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Table of Contents
Year : 2021  |  Volume : 4  |  Issue : 3  |  Page : 147-163

Intraoperative electrophysiological principles in neurooncological practice

1 Department of Neurosurgery, Yashoda Hospital, Secunderabad, Telangana, India
2 Department of Neuro-anaesthesia, Yashoda Hospital, Secunderabad, Telangana, India
3 Associate Staff Physician Anaesthesiologist, Anaesthesiology Institute, Cleveland Clinic, Abu Dhabi, UAE

Date of Web Publication02-Nov-2021

Correspondence Address:
Dr. Anandh Balasubramaniam
Department of Neurosurgery, Yashoda Hospitals, Alexander Road, Shivaji Nagar, Secunderabad - 500 003, Telangana
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/IJNO.IJNO_421_21

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Intraoperative neurophysiological monitoring (IOMN) is an important adjunct in modern day neurosurgical practice. There has been a paradigm shift from functional preservation to maximal safe or total excision of a tumor along with functional preservation, aiming for a better quality of life to the patients. In neurosurgery, like in any other specialty, we have two extremes of tumors, benign and malignant. In malignant tumors, the extent of resection, along with molecular genetics of the tumor, play an important role in the survival of patients. Thus, one should target for complete resection, whenever feasible, in these types of tumors. In benign tumors, such as World Health Organisation (WHO) grade 2 gliomas, a good chance of long-term survival exists. IOMN is a valuable adjunct in neurosurgical practice that guides the surgeon and warns him/her of the important neurological structures in the vicinity, during surgery. The IOMN procedures, however, have their own limitations that everyone should be aware of. The technique has been used along with other adjuncts like a preoperative MRI (including the functional magnetic resonance imaging [MRI], diffusion tensor imaging of long tracts and perfusion studies), neuronavigation and intraoperative imaging to maximize the chances of a better outcome in the form of onco-functional balance. In this review, an overview of IONM has been discussed.

Keywords: Neuro-electrophysiology, intraoperative neurophysiological monitoring, neurooncology, brain tumors, surgery

How to cite this article:
Kumar G K, Pradeep K, Rajesh B J, Bhaire VS, Manohar N, Balasubramaniam A. Intraoperative electrophysiological principles in neurooncological practice. Int J Neurooncol 2021;4, Suppl S1:147-63

How to cite this URL:
Kumar G K, Pradeep K, Rajesh B J, Bhaire VS, Manohar N, Balasubramaniam A. Intraoperative electrophysiological principles in neurooncological practice. Int J Neurooncol [serial online] 2021 [cited 2022 May 27];4, Suppl S1:147-63. Available from: https://www.Internationaljneurooncology.com/text.asp?2021/4/3/147/329816

  Introduction Top

Maximal safe resection has become the gold standard in neurooncological surgery. This is done by utilizing a plethora of armamentarium to address preoperative, intraoperative and postoperative aspects. This article emphasizes the use of intraoperative neurophysiological monitoring (IONM). The most relevant oncological impact, both in terms of the amount of tissue removal and the functional preservation, could be achieved by the introduction of the brain mapping and monitoring techniques. The basic principle in functional guided neurosurgery is that before structural damage occurs, there will be alterations in physiology that can be rectified in a time-reversible fashion.

From the conventional brain organization of 'eloquence and dominance', there is a gradual shift to a global phenomenon inspired by the works on 'parcellation and connectomics'. Both these terms emphasize the concept that all parts of the brain, including the cortical and subcortical white matter, are important. However, it is beyond the scope of this article to describe the principles of IONM in entirety, and hence, the conventional concepts of IONM with an overview of the evolving concepts are described.

This article describes in a concise manner, the various modalities, their applications and flaws with a focus on practical issues of the most relevant and frequently performed IONM procedures. Some insights regarding the electrophysiological basis of stimulation and recordings, and techniques such as electromyography (EMG), electroencephalogram (EEG) and electrocorticogram (ECoG), evoked potentials including somato-sensory (SSEPs), auditory (BAEPs), visual (VEPs) and motor (MEPs), as well as stimulated EMG are provided.

