**3.6. EMG recording in Intraoperative neuromonitoring**

partially dependent upon the subject height. A significant interside difference indicates a

**Figure 9.** Left: MEP recorded from abductor pollicis brevis muscle. The top trace shows the MEP evoked by single pulse TMS over the corresponding M1. The lower trace shows the MEP elicited by ipsilateral cervical (motor root) stimula‐ tion. Right: MEP recorded from extensor digitorum brevis muscle. The top trace shows the MEP induced by cortical

*Amplitude* is often measured from peak-to-peak amplitude. MEP amplitude can also be measured from baseline EMG activity to the first positive or negative deflection. Amplitude of MEP reflects the integrity and excitability of motor cortex, corticospinal tract, nerve roots and peripheral motor pathway to the muscles [64]. Dispersion of the alpha-motoneuron response to the descending volley in the corticospinal tract, leads to a broad range of normal values. The triple stimulation technique (TST) provides a more precise assessment of cortico‐ spinal tract conduction by suppressing desynchronization of MEPs. The TST involves three stimuli (transcranial, distal and proximal on the peripheral nerve) timed to produce two collisions. The TMS descending impulses collide with the antidromic impulses from the distal stimulus. Proximal stimulation on the nerve evokes orthodromic impulses, which cancel out any uncollided impulses from the distal stimulus. The response from the third stimulus

Lengthening of MEP latency and CCT suggests impairment of the white matter fibers, while abnormalities of MEP amplitude or absence of responses are more suggestive of loss of neurons or axons. TMS has the potential to facilitate early diagnosis of myelopathy by detecting signals of demyelination of the pyramidal tract [66,67], plexus entrapment and injuries [62]. Moreover, MEP abnormalities may be useful objective markers of progression of amyotrophic lateral sclerosis (ALS) [68], and effective parameters in spinal pathology for deciding the timing of

TMS can however be performed using single pulse or pair pulse paradigm in order to explorer the reactivity of the motor cortex. Since motor threshold (MT) is believed to reflect membrane examine of corticospinal neurons, motor neurons in the spinal cord, NMJs and muscle [70], it

lateralized prolonged CCT even if still within normal values (Fig 9).

12 Electrodiagnosis in New Frontiers of Clinical Research

stimulation. The lower trace shows the MEP elicited by ipsilateral lumbar stimulation.

therefore reflects the number of peripheral neurons activated from TMS [65].

the surgical intervention [69].

Intraoperative neuromonitoring includes mapping and true monitoring techniques. Mapping techniques are used intermittently during surgery for functional identification and preserva‐ tion of anatomically ambiguous nervous tissue. On the other hand, true monitoring techniques permit a continuous assessment of the functional integrity of neural pathways [82].

In posterior fossa and brainstem surgeries, mapping the floor of the fourth ventricle allows the surgeon to find a safe entry to the brainstem, and therefore, helps to identify and preserve cranial nerves and their motor nuclei. Traditionally, Intraoperative monitoring of the facial nerve has been employed in operations for acoustic tumors to reduce the risk of neural damage. To date, EMG recording of the activity of selective cranial nerve muscles is currently included in the intraoperative set during surgical manipulation of the brainstem. [83,84].

During brain surgery, neurophysiological mapping techniques have been employed in the identification of eloquent areas such as the motor areas. In addition, these techniques have been introduced in surgery for deep-seated gliomas, insular tumors and lesions involving the cerebral peduncle [82,84]. The goal is an aggressive resection of such lesions to the greatest extent as possible, to improve the patient's survival chance and the postoperative life quality. With this aim, monopolar or bipolar stimulation of cortical and subcortical areas is applied carefully. Visual detection of the elicited movement of the limb contralateral to the operative side is usually employed during the mapping of motor areas. However, it is difficult to detect visually a subtle twitch over an entire contralateral limb at once, specially during awake surgery (because of the specific patient positioning). EMG recording is more sensitive than the visual detection of muscle twitch. EMG signals precede the visually observed motor activity, since the applied stimulation may weakly activate motor pathways enough to elicit EMG responses and yet not recruit a sufficiently large pool of motor neurons to produce visible muscle movement. Moreover, multichannel EMG recording has three important advantages: First, it facilitates the monitoring of the face, upper and lower extremities simultaneously, detecting motor responses that may not be observed during gross inspection (Fig.10). This advantage is particularly important during mapping of subcortical pathways. Second, EMG recording also improves the ability to detect subclinical ictal events (EMG activity elicited by stimulation that persists after the end of the stimulation). In addition, EMG—complementary to electrocorticography— allows the early detection of spreading muscle activation over a limb as a sign of seizure. The immediate removal of stimulation diminishes the likelihood of the progression of a seizure. Third, the sensitivity of EMG recording allows the application of lower intensity and duration of stimuli during mapping procedures [85-87].

