How Single Fibre EMG Moved Into Clinical Routine

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					N E U R O P H Y S I O L O G Y A RT I C L E

                                                   How Single Fibre EMG
                                                   Moved Into Clinical
                                                   History                                                results in a change in organisation of the muscle
                                                   In the late 1950s, conventional EMG had been           fibres that now occur in small or large groups
                                                   proven to be a useful complement in the evalua-        together. On biopsies this is observed as so called
                                                   tion of patients with neuromuscular disorders.The      fibre type grouping. Also in myopathies, the organ-
                                                   method had then been tested in some laborato-          isation is different from normal due to splitting,
                                                   ries for a decade. At this time Dr Jan Ekstedt and     degeneration-regeneration. This parameter is a
                                                   myself were introduced to research in the Dept of      good complement to the conventional EMG when
                                                   Pharmacology, Uppsala. Our mentor, Prof Bárány,        the picture is difficult to interpret.
Prof Erik Stålberg                                 suggested to us to study fatigue of skeletal muscle.      The other parameter is the neuromuscular jit-
developed SFEMG and a number
of other EMG methods such as                       Could this be reduced pharmacologically in the         ter. This is, in the individual motor end-pate, the
Macro EMG, Scanning EMG,                           same way as the heart muscle can be strength-          variability of the trans-synaptic time for a nerve
Multi-MUP analysis. Quantitative                   ened? For that we needed techniques to measure         pulse to activate a muscle fibre. This particular
EMG analysis in EMG for the                                                                               parameter is the most important contribution
                                                   fatigue. Trials with mechanical force measure-
understanding of
pathophysiology in nerve and                       ments with strain gauges in combination with var-      from SFEMG.
muscle disorders is his main                       ious recordings of electrical muscle activity, elec-
research interest.                                 tromyography (EMG), were the basic methods to          Screening the usefulness of SFEMG in
                                                   be used. With insufficient information from these      various neurological disorders
Correspondence to:
Erik Stålberg,                                     routine methods, we concentrated on the                The early steps in the development were to test
Dept Clin Neurophysiology,                         improvements of the EMG methods and tried sur-         the method in a variety of nerve and muscle dis-
Uppsala University,                                face EMG, wire electrodes and conventional con-        orders. It was clear that the jitter parameter was
Sweden.                                                                                                   directly applicable in the diagnosis of myasthenia
                                                   centric needle EMG. Inspired by multielectrode
                                                   studies by Buchthal and collaborators,1 we started     gravis, MG, and other disorders with disturbed
                                                   to construct multielectrodes, but smaller and          neuromuscular transmission. It also contributed
                                                   more selective than those previously used. This        to the understanding of reinnervation dynamics
                                                   ended with various multielectrodes with the            and has found a place in the neurogenic dis-
                                                   recording wires sized 25µm in diameter, that is        eases. In myopathies, special new aspects have
                                                   smaller than a normal muscle fibre, exposed in a       evolved particularly in the field of chan-
                                                   side-port close to the tip of a small cannula. We      nelopathies (myotonic disorders). The results
                                                   recorded brief signals, the nature of which finally,   have been published in a large number of publi-
                                                   after experiments with ischaemia, curare, simula-      cations.4,5 The interested reader is referred to a
                                                   tions, special electrode configurations, were          monograph on SFEMG6 with a new edition under
                                                   proven to represent activity from single muscle        preparation (Stålberg, Trontelj, Sanders, 2010).
                                                   fibers. This resulted in two PhD theses, one on the
                                                   recording method itself and the description of         Technical improvements
                                                   the jitter phenomenon2 and one the measure-            The jitter is based on measuring time variability of
                                                   ments of propagation velocity and “fatigue” in sin-    the order of four to a few hundred µsec, normally
                                                   gle muscle fibres.3 Without substantial work on the    below 50µsec. Initially, the electrodes and ampli-
                                                   original project on fatigue we left the institution    fiers were home built, and the measuring devices
                                                   and moved to the clinical side, Ekstedt to neurolo-    were dependent on separate timers for short time
                                                   gy, and I to clinical neurophysiology. I now had the   intervals (Figure 1). Since the wide testing of the
                                                   chance to test this new method in patients, and        SFEMG in various pathological conditions succes-
                                                   successively implement it for routine use.             sively showed the clinical usefulness, commercial
                                                                                                          electrodes became available and all high level
                                                   What do we measure                                     EMG equipment now have inbuilt software for jit-
                                                   The method of single fibre EMG is much more            ter analysis and other software based on the
                                                   selective than conventional EMG. There are two         results from SFEMG studies (Figure 2). The tech-
                                                   parameters that have a clinical application. One is    nique to obtain single fibre potentials takes some
                                                   the assessment of fibre concentration within a         training and manual skill. It has therefore been of
                                                   motor unit, a parameter called Fibre Density (FD).     utmost importance that the online signal analysis
                                                   The background is that muscle fibres, innervated       can be made with user friendly software.
                                                   by the same neuron with connected axon, are ran-
                                                   domly distributed within an area of 5-15 mm, and       Applications and Present indications
                                                   separated from each other by about 200µm. In           The jitter parameter is the most sensitive physio-
                                                   cases of reinnervation the surviving motor unit        logical test of neuromuscular transmission.4 It is
                                                   will innervate denervated neighboring muscle           possible to detect even subclinical disturbance,
                                                   fibres. This so called collateral reinnervation        i.e. before the patient has symptoms in that mus-

