nifedipine in the treatment ofmyotonia in myotonic dystrophy

Journal of Neurology, Neurosurgery, and Psychiatry 1987;50:199-206

Nifedipine in the treatment of myotonia in myotonicdystrophyR GRANT,* D L SUTTON,t P 0 BEHAN,* J P BALLANTYNE*

From the Department ofNeurology*, Institute ofNeurological Sciences, Southern General Hospital, and theDepartment of Clinical Physics and Bioengineering,t Glasgow, UK

SUMMARY Abnormal calcium transport may be implicated in the membrane defect in myotonicdystrophy. A single blind crossover trial of placebo (t.i.d.), nifedipine 10mg (t.i.d.) and nifedipine20mg (t.i.d.), was performed in 10 patients with myotonic dystrophy. The severity of myotonia wasassessed by measuring finger extension time after maximum voluntary finger flexion. A significantimprovement in myotonia, after nifedipine, was recorded by this technique and supported by asubjective improvement in 50% of patients and clinical improvement of greater than 20% in fivepatients. Initial grip strength and muscle fatiguability measured by grip strength ergometry were notsignificantly altered.

Myotonic dystrophy is an autosomal dominant disor-der characterised by myotonia and a predominantlydistal distribution of muscular wasting. Mytonia,manifest by slow relaxation of muscle after con-traction, is clinically most obvious in relaxation of thegrasp, and tends to improve with exercise. Abnormal-ities of the cardiac conduction system, eyes, testes andcentral nervous system are found in some patients.'

There is no satisfactory animal model for myotonicdystrophy. In patients with this disorder, chlorideconductance of muscle membrane is not significantlyaltered but resting muscle membrane potential isreduced.23 Abnormalities of erythrocyte and lym-phocyte membrane are also described suggesting awidespread cell membrane defect.4 5

Treatment of myotonia has been directed at "sta-bilising" the muscle membrane. In 1939, Harveyrecorded an improvement in myotonia in patientstreated with quinine.6 Since then procainamide,7corticosteroids8 and phenytoin,9 have also beenfound to be beneficial in some cases. A small trialrevealed that 56% of cases with myotonia improvedon phenytoin and a similar percentage on pro-cainamide; however, phenytoin produced fewer sideeffects.9 More recently tricyclic antidepressants,'0lithium" and verapamil'2 have been successful inimproving myotonia refractory to more established

Address for reprint requests: Dr R Grant, Magnetic ResonanceImaging Unit, Southern General Hospital, 1345 Govan Rd, GlasgowG51 4TF, UK.Received 3 April 1986 and in revised form 8 July 1986.Accepted 10 July 1986

treatment. In an uncontrolled trial, the calcium chan-nel blocker, verapamil lessened myotonia in five casesresistant to phenytoin. Verapamil may worsen car-diac conduction abnormalities and is not recommen-ded in patients with myotonic dystrophy of whom40% have first degree heart block, over 30% haveinterventricular conduction defects,'3 both of whichmay progress to complete heart block.'4The calcium channel blocker, nifedipine, does not

affect cardiac conduction in man, and side effects areminor and relatively infrequent.'5 A single blindcrossover trial of placebo, nifedipine 1Omg and nif-edipine 20mg was performed in patients with myo-tonic dystrophy to assess the effect on the myotonia,muscle power and muscle fatigue.

Methods

Finger Extension Dynamometer This consisted of a simplegoniometer with battery powered potentiometer, which wasmechanically coupled, via a miniature universal joint to alightweight copper tube 150mm long. A Velcro strap to fixthe finger was connected to the end of the copper tube viaanother miniature universal joint. The finger/tube arrange-ment therefore had freedom of movement in any plane. Thepotentiometer shaft was only rotated by finger extension andflexion but not by lateral movements. The displacementtrace of relaxation and contraction was measured by con-necting the potentiometer output to a modified chartrecorder. The mechanical rotation of the potentiometer waslimited to 290° + 100 (effective rotation 2650 ± 15°).Grip Strength Ergometer Grip strength was recorded by astrain gauge ergometer. This comprised of a handgrip,bonded strain gauge, a strain gauge amplifier and a single

