POSTGRAD. MED. J. (1965), 41, 325

ELECTRODIAGNOSIS OF MYOPATHICDISORDERS

D. A. H. YATES, M.D., M.R.C.P., D.Phys.Med.Chief Assistant, Department of Physical Medicine, St. Thomas's Hospital, London.

ELECTRODIAGNOSTIC examination (EMG) can beof considerable help in the diagnosis of variousmuscle disorders. However, the electricalabnormalities of diseased muscle are rarelypathognomonic of one particular conditionand, as with other specialised techniques, theresults are of most value when correlated withthe results of other investigations.The examiner should fully appraise the

history, the clinical state of the patient andother available data in order to decide whichmuscles to study and which procedures willproduce the most information with the leastdisturbance to the patient.EquipmentA standard electromyograph contains a

muscle stimulator unit, a nerve stimulator anda high gain multi-stage amplifier with a highdiscrimination ratio and a high input imped-ance. The muscle stimulator unit deliversrectangular pulses of variable amplitude andpreset duration at regular intervals. The nervestimulator unit is isolated from earth tominimize the shock artefact in the recording,and is designed to trigger the sweep of thedisplay oscilloscope, thus providing for theaccurate measurement of the latency of themuscle response.For recording electrical potentials in muscle,

unipolar concentric electrodes are generallyused. In structure these consist of a hypo-dermic needle down the centre of which isthreaded a fine insulated wire, the tip of thiswire being the active recording surface.Following amplification the signals are dis-played on a cathode-ray oscilloscope with atime scale providing 10 and 1 milliseconddivisions and an oscilloscope camera producespermanent records for more detailed analysis.The amplified signals are also fed into a loudspeaker for auditory monitoring.

Electrodiagnostic Procedures1. Nerve Stimulation.

-Threshold of Excitation.-Anomalous Innervation.-Response to Repetitive Stimulation.

2. Direct Stimulation of Muscle.-Intensity-duration Relationships.-Briskness of Response.

3. Electromyographic Exploration of Muscle.-Spontaneous Activity at Rest.-Volitional Activity.-Mean Potential Duration Estimation-Motor Nerve Conduction Studies.-Multi-electrode Studies.

4. Pharmacological Tests.

These procedures, when used in combin-ation, will indicate whether the lesion affectsprimarily the muscle fibres as in the myo-pathies, the neuromuscular junction as inmyasthenia and myotonia, the lower motorneurone as in the neuropathies, or the spinalcord as in the myelopathies.Nerve StimulationThe response to the stimulation of peri-

pheral nerves often gives useful information,and should be performed routinely. If there is alesion of one peripheral nerve its threshold tostimulation may be found to be considerablyhigher than that of other nerves, while inpolyneuritis the thresholds of all peripheralnerves may be raised. The pattern ofmuscle response may reveal anomalous in-nervation or indicate a primary agenesis ofan individual muscle. Repetitive stimulationat different rates up to that producing asustained tetanus is used in investigating themyasthenic disorders and the effects of variouspharmacological agents can be observed.

Direct Stimulation of MuscleSkeletal muscle which has been partially or

totally deprived of its nerve supply shows an

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A

BFIG. 1.-A. Fibrillation and positive potentials.

B. High-frequency discharge.Time scale in 1 and 10 msec. divisions.

increased threshold to short-duration stimuli.The plotting of intensity-duration curves canbe time-consuming and Bauwens (1943) hasadvocated an abbreviated form of this investi-gation to enable more muscles to be examined.He has shown in normal muscle, using current-stabilised pulses, that the threshold for a 1msec. duration stimulus does not exceed twicethe threshold to stimuli of 10 msecs. and 100msecs. duration, the latter being the equivalentof the rheobase. In partially denervated musclethe 1 msec. threshold exceeds twice therheobase, while in totally denervated muscle itmay be so high that a response cannot beobtained. When a muscle is severely fibrosedor agenetic no response can be obtained evento the long duration stimulus.

