Electromyography (EMG)

Electromyography is the recording of electrical activity from muscle tissue and represents an important diagnostic tool to monitor nerve root function and motor neuron activity. Motor neurons generate action potentials during voluntary movement, when electrically stimulated, and in response to disease or injury. An electromyograph can detect these changes in electric potential.

The Neuromuscular Junction

Individual motor neurons from the anterior horn of the spinal cord and from the motor nuclei in the brainstem send axonal projections to the periphery where they form synaptic connections with multiple muscle fibers. This interface between an axon terminal of the motor neuron and a single muscle fiber is known as the neuromuscular junction. The alpha motor neurons release acetylcholine as a neurotransmitter. Acetylcholine binds to nicotinic acetylcholine receptors located on the cell membrane of the muscle fiber, a process that can lead to depolarization and muscle contraction.

Electromyography Recording

EMG activity can be recorded using different types of electrodes, including monopolar needles, concentric needles, bipolar needles, and single-fiber needles. Without the use of high and low frequency filters, EMG signals would be very noisy and difficult to interpret. The low frequency filter should be set to 10-30 Hz and the high frequency filter to 10-20 kHz, for example.

1. Spontaneous EMG. After the recording electrodes are inserted into the muscle tissue, the background EMG activity is stable and quiet under healthy conditions. However, in the presence of an injury or a pathology, spontaneous EMG activity is generated by the motor neuron or post-synaptically at the level of the muscle fiber. Examples of spontaneous activity arising from the muscle fiber include fibrillation potentials, positive sharp waves, myotonic discharges, and complex repetitive discharges. Examples of spontaneous activity arising from the motor neuron include neuromyotonic tonic discharges, myokymic discharges, and tremors.
2. Stimulated EMG. Electrically stimulated EMG activity is used in IONM to monitor motor neuron or cranial nerve excitation by determining the stimulus threshold required to elicit a compound muscle action potential (CMAP). We can determine the stimulus threshold by ramping up the electrical current slowly until a CMAP is recorded. A CMAP is typically a large amplitude event and represents the cumulative activation of multiple muscle fibers.
3. Single Fiber EMG

Cranial Nerves

For IONM involving the cranial nerves, the purpose of electrical stimulation is to identify the location of the nerves in the tissue. With that knowledge, the surgeon can then avoid making contact with them.

1. Facial Nerve.
2. Cranial Nerve III IV and VI
3. Other Cranial Nerves

Intraoperative Monitoring

1. Pedicle screws
2. Peripheral Nerves
3. Brachial Plexus
4. Dorsal Rhizotomy
5. Skull Base Tumor
6. Others

Anesthesia and Other Factors

1. Anesthesia Methods
2. Muscle Relaxants
3. Temperature
4. Tourniquet
5. Others