A useful electrophysiological test for diagnosis of minimal conduction block

Single fiber electromyography and measuring jitter with concentric needle electrodes

This monograph contains descriptions of the single fiber electromyography (SFEMG) method and of the more recently implemented method of recording jitter with concentric needle electrodes (CNEs). SFEMG records action potentials from single muscle fibers (SFAPs), which permits measuring fiber density (FD), a sensitive measure of reinnervation, and jitter, a sensitive measure of abnormal neuromuscular transmission (NMT). With voluntary activation, jitter is measured between two SFAPs with acceptable amplitude and rise time. With activation by axon stimulation, jitter is measured between the stimulus and individual SFAPs. Pitfalls due to unstable triggers and inconstant firing rates during voluntary activation and subliminal stimulation during axon stimulation should be identified and avoided. In CNE recordings, spikes with shoulders or rising phases that are not parallel are produced by summation of SFAPS; these should be excluded and reference values for CNE jitter should be used. CNE and SFEMG have similar and very high sensitivity in detecting increased jitter, as in myasthenia gravis and other myasthenic conditions. However, jitter is also seen in ongoing reinnervation and some myopathic conditions. With SFEMG, these can be identified by increased FD; however, FD cannot be measured with CNE, and conventional electromyography should be performed in muscles with increased jitter to detect neurogenic or myogenic abnormalities. Jitter is abnormal after injections of botulinum toxin, even in muscles remote from the injection site, and can persist for 6 mo or more. This can complicate the detection or exclusion of abnormal NMT.

Single fiber EMG and measuring jitter with concentric needle electrodes

This monograph contains descriptions of the single-fiber electromyography (SFEMG) method and of the more recently implemented method of recording jitter with concentric needle electrodes (CNE). SFEMG records action potentials from single muscle fibers (SFAPs), which permits measuring fiber density (FD), a sensitive measure of reinnervation, and jitter, a sensitive measure of abnormal neuromuscular transmission (NMT). With voluntary activation, jitter is measured between two SFAPs with acceptable amplitude and rise time. With activation by axon stimulation, jitter is measured between the stimulus and individual SFAPs. Pitfalls due to unstable triggers and inconstant firing rates during voluntary activation and subliminal stimulation during axon stimulation should be identified and avoided. In CNE recordings, spikes with shoulders or rising phases that are not parallel are produced by summation of SFAPS; these should be excluded and reference values for CNE jitter should be used. CNE and SFEMG have similar and very high sensitivity in detecting increased jitter, as in myasthenia gravis and other myasthenic conditions. However, jitter is also seen in ongoing reinnervation and some myopathic conditions. With SFEMG, these can be identified by increased FD; however, FD cannot be measured with CNE, and conventional EMG should be performed in muscles with increased jitter to detect neurogenic or myogenic abnormalities. Jitter is abnormal after injections of botulinum toxin, even in muscles remote from the injection site, and can persist for 6 mo or more. This can complicate the detection or exclusion of abnormal NMT.

Sustained atypical myokymia of the abductor pollicis brevis with a focal slowing of the median nerve motor axons at the wrist

OBJECTIVE

We report a case of sustained atypical myokymia associated with short bursts of neuromyotonic discharges involving the abductor pollicis brevis (APB) muscle and describe a useful way of detecting a focal slowing involving a small number of median nerve motor fibers with a concentric needle using the filter setting for single fiber electromyography (EMG).

METHODS AND RESULTS

A 62-year-old woman developed right thumb twitches at regular interval of 1.7-3.3 s (0.6-0.3 Hz), which continued for more than four months. Muscle twitches remained the same during altered hand position, psychological stress, or sleep. A concentric needle inserted in the active zone of the APB muscle revealed myokymic bursts with a characteristic of neuromyotonic discharges. Inching study, stimulating at 5 mm increment along the median nerve and recording with a concentric needle using a filter setting for single fiber EMG, revealed a focal slowing of the motor fibers at a point 5-10 mm distal from the distal crease of the wrist, an entrapment site occasionally seen in the carpal tunnel syndrome. One injection of botulinum toxin type A eliminated the myokymia, which then recurred two and a half years later, showing less prominent muscle twitches.

