Abstract

Myotonic dystrophy type 1 (DM1) is a multisystem, dominantly inherited disease caused by an unstable CTG repeat expansion in the 3′ nontranslated region of the DM1 gene on chromosome 19.¹ Myotonia, weakness of facial and distal limb muscles, and cataracts are core clinical findings.^2,3 Using various methods to measure myotonia, previous studies have reported antimyotonic effects for quinine,^4,5 sodium channel blockers (phenytoin, procainamide, tocainide, mexiletine, carbamazepine),^4,6–11 antidepressants (amitriptyline, clomipramine, imipramine),^12–14 calcium channel blockers,¹⁵ benzodiazepines,¹⁶ prednisone,⁴ dehydroepiandrosterone,¹⁷ taurine,¹⁸ acetazolamide,¹⁹ and others. Recent studies have focused on the class Ib antiarrhythmic lidocaine analogs mexiletine^7,20–22 and tocainide.^7,8,10,23,24

One randomized study suggested that mexiletine at 400 to 600 mg daily is a safe and effective antimyotonia medication.⁷ (Although tocainide has also been reported to be effective in treating myotonia,⁸ bone marrow suppression^8,25 limits its use.) While this study⁷ was an important step forward, it had some deficiencies: short duration (4 weeks), a heterogeneous patient group, a single-blind study design, heterogeneous treatment group sample sizes, and incomplete validation of the methods used to measure myotonia. Moreover, a recent Cochrane review concluded that none of the myotonia treatment studies reported as of 2006 were of sufficiently high quality to conclude that any available drug treatment of myotonia is effective and safe.²⁶

Given the need for a more definitive antimyotonia treatment trial, we prospectively evaluated the effect of mexiletine on grip myotonia and cardiac conduction in DM1 in 2 randomized, double-blind, placebo-controlled crossover trials, the first using a dosage of 150 mg 3 times daily (TID) and the second using 200 mg TID.

METHODS

Randomization and enrollment.

We carried out 2 randomized, double-blind, placebo-controlled crossover trials (figure 1). In each trial, the 2 treatment periods were 7 weeks in duration separated by a 4- to 8-week washout period. Those who participated in both trials (n = 10) were required to wait at least 8 weeks after the end of the first trial to enroll in the second trial. Participants were randomly assigned in each trial to 1 of 2 treatment sequences: mexiletine/placebo or placebo/mexiletine. The computer-generated randomization plans included blocking to ensure approximate balance between the 2 treatment sequences. A programmer in the Department of Biostatistics (Rochester, NY) generated the randomization plans and sent them to the pharmacy where drug packaging and labeling took place. All drug was labeled with a participant ID number. Only the biostatistics programmer and the pharmacist had access to the treatment assignments. Drug was assigned sequentially. The study coordinator was required to check all baseline forms for completeness and verify eligibility criteria before calling the pharmacy to officially randomize a participant.

Figure 1 Consort flow diagram

Myotonia testing.

The right arm was used for measuring grip relaxation times (RTs) after a maximal voluntary isometric contraction (MVIC) as described previously.^28,29 In brief, each trial consisted of 6 maximal voluntary contractions, each lasting 3 seconds, with a 10-second rest period between each contraction. Three sets of measurements (trials) were performed with 10-minute intervals of rest between trials.

An automated computer program first determined peak force (PF) in kg units and then placed cursors on the declining, relaxation phase of the force recording at various levels of PF: 90%, 50%, and 5% (figure 2).^28,29 The RTs reported here are the times required for decline in force from 90% to 5%, 90% to 10%, and 50% to 5% of PF.

Figure 2 Grip relaxation time (myotonia)

Grip RT and PF were calculated from the initial contraction from each of the 3 trials on the first 2 days of admission (6 trials total). The additional contractions 2–5 were analyzed for study of the warm-up phenomenon as previously reported.²⁸ The mean PF and RT were determined by averaging these values from the 6 trials over the 2 days of evaluation.

RESULTS

The participants in both studies were middle-aged with a slight male predominance (table 1). Symptoms had been present for a mean of about 2 decades, and median CTG repeat sizes were approximately 500 in both groups with a minimum of 169. Four participants dropped out of the study after the initial General Clinical Research Center visit: 2 participants in the 150 mg TID trial (1 on placebo and 1 on mexiletine, both due to family reasons and inability to fulfill the required time commitment) and 2 participants in the 200 mg TID trial (1 on placebo due to perceived lack of therapeutic benefit and 1 on mexiletine due to diarrhea).