Electromyographic Exploration of MuscleThe insertion of a needle electrode into a

denervated portion of muscle at rest mayevoke short duration diphasic fibrillation

potentials which can be recognised by theircharacteristic regular clicking sound on theloud speaker. Another characteristic dischargeof denervated muscle is the "positive"potential which is a monophasic spike oflonger duration and is heard as a seriesof regular, dull thuds (Fig. la). In themyotonic disorders the neuromuscular junctionappears to be hyper-excitable. Any mechanicalstimulation of the muscle will evoke a trainof high frequency discharges which producea characteristic "dive-bomber" sound. (Fig. lb).The functional unit in reflex or voluntary

activity is the motor unit which comprises ananterior horn cell, its neurone and branchesand the group of muscle fibres which they inner-vate. Conduction along the fine intramuscularbranches of the lower motor neurone occursso rapidly that all the muscle fibres in a motorunit are activated almost simultaneously. Themean number of muscle fibres per motor unitvaries considerably from muscle to muscle,

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13 r

NORMALT

\300.V ';12

4-I. m.secs--I........1..... II....1....

19MYO PATHY 3

M300)LV ii-,

7. m.secs..... ..... i.. I.. ...II

FIG. 2.-Diagrammatic representation of individualmuscle potentials recorded at minimal volitionto show technique of duration measurement.Time scale 1 and 10 msec.

those of gastrocnemius average about 1600muscle fibres while those of the ocular musclescontain less than 10 fibres. In the limb musclesthese fibres are grouped in 50 to 100 sub-unitswhich lie interspersed with sub-units inner-vated by other anterior horn cells.The active core of a concentric needle

electrode will record spike potentials arisingwithin a distance of 0.5 mm. and within thisarea will lie some 10 to 20 different sub-units.The electrical potentials produced by thesesub-units, somewhat confusingly termed motorunit potentials, summate during increasingvolitional effort as increasing numbers areactivated to a complete interference pattern.The electrical activity produced by increasingvolitional effort indicates whether normal andfull recruitment of motor units occurs. At mini-mal volitional effort it is possible to study theduration, configuration and amplitude ofindividual motor unit potentials, particularlyif photographic records are made. A usefuldiagnostic parameter is to measure thedurations of at least twenty different potentialsand obtain the mean of these durations. (Fig.2). The normal range of mean potentialduration (MPD) can be obtained for indivi-dual muscles by studying control subjects ofdifferent ages, and Fig. 3 shows that the MPDincreases with age. The measurement isaffected by other factors, particularly by theelectrode characteristics, so that independentcontrol figures must be established by each

Control limits at ± 3 X S.E.r=.64 (P<. l)

...-

Control limits at + 3 X S.E.r=.64 (P<.O I)

//'i20 30 40 so 60 70 80

AGE

FIG. 3.-Relation between age (in years) and meanpotential duration (MPD) in deltoid muscle in28 control subjects. Equation of regression lineis: MPD = 0.020 X age + 10.21.Broken lines are 3 standard deviations (3 X 0.40msec. = 1.20 msec.) either side of the regressionline; these are taken as the normal limits.

laboratory (Buchthal, Gold and Rosenfalck,1954).Since the velocity of conduction along a

nerve is proportional to its diameter, a local-ised constriction of a peripheral nerve or adiffuse thinning of its fibres will reduce therate of conduction. The rate of motor nerveconduction can be calculated from the latencyof the muscle response when its nerve isstimulated at different levels. In a normalperipheral nerve the fibres have conductionrates of the same order and evoke a synchro-nised response in the muscle. However, if forany reason the conduction rate in some of thenerve fibres is reduced, this will produce atemporal dispersion of the nerve impulseswhich will be recorded as a scatter of theresponse in the muscle.By using a multi-electrode probe, Buchthal,

Erminio and Rosenfalck (1959) were able tomeasure the spatial distribution of musclefibres innervated by a single anterior horn cell,the motor unit territory.Electrodiagnostic Changes in Muscle Fibre

LesionsIn primary muscle-fibre disease the peri-

pheral nerves and muscles usually respondnormally to electrical stimulation unless gross