Current concepts of IONM in functional neuro-oncology

The function-based surgical resection should be performed up to the individual functional boundaries, at both cortical and subcortical levels. The concept of detecting and preserving the essential functional cortical and subcortical structures according to functional boundaries is called as the “functional neuro-oncological approach”. Thus, there is a gradual shift from image-based surgery to function-based surgery. The functional approach exploits the functional reorganization of brain, permitting the removal of as much tumor as feasible, possibly extending the resection far beyond the visible tumor margins and detectable by conventional MR images, while preserving functional integrity.[1],[2]

Optimizing the onco-functional balance indicates that the extent of resection (EOR) will depend on whether or not neural circuits critical for brain functions are involved by the tumor.[3] When functionally possible, a complete supratotal resection is currently recommended.[4],[5] Neurophysiological monitoring can be broadly categorized into two groups, mapping and monitoring.[6],[7] Both these methodologies overlap in clinical practice and are used at different instances during surgery. The main difference between these two basic techniques is the duration of study, its objective and the inference derived by the surgeon from the results.

Functional mapping consists of topographic assessment of the cortical and subcortical structures to determine their function.[8] It also helps to understand the electrical activity (such as the seizure focus) in and around the lesion. Mapping is usually stopped as soon as the function is positively identified. This functional identification may include a positive effect (that is, eliciting a function) or a negative effect (that is, suppressing a function). Mapping helps to identify the functional area and its relationship to tumor, thereby allowing surgeons to identify the site of incision and to restrict the extent of surgery from the functional areas. The positive sites need to be marked and labelled. The two most important areas that need to be mapped on the cortex are the sensori-motor and language areas.

The main principle of functional monitoring is to perform surveillance of the functional state of structures. As these areas need to be monitored throughout surgery, the procedure takes a longer time, in contrast to the functional mapping procedure.[8] The former procedure also needs more expertise and commitment from the anesthetist, the neurophysiologist and the surgeon. Hence, the objective of functional monitoring is not just monitoring, but also involves mapping all through the surgery to preserve the functional integrity of structures in close vicinity that are likely to be compromised during the surgical procedure.

  IONM in Neuro-Oncological Tumors Top

Various types of IONM modalities are explained in [Figure 1]. Each of the modalities has been explained in brief and its application in the excision of neurooncological tumors has been defined[9],[10],[11],[12],[13],[14],[15],[16],[17] in [Table 1]. The stimulation and recording parameters with the alarm criteria of each of these modalities have been explained in [Table 2]. International standard 10-20 montages are utilized for cortical surface IONM [Figure 2].[18]
Figure 1: Intraoperative neurophysiological modalities

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Table 1: Intraoperative neurophysiological monitoring modalities in relation to tumors

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Table 2: Intraoperative neurophysiological monitoring modalities, parameters, and alarm criteria

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Figure 2: Diagrammatic representation of the international standard 10-20 montage for stimulation and recording electrodes Fz (blue) for reference; C3', C4', Cz' (green) for stimulating MEP; C3, C4, Cz (orange) for recording SSEP; O1, O2, Oz (yellow) for recording VEP

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Free running response

Electrocorticogram (ECoG)

Electrocorticogram is a measure of the electrical activity of the brain measured from the cortex. It can be done by using surface (strip or grid electrode) or depth electrodes. In neuro-oncological practice, it is used in tumors associated with seizures. Epileptogenic activity is noted in and around the tumor area. After tumor resection, the electrodes are placed again and electrical activity of the cortical region is recorded for at least for 5 minutes to confirm the completeness of resection of the epileptogenic foci. Apart from the ECoG, an electroencephalogram (EEG) can also be used to measure the depth of anesthesia, monitor the intraoperative seizure activity, and also assess for ischemic insults in the form of slowing of electrical waves.[19]

Free running electromyogram (EMG)

Free running EMG consists of continuous recording of the muscle activity without any stimulation. The corresponding muscle groups, chosen on the basis of the nerve root at risk, are continuously monitored and any mechanical irritation occurring during the resection can be recorded. EMG spikes, bursts, trains, and neurotonic discharges are used to alert the surgeon during tumor resection, regarding an impending neurological deficit caused by the surgical intervention [Figure 3].
Figure 3: Raw EMG. Blue arrow showing EMG bursts

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Evoked potentials

Somatosensory evoked potentials (SSEP)

SSEPs are used to continuously monitor the integrity of large fiber sensory system, which constitutes the dorsal column (within the spinal cord), dorsal spinocerebellar tract (within the brainstem), the ventrolateral tract (within the subcortex) and the post-central gyrus (within the cortex) [Figure 4]. Stickers or needle electrodes placed over the peripheral nerves (median, ulnar, posterior tibial, or trigeminal nerve) are stimulated and the evoked potentials are recorded over the cervical spine and scalp through corkscrew electrodes. SSEPs are altered during damage in the pathway due to vascular compromise, compression, stretching, edema, inhalational anesthetics, hypothermia, acidosis and hypotension. It is not affected by muscle relaxation and total intravenous anesthesia (TIVA).[11],[12],[17],[20],[21].
Figure 4: SSEP of both upper and lower limbs; blue line shows latency, and orange line the amplitude

