Invited review
The role of different EMG methods in evaluating myopathy

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Abstract

For the diagnosis of myopathy, EMG may have an important role along with blood tests, muscle biopsies and genetic testing. This review evaluates different EMG methods in the diagnosis of myopathy. These include manual analysis of individual motor unit potentials and multi-motor unit potential analysis sampled at weak effort. At high effort, turns–amplitude analyses such as the cloud analysis and the peak ratio analysis have a high diagnostic yield. The EMG can seldom be used to differentiate between different types of myopathy. In the channelopathies, myotonia, exercise test and cooling of the muscle are helpful. Macro-EMG, single-fibre EMG and muscle fibre conduction velocity analysis have a limited role in myopathy, but provide information about the changes seen. Analysis of the firing rate of motor units, power spectrum analysis, as well as multichannel surface EMG may have diagnostic potential in the future. EMG is of great importance in the diagnosing of patients with myopathy, preferably a needle electrode and quantitative analyses should be used. A combination of a method at weak effort as well as a method at stronger effort seems optimal.

Introduction

For more than 50 years, EMG has been used as a diagnostic tool in patients with myopathy. Following the introduction of the manual motor unit action potential (MUP) analysis at weak effort, several other methods have been introduced, including automatic analysis of MUPs and analysis of interference pattern at stronger effort. Today the EMG still has an important role in the evaluation of myopathy along with clinical findings, muscle biopsies and gene analyses.

To evaluate an EMG method correctly, a well-described patient material diagnosed by other means such as clinical findings, blood tests and muscle biopsies is required as well as a comparable control material. If a new method is applied on patients in a very advanced stage, the sensitivity may be high, but this is not very useful. When introducing a new method, a comparison to a well-known method is important in order to assess the severity of the changes in the muscle. In order to obtain information on specificity, a group of patients with neurogenic muscle lesion should also be examined. In the following, the author has reviewed the literature on the role of EMG in evaluating myopathy, selected the articles as judged relevant by the author, and, when possible, taken into account the above considerations in the evaluation of the significance of a study. The clinically most used quantitative EMG methods, i.e. MUP analyses and turns–amplitude analyses, are described in more detail than quantitative methods or less used methods. An evidence-based medicine analysis has not been performed, as this review is not a guideline programme. A guideline programme has to be built on an evidence-based medicine analysis that includes a complete computer research of the existing literature. However, most of the papers on diagnostic EMG do not score more than class IV for evidence support, i.e. they do not include a prospective design, a gold standard, or a blind evaluation (Dubinsky et al., 2003, Marciniak et al., 2005). In future, studies on diagnostic EMG methods are suggested to use the recently published STARD initiative for diagnostic tests with 25 different checkpoints (Bossuyt et al., 2003).

One should realise that while EMG is useful in detecting myopathic changes in general, it is most often of no help in the differentiation of different forms of myopathy. For example, in a limb girdle muscular dystrophy group where genetic studies have determined the molecular basis of a number of these clinical syndromes, the EMG cannot differentiate between syndromes but show typical signs of myopathy (Fawcett and Chinnery, 2003). Some disorders may show myopathic EMG in association with normal muscle biopsy as in thyrotoxic myopathy (Ramsay, 1974) or systemic lupus erythematosus (Vilppula, 1972). Other disorders may show myopathic or neurogenic muscle changes, or both, as in mitochondriopathy (Finsterer and Fuglsang-Frederiksen, 1999, Girlanda et al., 1999, Torbergsen et al., 1991) or in myotonic dystrophy (Jamal et al., 1986, Mondelli et al., 1993). A few indications of when a method may be useful in the differentiation of some forms of myopathy are included in the following, when appropriate for the method. A more detailed description of the EMG changes in the different forms of myopathy is not included in this review. Many of the studies on diagnostic EMG in myopathy are on a mixture of diseases. It would be more informative if one method could be tested separately on different types of myopathy. However, often these types of myopathy are so rare that this is not possible in practice.

Although, the main focus of this review is on EMG in myopathy, other methods are mentioned when valuable additional diagnostic information can be obtained as, e.g. exercise test in channelopathies and direct muscle stimulation in critical illness patients.

Section snippets

Fibrillation potentials

Fibrillation potentials are spontaneous di-triphasic potentials with a positive start, a duration of 2–3 ms, and with an amplitude of 100–200 μV (Buchthal and Rosenfalck, 1966), and are, together with positive sharp-waves, called denervation potentials. Fibrillation potentials have been believed to be specific for chronic denervation (Denny-Brown and Pennybacker, 1938). However, the finding of fibrillation potentials is unspecific, as they may be seen in 5% of sites examined in a normal muscle as

MUP analysis

The MUPs can be analysed using qualitative visual inspection of MUPs on the screen or quantitative manual or automatic analysis of MUPs from a representative number of motor units. Qualitative visual inspection of MUPs on the screen may be influenced by bias. Qualitative methods may be reliable when the disorder is at an advanced stage but not in borderline cases.