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300ULV

A B

FIG. 4.-Full volitional activity patterns.A. Normal muscle.B. Myopathic muscle.Time scale in 1 and 10 msec. divisions.

fibrosis and contracture have occurred.Occasionally fibrillation potentials, the hall-mark of denervation, may be detected inpolymyositis, and rarely in the musculardystrophies, and this is probably due to somesecondary involvement of terminal nerve fibres.Profuse high-frequency discharges are detectedin dystrophia myotonica and myotonia con-genita but may also be found to a lesserextent in polymyositis, hyperkalaemic periodicparalysis and myxoedema.The various myopathic disease processes

pick off individual muscle fibres from the sub-units which consequently produce actionpotentials that are shorter in duration, andsmaller in amplitude, a greater proportionthan normal are polyphasic (Fig. 4), but thesepotentials can still summate to a semblanceof an interference pattern until the late stagesof the disease cause gross muscle destruction.These changes increase the high-frequencycomponents in the electrical activity producedon volition which can be detected as a "crack-liness" on the loud speaker or by electronicfrequency analysis. However, these changesare frequently only detectable in some areasof a myopathic muscle while other areas ofthe same muscle seem normal. A quantitativeand objective estimate of these changes is thusdesirable but this is difficult to obtain fromfrequency analysis. The MPD has been shownto be shortened in myopathic disorders (Kugel-berg, 1949), in polymyositis (Buchthal andPinelli, 1953) and in the myopathies comp-licating thyrotoxicosis, Cushing's Syndrome and

corticosteroid therapy (Yates, 1963). Thismethod produces a quantitative result, andFig. 5 shows the results obtained in sixteen casesof myopathy due to various causes. The resultsof serial examinations can be compared as in thereversible myopathies complicating endocrinedisorders (Fig. 6). where the MPD tends toreturn to normal with recovery.

Similar electromyographic changes occur inthe myopathies occasionally encountered inother diseases, such as carcinoma, dissemin-ated lupus erythematosis, polyarteritis nodosaand rheumatoid arthritis but the EMGprovides no particular differentiating signs.

Buchthal, Rosenfalck and Erminio (1960)have shown a reduction of motor-unit terri-tory in myopathic disorders but the techniquerequires special equipment and is too time-coqsuming for routine use. Other methods areavailable by which the electrical changesoccurring in myopathy can be quantitated. Ifthe complex electrical activity produced onvolition is passed through an integrating cir-cuit following amplification the mean voltagecan be measured. Lenman (1959) has related theintegrated electrical activity produced by amuscle to the mechanical tension achieved andhas shown that myopathic muscle producesrelatively less contractile force than normalmuscle at a similar level of electrical activity.Using a special tracing device Willison (1964)has shown that in a myopathic muscle theelectrical activity pattern shows a higher spikefrequency and sometimes a lower mean spike

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12

2'

%-10

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7

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0

.

.).U

20 30 40 5Q

AGE

FIG. 5.-Mean potential duration in deltoid muscle insixteen cases of myopathy.

amplitude than in a normal muscle operatingagainst the same resistance. Further auto-mation of both these techniques is requiredbefore their application to routine diagnosticwork can be assessed and their reliability indetecting minor degrees of myopathic involve-ment can be estimated. At present theestimation of MPD seems to be the mostreliable method that can be performed onstandard electromyographs.In the later stages of dystrophia myotonica

muscle fibre degeneration occurs and the EMGchanges of myopathy develop in addition tothe myotonia. Sarcoidosis may be complicatedby a myopathy (Crompton and MacDermott,1961), but in addition abnormal nerve con-duction studies and signs of denervation inperipheral muscle may indicate a peripheralneuritis.Muscle wasting occurs as a result of disuse

particularly in relation to painful joints. Theonly EMG abnormality that can be detected inatrophic muscle is an inability to produce acomplete interference pattern on volition,while the individual motor unit potentials havea normal duration.

