Myopathy

, Ali T. Ghouse² and Raghav Govindarajan³

(1)

Parkinson’s Clinic of Eastern Toronto and Movement Disorders Centre, Toronto, ON, Canada

(2)

McMaster University Department of Medicine, Hamilton, ON, Canada

(3)

Department of Neurology, University of Missouri, Columbia, MO, USA

Electromyography (EMG) is a procedure for screening patients with myopathies, and it remains the most common and the most important technique for assessing the course of these diseases over time. For a definitive diagnosis, molecular genetics and muscle biopsy are required. Fibrillation potentials, myotonic or complex repetitive discharges, positive sharp waves, and polyphasic potentials are nonspecific and can occur in both myopathic and neurogenic lesions.

The most sensitive and specific parameter for myopathy in conventional electrodiagnosis is the EMG pattern generally referred to as “myopathic.” This is one of brief duration, small amplitude, and abundant recruitment of motor unit action potentials upon slight voluntary effort. However, this pattern can also be observed in disorders of the neuromuscular junction or the terminal motor fibers. More advanced techniques, such as macro EMG, single-fiber EMG, scanning EMG, and turns/amplitude analysis have revealed additional possibilities for analysis of the motor unit and the interference pattern see Table 20.1 for differences between neuropathic and myopathic potentials.

Figure 20.1

Myopathic motor unit potential

Table 20.1

Electromyographic findings in various conditions

Abnormality	Fibrillation	Polyphasic motor unit potential (MUP)	Brief small-amplitude polyphasic MUP
Myopathy	+	+	+
Acute denervation	+	–	–
Chronic denervation	±	+	–
Neuromuscular junction disorders	±	–	±

Myopathies with Myotonia

Myotonia is described clinically as the occurrence of delayed relaxation of a muscle after a voluntary contraction or percussion. Myotonia is often easier to recognize on EMG examination than on neurological examination. Myotonic potentials are caused by chronically depolarized muscle membranes and are noted as spontaneous, painless discharges, with waxing and waning of both amplitude and frequency producing a characteristic audio profile, described as sounding like a dive-bomber.

Myotonic potentials are repetitive discharges at a rate of 20–80 Hz. They can be biphasic spike potentials less than 5 ms in duration, or they can be described as positive waves of 5–20 ms in duration that resemble positive sharp waves. A single myotonic potential would look and sound just like a fibrillation potential or a positive sharp wave. Needle insertion and movement, muscle contraction, or tapping the muscle will bring about myotonia.