Evoked isometric muscle contractions in myopathies: analysis of pathophysiological properties by different stimulus patterns

Neuromuscular excitability properties in myotonic dystrophy type 1

OBJECTIVE

To study neuromuscular excitability in patients with dystrophia myotonica type 1 (DM1).

METHODS

The neuromuscular recovery cycle following motor nerve stimulation was assessed in 16 DM1 patients who had no sign of peripheral neuropathy or diabetes. Compound muscle action potentials were recorded from the adductor digiti minimi muscle to ulnar nerve stimulation at the wrist. Paired pulses were delivered, consisting of a conditioning stimulus of supramaximal intensity, followed by a submaximal test stimulus. Interstimuli intervals (ISIs) ranged between 1 and 8ms. Durations of the absolute and relative refractory periods (ARP, RRP) and percentages of refractoriness and supernormality at ISIs of 2.6 and 7ms, respectively, were computed using a subtraction method. The results obtained in the series of DM1 patients were compared to those obtained in six patients with other forms of myotonia and to normative values established in a series of age-matched healthy subjects. Correlations were made between excitability parameters, the number of cytosine-thymine-guanine (CTG) repeats, and the severity of myotonia, scored clinically.

RESULTS

Compared to controls, DM1 patients presented prolonged durations of ARP and RRP, increased refractoriness and reduced supernormality. The decrease in refractoriness correlated with both the number of CTG repeats and the severity of myotonia.

CONCLUSIONS

Changes in the recovery cycle following supramaximal motor nerve stimulation revealed the existence of subtle alterations of neuromuscular excitability in DM1 patients.

SIGNIFICANCE

Increase in refractoriness together with a reduced supernormality was consistent with a process of membrane depolarization. Such a depolarization may be related to the loss of chloride channels or to alterations in sodium conductance in the motor axon or the muscle fiber.

Mechanomyographic determination of post-activation potentiation in myopathies

OBJECTIVE

There is a need to provide an index of muscle contractility in evaluating myopathies especially in the clinical setting. This study was conducted to investigate if the mechanomyogram post-activation potentiation (MMG-PAP) can be used as an index of muscle contractile force potentiation (force-PAP), if it differs between normal and myopathic muscles, and if it can reflect abnormalities in muscle fiber anatomy.

METHODS

The correlation between MMG-PAP and force-PAP was evaluated in 12 normal subjects after maximum voluntary contraction (MVC) of the biceps brachii muscle. The same method was then applied to study MMG-PAP in 16 patients with myopathies, 16 disease and 25 normal controls. Mean fiber diameters and the proportions of type 1 and 2 fibers in biopsied biceps brachii muscle were determined and compared with MMG-PAP values.

RESULTS

There was a significant positive correlation between force-PAP (197 +/- 148%) and MMG-PAP (135 +/- 68%) immediately after MVC (P < 0.05). The mean MMG-PAP in myopathies (66 +/- 53%) was significantly lower than those of the disease (128 +/- 34%; P < 0.005) and normal controls (120 +/- 56%; P < 0.005). Patients with non-dystrophic myopathies, including those with myositis, had significantly lower MMG-PAP values (38 +/- 20%; P < 0.005) than those with muscular dystrophy (148 +/- 23%). MMG-PAP did not clearly correlate with either type 2 fiber atrophy or type 2 fiber disproportion based on muscle biopsy analysis of myopathic patients.

CONCLUSIONS

This study shows that MMG-PAP can be used as an index of muscle contractility and that it is significantly lower in non-dystrophic myopathies compared to normal subjects. MMG-PAP does not seem to reflect abnormal muscle fiber anatomy.

SIGNIFICANCE

MMG-PAP may become a valuable non-invasive tool in augmenting routine clinical electrophysiologic studies especially in evaluating muscle contractility in myopathies.

Decreased EMG inhibition following electrical stimulation over muscle tendons in myopathies

OBJECTIVE

To assess the central EMG inhibitory action of tendon afferent input in muscle diseases.

METHODS

The EMG inhibition elicited by electrical stimulation over muscle tendons was tested in 13 healthy voluntary subjects and 8 patients who had a primary muscle disease with a mild force deficit. Electrical stimuli were delivered to the tendon of the extensor carpi radialis muscle at the wrist during tonic voluntary isometric contraction at 50% of the maximum EMG level. The EMG signal was recorded by surface electrodes over the extensor carpi radialis muscle.

RESULTS

The prestimulus background EMG level was reduced in 7 out 8 of the patients. Both groups had the same phases of EMG modulation following tendon stimulation (TE1, TI1, TE2) and their latency and amplitude did not differ significantly. Conversely, the area of TI1 was significantly larger (i.e. the inhibition decreased) in patients ([mean+/-SD] absolute area: controls=4.1+/-1.6 mVms, patients=6.9+/-2.9 mVms, P<0.05).

CONCLUSIONS

In muscle dysfunction there are serial 'upstream' changes of central inhibitory systems, probably to maximize the residual muscle power of the affected muscle.