Computer-aided analysis of muscle fibre conduction velocity in neuromuscular diseases

Muscle fiber conduction velocity in situ revisited: A new approach to an ancient technique

The aim of this study was to measure the muscle fiber conduction velocity (MFCV) in situ in the tibialis anterior muscle in healthy subjects. A total of 36 subjects matched for age and sex were studied. The MFCV was measured with a concentric needle by intramuscular monopolar needle electrical activation at a distance of 50 mm. The mean consecutive difference (MCD) of <5 μs was obtained after a median of 62 muscle fiber action potentials (MFAPs), confirming a direct muscle fiber activation. The measuring latency was at the median point of ascending depolarizing line of the MFAP. The calculated MFCV from 784 MFAPs was 4.10 ± 0.66 m/s, 3.99 ± 0.57 for female subjects (95%, 2.85 to 5.13), and 4.20 ± 0.73 for male subjects (95%, 2.74 to 5.67). The MFCV was 5.22% faster in male subjects. The calculated fast-to-slow MFCV ratio (F/S ratio) was 1.47 for female subjects (95%, 1.27 to 2.54) and 1.67 for male subjects (95%, 1.31 to 3.74). Aging significantly increased the F/S ratio. As the MFCVs mainly depend on the muscle diameter, their assessment is a quick and helpful tool for estimating it. Its variability by the F/S ratio is also a powerful tool in the follow-up of some neuromuscular disorders.

Is thyrotoxic periodic paralysis a disease caused by muscle membrane dysfunction?

INTRODUCTION

Thyrotoxic periodic paralysis (TPP) is characterized by recurrent episodes of reversible paralysis with hyperthyroidism. It is clinically similar to hypokalemic periodic paralysis (HOPP), which features significant ion-channel dysfunction and reduced muscle fiber conduction velocity (MFCV). However, the muscle membrane function in TPP is not known.

METHODS

For 13 patients with TPP and 15 age-matched controls, clinical assessment and serial neurophysiological testing, including nerve conduction, prolonged exercise (PE) testing, and MFCV. were performed.

RESULTS

MFCV values were elevated up to 1 year from the paralytic attack in TPP patients. In the group with a positive PE test, MFCV values were higher. There was no significant relationship between MFCV values and either hypokalemia or hyperthyroidism.

CONCLUSIONS

Although clinical manifestations in TPP are similar to those observed in HOPP, TPP appears to feature an alternate pathogenic mechanism. Specifically, MFCV values increased rather than decreased. Further studies are needed to support these findings. Muscle Nerve, 2016 Muscle Nerve 56: 780-786, 2017.

Optimizing testing methods and collection of reference data for differentiating critical illness polyneuropathy from critical illness MYOPATHIES

INTRODUCTION

In severe acute quadriplegic myopathy in intensive care unit (ICU) patients, muscle fibers are electrically inexcitable; in critical illness polyneuropathy, the excitability remains normal. Conventional electrodiagnostic methods do not provide the means to adequately differentiate between them. In this study we aimed to further optimize the methodology for the study of critically ill ICU patients and to create a reference database in healthy controls.

METHODS

Different electrophysiologic protocols were tested to find sufficiently robust and reproducible techniques for clinical diagnostic applications.

RESULTS

Many parameters show large test-retest variability within the same healthy subject. Reference values have been collected and described as a basis for studies of weakness in critical illness.

CONCLUSIONS

Using the ratio of neCMAP/dmCMAP (response from nerve and direct muscle stimulation), refractory period, and stimulus-response curves may optimize the electrodiagnostic differentiation of patients with critical illness myopathy from those with critical illness polyneuropathy.

Genioglossus fatigue in obstructive sleep apnea

Obstructive sleep apnea (OSA) is a prevalent disorder that may cause cardiovascular disease and fatal traffic accidents but the pathophysiology remains incompletely understood. Increased fatigability of the genioglossus (the principal upper airway dilator muscle) might be important in OSA pathophysiology but the existing literature is uncertain. We hypothesized that the genioglossus in OSA subjects would fatigue more than in controls. In 9 OSA subjects and 9 controls during wakefulness we measured maximum voluntary tongue protrusion force (Tpmax). Using surface electromyography arrays we measured the rate of decline in muscle fiber conduction velocity (MFCV) during an isometric fatiguing contraction at 30% Tpmax. The rate of decline in MFCV provides an objective means of quantifying localized muscle fatigue. Linear regression analysis of individual subject data demonstrated a significantly greater decrease in MFCV in OSA subjects compared to control subjects (29.2 ± 20.8% [mean ± SD] versus 11.2 ± 20.8%; p=0.04). These data support increased fatigability of the genioglossus muscle in OSA subjects which may be important in the pathophysiology of OSA.