                                                                                                                                    N E U R O P H Y S I O L O G Y A RT I C L E

                                                                                                                                                      Figure 2: The author and an
                                                                                                                                                      engineer (2009) from the
                                                                                                                                                      Dept of Clinical
                                                                                                                                                      Neurophysiology using
                                                                                                                                                      modern EMG equipment,
                                                                                                                                                      developed in the
                                                                                                                                                      Department (by S Stålberg
                                                                                                                                                      and the staff, and by Dantek
                                                                                                                                                      A/S, Copenhagen,
                                                                                                                                                      Denmark). Note the size
                                                                                                                                                      reduction of this compact,
                                                                                                                                                      multi purpose EMG equip-
                                                                                                                                                      ment for routine use with
                                                                                                                                                      inbuilt computer connected
                                                                                                                                                      to a laboratory network.
Figure 1: The author and his engineer (1977) in front of their
assembled EMG recording system. Large, multi-knobbed and
very flexible equipment connected to a separate computer.        Reinnervation dynamics (larger jitter in the        •   SFEMG should be applied in patients with
                                                                 phase of ongoing reinnervation, larger jitter in        suspicion of neuromuscular disturbance,
cle. The method has therefore become a rou-                      active than in chronic myositis).                       particularly when RNS has been negative.
tine method in many laboratories all over the                    FD                                                  •   SFEMG can be used to assess the dynamics
world, and is recommended to be used when                        Reinnervation                                           of neurogenic and myogenic disease. An
the repetitive nerve stimulation (RNS) test is                     Degree of involvement.                                increased jitter indicates active reinnerva-
negative. It should be stated that increased jitter                Complement to EMG when results are                    tion after denervation that has taken place
is not equal to MG, but indicates disturbed neu-                   uncertain.                                            during the last 3-6 months.
romuscular transmission. One such situation is                   Myopathy                                            •   The electromyographer may also use
during ongoing reinnervation. The sensitivity in                   When conventional EMG is uncertain.                   SFEMG in situations when conventional
MG has been tested by many authors. The                          Special indications                                     EMG has given uncertain results. The addi-
largest study is published by Sanders and col-                     For spike triggering (motor unit counting,            tional information may then help under-
laborators.7 They find a correlation to severity                   Macro EMG, Scanning EMG), Firing rate of              stand the condition better.
of the MG. In 503 MG patients, the sensitivity of                  individual motor units.                           •   SFEMG can be performed in all muscles
SFEMG was 97% in ocular MG if two muscles                                                                                where conventional EMG can be per-
are studied and 99% in generalised. They have                    New developments                                        formed.
also shown that the degree of jitter follows the                 Over the last 10 years, the use of re-sterilised    •   Small children, unconscious patients can
clinical situation and that the method can be                    products has been discouraged or abandoned              be studied by using stimulation SFEMG.
used in monitoring over time.                                    due to potential risk of prion infections. The
   Usually SFEMG recording is performed                          SFEMG electrodes can be used for hundreds
under slight voluntary activation of the mus-                    of investigations if properly maintained, but
cle. Some investigators prefer to use stimula-                   are too expensive for single use. Therefore
tion SFEMG. Here a monopolar electrode is                        there has been a great interest in alternatives     REFERENCES
inserted in the muscle as stimulating cathode                    with disposable electrodes. One such alterna-       1. Buchthal F, Guld C, Rosenfalck P. Multielectrode study of
and the individual endplates are studied.8 The                   tive is the conventional concentric electrode          the territory of a motor unit. Acta Physiol Scand
advantage is that no patient cooperation is                      used with special filter settings on the ampli-        1957;39:83-103.

