(EMG)

Fig. 8.1

A motor unit is shown with a recording electrode in proximity. Individual muscle fiber potentials are recorded in sum as the motor unit action potential (MUAP)

The neuromuscular junction is the specialized synapse between the motor neuron and the muscle endplate (Fig. 8.2). The transmission of an excitatory signal is a result of the calcium-dependent release of the excitatory neurotransmitter acetylcholine by the presynaptic element into the synaptic cleft. Acetylcholine binds ligand-gated receptors on the muscle endplate opening nonselective cation channels and resulting in depolarization of the endplate . If you recall from Chap. 2, individual postsynaptic potentials summate either spatially or temporally and may result in depolarization of the endplate to threshold and the generation of an action potential.

../images/310119_2_En_8_Chapter/310119_2_En_8_Fig2_HTML.jpg — Fig. 8.2

The neuromuscular junction. (1) Action potential travels down from the axon to the endplate. (2) Calcium enters the endplate. (3) Acetylcholine is released. (4) Acetylcholine travels down the synaptic cleft. (5) Acetylcholine attaches to ligand-gated receptor. (6–7) © VectorMine|Dreamstime.com

EMG recordings are made from either surface electrodes or needles placed directly into the muscle(s) of interest [8]. Intraoperative EMG testing can involve passive muscle recordings for the purposes of detecting cranial nerve or nerve root irritation (known as spontaneous EMG or S-EMG) or may involve electrical stimulation of neural elements or hardware for the purposes of assessing function (known as triggered EMG or T-EMG). Reporting s-EMG and obtaining t-EMG responses provide the surgeon with real-time information concerning the function of cranial nerves or nerve roots [9]. In order to convey accurate information to surgeons, it is important for IOM technologists to understand the EMG innervations of these cranial nerves or nerve roots. For example, technologists should know that firing in quadriceps corresponds to L2, L3, and L4 nerve root activation , while concurrent firing in tibialis anterior and biceps femoris involves activation of the L5 nerve root (Tables 8.1 and 8.2).

Table 8.1

Muscles used for nerve root monitoring by region

Region	Nerve root	Muscle monitored
	C3	Trapezius
	C4	Trapezius
	C5	Deltoid
	C6	Biceps
	C7	Triceps
	C8/T1	Abductor pollicis brevis/flexor carpi ulnaris
	T2–6	Intercostals
	T7–9	Upper rectus abdominis
	T10–12	Lower rectus abdominis
	L1	Sartorius, iliopsoas
	L2	Rectus femoris, vastus lateralis
	L3	Rectus femoris, vastus lateralis
	L4	Tibialis anterior, rectus femoris
	L5	Tibialis anterior, biceps femoris
	S1	Gastrocnemius, biceps femoris
	S2	Gastrocnemius
	S3	Anal sphincter
	S4	Anal sphincter

Table 8.2

Recording parameters for EMG monitoring

Bandwidth	Sensitivity	Time base
5 Hz–5 kHz	50–100 mV	50 ms

Spontaneous EMG

Spontaneous EMG (s-EMG) is used as a means of monitoring cranial and spinal nerves during surgery. The premise is that impending injury to these structures by stretch, compression, or other forms of mechanical irritation will cause them to increase firing which is detectable as CMAPs in the monitored muscle groups [9–13]. Rare firing may occur from heat/cold exposure, while ischemia usually does not induce action potential firing and thus is poorly detected by EMG. Proper selection of muscles to monitor is key to the success of S-EMG monitoring (see Table 8.1).

Many of the cranial nerves that are routinely monitored with EMG have sensory or autonomic components in addition to the monitorable motor component. In these cases, EMG is used as a sentinel for function of the entire nerve, even if the motor component is the smallest functional component of the nerve. If the motor division of the cranial nerve includes branches, it is appropriate to monitor the muscles innervated by each branch whenever possible (review Table 2.1 in Chap. 2).

Spinal nerves are mixed (sensory and motor) nerves that may be monitored for irritation with spontaneous EMG [9, 11, 13]. Spontaneous EMG monitoring differs from other intraoperative neuromuscular monitoring modalities in that the expected or normal state is the lack of response due to the absence of any muscle activity [11]. This indicates that a normal healthy nerve has not become activated as a result of surgical stimulation. Hence, reporting s-EMG to the surgeon is considered a neurological event or change.

Interpretation of intraoperative EMG depends on a familiarity of the various types of firing patterns commonly seen . Some patterns of spontaneous activity are suggestive of nerve root irritation or injury. If pre-existing nerve root irritation is present, the baseline EMG recording will often contain low-amplitude periodic firing patterns [1] (Fig. 8.3).

../images/310119_2_En_8_Chapter/310119_2_En_8_Fig3_HTML.jpg — Fig. 8.3

Abnormal spontaneous activity . (A) Fibrillations (∗) and positive sharp waves (∗∗) in an acutely denervated hand muscle. (B) Single, doublet, triplet, and multiplet motor unit neuromyotonic discharges. Bursts of discharge are irregular in frequency and the intra-burst frequency of discharge is up to 200 Hz. (C) Fasciculations in the tongue in a patient with amyotrophic lateral sclerosis. The single discharges are irregular and occur on a background of ongoing EMG activity caused by poor relaxation. (D) Myotonic discharges in a patient with dystrophia myotonica . There is a characteristic waxing and waning in frequency. (Reproduced from Journal of Neurology, Neurosurgery & Psychiatry, Mills K, 76, ii32–ii35, Copyright 2005, with permission from BMJ Publishing Group Ltd)

The clinical significance of the EMG firing pattern can be generally considered proportional to the frequency, amplitude , and persistence of the firing. Waveforms occurring at high frequency and amplitude indicate multiple motor units involved and a higher likelihood that the firing pattern is a warning of an impending injury. The correlation of EMG activity with a surgical event (such as retractor placement or hardware insertion) suggests a causative event, and reversal or cessation of the event should result in a return to the baseline EMG pattern. Persistence of EMG firing beyond cessation of the causative event is worrisome and suggests that injury to the nerve may have already occurred.

Random activation of one or a few motor units during surgery may occur with incidental contact with the neural elements and is not considered clinically significant. These waveforms are termed spikes when the activity of one motor unit is recorded or bursts when the waveform is generated by activation of several motor units (Fig. 8.4). It is important to remind ourselves that the MUAP or spike is actually the recording of a compound action potential consisting of the individual muscle fiber action potentials. As such, its morphology is distinctly different than a single action potential generated by a muscle cell or a neuron. Specifically the duration of the event is longer, often several milliseconds. These waveforms are also polyphasic as opposed to biphasic. Spiking or bursting in the EMG indicates proximity to the neural elements and may be a useful information to the surgeon while navigating the field.

../images/310119_2_En_8_Chapter/310119_2_En_8_Fig4_HTML.jpg — Fig. 8.4

An example of EMG spikes (*upper panels*) and bursts (*lower panels*)

Sustained activation of multiple motor units results in firing patterns with a greater degree of clinical significance. EMG “trains” are repetitive prolonged firing of one or more motor units, lasting from seconds to minutes [8]. The length of time a nerve is activated is dependent on the degree of nerve irritation [14]. Significant nerve irritation or nerve damage can produce neurotonic discharges in which no individual muscle action potentials are distinguishable [12] (Fig. 8.5). These two patterns of activity are ubiquitously recognized as warning criteria for nerve or nerve root injury and should be reported to the surgeon immediately.