials leading to muscle stiffness. The current conducted through the reopening Na⁺ channels may be responsible for the Na⁺ accumulation, which was visualized with the 2D radial MR technique.

Moreover, effects of a specific blockage of the pathologic PC Na⁺ channels were examined. Muscle stiffness and weakness are prevented by Na⁺ channel blockers such as mexiletine (2). Our preliminary data showed that effects of a specific blockage of the pathologically altered Na⁺ channels could be monitored in vivo in patients with PC. After a 4-day period of medication with mexiletine, the provocative test caused much smaller Na⁺ accumulation and less weakness; thus, the beneficial effect of a drug that exerts its effects at the molecular defect of the disease could be visualized.

To date, reports on ²³Na MR imaging have been restricted mainly to pathologic conditions in the heart, such as ischemia (4–5), and those in the brain, such as stroke (14) and tumors (11,15,16), or have focused on imaging of larger organ systems (17,18). To our knowledge, only two studies have dealt with ²³Na MR imaging in patients with a muscle disease, myotonic dystrophy. In two (7) and seven (6) patients, the observed myoplasmic Na⁺ accumulation correlated with muscle degeneration. Compared with the results of these studies, which suggest that an increased Na⁺ concentration in the muscle reflects a sign of irreversible cell necrosis, our results show that even a striking intracellular Na⁺ accumulation can be observed in muscles of patients with channelopathies, such as PC, that were normal or only mildly degenerated on ¹H MR images.

Limitations of our study were that findings at ²³Na MR imaging were correlated with subjective assessment of muscle strength by nonblinded observers by using the British Medical Research Council score. Furthermore, absolute Na⁺ concentrations within the muscle tissue were not calculated. We considered the signal-to-noise ratio provided by the measurements at 1.5-T MR as too low, together with the B₁ field inhomogeneity caused by the surface coil, for applying the necessary corrections on the measured data to calculate valid tissue Na⁺ concentrations.

The results of our study show that ²³Na MR imaging can enable visualization of pathologic Na⁺ accumulation in muscle cells that is associated with muscle weakness in patients with the inherited Na⁺ channel disease PC. Furthermore, effects of a specific Na⁺ channel blockage can be visualized. In our institution, ²³Na MR imaging sequences have been introduced to integrate ²³Na MR imaging into clinical work-up, thus indicating its potential to evolve from a research topic to a clinically feasible diagnostic tool.

	ADVANCES IN KNOWLEDGE

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 

• ²³Na MR imaging can depict pathologic myoplasmic Na⁺ accumulation associated with muscle weakness in the muscular Na⁺ channelopathy paramyotonia congenita.
  
• ²³Na MR imaging can depict the beneficial effect of an Na⁺ channel blocking agent.
  
• A striking intracellular Na⁺ accumulation can be observed in muscles in those patients who had normal findings or only mild degeneration at ¹H MR imaging.
  

	ACKNOWLEDGMENTS

 
The authors thank Ivan Zuna, PhD, for the statistical analysis and Renate Jerecic, PhD, and Heinz-Peter Schlemmer, MD, for their help with the preliminary experiments and the study concept. The support of Achim Bankamp, PhD, with the development of the 2D radial MR data reconstruction is gratefully acknowledged.

	FOOTNOTES

 

Abbreviations: FID = free induction decay • FLASH = fast low-angle shot • PC = paramyotonia congenita • 2D = two-dimensional

Authors stated no financial relationship to disclose.

Author contributions: Guarantor of integrity of entire study, M.A.W.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, M.A.W., S.N., F.L., H.M.M.; clinical studies, M.A.W., S.N., H.B.H., J.C.W., K.J., F.L., H.M.M.; statistical analysis, M.A.W., S.N., F.L.; and manuscript editing, M.A.W., S.N., J.C.W., K.J., F.L., L.R.S., H.M.M.

	References

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE References

 

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