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Visual evoked potential (VEP)

Visual evoked potentials are obtained using light stimulation (using a light emitting diode [LED]) through the retina, and the response is recorded from the occipital cortex, to monitor the excision of tumors that are involving the visual pathway, such as suprasellar and anterior skull base tumors. An electroretinogram (ERG) is recorded simultaneously with a VEP to ascertain that the flash stimuli to the retina has been adequately delivered [Figure 5]. Inability to identify the laterality, lack of a standard alarm criteria, non-reproducibility of the procedure, and a high anesthetic sensitivity, are some of the drawbacks of this modality. [22],[23]
Figure 5: VEP: Blue arrow shows P100 in the visual evoked potential waveform; and, orange arrow shows the electroretinogram, in a patient with anterior skull base tumor

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Brainstem auditory evoked potentials (BAEP)

Brainstem auditory evoked potentials are obtained by applying sounds or clicks over the external auditory meatus and the response recorded from vertex. This brief auditory stimulus can assess conduction through the auditory pathway up to the midbrain. It comprises of 5 waves in 10 millisec, waves I to V. Parameters such as amplitude, interpeak latency, amplitude ratio of the V/I, or IV and V/I, and the inter-ear peak differences and interpeak latencies (I-V, I-III and III–V latencies) are commonly measured. While the main advantage of BAEP is that the waves are resistant to anesthetic agents, it does have a lot of disadvantages, such as, the presence of complex wave forms, the difficult interpretation, and the interference induced in them during surgical drilling and cauterization. [11],[12],[17],[20],[21],[24],[25]

Central sulcus mapping

Phase reversal is the technique used to identify the central sulcus electro-physiologically [Figure 6]. A change in polarity occurs across the central sulcus, when SSEPs are recorded directly on the cortex and when a strip or grid electrode is placed perpendicularly across the central sulcus. Though the lesions or its oedema distorts the anatomy, this technique allows for a precise assessment of functional-anatomic relationship. This modality can be used to preserve eloquence while operating on lesions around the central sulcus. [26],[27]
Figure 6: Phase reversal for central sulcus mapping marked by red circle peak and trough, from 2 and 3 contact points of strip electrodes, respectively, placed across the central sulcus with posterior tibial nerve stimulation

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Dorsal column mapping

Inability to identify the neurophysiological midline while operating on an intramedullary spinal cord tumors, cord edema, cord rotation, or due to displacement of tracts, can be overcome using this modality. Bilateral posterior tibial nerves are stimulated and recorded over the width of spinal cord at the level of lesion. Physiological midline is the point in between the two highest amplitude waveforms obtained from a miniature strip electrode[28],[29],[30],[31].

Motor evoked potential (MEP)

MEPs are used to assess the integrity of corticospinal tract, triggered by transcranial electrical or magnetic stimulation [Figure 7] and [Figure 8]. Transcranial magnetic stimulation is very susceptible to anesthetics, thus transcranial electrical stimulation is used for intraoperative neuromonitoring. A constant electrical current is delivered transcranially (using corkscrew electrode: tcMEP); using direct cortical stimulation (using strip electrode: dcMEP); or, subcortically (using monopolar electrode: scMEP). This depolarizes the lateral corticospinal tract and action potentials are recorded from distal muscle groups. This modality can be used for spinal cord, supratentorial and infratentorial tumors. Direct cortical and subcortical stimuli use current intensity upto a maximum of 25mA, avoiding the deeper stimulation of motor tracts. This technique is useful for mapping motor tracts at the precentral gyrus, internal capsule, brainstem and spinal cord. Corticobulbar MEPs are used to monitor the functional integrity of the corticobulbar tract as well as the cranial nerves. In order to avoid stimulating nerves surrounding the tumor directly, which can happen when using a single train stimulus, a double train stimulus with a low current is given (so that the electricity is not dispersed).
Figure 7: Normal bilateral lower limb MEP

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Figure 8: Blue arrow shows the corticobulbar MEP of lower cranial nerves in the cerebellopontine angle and brainstem tumors