Table 1 Demographic and clinical characteristics at baseline

Trough blood levels of mexiletine were somewhat higher with 200 mg TID (0.86 ± 0.48 μg/mL, range 0–1.70 μg/mL) than with 150 mg TID (0.54 ± 0.28 μg/mL, range 0–1.15 μg/mL). Mexiletine blood levels reached the therapeutic range for treating arrhythmia (0.5–2.0 μg/mL) in 13 patients (68%) with 200 mg TID and in 10 patients (55%) with 150 mg TID. Three participants (1 in the 150 mg TID trial and 2 in the 200 mg TID trial) had a mexiletine blood level of 0. When these possible noncompliers were removed, the blood levels achieved the therapeutic range in 76% of those on 200 mg TID and 59% of those on 150 mg TID.

Mexiletine was generally well-tolerated and there were no significant adverse events at either dosage. Adverse events were mild (table 2), and slightly more common with mexiletine treatment than with placebo treatment. Adverse events that seemed to be more common with mexiletine were mild upper gastrointestinal distress and lightheadedness. As noted above, mexiletine treatment was discontinued due to an adverse event (diarrhea) in only 1 participant while taking 200 mg TID mexiletine.

Table 2 Adverse events

In both trials, there were no significant effects of mexiletine on EKG measurements (table 3, table e-1 on the Neurology® Web site at www.neurology.org). The mean PR, QRS, and QT_c intervals remained stable over time with mexiletine and placebo treatment.

Table 3 Treatment effects

The MVIC grip traces showed prolonged 90%–5% RTs, particularly in the terminal portion of the relaxation phase (figure 2B). In both trials, the RTs were elevated at baseline (table 1) and during placebo treatment (table 3; figure 2) with mean 90%–5% RTs of over 2 seconds. Mexiletine at the 150 mg TID and 200 mg TID dosages was associated with a significant reduction in RT, with mean reductions of 48% (90%–5% RT), 48% (90%–10% RT), and 55% (50%–5%) with 150 mg TID and mean reductions of 52% (90%–5% RT), 51% (90%–10% RT), and 58% (50%–5%) with 200 mg TID (table 3, table e-2, figure 2). In both the 150 mg TID and 200 mg TID trials, 17 of the 18 participants (94%) had a shorter 90%–5% RT on mexiletine than on placebo. There was a significant improvement in peak grip force with 150 mg TID mexiletine compared to placebo (table 3), but this was not the case with 200 mg TID mexiletine (table 3).

DISCUSSION

The major finding in these randomized, double-blind, placebo-controlled trials of mexiletine in DM1 is that dosages of 150 and 200 mg TID for 7 weeks significantly improve myotonia, as measured by hand grip relaxation time, but do not cause significant adverse events or produce EKG conduction abnormalities in the short term.

These trials have several advantages over previous myotonia treatment trials in patients with DM1. 1) These studies are the largest we are aware of in that 20 genetically defined DM1 patients participated in each of the 2 trials. Previous trials have included relatively small numbers of DM1 patients, have often included participants with different forms of myotonic myopathy, and none have provided genetic confirmation. 2) These trials were double-blind; previous trials of mexiletine or tocainide for myotonia in DM1 have been single blind⁷ or unblinded.⁸ 3) The improvement in myotonia noted in our first trial was corroborated by the second. 4) Plasma concentrations of mexiletine were measured, a requirement for the tocainide study,⁸ but not in the previous mexiletine study.⁷ 5) These trials employed a carefully tested method of measuring grip myotonia^28,29 that has good test-retest reproducibility, and yields grip relaxation times that increase and grip peak forces that decrease as CTG repeat lengths increase. Ergonomic techniques have been used to measure myotonia in patients with myotonic dystrophy for some time,^9,30 but they have not been as carefully tested. The previous study of mexiletine⁷ utilized timed measurements of eye and hand opening, stair climbing, and the opponens pollicis, surface EMG-recorded myotonic afterdischarge that have merit as outcome measures, but again, have not been as carefully validated as our method.