                                                                 fiers. A small facial needle electrode seems to     2. Ekstedt J. Human single muscle fiber action potentials.
necessary (small children, unconscious
                                                                                                                        Acta Physiol Scand 1964;61:1-96.
patients, patients with movement disorders). In                  be a reasonable replacement.10 Reference val-
                                                                                                                     3. Stålberg E. Propagation velocity in single human muscle
addition, the neuromuscular junction can be                      ues have been established in a few individual          fibres. Acta Physiol Scand 1966;suppl 287:1-112.
studied during different stimulation frequen-                    laboratories (for summary see reference 11)         4. Trontelj JV, Stålberg E. Single Fiber and Macro-
cies, one way to differentiate presynaptic from                  indicating that the jitter values for concentric       Electromyography. In: Bertorini TE, ed. Clinical Evaluation
                                                                                                                        and Diagnostic Tests for Neuromuscular Disorders.
postsynaptic defects. Stimulation SFEMG has                      recordings are about 3-5µsec lower than those          Woburn, MA, USA, Butterworth-Heineman / Elsevier
been used in the study of congenital myas-                       published from a multicenter study with regu-          Science; 2002. pp 417-47.
thenic syndromes.9 Stimulation is performed                      lar SFEMG.12 This new possibility is therefore      5. Sanders DB, Stålberg E. Single-fiber electromyography.
with a small needle electrode near the facial                    accessible for all electromyographers, since           Muscle Nerve 1996;19:1069-83.

nerve, and recording is performed in the                         the electrode has the same low price as other       6. Stålberg E, Trontelj JV. Single Fiber Electromyography in
                                                                                                                        Healthy and Diseased Muscle. New York: Raven Press;
frontalis or orbicularis oculi muscle.                           disposable electrodes. Great care should be            1994. 1-291 p.
   The fibre density, FD, parameter is used to                   taken during recording and interpretation, and      7. Sanders DB. Clinical impact of single-fiber electromyogra-
evaluate the organisation of the motor unit, as                  some more work is necessary to define some             phy. Muscle Nerve 2002;Supp11:15-20.
a sensitive indicator of abnormality, neurogenic                 details of the technique.                           8. Trontelj JV, Stålberg E. Jitter measurement by axonal stim-
                                                                                                                        ulation. Guidelines and technical notes.
or myogenic. No special measuring software is                                                                           Electroencephalogr Clin Neurophysiology 1992;85:30-7.
necessary. FD measurements can be performed                      Summary                                             9. Tidswell T, Pitt MC. A new analytical method to diagnose
in all EMG equipment where the sweep can be                      • The principles of SFEMG should be known              congenital myasthenia with stimulated single-fiber elec-
triggered. This is often a parameter that is                       by all electromyographers, since the single          tromyography. Muscle Nerve 2007;35:107-10.
included in the complete SFEMG study.                              fibre action potentials are the basic compo-      10. Stålberg EV, Sanders DB. Jitter recordings with concentric
                                                                                                                         needle electrodes. Muscle Nerve 2009;40:331-9.
The SFEMG indications can be briefly sum-                          nents of the motor unit potentials. The jitter
                                                                                                                     11. Kouyoumdjian JA, Stålberg E. Reference jitter values for
marised as follows:                                                seen in SFEMG can also be observed in the             concentric needle electrode in voluntary activated Extensor
Jitter                                                             motor unit potentials as variability in its           digitorum Communis and Orbicularis Oculi muscles.
                                                                                                                         Muscle Nerve 2008;37:694-9.
Neuromuscular transmission disorders                               shape at consecutive discharges. Thus, this
                                                                                                                     12. Gilchrist J, Barkhaus PE, Bril V, Daube JR, DeMeirsman J,
   diagnosis,                                                      should be interpreted as an indicator of dis-
                                                                                                                         Howard J, Jablecki C, Sanders DB, Stålberg E, Trontelj JV,
   distribution between muscles,                                   turbed neuromuscular transmission, a use-             Pezzulo J. Single fiber EMG reference values: a collabora-
   monitoring.                                                     ful observation from conventional EMG.                tive effort. Muscle Nerve 1991;15:151-61.

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