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PRACTICAL NEUROLOGY 96

2003 Blackwell Publishing Ltd

as the older tests such as acetyl choline receptorantibodies, repeti tive nerve stimulation and thetensilon test.

THE PARAMETERS OF SINGLE FIBRE EMGTo detect abnormal neuromuscular transmis-sion, two parameters are analysed: jitter asan indicator of irregularity of neuromusculartransmission, and blocking as an indicator of failure of transmission (Stlberg and Trontelj1994). Each lower motor neurone arising froman anterior horn of the spinal cord connectswith a number of muscle bres via the termi-nal branches of its axon. With discharge of themotor neurone, all the connected muscle bresre more or less simultaneously. Conduction

of the discharge along the axon is fast but neu-romuscular t ransmission from axon terminal tothe muscle bre at the motor end plate is slower.And the transmission time across the motor endplate varies from one end plate to another. Evenin the same motor end plate, it can differ fromone ring to another. This variation is what ismeant by jitter (Fig. 1a). While the variation intransmission time is least during regular ringof a muscle bre, it increases with irregular r-ing, and is most pronounced when the motorend plate is defective, as in myasthenia gravis

HOW TO DO IT

Mustafa ErtasDepartment of Neurology,

Istanbul Faculty of Medicine,Istanbul, Turkey; E-mail:

[email protected] Practical Neurology , 2003,

3, 9699

INTRODUCTIONDr Donald B. Sanders, when introducing ErikStlberg who invented single bre electromyog-raphy (EMG), at a meeting in Uppsala in 2001,

told a true story about a lady with puzzlingsymptoms, suspected of having myastheniagravis. Finally after the diagnosis of myastheniawas made on the basis of single bre EMG, thelady said to Dr Sanders, Doctor, I just thank theLord he made someone smart enough to gureall this out, so you doctors could help me! Sin-gle bre EMG practitioners are used to hearingwords like this because the technique can be theonly test to show an abnormal result when re-petitive nerve stimulation, the tensilon test andacetylcholine receptor antibodies are negative in

myasthenia gravis. Although the major clinicalrole of single bre EMG is in the diagnosis of myasthenia gravis, it is helpful in some otherconditions, such as the congenital myasthenias,LambertEaton myasthenic syndrome (LEMS)and botulism.

Single bre EMG is abnormal in almost everypatient with generalized myasthenia gravis, andin 90% with ocular myasthenia if one muscleis examined, and in 99% if two muscles areexamined (Sanders 2002). However, althoughit is the most sensitive test, it is not as specic

Single breelectro

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(i.e. there is increased jitter, Fig. 1b). Sometimesthe transmission may be too slow to re themuscle bre at all, so blocking neuromusculartransmission (Fig. 1c).

Another parameter of single bre EMG, un-related to neuromuscular transmission, is bredensity, which indicates the number of musclebres belonging to the same motor unit withinthe recording area of a single bre electrode(Stlberg 1990). This number increases with col-lateral reinnervation (the electrophysiologicalequivalent of type specic grouping in a musclebiopsy from a patient with a neuropathy), andwith muscle bre atrophy in myopathies.

TECHNIQUE

Single bre EMG is performed with a specialsingle bre needle electrode. The recordingpole on the side of the electrode is small enoughto record the action potential of a single musclebre the single bre action potential. TheEMG machine has to be set up to record thesepotentials by using a suitable lter. The low cutlter, at 5 Hz to allow recording of motor unitpotentials in conventional concentric needleEMG, has to be increased to 500 Hz to lter outany slow potentials coming from sources otherthan a single muscle bre. The criteria (AAEM

yography

Figure 1 Consecutive discharges of the same motor unit, recorded from extensor digitorum communis by a single bre needle electrode during voluntary contraction.(a) Traces from a normal subject. Calculated jitter: 24 s. (b) Traces from a myasthenicpatient. Calculated jitter: 56 s. (c) Traces from a myasthenic patient. Impulse blockade insecond trace. Calculated jitter: 91 s IPI: interpotential interval

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be high. Therefore, jitter analysis is done dur-ing stable r ing of the motor unit, usually a fewseconds after the start of ring.