Mexiletine is a lidocaine analog.³¹ Both drugs are type Ib anti-arrhythmics and produce state-dependent block of inactivated and to a lesser extent open (but not resting) sodium channels in cardiac and skeletal muscle; therefore, the channel block produced by these medications is mainly action potential-dependent. The 2 drugs differ in that sodium channel block induced by mexiletine is slightly slower to recover than that from lidocaine. In addition, hepatic metabolism is greatly reduced for mexiletine, which makes it suitable as an oral medication. Mexiletine is effective in cardiac patients with ventricular arrhythmias³¹ and long QT syndrome³² and in neuromuscular patients with myotonic disorders such as potassium aggravated myotonia or paramyotonia congenita.^33,34 These myotonic disorders are skeletal muscle sodium channelopathies which typically manifest a slowed rate of fast sodium channel inactivation. This results in abnormally persistent sodium currents leading to repetitive firing of muscle fibers and delayed muscle relaxation (myotonia).^33–35

Mexiletine ameliorates myotonia in sodium channelopathies by enhancing fast-inactivation of the sodium channels, resulting in use-dependent sodium channel block.^33,34,36,37 In addition, mexiletine may also produce open channel block of late-opening channels³⁸ at lower serum concentrations than those required to block channels that are closed and inactivated. By contrast, mexiletine is not known to affect skeletal muscle chloride channels, and its beneficial effects in patients with chloride channel myotonic disorders, such as DM1 or myotonia congenita, probably result from block of normal sodium channels which thereby decrease repetitive motor unit discharges.^33,34 Our data show that even moderate oral dosages of 150 mg TID with drug levels in the low to mid-therapeutic range (by cardiac standards) reduce handgrip myotonia in DM1. Importantly, although mexiletine produces use-dependent sodium channel blockade and could result in loss of muscle strength, there was no decline in grip peak force on mexiletine at either dosage. On the contrary, peak force significantly improved on mexiletine 150 mg TID, but this was not confirmed by the 200 mg TID trial. In any case, the preservation of grip force on mexiletine is consistent with the absence of significant cardiac hemodynamic compromise in patients on mexiletine for ventricular arrhythmia.³¹

Given that patients with DM1 may have abnormalities of cardiac conduction,³⁹ a major aim of these trials was to establish that mexiletine did not affect cardiac conduction in patients with DM1 with no baseline cardiac risk factors except first-degree AV block. It is recognized that lidocaine or mexiletine can potentially increase QRS duration particularly at higher heart rates, may shorten the QT interval, may compromise hemodynamic function in patients with heart failure,³¹ and can increase pacing threshold and the energy required for defibrillation.³¹ Still, treatment with these drugs in cardiac patients typically has little effect on PR, QRS, and QT duration, or on hemodynamic function.³¹ Similarly, previous studies of mexiletine and tocainide^7,8 in patients with myotonic myopathy of various types have also shown no significant changes in EKG measurements. Our study supports the view that mexiletine at dosages of 150 to 200 mg TID has no significant effect on cardiac conduction over a 7-week treatment period in patients with DM1 who have no significant cardiac risk factors except for first-degree AV Block. However, we recognize that our conclusions about the cardiac safety of mexiletine in DM1 are somewhat tentative given that 1) mexiletine is rarely associated with a pro-arrhythmic response that can aggravate underlying ventricular arrhythmias,⁴⁰ and 2) our studies involved a total of only 30 patients with DM1 followed for 7 weeks on mexiletine and excluded patients with significant heart disease. As a result, our practice is to obtain cardiac consultation before beginning mexiletine therapy for symptomatic myotonia in patients with DM1 who have current or remote cardiac symptoms or an abnormal baseline EKG.

In our trials, adverse events were observed in participants on placebo and on both dosages of mexiletine (table 2); none were considered serious, and only 1 participant discontinued the drug due to an adverse event. For those completing the trial, the occurrence of upper gastrointestinal distress and lightheadedness appeared to be more common with mexiletine than with placebo. Tremor occurred in only 1 participant taking mexiletine 200 mg TID. These results are similar to those observed previously.⁷

Overall, our data suggest that mexiletine at oral dosages of 150–200 mg TID may be considered as an effective, symptomatic treatment in patients with DM1 with functionally significant myotonia, and appears to be safe in the short term. Moreover, because the study required the presence of clinical myotonia, but did not specifically require moderate or severe myotonia as in 2 prior trials,^7,8 we believe that our data are generalizable to typical patients with DM1 with clinically evident myotonia. The percentage improvement in grip RT was not significantly greater in the 200 mg TID trial than in the 150 mg TID trial. We do not know why our study did not demonstrate a clear dose response effect of mexiletine on grip RT. This may be due to the different baseline characteristics of the 2 groups (e.g., the grip RTs were longer in the 200 mg TID than the 150 mg TID trial) or possibly that the effect of mexiletine on RT plateaus over the 150–200 mg TID dosage range.