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D Wave

Epidural MEP or D wave is used to assess the integrity of the corticospinal tract [Figure 9]. Transcranial stimulation of the motor cortex elicits a response in the descending motor tract recorded from the surface of spinal cord through an electrode placed in the epidural or subdural space. The amplitude of D wave depends on the thickness of the corticospinal tract; hence, it is less reliable at the lumbar region. The disadvantages of D wave are that it does not have a laterality and is unreliable below the thoracic 10 level. One can proceed with surgery using D wave recording without using MEP, as D waves are more specific and resistant to inhalational anesthetics (as they do not involve synapses). While resecting spinal cord tumors, the D wave above the lesion is taken as a reference wave.[32],[33],[34],[35],[36],[37],[38].
Figure 9: Blue arrow shows the D wave in a patient with a thoracic intramedullary spinal cord tumor

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Cortical and subcortical mapping

This modality is used during surgery for superficial cortical and subcortical tumors involving eloquent areas and the speech area. Speech mapping needs to be done in an awake state, while motor mapping can be done in both the awake state as well as general anesthesia. A constant current using a monopolar, bipolar or strip electrode placed over the cortex is delivered for mapping the motor area. A suction stimulator is used simultaneously to stimulate motor tracts while performing tumor resection in the subcortical area. Based on Coulomb's law, the rule of thumb is that with a 1 mA stimulation, the current spreads to a 1mm distance. Thus, one can assess the approximate distance of the surgical site from the vital structures using this technique. Certain disadvantages of this stimulation are the shock-like sensations that may be felt, a lack of cooperation by the subject, the occurrence of fatigue, exhaustion, or seizures, and the false alarms raised due to the subject's inability to perform the given tasks. During general anesthesia, motor mapping can done using the EMG recording over the respective muscle group; however, there is a high chance of negative mapping if all the muscle groups are not used for EMG recording.[39],[40],[41],[42],[43],[44],[45],[46],[47]

Triggered EMG

Current applied directly on the cranial motor nuclei or on the cranial nerve elicits a response in the corresponding muscle group. Monopolar probe stimulation is very sensitive, hence, it is used for identifying regional neural structures during tumor decompression; while bipolar probe stimulation is very specific and is used for identifying the precise location of neural structures, for example, stimulating on a fibre-like structure after tumour decompression [Figure 10]. It is used to assess the integrity, course and mapping of nerve roots and cranial nerves in the vicinity of tumors in areas such as the cerebellopontine angle and the brainstem.
Figure 10: Blue arrow shows triggered EMG of the facial nerve with 2.7mA current in a patient with a cerebellopontine angle tumor

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This modality can continuously monitor the function of peripheral nerves, the plexi, and the nerve roots.

Bulbocavernous reflex (BCR) indicate the patency of the lower sacral reflex arc (S2, 3, 4 nerve roots). It is elicited by stimulating the dorsal nerve of penis/clitoris and recording the reflex in the external anal sphincter. A unilateral stimulation results in the elicitation of bilateral BCR reflexes. It can be used for real-time monitoring during excision of spinal cord tumors involving the sacral roots, during detethering of the cord [Figure 11]. The lacunae in literature of proper protocols, the limited data available, and the lack of standard alarm criteria, are the drawbacks of this modality.[48],[49]
Figure 11: Blue arrow shows bilateral bulbocavernous reflex (BCR) with raw EMG with unilateral stimulation of dorsal nerve of penis/clitoris

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Blink reflex can monitor the integrity of the brainstem reflex arc with the sensory component of the reflex arc being the trigeminal nerve, and the motor component being the facial nerve. It is elicited by stimulating the supraorbital nerve to elicit a response in the orbicularis oculi muscle. This modality can be used during the resection of intrinsic brainstem lesions and cerebellopontine angle tumors.[50]

Laryngeal adductor reflex is the most crucial among all brainstem reflexes. It protects the airway from aspiration by adducting the vocal cords and by closing the laryngeal inlet. This modality can be used for brainstem tumors and cerebellopontine angle tumors.[51],[52],[53],[54],[55]

Masseter reflex is a trigemino-trigeminal reflex, which connects the midbrain and mid-pons. Unilateral stimulation of the masseter nerve causes jaw jerk. This modality can be during the excision of intra-axial brainstem tumors.[56],[57]

