**4.5. Spinal cord surgery**

pharyngeal wall for CMN IX, the cricothyroid or vocalis muscle for X and the intrinsic tongue muscles for CMN XII. Electrical stimulation of the floor of the fourth ventricle is delivered through a monopolar hand-held probe using the same parameters described above for the stimulated EMG technique. It is important to stress that during this procedure, the threshold

**Figure 4.** Brain stem mapping in a patient with a tumour locate on the floor of the fourth ventricle. (A) Frontal and sagittal MRI before surgery. (B) Stimulated EMG during identification of the right CN IX (above) and the left CN VII (below). (C) Identification of the left CN IX (above) and the right CN VII (below). (D) Brain stem monitoring compris‐

This technique is very valuable because although there has been a recent advancement of neuroimaging techniques, BSM remains the only way to reveal the surgical anatomy in the

ing SSEPs (top) and BAEPs (bottom).

intensity is essential for proper localization of the CMN.

230 Neurooncology - Newer Developments

## *4.5.1. Anatomical and surgical considerations*

Intramedullary spinal cord tumours (ISCTs) comprise 15% of intradural spinal tumours in adults [67]. Tumours of glial origin represent approximately 68% of ISCTs, most of which are ependymomas or astrocytomas [68].

Microsurgery plays a central role in the treatment of these tumours, and the main goal is the complete surgical removal. This aim is limited because preservation of spinal cord function with minimum neurological morbidity it is also desired, representing another challenge of these surgeries.

Microsurgicalresection and the surgical strategy may vary, depending on the histological type of each tumour. However overall, there are well-defined steps during the surgical approach to ISCTs: opening of the posterior median sulcus, exposure and initial tumour debulking with separation between the sidewall of the tumour and spinal cord tissue and complete removal of the tumour, which requires the dissection and coagulation of the vascular afferences from the anterior spinal artery.

#### *4.5.2. Particularities of IONM*

During the resection of ISCT, lesions in neural structures can inadvertently occur, potential‐ ly creating severe neurological deficits. IONM of the spinal cord plays an important role in facilitating the resection of these tumours [69].

As previously described, an effective strategy could be to adapt IONM to the steps of the surgery to protect the somato-sensory and motor pathways [70]. Therefore, monitoring with SSEP and MEP is indicated.

Additionally, for spinal cord surgery, D wave recording is widely used and recommended. In well-documented studies of more than 100 ISCT surgeries, a preserved D wave up to 50% of the original amplitude, with a complete loss of muscle MEPs, has been shown to result in only transient paraplegia [71].

Although it also has disadvantages, we generally consider that under those circumstances in which the mechanism of injury to the spinal cord is purely ischemic, D wave monitoring does not add a significantly value to muscle MEP monitoring. Grey matter is more sensitive than white matter to cord ischemia, and both clinical and experimental data suggest that both peripheral and myogenic MEP disappear earlier than the D wave when spinal cord vascular‐ isation is acutely compromised [72]. Regarding this issue, it seems logical to postulate that the nerve structures most likely to be affected during an ischemic alteration in ISCT surgery are the anterior horns (AH).

It is important to note that previous observations of EMG may improve the reliability of IONM during spinal cord surgery [73, 74]. Skinner and Transfeldt [75] have reported experience with EMG for monitoring ISCT in 14 patients. They described segmental and suprasegmental elicitation of neurotonic discharges that could anticipate the loss of MEP and predict a postoperative motor deficit. Moreover, some studies have associated spinal cord mechanical and thermal injury with EMG activity and motor conduction block (MEP loss) in animals [75, 76].

In our experience, we can define three different phases of the surgical approach to ISCTs that must be followed by the neurophysiological techniques used (**Figure 5**).(i) Phase A: during posterior median sulcus opening, it is important to protect the dorsal columns by monitor‐ ing the somato-sensory evoked potentials (SSEPs). (ii) Phase B: working at the cleavage plane, monitoring of motor evoked potentials (MEPs) is mandatory, mainly to protect the cortico‐ spinal tract in the lateral cords during the separation between the sidewall of the tumour and the spinal cord tissue. (iii) Phase C: the final phase of complete tumour removal during which, as previously mentioned, the anterior vascular supply is threatened and consequently there is a certain risk of injuring the AH. Thus, fEMG is critical during this phase.

**Figure 5.** Two different times during monitoring of ISCT surgery. (A) (Top) Microphotograph during incision of the dorsal median raphe. (Bottom) SSEPs for the left (blue) and right (red) sides. Times are shown on the left. Zero min indicates opening of the dura. Between 8 and 18 min, both SSEPs, displayed a decrease in amplitude and an increase in latency (right SSEPs at 8 min). (B) (Top) Microphotograph during tumour removal from the lateral walls. (Bottom) MEPs for the left (blue) and right (red) sides. Grey lines indicate significant changes in MEPs. Times are shown on the left.