One limitation of our trials is that they show relatively short-term benefits of mexiletine on handgrip relaxation. We do not know if this effect is durable over months to years, or if it is associated with improvement in quality of life. Our findings warrant long-term clinical trials with mexiletine in order to determine whether 1) its minimal risk and significant benefit in relieving myotonia are maintained over time, 2) it results in functional improvement in muscle-related activities of daily living, 3) it has a beneficial effect in maintaining muscle strength and mass, 4) it has a beneficial role in reducing or preventing musculoskeletal pain, and 5) it improves quality of life.

AUTHOR CONTRIBUTIONS

Statistical analysis was conducted by Dr. M.P. McDermott and Dr. A.T. Pearson.

ACKNOWLEDGMENT

The authors thank Tracy Forrester for help in manuscript preparation.

DISCLOSURE

Dr. Logigian serves on the editorial board of Muscle and Nerve. Mr. Martens and Mr. Moxley report no disclosures. Dr. McDermott has received honoraria from the Michael J. Fox Foundation; has served as a consultant to the New York State Department of Health, the American Epilepsy Society, the Michael J. Fox Foundation, and Teva Pharmaceutical Industries Ltd.; has received research support from Medivation, Inc., Boehringer Ingelheim, NeuroSearch Sweden AB, and Forest Laboratories, Inc.; and receives research support from the NIH (NS42372 [Co-I], HD44430 [Co-I], DE16280 [Co-I], NS52619 [PI], NS45686 [Co-I], NS46487 [Co-I], NS50095 [Co-I], NS49639 [Co-I], AR52274 [Co-I], NS50573 [Co-I], HL80107 [Co-I], RR24160 [Co-I], NS58259 [Co-I], and NS48843 [Co-I]), the US Food and Drug Administration, the Michael J. Fox Foundation, the SMA Foundation, and from the Muscular Dystrophy Association. Ms. Dilek, Dr. Wiegner, Dr. Pearson, Ms. Barbieri, and Ms. Annis report no disclosures. Dr. Thornton receives research support from the NIH (AR046806 [PI], AR049077 [PI], AR48143 [PI], and U54NS48843 [Co-I]), and from the Muscular Dystrophy Association. Dr. Moxley serves on scientific advisory boards for the NIH, the CDC, the Myotonic Dystrophy Foundation, Asklepios BioPharmaceutical, Inc., Acceleron Pharma, Wyeth, and Insmed Inc.; and receives research support from the FDA, the NIH (NIAMS N01-AR-0-2250 [PI], NCRR 5 MO1 RR00044 [Co-Director], NIAMS R01 AR49077 [Co-I], 9 U54NS48843 [PI], NCRR 1UL 1RR02416002 [Co-Director], N01-AR-5-2274 [PI], 1 R13 NS066630-01 [PI], and NIAMS 2 R01 AR049077 [Co-I]), the Saunders Family Foundation, and from the Muscular Dystrophy Association.

Supplementary Material

Notes

Address correspondence and reprint requests to Dr. Eric Logigian, Department of Neurology, Box 673, 601 Elmwood Ave, University of Rochester Medical Center, Rochester, NY 14642 eric_logigian@urmc.rochester.edu

Supplemental data at www.neurology.org

Study funding: This work comes from the University of Rochester Senator Paul D. Wellstone Muscular Dystrophy Cooperative Research Center (NIH/NS048843) and Clinical Research Center (NIH/NCRR MO1 RR00044) with support from the Food and Drug Administration (FD-R-001662). The work was also supported by the Saunders Family Neuromuscular Research Fund and NIH (AR49077).

Disclosure: Author disclosures are provided at the end of the article.

Received May 29, 2009. Accepted in final form February 3, 2010.

REFERENCES

Mexiletine is Effective in Reducing Myotonia in Myotonic Dystrophy Type 1

July 1, 2010

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Eric Logigian, MD, and Richard T. Moxley, III, MD, from the University of Rochester School of Medicine and Dentistry, discuss their paper “Mexiletine is an effective antimyotonia treatment in myotonic dystrophy type 1” that was recently published in Neurology (2010;74:1441-1448). They spoke with José Merino, MD, Science Editor of AAN.com

AAN.com: Among patients with myotonic muscular dystrophy (MD1), how debilitating are the myotonias? 

Logigian and Moxley: Admittedly, most patients with DM1 are bothered more by the consequences of muscle weakness than by those of delayed muscle relaxation or myotonia. However, in our experience, there are a significant number of DM1 patients who do have functionally debilitating myotonia manifested by muscle stiffness and inability to relax activated muscles for whom treatment with mexiletine is a therapeutic consideration.