Stimulation single bre EMGThe motor axons of the muscle, either at a pointoutside the muscle (extramuscular axonal stim-

ulation) or within the muscle (intramuscularaxonal stimulation) are stimulated by smallelectrical pulses (a few milliamper pulse inten-sity and a 0.05 ms or less pulse duration) deliv-ered regularly (usually ve pulses per second)by Teon coated needle electrodes. The activestimulation needle electrode (cathode) has to beat least 2 cm away from the recording electrode.As it may not be possible to stimulate only oneaxon alone, more than one single bre actionpotential may appear on the screen. These si-multaneously appearing potentials may or may

not belong to the same motor unit. Because ther ing patterns of these potentials are articiallyelicited, and the machine delivers the electricalpulses regularly, when the stimulation timepoint is used as the trigger any icker (j itter) of any potential on the screen reects its own neu-romuscular transmission change from stimulusto stimulus (Fig. 2). A pitfall in this technique isto deliver the electric shocks at only thresholdlevels for evoking single bre action potentials.In this situation the jitter value is articiallyhigh. To avoid this arti fact, the stimulus strength

must be increased by 1015% above threshold.

AnalysisTwenty different potential pairs (in voluntarycontraction single bre EMG) or potentials (instimulation single bre EMG) from different re-cording points of the same muscle are recordedand 20 jitter values are calculated. For each jitteranalysis, 50100 consecutive traces containingsingle bre action potentials are recorded. MostEMG machines are capable of automatic jitteranalysis. In voluntary contraction single bre

EMG, the time interval between each potentialpair is called the interpotential interval (Fig. 1a)and the mean value of consecutive differencesin the interpotential intervals is called the meanconsecutive difference (the jitter value) calcu-lated by the formula:

MCD = ([IPI1 IPI2] + [ IPI2 IPI3] + + [IPIn-1 IPIn])/(n1)

where MCD is the mean consecutive differenceand IPI is the interpotential interval.

In stimulation single bre EMG, the interpo-tential interval used in the jitter calculations is

Quality Assurance Committee 2001) for accept-ing a single bre action potential are: a stable shape; a rise time of less than 0.3 ms; an amplitude of more than 200 V (Fig. 1a).Although the recording can be taken from anymuscle, the most diagnostically helpful muscles

in myasthenia gravis are extensor digitorumcommunis, orbicularis oculi and frontalis.There are two techniques for activation of mus-cle: by the patients voluntary contraction, andby electrical stimulation of the motor axonbranches within the muscle using a stimulatingneedle electrode (Trontelj and Stlberg 1992).

Voluntary contraction single bre EMGRecordings are taken during slight voluntarycontraction of the examined muscle, main-tained in a steady state. After inserting the needle

into the muscle, the rst step at each recordingsite is to nd at least two separate single breaction potentials, satisfying the single bre po-tential criteria above, and ring simultaneously.Because simultaneously ring potentials have tobelong to the same motor unit except in someextreme conditions such as tremor, and ephap-tic transmission as in demyelinating conditions,the variation in time ji tter between the twopotentials reects changes in transmission atthe two motor end plates. Within the rst fewseconds of the contraction, the ring patterns

of the motor units are unstable and so ji tter may

Figure 2 Consecutive dischargesof the same muscle bre,

recorded from extensor digitorumcommunis in a myasthenic

patient by a single bre needleelectrode following 5 Hz electrical

stimulation. Calculated jitter:48 s.

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the time interval between the beginning of thestimulus arti fact and the evoked single bre ac-tion potential.

Another important parameter is the mean jitter value, which is the mean of the meanconsecutive differences (i.e. of the individual

jitter values). Because a split muscle bre (in

voluntary contraction single bre EMG) ordirect electrical stimulation of a muscle bre(in stimulation single bre EMG) may give riseto no or small ji tter, up to 5 s due to bypassingof the neuromuscular junction, individual jittervalues have to be more than 5 s to be accepted.

To label jit ter in voluntary contraction singlebre EMG as abnormally high, the upper limitgenerally used in daily practice is 55 s for anindividual j itter value and 36 s for a mean jit -ter value. However, the most detailed referenceupper limits for different muscles at different

ages have been dened by a collaborative multi-centre study, and these values are more accept-able (Bromberg and Scott 1994).