Myasthenic SyndromesVarious electro-pharmacological tests have

been introduced to investigate these conditionsbut the techniques and their interpretation arecomplicated. For detailed information the readeris referred to a recent review by Simpson

I ..20 30 40

AGE50 60 70 80

FIG. 6.-Mean potential duration in deltoid muscle in10 cases of thyrotoxicosis and four cases ofCushing's syndrome showing correlation betweenclinical weakness and shortening of meanpotential duration. Upward pointing arrowsindicate return of normality following success-ful treatment.

(1964) which gives a full bibliography andonly a summary will be given here.

Myasthenia GravisEMG recording shows that the loss of

power which occurs when the motor nerveto an affected muscle is repetitively andsupramaximally stimulated is accompaniedby a decrement in the size of the evokedmuscle action potentials. This phenomenoncan usually be prevented or reversed bythe intravenous administration of edrophoniumhydrochloride ('Tensilon'), a short-actinganticholinesterase substance. The muscles of amyasthenic subject are more resistant to theneuromuscular blocking action of decameth-onium iodide (C 10) than normal muscles.When a neuromuscular block is produced bythis agent its action can be reversed by edro-phonium in myasthenic muscle but not innormal muscle. (Churchill-Davidson andRichardson, 1952). This test should only beperformed with the full measures for assistedrespiration immediately available. When thedisease is long-standing permanent myopathicchanges develop in weakened muscles.

Carcinomatous MyastheniaAnderson, Churchill-Davidson and Richard-

son, (1952) reported a myasthenic syndrome

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330 POSTGRADUATE MEDICAL JOURNAL June, 1965

associated with a bronchial carcinoma present-ing as an undue sensitivity to succinylcholine.Lambert, Eaton and Rooke, (1956) describedsix cases with pulmonary neoplasm and as-sociated myasthenia. On repetitive nerve stimu-lation a marked increment (up to tenfold) wasobserved in the evoked muscle response. Tetanicfacilitation, as this phenomenon is called, isnot peculiar to this condition but when it isobserved thorough investigation includingtomography of both lung fields is indicated.

Fatigue phenomena which can be reversedby edrophonium and other anticholinesterasesare also sometimes encountered in a varietyof neurological disorders including poly-myositis, muscular dystrophy and motor-neurone disease.

Metabolic Diseases of MuscleHypokalaemic periodic paralysis is a

familial condition, characterised by attacks ofweakness lasting several hours, associated witha low plasma potassium and which are oftenprecipitated by heavy carbohydrate ingestion.During an attack the threshold of peripheralnerves and muscles to stimulation is markedlyraised and no electrical activity is recorded byintramuscular electrodes. Eventually the musclesmay show the EMG changes of myopathy inbetween attacks.Hyperkalaemic periodic paralysis is also a

familial condition but attacks are associatedwith a raised plasma potassium. During anattack the nerves and muscles are relativelyinexcitable but intramuscular electrodes recordfibrillation potentials and high frequencydischarges, and this feature helps to distinguishthis disease from hypokalaemic paralysis(Morrison, 1960). Here again, the muscles mayeventually develop myopathic changes.Weakness is a common complaint of patients

with myxoedema. Astrom, Kugelberg andMuller (1961) claimed to have detectedclinical and EMG evidence of myopathy in 5out of 7 cases. However, Herring and Yates(1965) in a detailed study of ten cases withtypical slow reflexes, found no clinical, electro-diagnostic or biochemical evidence of a truemyopathy, and muscle biopsy showed normalhistology apart from some infiltration of themuscle fibres with mucoid material in twocases.

In many of the myopathic disorderscorticosteroid therapy has a beneficial effect.However, it has been shown that, even inconservative dosage, this therapy may produce

the EMG changes of myopathy in previouslynormal muscles (Yates, 1936). To avoid con-fusion an EMG examination must be per-formed before treatment is commenced toprovide baseline data with which subsequentexaminations can be compared.