AAN.com: What is the cause of the myotonia in these patients?

Logigian and Moxley: Recent investigations have shown that toxic, triplet repeat-containing RNA produced from the DM gene accumulates in the myonucleus, resulting in abnormal splicing of the C1C-1 chloride channel pre-mRNA. This leads to a loss of hyper-polarizing chloride channel conductance in muscle fiber membranes and the development of spontaneous repetitive muscle fiber discharges (e.g., electrical myotonia), and slowed muscle relaxation.

AAN.com: What is the best way to elicit myotonia in the clinic?

Logigian and Moxley: Observing the speed with which patients can open their closed fist or their closed eyes is one way. Another is to observe the delay in muscle relaxation after reflex activation of thenar, finger, or wrist extensor muscles evoked by direct percussion of these muscles with a reflex hammer. Whether activated voluntarily or reflexively, myotonic muscles are slow to relax and become “hung up.”  In addition, DM1 patients exhibit a “warm up” phenomenon in which muscle relaxation improves with serially repeated muscle contractions.

AAN.com:  How have clinical trials evaluated myotonia? 

Logigian and Moxley: As we mentioned in our paper, ergonomic techniques using force transducers, similar to the method we have reported, have been used to measure myotonia in patients with myotonic dystrophy for many years. Other methods have utilized timed measurements of eye and hand opening, stair climbing, and surface EMG-recorded myotonic afterdischarges.

The method of measuring grip myotonia employed in our report has previously been shown by our group to have good test-retest reproducibility, and to yield grip relaxation times that increase and grip peak forces that decrease as CTG repeat lengths increase.

AAN.com: Please summarize the methodology of your study.

Logigian and Moxley: The right arm was used for measuring grip relaxation times (RTs) after a maximal voluntary isometric contraction. The hand grip force recordings were analyzed off line utilizing an automated computer program that first determined peak force (PF) in kg units and then placed cursors on the declining, relaxation phase of the force recording at various levels of PF: 90%, 50%, and 5%. The RTs we reported were the times required for decline in hand grip force from 90% to 5%, 90% to 10%, and 50% to 5% of PF.

AAN.com: Your study was a crossover trial. Why is this the best way to answer your question?

Logigian and Moxley: The advantage of a crossover design in a rare disease like DM1 is that it maximizes the power of a placebo-controlled trial when the number of patients in the trial is relatively small. For example, in the first treatment trial of mexiletine at 150 mg p.o. t.i.d. versus placebo in 18 patients, a standard, non-crossover trial would compare nine randomized placebo patients with nine randomized mexiletine patients. The crossover design allowed us to double the number to 18 in both the placebo and mexiletine groups.

AAN.com: What was the main outcome measure in your study? 

Logigian and Moxley: The primary outcome measure was time for isometric grip force to relax from 90 percent to 5 percent of peak force after a 3-second maximum grip contraction. Secondary outcome variables included other average RTs (90%–10% of PF, 50%–5% of PF) and average PF. EKG outcomes, including PR interval, QRS interval, and QTc interval, were of interest from the perspective of safety.

AAN.com: You wanted to “detect a treatment effect of at least 0.66 SD units.”  What does this change in mean relaxation time mean clinically? 

Logigian and Moxley: Detection of a treatment effect of 0.66 SD units was purely a statistical “target” that we employed to power the study. It is a legitimate question to ask whether such a treatment effect is clinically meaningful. We cannot answer that question currently, but hope to do so in a second study of mexiletine and DM1 patients utilizing a DM1 specific quality of life instrument.

AAN.com: You found a statistically significant reduction in grip relaxation time. Was this difference clinically significant?

Logigian and Moxley: Grip relaxation time declined by about 50 percent in both mexiletine treatment groups. The study did not include a functional outcome measure to determine if this dramatic reduction in grip relaxation time was clinically significant. We believe strongly that this difference is clinically significant, but we will require a second follow-up study to prove it unequivocally.

AAN.com: What were the major adverse effects of mexiletine and how can these be managed?

Logigian and Moxley: In our two small studies including a total of 30 DM1 patients, there were no major adverse effects, and only one patient discontinued the medicine due to an adverse event (e.g, diarrhea). Minor side effects were observed in both the placebo and the mexiletine groups. Mild gastrointestinal distress (e.g., heartburn, nausea, vomiting, diarrhea, abdominal pain) and lightheadedness seemed to be the most common adverse side effects in the mexiletine versus the placebo group. We believe that these side effects can be managed by keeping the mexiletine dose on the low side, and by encouraging patients to take the medicine with food.