199

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200chart recorder. The strain gauge was bonded on a 3 mmthick steel bar measuring 11Om x 15 mm. The bar wasmounted on a handgrip such that hand pressure on the gripcaused distortion of the steel bar and the strain gauge. Cali-bration of the grip strength dynamometer demonstrated that4-2 Kg of force at the midpoint of the steel bar produced1 cm deflection on the chart recorder. There was a linearrelationship between force and chart deflection in cm., overthe range used. The maximum distortion was adjustable bya screw/nut combination. The foil strain gauge (Showa) hada resting resistance of 119-7 ohm, which formed a variablearm of a conventional bridge circuit powered from a lowvoltage source. During muscle contraction therefore anunbalanced voltage, obtained across the bridge wasamplified by a low noise instrumentation amplifier circuit.The power for the amplifier was derived from the mainframe of the chart recorder via a DC/DC converter. Thisvoltage powered anbther circuit consisting of an oscillatordriving a piezo-electric sounder to give a I Hz metronome.The metronome prompted the subject to give regularmaximum voluntary contractions of the handgrip eachsecond.Chart recorder The chart recorder used was a single chan-nel ink jet ECG recorder (Mingo 14) modified to bypass itsinput stages (that is, ECG amplifier). The output from thestrain gauge amplifier was AC coupled to the driver stage ofthe ink jet galvonometer. The time constant was very high,approximately 100 seconds, to produce a DC response.Technique Before each procedure the chart recorder wascalibrated and the test demonstrated to the subject by theexaminer. After resting for 10-15 minutes at a room tem-perature of 22°C + 1°C subjects were comfortably seated at

.,,,'XTVA

Grant, Sutton, Behan, Ballantynea table and speed of finger flexion and extension was testedfirst using the finger extension dynamometer (fig 1). The sub-ject's hand was placed in a supine position and the Velcrofinger strap, with recorder arm attached, was tightly placedaround the distal phalanx of the index finger. A secondVelcro strap was placed across the palmar aspect of the handand forearm lightly restrained against the table to preventforearm pronation and wrist extension. It was emphasisedthat the important movements were speed of finger flexionand extension when requested and that the forearm shouldnot be pronated. The patient was asked to flex his fingersfully for 2 seconds and then to extend the fingers as quicklyas possible. After full extension for 2 seconds, a similar com-mand was repeated. Ten such cycles were recorded and chartrecorder speed maintained at 10mm/s. Both hands weretested. This active finger extension has been referred to byother authors as "Finger relaxation".9 16 17 Finger extensiontime ("Relaxation Time") after maximum voluntary con-traction was measured. This method provides a lowresistance method which produces an accurate recording ofclinical myotonia.

Grip strength was then assessed, using the ergometer, byasking the subject to squeeze the moulded handgrip astightly as possible every second (fig 2). The piezo-electricmetronome prompted the subject at second intervals and thehandgrip was fully released between auditory cues. Eachhand was tested for 60 maximal contractions and encour-agement given after each 10 second period.

In individual patients, recordings of myotonia and gripstrength were measured at the same time of day and at thesame time after drug administration.Subjects Control values were obtained from 20 age and sex

Fig 1 Finger extension dynamometer.

._,_,.,.;..4 .~ -,,, v

_.




Nifedipine in the treatment ofmyotonia in myotonic dystrophy

matched normal volunteers (10 males, 10 females) withoutclinical evidence of neurological or joint disease. Fingerextension time, initial grip strength and muscle fatigue afterexercise were measured. Ages ranged from 18-65 years. Themean age for males was 40 6 years and 35-2 years forfemales.Ten patients (six male, four female) with myotonic dys-

trophy were studied, aged from 28 to 64 years, mean 40 4years. Diagnosis was established by accepted clinicalcriteria2 and by electromyographic characteristics.'8 Therange of severity of clinical myotonia and dystrophy variedbut, in the past, all had taken phenytoin for myotonia with-out symptomatic relief. Prior to inclusion in the trial allpatients had full clinical neurological examination and elec-trocardiograph.Four patients had first degree heart block and two had