EMG as a monitoring technique has a high sensitivity but a low specificity. To date, some limitations of EMG recording have been overcome using multimodality intraoperative monitoring, including motor evoked potentials, somatosensory evoked potentials and some reflex responses (H- reflex, blink reflex, etc). Transcranial MEP elicited by transcranial multipulse electric stimulation of the motor cortex (TcMEP) are currently the most effective means of continuous monitoring of the functional integrity of corticospinal and corticobulbar

Overview of the Application of EMG Recording in the Diagnosis and Approach of Neurological Disorders

http://dx.doi.org/10.5772/56030

15

During spinal procedures, free running EMG (frEMG) and stimulus-triggered EMG (stEMG) are basic monitoring tools to assess the functional integrity of nerve roots, plexus and periph‐ eral nerves. Train activity or neurotonic discharges recorded by means of spontaneous EMG indicate excessive direct or indirect nerve contact during manipulation; therefore, adjustment should be made to avoid nerve injury. Recently, frEMG or stEMG have been included in the intraoperative set applied during minimally invasive surgeries such as transpsoas approaches. On the other hand, stEMG have been used to control the correct pedicular screw placement during orthopedic surgery [88]. Unlike the CMAPs recorded in neurophysiological laborato‐ ries, intraoperative CMAP are typically elicited using submaximal stimulation and are recorded as highly complex polyphasic responses with variable onset latencies and ampli‐ tudes. Stimulus threshold however can provide some information about the proximity to the

TMS is likewise an advantageous optional technique in planning brain surgery, based on noninvasive mapping. TMS mapping consists of locating where the largest MEP responses can be measured by using suprathreshold single stimuli applied to the assumed area (M1) of the optimal stimulation site [90]. There is some evidences of the reliability of this planning method in correlation with the gold standard "direct cortical stimulation" described previously [91,92]. Interestingly, a recent report has provided the first result of the reliability of TMS, in the assessment of the plasticity changes of the involved M1 concurrent with multistage surgery, in a patient with a diagnosis of low grade glioma. However, further studies should confirm the power of this non-invasive mapping technique, in regard to patient-specific variation, and

Virtually all primary neuromuscular diseases result in changes in the electric activity recorded from the muscle fibers. The pattern of abnormalities can usually mark the underlying pathol‐ ogy as neuropathic (e.g. disorders affecting the CNS, nerve roots, plexuses and peripheral nerves), myopathic, or NMJ disorder, etc. EMG recording allows measurement of the severity

In the field of intraoperative neuromonitoring, to date, EMG recording – despite its low specificity - continues to be a valuable tool included in a multimodal monitoring set during

pathways in diverse surgical procedures [82,84].

nerve root [89].

**4. Conclusion**

especially to functional anatomy [93].

of the injury, and provides prognostic information.

diverse neurosurgical and orthopedic procedures.

**Figure 10.** Multichannel EMG recording during mapping for glioma surgery. Stimuli artifact. ( ) Muscle activity ( ) recorded preceding visual muscle twitch ( ).

EMG as a monitoring technique has a high sensitivity but a low specificity. To date, some limitations of EMG recording have been overcome using multimodality intraoperative monitoring, including motor evoked potentials, somatosensory evoked potentials and some reflex responses (H- reflex, blink reflex, etc). Transcranial MEP elicited by transcranial multipulse electric stimulation of the motor cortex (TcMEP) are currently the most effective means of continuous monitoring of the functional integrity of corticospinal and corticobulbar pathways in diverse surgical procedures [82,84].

During spinal procedures, free running EMG (frEMG) and stimulus-triggered EMG (stEMG) are basic monitoring tools to assess the functional integrity of nerve roots, plexus and periph‐ eral nerves. Train activity or neurotonic discharges recorded by means of spontaneous EMG indicate excessive direct or indirect nerve contact during manipulation; therefore, adjustment should be made to avoid nerve injury. Recently, frEMG or stEMG have been included in the intraoperative set applied during minimally invasive surgeries such as transpsoas approaches. On the other hand, stEMG have been used to control the correct pedicular screw placement during orthopedic surgery [88]. Unlike the CMAPs recorded in neurophysiological laborato‐ ries, intraoperative CMAP are typically elicited using submaximal stimulation and are recorded as highly complex polyphasic responses with variable onset latencies and ampli‐ tudes. Stimulus threshold however can provide some information about the proximity to the nerve root [89].

TMS is likewise an advantageous optional technique in planning brain surgery, based on noninvasive mapping. TMS mapping consists of locating where the largest MEP responses can be measured by using suprathreshold single stimuli applied to the assumed area (M1) of the optimal stimulation site [90]. There is some evidences of the reliability of this planning method in correlation with the gold standard "direct cortical stimulation" described previously [91,92]. Interestingly, a recent report has provided the first result of the reliability of TMS, in the assessment of the plasticity changes of the involved M1 concurrent with multistage surgery, in a patient with a diagnosis of low grade glioma. However, further studies should confirm the power of this non-invasive mapping technique, in regard to patient-specific variation, and especially to functional anatomy [93].