CONCLUSIONS

Sustained atypical myokymia seen in our case represented bursts of neuromyotonic discharges originated from a focal demyelinating lesion involving a few median nerve motor fibers.

Effect of head and neck position on outcome of quantitative neuromuscular diagnostic techniques in Warmblood riding horses directly following moderate exercise

REASONS FOR PERFORMING STUDY

There has been growing interest in training techniques with respect to the head and neck position (HNP) of the equine athlete. Little is known about the influence of HNP on neuromuscular transmission in neck muscles.

OBJECTIVE

To test the hypothesis that different HNPs have effect on single fibre (SF), quantitative electromyographic (QEMG) examination and muscle enzyme activity directly after moderate exercise.

METHODS

Seven Warmblood horses were studied using a standard exercise protocol in 5 HNPs: HNP1: unrestrained; HNP2: neck raised; bridge of nose around the vertical; HNP4: neck lowered and considerably flexed, bridge of nose pointing towards the chest; HNP5: neck raised and considerably extended; bridge of nose in front of the vertical; HNP7: neck lowered and flexed; bridge of nose pointing towards the carpus. Mean consecutive difference (MCD) of single muscle fibre potentials and motor unit action potential (MUP) variables (amplitude, duration, area, turns and phases) were recorded in each fixed position directly after exercise at rest using commercial EMG equipment. Muscle enzyme activity was measured before and 4, 6 and 24 h after exercise.

RESULTS

Mean consecutive difference in all HNPs was higher than in HNP1 (22 µs, P < 0.001) of which HNP4 was highest with 39 µs compared to 30 µs in HNP2 (P = 0.04); MCD in HNP 5,7 was with 25 µs lower than in HNP 2 and 4 (P < 0.001). Odds ratio for MCD suggestive for conduction delay or block was 13.6 in HNP4 compared to HNP1 (P < 0.001). Motion unit action potential variables followed the same pattern as MCD. Lactate dehydrogenase (LDH) activity increased in HNP4 at 4 h (P = 0.014), 6 h (P = 0.017) and 24 h (P = 0.038) post exercise and in HNP5 and HNP7 at 4 h (P = 0.037; 0.029).

CONCLUSIONS AND CLINICAL RELEVANCE

HNP4 in particular leads to a higher rise in LDH activity, MCD and MUP variables, indicating that HNPs have effect on variables characterising neuromuscular functionality.

A novel approach to the measurement of motor conduction velocity using a Single Fibre EMG electrode

OBJECTIVE

To evaluate whether, for patients with suspected myelin impairment, the sensitivity of motor nerve conduction studies can be increased by using an SFEMG electrode which makes it possible to study conduction velocity in a small number of axons (SF-CV).

METHODS

We studied 22 consecutive patients with suspected neuropathy through conventional motor conduction study and through SF-CV. For each patient we selected a nerve that was normal at conventional neurography and studied it through SF-CV. Also, we performed SF-CV in 15 healthy subjects. We considered 36 m/s as the low limit of normal SF-CV (the normal value commonly accepted in the literature for the slowest alpha motor axons).

RESULTS

In the healthy subjects we never observed abnormal SF-CV values. Of the 22 patients, in 18 the conventional tests showed abnormal findings suggestive of neuropathy. The remaining 4 patients were completely normal at the conventional tests. Through SF-CV we studied 22 nerves that were normal at the conventional tests. Fourteen of 22 (64%) nerves presented pathological SF-CV test. Half of the patients with normal findings at the conventional tests showed pathological SF-CV test.

CONCLUSIONS

SF-CV evaluation may be useful in detecting early, mild, or partial myelin damage, because it makes it possible to detect nerve conduction slowing when conventional tests are normal.

SIGNIFICANCE

Increasing sensitivity of motor conduction evaluation.