AAN.com: Do other sodium channelopathies benefit from treatment with type Ib antiarrhytmic agents?

Logigian and Moxley: Yes, mexiletine is effective in several skeletal muscle sodium channelopathies associated with myotonia such as potassium aggravated myotonia and paramyotonia congenita.

AAN.com: Are other type Ib antiarrhytmic agents useful in MD1 patients?

Logigian and Moxley: Yes, several small studies have provided evidence that tocainide, another type Ib antiarrhythmic agent, improved myotonic symptoms in patients with various types of myotonic myopathy. However, bone marrow suppression and pulmonary fibrosis can occur with this medication, and therefore it is no longer available in the United States.

AAN.com: In your opinion, is there enough evidence to recommend the routine use of mexiletine for myotonia in patients with MD1?  Or should clinical trials that evaluate long-term functional effects be completed before the treatment is recommended?

Logigian and Moxley: Currently, we do not recommend routine use of mexiletine in patients with DM1. We reserve the drug for those who have functionally disabling symptomatic myotonia. If patients do not have clear-cut functional improvement in muscle specific activities of daily living on mexiletine, we do not continue therapy.

AAN.com: How do you identify patients who are likely to benefit from this treatment?

Logigian and Moxley: We consider mexiletine treatment in DM1 patients who have clinical evidence of disabling myotonia. Such patients typically have symptoms of “stiffness,” or myalgia, and signs of delayed hand grip relaxation or prominent percussion myotonia. By contrast, DM1 patients with prominent muscle weakness and minimal myotonia are not likely to benefit from mexiletine.

AAN.com: What are the contraindications to mexiletine? 

Logigian and Moxley: Other than drug allergy, we are most concerned about the pro-arrhythmic potential of mexiletine. In a DM1 patient with disabling myotonia and with no cardiac history, no cardiac risk factors, and a normal EKG, it could be argued that mexiletine could be prescribed without cardiology input. It is our practice to obtain cardiac consultation on all patients prior to starting the medication.

AAN.com: How do you start patients on the medication?

Logigian and Moxley: After reviewing the risks and benefits of the medication, obtaining a baseline EKG and cardiac consultation, we begin treatment with mexiletine. In our study, improvement in hand grip relaxation time was similar for the two dose regimens tested. Therefore, we would recommend beginning treatment with the lower mexiletine dose, using a dose escalation protocol as follows: 150 mg p.o. q.a.m. for three days, then 150 mg p.o. b.i.d. for three days, then 150 mg p.o. t.i.d. To minimize associated nausea or GI side effects, we suggest that the patient take the medicine with meals. We also encourage patients to keep track of certain target symptoms for which mexiletine was prescribed (e.g., buttoning, zipping, handwriting, ease of  walking, myalgia, etc.) in order to determine if the medication is providing benefit in muscle specific activities of daily living.

AAN.com: How often should clinicians monitor response to treatment?

Logigian and Moxley: We would expect improvement in targeted myotonic symptoms to begin within a week of initiating treatment. Typically patients can be seen back in the office within a few weeks of starting the medication, but we encourage patients to call us immediately for any potential adverse side effects.

AAN.com: Are there risks to the use of the drug for more than seven weeks?

Logigian and Moxley: We are not aware of long-term risks. Although we believe that a seven-week trial should uncover significant adverse events, a very low frequency adverse event might only become obvious with longer term therapy in a larger group of patients. Longer term studies are required to answer this question.

Clearly, a longer term clinical trial is required to prove that mexiletine enhances quality of life, that its beneficial effect is durable over the long term, and that significant adverse events do not occur or occur rarely. However, for a DM1 patient who is functionally impaired from his or her myotonia, and who has no significant cardiac risk factors, our study suggests to us that the risks and benefits of treatment with mexiletine could be presented to the patient as a therapeutic option, at least for short-term trial of the medication to see if it provides clinical benefit in muscle specific activities of daily living. However, we believe that treatment with mexiletine should not be used for nonfunctionally disabling symptoms of myotonia.

Author Disclosure

Dr. Logigian serves on the editorial board of Muscle and Nerve.

Within the past five years, Dr. Moxley has served as principal investigator for research projects funded by the Centers for Disease Control, Food and Drug Administration, Insmed Inc., Muscular Dystrophy Association, and National Institutes of Health.

Dr. Merino performed a one-time consultation with staff from Bell, Falla and Associates.