  Anesthesia and IONM Top

Inhalational anesthetics cause a dose-dependent increase in latency, and a decrease in amplitude by inhibiting the spinal motor neurons at the anterior grey column or by depressing the synaptic transmission in the cortex. A minimal alveolar concentration (MAC) value of more than 0.5 affects all evoked potentials. D waves are resistant to inhalational anesthetics as there are no synapses between the stimulating and recording electrodes. Intravenous opioids have little effect on evoked potentials. Propofol and thiopental depress the evoked potentials when given in a bolus, and have a minimal effect when given in a steady infusion state. Ketamine increases the amplitude of SSEP and MEP but has adverse effects such as raising the intracranial pressure and causing hallucinations. Muscle relaxants should be avoided if the muscle MEPs (EMG, MEP, reflex) are to be monitored. All evoked potentials, including MEP, SSEP, BAEP are sensitive to vascular compromise and mechanical compression. Stable hemodynamics, normal pH, normocarbia and normothermia should be maintained throughout the duration of performing the IONM procedure.[12],[58],[59]

  Special Consideration Top

IONM in awake patients

Patient selection, cooperation and good rapport between the patient and anesthesiologist are prime components for the conduction of a successful awake craniotomy. Anesthetist and neurophysiologist need to discuss in detail with the patient preoperatively the questions and the procedure that will be done during the procedure. Asleep-awake-asleep and monitored anesthesia care are the techniques that are used for tumors involving eloquent area and epilepsy surgeries. Scalp block with adequate analgesics and sedatives are titrated such that the patient is comfortable and co-operative throughout the procedure. Awake-asleep-awake is the most common technique used; however, whenever a longer duration surgery is performed, problems may arise in patient positioning, and in management of intraoperative seizures, blood loss and anxiety.[60],[61],[62],[63]

IONM in pediatric patients

It is challenging to do neurophysiological monitoring in the pediatric age group in view of the immature nervous system, as maturation occurs at about 13 years of age. Furthermore, the children are more prone to develop hypothermia, blood loss and hemodynamic fluctuations due to an immature nervous system, which can alter IONM responses.[13],[21],[24],[64],[65],[66],[67],[68],[69].

IONM in obstetric patients

Monitoring during pregnancy involves a multidisciplinary team approach involving neuroanesthesiologists, neurosurgeons, and obstetricians. Keeping the voltage minimum for MEP, reducing the number of MEP stimulation trains, and monitoring of fetal heart rate and uterine tone are some of the strategies considered in pregnant patients. Drugs crossing fetoplacental circulation and causing fetal depression should be used cautiously and only if needed to avoid unnecessary false alarm. Preoperative and postoperative fetal wellbeing should be documented. The risk of fetal loss and emergency cesarian section during the procedure should be explained.[70]

  Supratentorial Tumors Top

IONM in cortical surgery

IONM is indicated when the lesion is in or near an eloquent area, such as the sensorimotor cortex, the premotor cortex or the language area. Monitoring is used to identify the functional relationships between edge of the tumor and the surrounding areas. While ECoG is used to map and assess the electrical activity of the brain, SSEP is used to identify the central sulcus via phase reversal technique, and direct cortical stimulation is performed to map the vital structures and can be done either in awake or asleep state.[71]

The time used for cortical mapping includes the time spent on identification of functional landmarks, deciding working currents as well as entry points, and in the designing of extent of corticotomy. Consequently, the required time varies between 5 and 15 minutes.

Surgical nuances in cortical stimulation

Direct electrical stimulation of the brain produces excitation and inhibition in neuronal networks, synaptic connections and fibre tracts.[72] Subthreshold stimulation can falsely ''clear” tissue as being non-functional, leading to surgical deficits; and, suprathreshold stimulation may overstimulate the adjacent cortical areas to falsely produce functional signs or symptoms, leading to subtotal resection. A region might be considered resectable because its function would not have been tested by an appropriate intraoperative task.[73] Inadequate functional brain mapping has been associated with a lower incidence of gross total resection of tissue and a greater post-operative morbidity [74] The maximal current intensity applied during the stimulation that does not interfere with function or produce after-discharges (ADs) on ECoG is considered an endpoint for non-functional tissue. After-discharges detected by ECoG provide optimal stimulation intensity and signal a boundary where further current escalation could result in a seizure[75]. One common mistake that often occurs by an inexperienced team is that the team fails to reduce the current when stimulating the new adjacent area, or when stimulating after a break. Together, direct electrical stimulation and ECoG provide real-time electrophysiological feedback to neurosurgeons during the operation.[62] A meta-analysis showed evidence that cortical mapping revealed a significant decrease in the rate of severe persistent neurological deficits even in eloquent areas.[76] Mapping and monitoring under general anesthesia of the motor cortex and pyramidal pathways has become the standard of care for tumors in the vicinity of the Rolandic cortex or the insula.[77]