In stimulation single bre EMG, the normallimit for individual jitter is 40 s for extensordigitorum communis and 30 s for orbicularisoculi, and the normal limit for mean ji tter valuesis 25 s and 20 s, respectively.

INTERPRETATIONSingle bre EMG is the most sensitive test for ab-normalities in neuromuscular transmission, but

its specicity is low. Although it may be abnormalin all the myasthenic conditions as well as botu-lism, motor neurone diseases and motor neu-ropathies, the test is mostly used for the diagnosisof myasthenia gravis, especially the ocular form.But great care should be taken, both during thetest i tself and in interpretation (Padua et al . 2000)as technical errors and artifacts, or misinterpreta-tion, may result in labelling a person mistakenlyas having myasthenia gravis. This is especially thecase in suspected ocular myasthenia gravis andwhen the other tests are normal (acetylcholine

receptor antibodies, etc.). The orbicularis oculiand/or frontalis muscles should be examinedeven if extensor digitorum communis is normal.

Voluntary contraction single bre EMG is ali tt le more sensitive than stimulation single breEMG (Murgaet al . 1998) but requires good pa-tient co-operation for maintaining stable musclecontraction. With poor patient co-operation, ininfants and young children, and in patients withso much weakness that they cannot maintain astable contraction, stimulation single bre EMGis preferable. Stimulation single bre EMG isalso quicker because there is no need to look for

a single bre action potential pair. Stimulationsingle bre EMG is superior to voluntary con-traction single bre EMG in the LambertEatonmyasthenic syndrome and other presynapticmyasthenic conditions because it may be pos-sible to demonstrate improvement in abnor-mally high jitters by increasing the stimulation

frequency up to 10 Hz or more.In any muscle, with either single bre volun-tary contraction or electrical stimulation, 20 jitteranalyses are performed. According to generallyaccepted guidelines, a study should be consid-ered abnormal if more than 10% of the singlebre action potential pairs (i .e. more than 2 in 20)exceed the normal upper limit for jitter, or haveimpulse blockade, and/or mean jitter exceedsnormal limits. However, in manifest neuromus-cular transmission abnormality, if more thantwo abnormal individual jitter values have been

obtained, there may be no need to analyse further,even if the number of pairs is less than 20. Becauseanticholinesterase drugs may improve the jittersin neuromuscular transmission abnormalities,these kinds of drug should be withdrawn at least3 days before the neuromuscular jitter analysis.However, in serious conditions, in which it maybe dangerous to withdraw the drugs, single breEMG may be performed during their use. But, if the single bre EMG is normal, it should be re-peated after the medication has been stopped.

REFERENCESAAEM Quality Assurance Committee (2001) Practice pa-rameter for repetitive nerve stimulation and single berEMG evaluation of adults with suspected myastheniagravis or LambertEaton myasthenic syndrome: sum-mary statement. Muscle and Nerve ,24 , 12368

Bromberg MB & Scott DM (1994) Single ber EMG ref-erence values: reformatted in tabular form. AD HOCCommittee of the AAEM Single Fiber Special InterestGroup. Muscle and Nerve , 17 , 8201.

Murga L, Sanchez F, Menendez C & Castilla JM (1998)Diagnostic yield of stimulation and voluntary single-ber electromyography in myasthenia gravis. Muscle and Nerve , 21 , 10813.

Padua L, Stlberg E, LoMonaco M et al . (2000) SFEMGin ocular myasthenia gravis diagnosis. Clinical Neuro- physiology , 111 , 12037.

Sanders DB (2002) Clinical impact of single-ber electro-myography. Muscle and Nerve supplement ,11 , S1520.

Stlberg E & Trontelj JV (1994) Single ber electromyography: Studies in Healt hy and D iseased M uscle , 2ndedn. Raven Press, New York.

Stlberg E (1990) Use of single ber EMG and macroEMG in study of reinnervation. Muscle and Nerve ,13 , 80413.

Trontelj JV & Stlberg E (1992) Jitter measurement byaxonal micro-stimulation. Guidelines and technicalnotes. Electroencephalography and Clinical Neuro- physiology , 85 , 307.

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