Paul J. Blijham

Muscle-fiber conduction velocity and electromyography as diagnostic tools in patients with suspected inflammatory myopathy: A prospective study

Combinations of different techniques can increase the diagnostic yield from neurophysiological examination of muscle. In 25 patients with suspected inflammatory myopathy, we prospectively performed needle electromyography (EMG) and measured muscle‐fiber conduction velocity (MFCV) in a single muscle, using a technique with direct muscle‐fiber stimulation and recording. Results of MFCV were compared with final diagnosis, EMG, and needle muscle biopsy. Diagnostic accuracy of combined MFCV and EMG studies was 72%, compared to 60% for EMG alone. This improvement was due to a gain in specificity. The MFCV did not prove useful in discriminating inflammatory myopathy from other myopathies. Furthermore, we found a correlation of 92% between variability of MFCV and myopathic changes in muscle biopsy. We conclude that the utility of electrodiagnostic examination can be increased if EMG examination is combined with MFCV studies. Muscle Nerve 29: 46–50, 2004

Needle Electromyographic Findings in 98 Patients with Myositis

Background/Aims: Little is known about the distribution of electromyographic (EMG) abnormalities in myositis even though this is relevant in daily practice. Methods: A retrospective semiquantitative analysis of needle EMG findings was performed in a group of 98 patients with myositis. The frequency, type, and distribution of abnormalities were studied. The influence of the use of corticosteroids and the stage of the disease were evaluated. Results: In most patients, a myopathic pattern with spontaneous activity was found, although several clinically relevant exceptions were noted. Long-duration motor unit potentials were found in all three diagnostic groups and were not associated with disease duration. In the lower extremity a distal to proximal gradient was present, adding to the diagnostic confusion with neurogenic diseases, and spontaneous activity was absent in a relatively large group although none of the patients in the acute stage of the disease had a normal EMG. The use of corticosteroids reduced the number of abnormal findings in dermatomyositis and polymyositis, but not in inclusion body myositis. Conclusion: A myopathic pattern with spontaneous activity was most frequently found, although several clinically relevant exceptions were noted. These results illustrate the spectrum of EMG findings in myositis, and may aid the clinician in the interpretation of the EMG in these patients.

Early diagnosis of ALS: the search for signs of denervation in clinically normal muscles.

AIM AND METHODS We prospectively investigated whether early diagnosis of amyotrophic lateral sclerosis (ALS) could be facilitated by demonstrating signs of denervation in a muscle of a clinical and electromyographical unaffected region. Muscle fibre conduction velocity (MFCV) was determined in 18 patients in whom the diagnosis ALS was considered but not established beyond a level of clinically possible ALS according to the revised El Escorial criteria. A muscle biopsy was obtained from the same muscle, to demonstrate neurogenic changes. The study followed the guidelines from the STARD initiative. RESULTS AND CONCLUSION Results were analysed with respect to the final diagnosis. After a mean follow-up of 16 months, 9 patients developed probable or definite ALS. Sensitivity of abnormal MFCV for developing ALS was 89%. Muscle biopsy confirmed that denervation was the cause of abnormal MFCV. We concluded that MFCV can be used to detect denervation in muscles that show no clinical or electromyographical signs of lower motor neuron disease, and thus may contribute to early diagnosis of probable laboratory-supported ALS.

Meralgia paresthetica after strenuous exercise.

We present three patients with signs and symptoms of meralgia paresthetica (MP) after long‐distance walking and cycling. No other possible causes of MP, such as trauma or exogenous compression, were present. A neuropathy of the lateral femoral cutaneous nerve was confirmed in all patients with somatosensory evoked potentials. We propose that conduction block due to local ischemia during repetitive muscle stretching was the probable cause for the neuropathy. Muscle Nerve, 2005

Reduced muscle-fiber conduction but normal slowing after cold exposure in paramyotonia congenita.