Cortical language mapping

Language mapping is unique compared to motor mapping as it always needs to be done under awake circumstances. One has to confirm the language mapping area three times before labelling it as eloquent. While the surgeon resects a lesion, a series of language tasks are carried out through the length of surgery. A 1-cm margin of tissue should be measured and preserved around each positive language site to protect the functional tissue during resection.[78] When operating on language areas, awake mapping has resulted in a decrease in post-operative permanent aphasias to less than 2%.[79]

IONM in subcortical surgery

It represents the most critical phase of tumor resection, since it significantly affects the extent of resection and the functional result of surgery. During the subcortical IONM, resection must be associated with consistent mapping, carried out either in an alternate or continuous manner. In this phase, with the aid of subcortical mapping methods, the functional limits of the tumor are identified. Limits of the eloquent area are sought at the periphery of the planned resection area, starting from the cortical areas that were judged as nonfunctional during the cortical phase of the procedure. The surgeon must have in his/her mind, the architecture of the functional map of the networks surrounding the tumor, adopting and varying the tests to be performed during the intraoperative mapping, to identify and recognize each of these networks during various stages of the procedure. In this way, a functional disconnection of the tumor is progressively obtained, and the resection is also precisely achieved according to functional limits. During the resection phase, particular attention must be paid not only to the identification and sparing of functional sites surrounding the tumor to define its functional limits, but also to carefully respect the vascular architecture within and around the tumor mass. The subcortical time for IONM has a variable duration depending on the area and the extent of resection, ranging from 20 minutes to a few hours.

With growing technology, with the realization of importance of subcortical white matter, and with the advent of techniques such as tractography, fibre dissection techniques and connectomics, the functional assessment of subcortical fibres is increasingly being deployed during routine neurosurgical procedures. It has become a standard practice to do a detailed subcortical monitoring especially near language areas and the motor cortex. With the evolution of connectomics and the concept of existence of meta networks, subcortical functions need to be assessed and to be monitored in an awake state.[71]

Common language pathways detected with reproducibility in the dominant hemisphere using stimulations should be surgically preserved to avoid definitive speech disturbances. These include the the subcallosal fibres (ScF) (eliciting initiation disorders during the stimulation), the periventricular white matter (PVWM) containing motor and sensory fibres of the face (inducing anarthria, when stimulated), the arcuate fasciculus (AF) and the insulo-opercular connections (generating anomia during stimulations).

The same techniques that are applied to the cortical assessment can be used at the subcortical level. For a surgeon, this assessment is more important than cortical mapping and monitoring, as there are multiple issues that need to be understood. Only with a sound anatomical knowledge can the surgeon decide when to start the subcortical monitoring. The concomitant usage of imaging techniques such as tractography, and operative assistance such as navigation, also help the surgeon. The strategy of immediately locating functional margins from early stages of subcortical resection reduces the time of awakening and collaboration.[80] With the surgeon's sound skill, he can start the monitoring when approximately 1 cm thickness of the tumor wall remains after an internal decompression, while some studies advocate the usage of high currents, up to 15 -20 mA along with a suction stimulator, while decompressing the tumor and then to start reducing the current as one starts getting positive signals. The methodology of stimulation parameters remains the same as for cortical stimulation, during the subcortical stimulation; however, monopolar cathodal stimulations are preferred. [81]

The safety margin for tumor resection has not been defined yet but most agree that one to three mm margin is sufficient.[82] Functional results can be evaluated either at an earlier (within 1 week) or later (from 1 to 3 months) time after surgery. Evaluation is generally performed with a detailed neurological examination, accompanied by a neuropsychological and psycho-oncological evaluation. [83],[84]

The purpose of brain mapping techniques is to identify and preserve at the time of surgery, the cortical and sub-cortical functional sites. Resection is in fact stopped when language and/or motor, or visuospatial, or cognitive cortical or subcortical areas are encountered. Evaluation of motor or language deficits during the postoperative course and at the follow-up visit showed that the chances of developing a new deficit or the worsening of a preexisting one in the immediate postoperative period varies between 65% and 92.8%. This is not surprising when the surgery is tailored to reach functional boundaries. In this situation, the chance of inducing permanent deficits is significantly high, because the surgeon reaches these functional limits in all cases in whom surgery is performed.[85] Most of the immediate deficits are transient and disappear within 1 month after surgery. MEP monitoring can help in monitoring and preventing the appearance of motor deficits due to vascular injury.[86] When subcortical stimulation was systematically applied during the resection of low grade gliomas located within language areas or pathways, only 2.3% patients showed a long-term impairment.[87] Similar or even lower numbers were observed during the resection of gliomas close to motor areas or pathways (0.5% of deficits).[85] These data support the relevance of subcortical stimulation as a useful surgical adjunct during removal of lesions involving motor or speech areas, as further demonstrated by the high percentage (95.8%) of patients who returned to work, 3 months after surgery.[1],[88]