evidence of an interventricular conduction defect. Fingerextension time after maximum voluntary contraction, initialgrip strength and muscle fatigue after I minute of exercisewere measured, in each patient, at the onset of the trial andafter each treatment period of either placebo, one tabletthree times per day for 2 weeks; nifedipine 10mg threetimes per day for 2 weeks or nifedipine 20 mg three times perday for 2 weeks. As a precautionary measure, patients wererandomised to either placebo or nifedipine 10 mg. As no seri-ous side effects occurred at the lower dosage, the patientswere then given nifedipine 20mg. This was a single blindcrossover trial during which no drugs were taken. Erect andsupine blood pressures and ECGs were recorded after each 2

week period of medication. Drug compliance was assessedby counting the number of tablets returned at the end ofeach treatment period.The trial was performed with the informed and written

consent of the patients and the approval of the Ethics Com-mittee of the Institute of Neurological Sciences, Glasgow.

AnalysisCharacteristically the myotonia in myotonic dystrophyimproves with exercise. However, the initial finger extensionafter maximum voluntary contraction may not be accom-panied by the most prolonged myotonia. In order to over-come this variability the mean value of the first five exten-sion times was measured (m ET) (fig 3). In the majority ofpatients myotonia was not marked after the fifth repetition.When testing initial grip strength the mean of the first 10consecutive recordings was taken to represent initial max-imal voluntary contraction (m VC i) (fig 4). The mean of thelast 10 consecutive maximal voluntary contractions, that is,from 50-60 seconds, was taken to represent the maximumforce of voluntary contraction after exercise (m VC e). Thedegree of muscle fatigue at 1 minute was calculated as:

mVCi - mVCe x 100%

Statistical AnalysisTen patients were studied and myotonia measured in bothhands. Analysis of variance was performed to assess whetherthe variability between each hand was significantly different

Fig 2 Grip strength ergometer.

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Grant, Sutton, Behan, Ballantyne

Normal

isRight hand (dominant) II 1(10mm/s)

Contraction RelaxationH H

CH-

RH

Myotonic dystrophyIMale44 yeors (mytonic dystrophy)Righthand (dominont): Is (iOmm/s)

I I

Fig 3 Recording ofcontraction (flexion) and relaxation (extension) times, measured by dynamometry in a normal subjectand a patient with myotonic dystrophy.

from the variability between each patient. This was notsignificant and therefore the 20 individual results were usedfor analysis of myotonia.The Wilcoxon Signed Ranks test for non-parametric data

was applied to the 20 results to take account of largeimprovements in myotonia which might influence theanalysis.When analysing grip strength and muscular fatigue the

results from dominant and non-dominant hands were

assessed. Response to treatment was again tested byWilcoxon Signed Ranks analysis on each group of 10 results.P < 0-05 was considered significant.

Results

In normal subjects the mean finger extension time inmales and females was 0x448 seconds (SD 0 054) and0 470 seconds (SD 0x061) respectively and wassignificantly faster in the dominant hand in males(p < 0-01, Wilcoxon Signed Ranks test) and infemales (p < 0 01). The initial grip strength was

significantly greater in the dominant hand in bothmales (p < 0-05) and in females (p < 0.01). The meaninitial grip strength was 24-09 kg for males and18 40kg for females. Percentage muscle fatigue fornormal males was 13-12% and 23-48% for females(table 1).

Five patients noted a subjective improvement inmyotonia on nifedipine 10mg and five had asignificant improvement on nifedipine 20mg. No

patient described any worsening of myotonia whiletaking nifedipine. Two patients reported an apparentdeterioration and one an improvement on placebo(table 2). Drug compliance was reasonable with allpatients taking at least 88% of the number of tabletsprescribed.

In myotonics, mean finger extension time was

significantly prolonged in males and females withwide inter individual differences. Extension time wasnot significantly faster in the dominant hand in maleor female myotonics, in contrast to the findings in the

42 kgJ

Ergometer

LILJUs10 I-Rate 5mm/s

Fig 4 Grip strength ergometric recording in a normalsubject.

