Invited reviewThe role of different EMG methods in evaluating myopathy
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.
References (229)
- et al.
Neural network analysis of the EMG interference pattern
Med Eng Phys
(1996) - et al.
Spontaneous electrical activity of human muscle
Electroencephalogr Clin Neurophysiol
(1966) - et al.
Discriminant analysis of various concentric needle EMG and macro-EMG parameters in detecting myopathic abnormality
Clin Neurophysiol
(2002) - et al.
A quantitative electromyographic index that is independent of the force of contraction
Electroencephalogr Clin Neurophysiol
(1982) - et al.
Conduction study in human muscle fibers in situ—a useful technique for diagnosing myopathies
Electroencephalogr Clin Neurophysiol
(1984) - et al.
Power spectrum and turns analysis of EMG at different voluntary efforts in normal subjects
Electroencephalogr Clin Neurophysiol
(1986) - et al.
Processing of electrical activity in human muscle during a gradual increase in force
Electroencephalogr Clin Neurophysiol
(1984) - et al.
The relationship of motor unit size, firing rate and force
Clin Neurophysiol
(1999) - et al.
Oscilloscopic recording of muscle fiber action potentials. The window trigger and the delay unit
Electroencephalogr Clin Neurophysiol
(1969) - et al.
Motor unit firing rates and firing rate variability in the detection of neuromuscular disorders
Electroencephalogr Clin Neurophysiol
(1989)
A special purpose digital computer (Biomac 500) used in the analysis of the human electromyogram
Electroencephalogr Clin Neurophysiol
Limb girdle muscular dystrophy
Comparison of electrophysiological and histochemical methods for assessing the spatial distribution of muscle fibres of a motor unit within muscle
J Neurol Sci
Macro-EMG in mitochondriopathy
Clin Neurophysiol
Concentric needle EMG versus macro EMG I. Relation in healthy subjects
Clin Neurophysiol
Concentric-needle versus macro EMG. II. Detection of neuromuscular disorders
Clin Neurophysiol
Turn/amplitude parameter changes during sustained effort
Electroencephalogr Clin Neurophysiol
Peak-ratio interference pattern analysis in the detection of neuromuscular disorders
Electroencephalogr Clin Neurophysiol
EMG power spectrum, turns–amplitude analysis and motor unit potential duration in neuromuscular disorders
J Neurol Sci
Electrical muscle activity during a gradual increase in force in patients with neuromuscular diseases
Electroencephalogr Clin Neurophysiol
Pattern of electrical activity and force in normal and pathological muscle: S-index of turns and amplitude
Electroencephalogr Clin Neurophysiol
Turns analysis (peak ratio) in EMG using the mean amplitude as a substitute of force measurement
Electroencephalogr Clin Neurophysiol
Motor unit firing intervals and other parameters of electrical activity in normal and pathological muscle
J Neurol Sci
Variation in performance of the EMG examination at six European laboratories
Electroencephalogr Clin Neurophysiol
Continuous EMG interference pattern analysis and computerised tomography of muscles
Electroencephalogr Clin Neurophysiol
Automatic analysis of the electromyographic interference pattern using the turns: amplitude ratio
Electroencephalogr Clin Neurophysiol
Electrophysiological study of neuromuscular system involvement in mitochondrial cytopathy
Clin Neurophysiol
Quantitative electromyography using automatic analysis. A comparative study with a fixed fraction of a subject's maximum effort and two levels of thresholds for analysis
J Neurol Sci
The declining electrical response of muscle to repetitive nerve stimulation in myotonia
Neurology
Methods for computer-aided measurement of motor unit parameters
Electroencephalogr Clin Neurophysiol Suppl
Adult-onset centronuclear myopathy: evidence against a neurogenic pathology
Acta Neurol Scand
On the selection of concentric needle electromyogram motor unit action potentials: is the rise time criterion too restrictive?
Muscle Nerve
Quantitative EMG in inflammatory myopathy
Muscle Nerve
Quantitative electrophysiologic studies in sporadic inclusion body myositis
Muscle Nerve
Adult acid maltase deficiency
Muscle Nerve
Acoustic and surface EMG diagnosis of pediatric muscle disease
Muscle Nerve
Computer assisted on line measurement of motor unit potential parameters in human electromyography
Electromyography
Centronuclear myopathy
J Neurol
Reference values of motor unit action potentials obtained with multi-MUAP analysis
Muscle Nerve
Muscle-fiber conduction velocity and electromyography as diagnostic tools in patients with suspected inflammatory myopathy: a prospective study
Muscle Nerve
Decomposition-based quantitative electromyography: effect of force on motor unit potentials and motor unit number estimates
Muscle Nerve
The STARD statement for reporting studies of diagnostic accuracy: explanation and elaboration
Clin Chem
Number of potential reversals (turns) and amplitude of the pattern of electrical activity of the abductor pollicis brevis muscle in patients with neurogenic diseases
Acta Neurol Scand
A comparison of two commercial quantitative electromyographic algorithms with manual analysis
Muscle Nerve
Muscle weakness after rest in myotonic disorders; an electrophysiological study
J Neurol Neurosurg Psychiatry
Congenital myotonia in the goat
Brain
An introduction to electromyography
Diagnostic significance of the myopathic EMG
Electrophysiological signs of myopathy as related with muscle biopsy
Acta Neurol (Napoli)
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