  Infratentorial Tumors Top

Cerebello-pontine (CP) angle tumors

IONM techniques required for tumors located in the CP angle are free-run EMG and triggered EMG of 5th, 6th, 7th and lower cranial nerves, MEP that include both corticospinal and corticobulbar stimulation, SSEP, BAEP, and reflexes like the blink, masseter and laryngeal adductor reflex. Most common tumors in this location are vestibular schwannomas, epidermoids, and CP angle meningiomas. Baseline MEP, SSEP and BAEP monitoring should be done before proceeding with the actual surgery. Before dissecting the arachnoid from these extraaxial lesions, triggered EMG of 7th and lower cranial nerves should be done. During decompression of the tumor, the free-run EMG should be monitored. During tumor dissection, if there is presence of free-run EMG activity for ≥100 msec that indicates a particular cranial nerve in the vicinity, the surgeon should be alerted, and triggered EMG should be conducted using a constant-current stimulator to trace the course of the nerve (once the firing of free-run EMG has subsided)[89],[90] The BAEP and SSEP waveforms are continuously monitored, which are obtained from the neuronal pathways. If there is compromised vascular supply or traction in the proximity of the resection area, there will be reduction in amplitude and increased latency of the waveforms. MEP is carried out at every 5-30 min interval. Blink, masseter and laryngeal adductor reflex indicate the integrity of the local reflex arc in the brainstem.

Intrinsic brainstem lesions and fourth ventricular tumors

Mapping of the facial colliculus, the vagal and hypoglossal triangle should be done before making an entry into the floor of fourth ventricle for intrinsic brainstem lesions. After making an entry into the brainstem and during dissection of the tumor, both corticospinal and corticobulbar MEPs, SSEP and BAEP are monitored along with laryngeal adductor and blink reflex. For the fourth ventricular tumors like medulloblastoma/ependymoma, continuous SSEP and BAEP are monitored, and intermittently MEP of the corticospinal and corticobulbar tracts should also be done.

Performing IONM during the resection of infratentorial tumors detects early electrophysiological changes, which help in preventing permanent neurological deficits. Cranial nerves are very sensitive to any kind of direct or indirect manipulation, such as the use of mechanical forces causing stretching or compression; or the performance of irrigation, bipolar cauterization, or tumor removal with an ultrasonic surgical aspirator. These surgical techniques are very likely to result in nerve conduction block and/or any kind of spontaneous activity in free run EMG.[91]

Skull base tumors

For endonasal transsphenoidal approach for pituitary macroadenomas, VEP is monitored; however, it is not recommended as a routine procedure due to inconsistencies found during its conduction and because of the variable results obtained.[92] For clival chordomas, an extraocular muscle EMG, along with MEP and SSEP monitoring, has to be done. For petroclival meningiomas, the extraocular muscles, as well as the 5th, 6th 7th and lower cranial nerve activity should be monitored, along with MEP, SSEP and BAEP. The incidence of cranial nerve deficits without IONM during the resection of skull base tumors is reported to vary from 8-70% versus 2.8% when IONM is simultaneously performed.[69]

Spinal cord tumors

IONM is done for intramedulary, intradural extramedullary (IDEM), and extradural tumors. MEP and SSEP are done for all types of spinal cord tumors and the baseline waveforms should be monitored before and after positioning the patient. For monitoring during surgery for intramedullary and IDEM tumors, baseline direct spinal motor evoked potentials (D-waves) are measured before opening the dura. The D-wave electrode should be placed below the tumor in the epidural space for obtaining the baseline waveforms, and these waveforms should be compared with the ones obtained during and after the tumor resection. For intramedullary tumors, dorsal column mapping is done before conducting the midline myelotomy. These lesions distort the normal midline and one tends to injure the dorsal column while making the myelotomy, unless an IOMN is simultaneously performed. Triggered EMG is also of great use in spinal tumors during the stage of tumor resection. It is applied to monitor selective nerve root function, and it is a “real-time” recording from the peripheral musculature. This monitoring also prevents the development of postoperative radiculopathy during spinal instrumentation surgery, including pedicle screw placement. Anal sphincter reflex and bulbo-cavernous reflex are mainly done for tumors involving the conus and cauda equina. The role of IONM in spinal tumors has been well-documented for predicting the adverse events and for obtaining better neurological outcomes.[93],[94],[95],[96]