H- I I

I II I

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Nifedipine in the treatment ofmyotonia in myotonic dystrophy

Table 1 Extension times grip strength and % fatigue in normal individuals andpatients with myotonic dystrophy

Mean Extension Mean initial Mean percentageTime (s) Strength (kg) Fatigue (%)

2SD 2SD 2SDNormal subjects (n = 20)

Males dominant 0-436 + 0 106 24 95 + 1 01 10-78 + 18 03Non-dominant 0-461 + 0-098 23-23 ± 5-01 15-43 + 19 54

Females dominant 0-465 + 0 127 19 74 + 4 81 23 02 + 17 62Non-dominant 0476 ± 0-111 1705 + 4-24 23-96 + 2315

Mytonic dystrophy (n = 10)Males 1 800 + 2-542 10-25 + 12 30 12 00 + 28 15Females 1 497 + 1-436 10-78 + 901 15 00 + 12-19

control population. Initial grip strength wasdecreased in patients with myotonic dystrophy, themean initial grip strength in this sample group beinggreater in females than in males. This was in keepingwith the more marked severity of disease in our malepatients. Percentage muscle fatigue was similar inmales and females with myotonia.

Results for mean extension time before and aftermedication are shown in fig 5. The group mean exten-sion time prior to commencing any medication was1-76 s, after placebo t.i.d. this was 1 70 s and after nif-edipine 10mg t.i.d. and nifedipine 20mg t.i.d. was1-44 s and 1-47 s respectively. Applying the WilcoxonSigned Ranks test, mean finger extension time wasnot significantly different between initial and placebot.i.d. but there was a highly significant improvementin myotonia when initial relaxation time was com-pared with nifedipine 10mg t.i.d. (p < 0-01) and nif-edipine 20mg t.i.d. (p < 0-01). Comparing placebot.i.d. with nifedipine 10mg t.i.d. the results just fail toreach significance but did reach significance at the 5%level when compared with nifedipine 20mg t.i.d.When nifedipine 10mg t.i.d. was compared with20mg t.i.d. there was no significant difference infinger extension time. Seven patients showed a greaterthan 10% improvement after nifedipine 10mg t.i.d.on testing, and six showed a greater than 10%improvement after nifedipine 20mg t.i.d. Myotoniain five patients lessened by more than 20% following

nifedipine 10mg t.i.d. and nifedipine 20mg t.i.d.when compared with placebo. Those patients who feltsubjectively improved, in general, also improved ontesting. Of the six patients who showed animprovement in myotonia of greater than 20% ontesting while taking nifedipine, five felt the clinicalimprovement worthwhile enough to remain on medi-cation. The Wilcoxon Signed Ranks test was appliedto compare grip strength prior to medication and gripstrength after placebo t.i.d., nifedipine 10mg t.i.d.and nifedipine 20mg t.i.d. There was no significantincrease or decrease in power after any treatmentstage and these drugs had no significant effect onmuscle fatigue.

Side EffectsNifedipine is well tolerated. Minor side effects such as

headache and lethargy are usually associated withvaso-dilation and invariably disappear with con-tinued treatment. Two patients complained of head-ache and lethargy while on nifedipine 20mg t.i.d.,both of whom developed mild ankle oedema. Onepatient showed slight T wave flattening on nifedipine10mg t.i.d. progressing to T wave inversion in leads 1

and AVL on nifedipine 20mg t.i.d. All patients had adrop in systolic blood pressure between 5 and20 mmHg but none developed symptomatic hypo-tension. Nifedipine should not be used in women ofchildbearing age wishing to have a family.

Table 2 Subjective responses

Placebo Nifedipine 10mg tds Nifedipine 20mg tds

MyotoniaNo change 7 5 5Worse 2 - -

Improved 1 5 5

StrengthNo change 7 7 5Worse 3 2 2Improved - 1 3

203




204

9 3m

c

x

c 2-8

1-

01

* Individualx Group

0

0

* 0

: *

* j0

0

0* 0

0 0S0~~~~~~~~~~~~

X

I0 0

.: 100 i0*

I

Initial Placebo Nifedipine10mg

Nifedipine20mg

Fig 5 Myotonic dystrophic individual andgroup meanextension times before and after medication.

Discussion

It is accepted that the underlying defect in myotonicdystrophy is probably a generalised membrane defectand drugs such as phenytoin which "stabilise" themuscle membrane have been used successfully intreatment of myotonia. Phenytoin is thought to havecalcium channel blocking properties'9 20 and it seemspossible that this may improve myotonia by blockingcalcium transport at a site on the surface membraneor by preventing signal transmission at the tubular-sarcoplasmic reticulum junction. As yet it is uncertainwhether "charge movement" opens calcium channelsin the junctional region or a diffusible chemical trans-mitter such as calcium acts directly or indirectly totrigger Ca2 + release from the sarcoplasmic reticulum.