Fibre density and jitter

The fibre density obtained by a multielectrode was normal in patients with muscular dystrophy, myotonic dystrophy, and polymyositis (Buchthal et al., 1960). The fibre density obtained by a single fibre electrode may be increased in patients with myopathy (Fawcett et al., 1985, Hilton-Brown and Stålberg, 1983a, Hilton-Brown and Stålberg, 1983b, Salmi, 1985). Similarly, an increased fibre density obtained by a macro-EMG electrode is seen in some patients with myopathy (25–75%) (Barkhaus et al.,

Analysis of motor unit firing rate

Visual qualitative analysis of the firing rate of motor units has been related to the degree of muscle contraction (Petajan, 1974, Petajan, 1991). In myopathy, early recruitment may be seen, i.e. too many MUPs are present for the level of muscle contraction compared to controls. The impression of an increased firing rate in patients with myopathy is possibly false, at least at low force levels. Owing to the weakness of the muscle, the patient has to use a relatively higher force associated with

Quantitative interference pattern analysis

In patients with myopathy, the interference pattern at maximal voluntary contraction may be full with decreased amplitude even in a muscle with decreased force. In clear-cut myopathic cases, visual analysis of the interference pattern may be diagnostic while in less clear-cut cases the use of a quantitative method is recommended. In muscles from patients with myopathy at a very advanced stage, the interference pattern may show reduced recruitment due to pronounced loss of muscle fibres

Surface EMG

There have been many attempts to use surface EMG in the diagnosis of myopathy. The advantage of surface EMG, especially in children, is that it is painless. The surface electrode has, however, an inherent problem with respect to diagnosis of patients with myopathy. The distance and the tissue between the muscle fibres and the skin will decrease the amplitude of the EMG signal and react as a tissue filter decreasing the high-frequency component of the EMG signal (Christensen and

Muscle fibre conduction velocity

Propagation in conduction velocity along the muscle fibre in normal subjects has been determined during voluntary effort (Denslow and Hassett, 1943, Stålberg, 1966) and by stimulation (Buchthal et al., 1955). In patients with myopathy, the propagation velocity or the conduction velocity of the muscle fibres has been obtained by surface electrodes during voluntary contraction or with needle electrodes using electrical stimulation of muscle fibres.

Direct muscle stimulation

In critical illness patients with paralysis, it may be difficult to evaluate the EMG as the patient may not be able to cooperate. Fibrillation potentials may be seen both in critical illness neuropathy and in critical illness myopathy. In order to differentiate between neuropathy and myopathy in these cases, a method using direct muscle stimulation was suggested (Rich et al., 1996, Rich et al., 1997). In muscles with myopathy, the electrical muscle excitability may be decreased while it is

Decrement test and short exercise test in patients with myotonic syndromes

In most patients with myotonic syndromes, repetitive stimulation of a nerve at, e.g. 5–10 Hz will show a decrement of the compound muscle action potential, probably caused by transient inexcitability of the muscle fibre membrane (Aminoff et al., 1977, Brown, 1974, Colding-Jørgensen et al., 2003, Lambert et al., 1952, Streib, 1984). The repetitive stimulation may in addition provoke spontaneous myotonic activity (Lambert et al., 1952). In patients with myotonic dystrophy with most pronounced

Advantages, limitations and pitfalls of different EMG methods

Qualitative EMG analysis with visual and auditory evaluation of the properties of a number of motor unit potentials sampled at weak effort, the motor unit firing rate and the interference pattern at maximal effort seems of value in evaluating myopathic changes in the muscle if the changes are clear-cut or the examiner is experienced, or both. The drawbacks with qualitative visual/auditory EMG analysis are: (1) the judgment of the properties of the MUP may be biased by the examiner's

Practical recommendations

It is a fact that some physicians use quantitative EMG and others use qualitative EMG for analysis of MUP properties and interference pattern, while all physicians analyse the spontaneous activity, qualitatively (Fuglsang-Frederiksen et al., 1995). What is preferable depends on the disorder and the chosen strategy of the examination. Qualitative visual analysis of MUPs and the interference pattern may be diagnostic in patients with clear-cut changes, but may be biased and misleading in patients

Acknowledgements

The author would like to thank Peter Fawcett, Newcastle, for help with the English language.

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