Introduction

Electrodiagnosis is a medical examination that utilizes the electrical properties of nerves and muscles to record responses from the stimulation of electrical currents, or to record the electrical activity of muscles using needle electrodes. This examination helps diagnose nerve and muscle disorders. Electrodiagnosis mainly consists of three parts: 1) Nerve Conduction Studies, 2) Needle Electromyography, and 3) Evoked Potentials. These examinations can help diagnose central, peripheral, and muscle diseases through the central nervous system, peripheral nerves, and muscle tissue. In particular, nerve conduction studies and needle electromyography can help detect the nature, location, and severity of abnormal conditions in the lower motor neurons, nerve roots, nerve trunks, neuromuscular junctions, and even muscles, assisting in correct clinical diagnosis, treatment selection, and evaluation of effects and prognosis. 

• Nerve Conduction Study 

Nerve conduction studies are conducted by stimulating nerves with electrodes and recording electrical activity in the sensory nerves or muscles under their control to obtain sensory nerve action potentials, compound muscle action potentials, and the examination of special reflexes such as H-reflexes and F-responses. The examination method uses high electrical stimulation (supramaximal stimulation) to stimulate the tested nerve (except for H-reflexes) to excite all the axons of the nerve at the same time to obtain the maximum response wave. By comparing the latency, amplitude, surface area, and nerve conduction velocity of this maximum response wave with normal values, axonopathy or demyelination can be distinguished. For example, demyelination can cause latency prolongation or slow nerve conduction velocity, while axonopathy or fiber loss can lead to decreased amplitude or surface area. It should be noted that some factors can affect the parameters obtained during the examination, including the examiner's technique, the patient's age, and skin temperature, etc. These factors must be considered to obtain accurate results.

The F response and H reflex are measures of nerve function. The F response measures the strength of a muscle response to a high-voltage stimulation of the nerve, while the H reflex measures the strength of a muscle response to a low-voltage stimulation of the nerve. Repetitive nerve stimulation checks the health of nerve-muscle junctions by using low-frequency (2-3 Hz) or high-frequency (10-20 Hz) electrical stimulation of the nerve and recording the muscle's response. A decrement response in the low-frequency stimulation indicates a serious case of muscle weakness (myasthenia gravis), while an increment response in the high-frequency stimulation indicates a myasthenic syndrome.

• Needle Electromyography

EMG (electromyography) is a diagnostic test that involves inserting electrodes into muscles to record their electrical activity in various states, to determine the characteristics, location, and severity of nerve and muscle diseases. The commonly used electrodes are coaxial electrodes (durable, less interference, but more painful) and single-pole electrodes (larger recording area, less painful, but more interference and easily damaged). The routine EMG examination includes four steps, observing the following electrical activities in order: 1. Insertional activity, 2. Spontaneous activity, 3. The shape of motor unit potential (MUP) during mild spontaneous contraction, and 4. Recruitment and interference pattern of motor unit potential during maximum force contraction.

The shape of the individual motor unit action potential is observed during mild muscle contraction. A motor unit is a functional unit of muscle contraction and consists of a motor neuron, its axon, and the muscle fibers it innervates. When a motor neuron's nerve impulse reaches the muscle fibers it controls, all the fibers contract and an integration of this results in the motor unit action potential. The main parameters for interpreting this include: 1. Amplitude: the potential difference between the highest positive and negative phase wave, which is normally between 200 μv and 5000 μv, and abnormally high or low is abnormal. 2. Duration: the time from when the potential wave leaves the baseline to when it returns to the baseline, which is related to the number of muscle fibers recorded and is normally between 2-15 ms. 3. Surface area: the area contained within the potential wave. 4. Phase: the number of times the waveform crosses the baseline, representing the density of muscle fibers and the integration of discharge, which is normally no more than 4 phases, otherwise known as a multiphasic wave, with each muscle having about 5-15% of multiphasic waves. 5. Turn: the number of times the waveform polarization direction changes. 6. Rise time: the extent to which the needle is close to the muscle fiber. 7. Firing rate: the abnormal motor unit action potentials seen in different diseases and different stages of the disease are different. For example, in the acute phase of neurodegeneration, residual normal motor unit action potentials and some normal duration multiphasic waves can be seen. When nerve endings regenerate, small multiphasic waves can be seen; if there is branch regeneration, a late component appears; in chronic nerve regeneration, it presents as long-duration, high-amplitude multiphasic waves. As for typical abnormal changes in muscle disease, it is characterized by increased discharge frequency, short duration, and low amplitude multiphasic waves.

Conclusion

With the advancement of computer technology, the techniques for electromyography (EMG) diagnostic examinations have also been updated, greatly enhancing its position and value in clinical applications. Whether it be central nervous system or peripheral nerve lesions, or even muscle lesions, EMG can assist with clinical diagnosis and disease localization. This article provides a simple introduction to the basic principles, general operating procedures, and clinical interpretation and applications of EMG, focusing on three main examinations, as a reference for beginners.