Grant, Sutton, Behan, BallantyneThe duration of the myotonia may vary depending

on previous work load, time of day, temperature andforce of initial contraction. The assessment of myo-tonia is difficult and various methods have been usedincluding timing the relaxation with a stopwatch,17measuring EMG after discharges following maximalvoluntary contraction,"6 intra-arterial potassiuminfusions to detect changes in myotonia,2' and musclerelaxation time after maximum voluntary contractionusing an isometric hand dynamometer.22

In this study finger extension time from the releaseof a maximum voluntary flexion contraction to fullfinger extension was measured as we are more con-cerned with clinical rather than the electro-physiological assessment of myotonia.

Muscle relaxation rate is determined by metabolicand other factors such as rate of crossbridge cyclingand free energy available for calcium reuptake by thesarcoplasmic reticulum. Wiles and Edwards considerthat relaxation rate may be an index of the capacity ofmuscle to liberate energy23 and this has been used formeasuring rate of energy turnover.24 It has been sug-gested that only part of clinical myotonia isaccounted for by the electrical after-discharge, andthere may also be a defect in relaxation of cross-bridge linkage.25 Calcium uptake is influenced by theCa2 + dependent ATPase which is present in high con-centrations in the membrane of the sarcoplasmicreticulum.26 Patchy abnormalities in the sarcoplasmicreticulum in myotonic dystrophy are early electronmicroscopic features and ultrastructural evidencesuggests that intracellular tubular changes antedatedegeneration of myofibrils.27 High concentrations ofCa2 + have been found in red blood cells and skeletalmuscle in myotonic dystrophy.' If there were anabnormality in the calcium dependent ATPase, or inthe membrane phospholipids, which are necessary forthe optimum activity of the calcium dependentATPase, free calcium may then accumulate in themuscle cell. Increased intracellular calcium may betoxic to mitochondria by activating membrane boundphospholipases which could cause further breakdownof the muscle membrane or by activating neutral pro-teases in the myoplasm would cause breakdown ofmuscle fibres.28 The characteristic damage attributedto calcium activated proteases is selective loss of the Zline material on electron microscopy and this is anearly finding in myotonic dystrophy and had alsobeen demonstrated after prolonged muscular con-traction in normal muscle.29 More recently calciumhas been found to stimulate prostaglandin productionwhich might activate lysosomal proteases or releasefree radicals which cause damage by auto-oxidation.30

Since nifedipine blocks calcium channels in the sur-face membrane of skeletal muscle this may reduce




Nifedipine in the treatment ofmyotonia in myotonic dystrophyintracellular calcium and obviate the above postu-

31 32lated harmful consequences.In this study, 50% of patients reported a subjective

improvement in myotonia after nifedipine and fivepatients showed a greater than 20% improvement ontesting, supporting the findings of Cook et al2 thatcalcium channel blockers are of value in myotonia.No obvious effects on grip strength or muscle fatiguewere apparent after the short treatment course of 2weeks. The longterm effects have yet to be studied.Interestingly in animal models, calcium entry block-ers have been found to be of some benefit in retardingdystrophic symptoms expressed in chickens andcats.33 34 However, a trial of nifedipine in Duchennemuscular dystrophy found it to be safe but to have nosignificant effect.35 Muscle fatigue was not greater inpatients with myotonic dystrophy than in normalcontrols. Whether calcium channel blockers will leadto an arrest of the associated dystrophy is as yetundetermined although the response to these drugsin other forms of muscular dystrophy is notencouraging.

The authors thank Dr Stig Hansen for his help withstatistical analysis, Professor TA Simpson for adviceand use of the ergometer, Eileen McMeekin andCharles Orr of Medical Illustration Department forphotography and art work and Christine Fraser-Campbell for secretarial help.

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myotonia in myotonic dystrophy.Nifedipine in the treatment of

R Grant, D L Sutton, P O Behan and J P Ballantyne

doi: 10.1136/jnnp.50.2.1991987 50: 199-206 J Neurol Neurosurg Psychiatry

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nifedipine in the treatment ofmyotonia in myotonic dystrophy

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