Electromyography (EMG) and nerve conduction studies (NCS)
These are tests of the function of muscles (EMG) and the peripheral nerves (NCS). They are useful in the diagnosis of muscle disease, diseases of the neuromuscular junction, peripheral neuropathies and anterior horn cell disease.
A needle electrode is placed into muscles and insertional, resting and voluntary electrical activity is studied, using a computer screen and a speaker.
· Denervated muscle shows prolonged insertional activity, fibrillation potentials and positive sharp waves.
· Peripheral neuropathies and anterior horn cell disease lead to a reduced number of motor units, which fire rapidly.
· Anterior horn cell disease – large motor units form, causing visible fasciculations, which can also be shown by EMG.
· Myotonic discharges can be seen, e.g. in myotonic dytrophy, and myopathic changes may occur with any primary muscle disorder. However, a normal EMG does not exclude a muscle disorder.
Surface electrodes or occasionally needles are used both for stimulation and recording:
· Motor and sensory conduction velocities (slow in demyelinating disorders).
· Action potential size – in axonal neuropathies, the conduction velocity and latencies are normal, but the total action potential is reduced in amplitude.
· F wave latencies and velocities are also useful – these are like an ‘echo’ which occurs as a nerve that is stimulated peripherally, the action potential travels up to the spinal cord, and back down – it allows the evaluation of brachial and lumbosacral plexus and nerve roots.
· Repetitive stimulation is useful in the diagnosis of neuromuscular junction disorders – see myasthenia gravis, Eaton–Lambert syndrome.