Sensitivity of conventional motor nerve conduction examination in detecting patchy demyelination: A simulated model

OBJECTIVE

To evaluate, in 5 simulated motor nerves with patchy demyelination: (1) the sensitivity of the conventional motor conduction examination; (2) the conduction velocity of single axons (SA-CV).

METHODS

Four damaged segments were simulated in each nerve. Myelin impairment was generated by varying two parameters: (1) percent reduction in conduction velocity, i.e. degree of damage (DEGREE); (2) percentage of affected axons, i.e. extent of damage (EXTENT). Myelin impairment was simulated in axons with different diameters. We evaluated: (1) conduction velocity; (2) temporal dispersion of the negative phase of compound motor action potential (CMAP); (3) amplitude decay of CMAP; (4) SA-CV of 20 randomly-chosen axons.

RESULTS

When the damage involved both large and small axons, the conduction velocity was pathological only when severe myelin damage involved a large number of axons. Temporal dispersion and amplitude decay were more sensitive than conduction velocity in detecting the damage. In damage involving only large axons or only small axons, all parameters remained in the normal range. SA-CV evaluation was much more sensitive than the conventional studies, regardless of the diameter of the damaged axons.

CONCLUSIONS

Conventional studies are not sensitive in detecting minimal myelin damage. Decomposing the CMAPs and randomly studying 20 SA-CVs would increase the sensitivity of damage detection.

SIGNIFICANCE

These results contribute to a better understanding of the relationship between axonal properties and neurophysiological findings in motor nerve demyelination.

Occurrence of nerve entrapment lesion in chronic inflammatory demyelinating polyneuropathy

OBJECTIVE

To evaluate the occurrence of nerve entrapment syndrome in chronic inflammatory demyelinating polyneuropathy (CIDP).

METHODS

We retrospectively evaluated neurophysiologic results of 41 (25 male and 16 female, mean age 49.8, range 11-87) patients with CIDP. We evaluated the frequency of focal neurophysiologic lesion at entrapment site distinguishing two kinds of lesion: (a) true entrapment; and (b) false entrapment on the basis of nerve conduction results.

RESULTS

Occurrence of focal aggression within the entrapment site is similar to that out of the entrapment site in all examined nerves.

CONCLUSIONS

The entrapment sites are not an elective zone of focal autoimmune aggression in CIDP. Therefore, in CIDP patients a true entrapment, neurophysiologically demonstrated, could be a concomitant pathology and if a severe and persistent entrapment worsens functional deficit and symptoms, a surgical decompression could be useful.

Nerve conduction studies in selected peripheral nerve disorders

PURPOSE OF REVIEW

The physiological properties of nerve and muscle are influenced by pathological changes and the aim of this review is to discuss recent contributions of electrophysiological studies to the understanding and diagnosis of selected peripheral nerve disorders. The relationships between pathology and physiology emphasize the close interdependence between electrophysiological studies, clinical deficits and other laboratory information. Attention should be paid to the strengths and limitations of electrophysiological methods, considering their impact on diagnosis and treatment of patients.

RECENT FINDINGS

Several studies have shown particular pathophysiological profiles associated with different antibody subtypes in autoimmune peripheral neuropathies and this association further supports the suggestion of pathological specificity in both acute and chronic neuropathy. The sensitivity and specificity of physiological profiles therefore become increasingly important since some of these neuropathies are accessible to treatment. On the other hand, the pathophysiological and clinical profiles may be heterogeneous in patients with some disorders. This could be related to a more indistinct division between different types of pathology with increased understanding of pathogenetic mechanisms. Moreover, new insights into disturbed axonal function have stimulated attempts to develop methods to explore normal and diseased human nerve function.

SUMMARY

The exploration of axonal membrane and ion-channel function has become accessible using studies of excitability and are of potential value where conventional studies only provide nonspecific evidence of the number of fibers and the integrity of myelin. These studies will presumably become increasingly important in the years ahead considering the lack of understanding of the functional disturbances in axonal neuropathies.