  Future of IONM Top

The functions to be mapped and preserved are not only limited to language or motor ones, but expand to haptic, visuospatial, visual, and cognitive functions, in order to maintain complete patient integrity and preserve his/her quality of life. Consequently, performing only the intraoperative motor and language mapping is not enough while removing a cerebral tumor. Identifying and sparing neural networks that subserve cognition (movement control, visuospatial cognition, executive functions, multimodal semantics, metacognition) and mentalization (theory of mind, which plays a key role in social cognition) in awake patients is essential to resume a normal way of life.

Using a functional approach, the management of a patient entering the out-patient clinic with a presumptive diagnosis of glioma must be aimed at defining the degree of functional reorganization achieved by the patient's brain surrounding the tumor.[2],[97] The general treatment plan is to be decided and tailored to the specific situation taking into consideration the patient symptoms, signs, and the nature of tumor, thereby assessing the feasibility of resection and the possible degree of tumor removal that can be achieved in that particular patient. A detailed neuropsychological evaluation needs to be integrated with the imaging. This knowledge should be utilized during the surgical resection of the tumor, with the simultaneous usage of intraoperative monitoring tailored to the needs of the patient, also being done to ensure a safe yet adequate resection of the tumor.

The disorders generated by surgical injuries of circuits underpinning nonmotor and nonspeech functions are usually not immediately visible on the postoperative standard clinical examination, leading the physician to believe that the patient has had no deficit following surgery. Yet, cognitive or emotional disturbances may subsequently prevent the patient's return to an active life and to his/her full-time working. A new connectome-based surgical approach should be more systematically considered in the management of cerebral tumors to preserve or to improve quality of life.

Redefining concepts of oncofunctional resections

An oncofunctional balance needs to be maintained by the neurosurgeon based on the patient's needs, imaging, and the genetic characteristics of the tumor. Patients should clearly formulate his wishes regarding the kind of functions that he/she wishes, should be primarily preserved. This will depend on the way of life, including profession, hobbies, and social interactions.[98] The degree of resection, tailored according to the patient's lifestyle, may be different for a patient who uses skills other than the pure motor or language ones, and a patient who is content with motor and language skills alone.

Troubleshooting and complications

The troubleshooting steps,[20],[25],[99],[100],[101] in the case of drop of any of the responses, have been depicted in [Figure 12]. Complications include tongue bite (0.2%); thermal skin and neural injury, burns around the electrode; the precipitation of seizures, muscle pain and hematoma, transient blood pressure changes, cardiac arrhythmia; wrong electrode placement; and displacement of electrodes due to patient movements.[102] The relative contraindications during the MEP monitoring include the presence of epilepsy, cortical lesions, skull defects, and the presence of intra-cranial electrodes, vascular clips or shunts, and biomedical implants.[103] Preexisting structural or physiological damage to eloquent areas/long tracts makes the IONM unreliable. For instance, if the grade of muscle power in the patient is less than 3, eliciting an MEP response will be difficult and unreliable. The drawback of IONM is that the simultaneous monitoring of all modalities is difficult and might miss alarming electrophysiological responses at critical points of resection.
Figure 12: Troubleshooting in IONM

Click here to view

We conclude that IONM is more relevant to achieve a good onco-functional neurosurgical outcome. An intraoperative MRI (IOMRI) as an adjunct to IONM, increased the extent of safe resection.[104] The widespread usage of and an improvement in these techniques have allowed a greater and safer removal of tumors, thereby reducing the risk of permanent postoperative neurological deficits and obtaining a better functional outcome.[105],[106],[107],[108]


We acknowledge (1) Mr. Bomma Umesh, Neurosurgery OT Electrophysiology Technician, Department of Neuroanaesthesia, Yashoda Hospitals, Secunderabad, [email protected], 9912152902, and (2) Mr. Chaitanya Srinivas Lanka Venkatachalam, PhD Student, Neurotech Lab, Biomedical Engineering Department, Indian Institute of Technology, Hyderabad, [email protected], 8297051156.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12]

  [Table 1], [Table 2]


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