It is important to avoid biopsy of a musclewhich has recently been studied by EMGbecause the insertion of needle electrodescauses a traumatic myositis which obscuresthe histological picture for some time. (Woolf,1962).As stated previously, the EMG findings

cannot be reliably interpreted in isolation fromother data. The results should be correlatedwith the clinical features, with the biopsyhistology and with the results of biochemicaltests which should preferably include the serumaldolase and creatine phosphokinase levels.Nevertheless, in a recent study of polymyalgiain which these factors were correlated,Chalmers, Alexander and Duthie (1964) con-cluded that EMG was the most useful singleinvestigation in the diagnosis of myositis.

I am grateful to the Editor, British MedicalAssociation Special Journals for permission toreproduce Figs. 2, 3, 5 and 6 from the articles inwhich they originally appeared.

REFERENCESANDERSON, H. J., CHURCHILL-DAVIDSON, H. C., andRICHARDSON, A. T. (1953): Bronchial Neoplasmwith Myasthenia, Lancet, ii, 1291.

ASTROM, K. E., KUGELBERG, E., and MULLER, R.(1961): Hypothyroid Myopathy, Arch. Neurol.,5, 472.

BAUWENS, P. (1943): Electrodiagnostic Interpreta-tions in Nerve Lesions, Proc. roy. Soc. Med., 37,25.

BUCHTHAL, F., and PINELLI, P. (1953): Muscle ActionPotentials in Polymyositis, Neurology (Minneap.),3, 424.

BUCHTHAL, F., GOLD, C., and ROSENFALCK, P. (1954):Action Potential Parameters in NormalMuscleand their Dependence on Physical Variables,Acta. physiol. scand., 32, 200.

BUCHTHAL, F., ERMINIO, F., and ROSENFALCK, P.(1959): Motor Unit Territory in Different Muscles,Acta. physiol. scand., 45, 72.

BUCHTHAL, F., ROSENFALCK, P., and ERMINIO, F.(1960): Motor Unit Territory and Fibre Densityin Myopathies, Neurology (Minneap.), 10, 698.

CHALMERS, T. M., ALEXANDER, W. R. M., andDUTHIE, J. J. R. (1964): Polymyalgia: Problemsof Differential Diagnosis, Ann. rheum. Dis., 23,123.

CHURCHILL-DAVIDSON, H. C., and RICHARDSON, A. T.(1952): The Action of Decamethonium Iodide(C10) in Myasthenia Gravis, J. Neurol. Neurosurg.Psychiat., 15, 129.

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CROMPTON, M. R., and MACDERMOTT, V. (1961):Sarcoidosis Associated with Progressive MuscularWasting and Weakness, Brain, 84, 62.

HERRING, A. B., and YATES, D. A. H. (1965):Muscular Changes in Myxoedema (in preparation).

KUGELBERG, E. (1947): Electromyogram in MuscularDisorders, J. Neurol. Psychiat., 10, 122.

LAMBERT, E. H., EATON, L. M., and ROOKE, E. D.(1956): Defect of Neuromuscular Conduction

Associated with Malignant Neoplasm, Amer. J.Physiol., 187, 612.

LENMAN, J. A. R. (1959): Quantitative Electromyo-graphic Changes Associated with Muscular Weak-ness, J. Neurol. Neurosurg. Psychiat., 22, 306.

MORRISON, J. B. (1960): Electromyographic Changesin Hyperkalaemic Familial Periodic Paralysis, Ann.Phys. Med., 5, 153.

SIMPSON, J. A. (1964): Myasthenia Gravis andMyasthenic Syndromes in "Disorders of VoluntaryMuscle", p. 336, ed. J. N. Walton, London:J. & A. Churchill.

WILLISON, R. G. (1964): Analysis of ElectricalActivity in Healthy and Dystrophic Muscle inMan, J. Neurol. Neurosurg. Psychiat., 27, 386.

WOOLF, A. L. (1962): "Modern Trends in Neuro-logy", 3rd series, p. 11, ed. D. Williams, London:Butterworths.

YATES, D. A. H. (1963a): The Estimation of MeanPotential Duration in Endocrine Myopathy, J.Neurol. Neurosurg. Psychiat., 26, 458.

YATES, D. A. H., (1936b): Muscular Changes inRheumatoid Arthritis, Ann. rheum. Dis., 22, 342.

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