In this study we investigated a family with paramyotonia (PC) congenita caused by a Gly1306Val mutation in the voltage‐gated sodium‐channel gene SCN4A. A previous study showed that exposure to cold aggravates the muscle stiffness in patients with this mutation. However, the mechanism behind cold sensitivity and the sodium‐channel defect remained unclear. In order to gain a better understanding of sarcolemmal propagation in these patients, we measured muscle‐fiber conduction velocity (MFCV) invasively. We studied four PC patients and four healthy subjects at room temperature. After the muscle was cooled, MFCV was measured again in the two PC patients and four control subjects. MFCV was significantly lower in the PC patients at room temperature, compatible with dysfunctional sodium channels. After cooling, MFCV was significantly lower in both groups as compared with room temperature. The relative slowing was 1.4% per °C for PC patients and 1.5% per °C for healthy subjects. These results indicate that, in these PC patients, mutant and wild‐type sodium channels respond equally to cold exposure. Thus, MFCV is abnormal in these patients, but the aggravation of muscle stiffness cannot be explained by an abnormal sarcolemmal response to cold. Muscle Nerve, 2007

Recognising F-response interference as a source of increased jitter in stimulated single fibre EMG

OBJECTIVE To determine the source of an abnormal pattern of latency shifts leading to falsely high jitters in single fibre electromyography (SFEMG). METHODS We observed a sudden shortening of the latency to an individual single fibre spike component followed by a gradual return to baseline values during stimulation single fibre electromyography (SFEMG) of the facial muscle. The pattern could be reproduced in healthy controls. RESULTS The sudden decrease in latency proved to follow an additional discharge of the muscle fibre, not due to the external stimulus. This additional discharge was identified as an F-response. CONCLUSIONS The mechanism is thought to be a higher muscle fibre conduction velocity resulting from a temporary increase in stimulus frequency, in the form of an extra impulse along the muscle fibre represented by the F-response. SIGNIFICANCE The typical abnormal pattern should be recognised because it can falsely increase the mean jitter. We advice to increase the time base to 50 ms if this pattern is observed and to exclude the affected potentials from jitter measurements.

Diagnostic yield of muscle fibre conduction velocity in myopathies

We prospectively assessed diagnostic yield of muscle fiber conduction velocity (MFCV) studies in patients with signs and symptoms suggestive of a myopathy. Results were analysed with respect to the final diagnosis, and compared to the reference standard, which was qualitative electromyography (EMG), turns-amplitude analysis (TAA), and muscle biopsy. We included 125 patients, in whom a myopathy was diagnosed in 71, and a neuromuscular disorder was excluded in 54. Sensitivity of MFCV for the presence of a myopathy was 84%, and specificity 83%. Diagnostic yield of MFCV was superior to EMG, TAA, and muscle biopsy in patients with metabolic myopathies, non-dystrophic myopathies, and channelopathies. We concluded that measurement of MFCV is a quantitative EMG technique with a high diagnostic yield. In certain myopathies, MFCV may be more informative than conventional EMG examination.

FC4.2 Conduction velocity changes during stimulated single muscle fibre fatigue

Background: The purpose of this study was to develop a test that measures the changes in propagation of the sarcolemma during fatigue at the level of single muscle fibres. Methods: We have measured muscle fibre conduction velocity (MFCV) of single fibres using an invasive technique described earlier, at 20 Hz stimulus rate to induce muscle fatigue. The following consecutive measurements were made in the biceps brachii of 13 healthy controls: measurements with 1 Hz stimulation (baseline), after a short stimuli train of 20 Hz, after 60 s of 20 Hz stimulation (fatigue), and after 30 s of rest (recvovery). The MFCV of both fast and slow fibres were analysed. Results: Results of the short 20 Hz stimuli showed supernormality: a significant mean increase of 17% of baseline MFCV of the fast fibres, while a non-significant mean increase was found in the slow fibres. During the fatigue test, the amplitude of most fibres decreased to below 25% of baseline values, but some of the faster fibres, usually 1 or 2 per subject, showed a much smaller decline. The initial supernormality of MFCV disappeared during the fatigue test, and the MFCV of some of the slower fibres decreased to below baseline level. All controls showed full recovery of MFCV after 30 s of rest. Discussion: Our results are in concordance with those from in vitro experiments, although the difference in supernormality of fast and slow fibres has not been reported. Further studies on patients with neuromuscular disorders or complaints of fatigue are needed to test the diagnostic value of this procedure.