Embolization is a useful option to treat hypervascular lesions of the spine. However, it presents with a risk of spinal cord ischemia with a high risk of neurological motor and sensory deficits. Intraoperative Neurophysiologic Monitoring (IOM), specifically Somatosensory Evoked Potentials (SSEPs) and Motor Evoked Potentials (MEPs) have been used in various surgical disciplines to prevent possible neurological deficits, including ischemic injury of the spinal cord. SSEPs test is designed to evaluate the functional integrity of the dorsal column/medial lemniscus sensory pathway. However, to confidently assess the anterior-lateral/corticospinal tract the MEPs test should be employed. Therefore, both tests are usually employed for complicated neurosurgical, orthopedic, and endovascular procedures. The specific contribution of IOM to the safety of endovascular procedures is the use of (MEPS) and (SSEPS) during the provocative tests. Injection of short-acting anesthetic agents (barbiturates and/or lidocaine), while evaluating pre- and post-injection IOM results, allows the surgical team to determine with confidence whether the vessel tested feeds the hypervascular lesions or the functional area of the spinal cord. Here we present 2 case studies to illustrate the role of IOM in transvascular embolization procedures.
Patients and Methods
The study covers two provocative interventional neurovascular surgeries performed at a hospital on Long Island, New York, USA. IOM was performed in both procedures. IOM data and provocative testing results were obtained and analyzed.
Intraoperative Neurophysiologic Monitoring
In all procedures, IONM was performed to assess the neural integrity of central and peripheral nervous systems at the surgical site, and in so doing, determine whether one vessel feeds the vascular malformation. Somatosensory Evoked Potentials (SEP) and Motor Evoked Potentials (MEP) were the modalities included in the multimodality monitoring protocol. (Toleikis et al, 2005).
Each patient was met in the pre-operative holding area. A brief history-taking of the present illness was conducted and documented. Informed consent for IONM was obtained from the patient after a thorough description of all modalities employed. Both patients did not have any pre-existing contraindications to MEPs ( MacDonalds, DB et al 2013).
Motor Evoked Potentials
For transcranial electrical MEP, scalp stimulating corkscrew electrodes were placed at C3, C1, C2, and C4, with interhemispheric montages C1-C2 or C3-C4 and recording with subdermal needle electrodes bilateral brachioradialis, abductor policis brevis / abductor digit minimi, tibialis anterior and abductor hallucis according to the International 10-20 Position system. (ASNM position statement on MEPs; MacDonalds, DB et al 2013). Stimulation intensities ranged from 250V-275V, at a duration of 0.5ms, repetition rate of 1Hz, 330 pulses per second, and 7 trains. MEPs were acquired at the following points of the procedure: pre-incision, insertion of the guiding wire, pre-injection and post-injection, and final trace.
Somatosensory Evoked Potentials
Subdermal needle electrodes were placed at the following 10-20 positions for recording SSEPs following stimulation of the ulnar nerves (for upper extremities) and posterior tibial nerves (for lower extremities at 55mA intensity, 0.3ms duration, 2.71Hz frequency): Cz; Fpz; C1’; C2’; C3’; C4’; Cv. To record peripheral responses for upper SSEPs and lower SSEPs, respectively, subdermal needle electrodes were placed at the left and right Erb’s points as well as bilateral popliteal fossa. Adequate electrical grounding was applied for both upper and lower extremities. All electrodes were connected to a 32-channel neurophysiologic monitoring system (NIM Eclipse, Medtronic, Jacksonville, Florida, USA). SSEPS were continuously monitored during the procedure.
Total intravenous anesthesia was employed. Neuromuscular blocking agents were used for induction only, just enough to wear off before incision. Neuromuscular junction functionality was assessed and confirmed by a train-of-four (TOF) technique from posterior tibial nerve stimulation and recording from the abductor hallucis.
Provocative testing was conducted just before the embolization to evaluate the effect of the embolic agent (Brevital plus or minus Xylocaine) on the neurological function of the spinal cord. Preservation of SSEPs and MEPs will indicate that no potential neurological sequelae is resulting from the injection of the embolic agents and allows the surgical team to proceed with embolization. The change in IOM data (either SSEPs or MEPs) is indicative of a possible ischemic event and points to a high probability of a new neurological deficit, allowing the surgeon to decide whether to proceed with or abort the procedure.
Patient 1: The patient is a 63-year-old female who presented with a T4-5 arteriovenous fistula. She complained of “severe” pain in her back, as well as generalized weakness in her bilateral upper and lower extremities. However, the patient was still able to move all extremities in the absence of resistance. She was able to ambulate with some difficulty. The patient stated a history of hypertension. The surgical details of the endovascular treatment strategies are described by Berlis et al. (2005).
Replicable IOM data for SSEPs and MEPs were acquired at the beginning of the case (Figures 1 and 2) as described in the Methods section of this paper. SSEPs data remained unchanged throughout the entire procedure (Fig. 1). However, right after the injection of the provocative agent (Brevital, Lidocaine), MEPs were diminished on the left tibialis anterior and left foot (abductor hallucis) (Fig. 2). The surgeon was alerted and embolization was aborted at that time. The provactive agent washed out and recovery of MEPs data was observed. Description of the surgical procedure for this patient was published earlier this year by S. Mangla and colleagues ( 2020).
Patient 2: The patient is a 77-year-old male who presented with weakness of his right leg and perineal numbness. A cancer survivor, he was known to have renal and prostate cancer. A hypervascular tumor to T9 with mass effect and spinal cord edema was found. The patient also reported a history of hypertension, coronary artery bypass grafting, and cardiac stenting. The patient denied a history of seizures, metal implants, or any other significant neurological diseases. The surgical details of the embolization procedure are described by Choi et al (2020). IOM was essential for safe treatment: the hypervascular blush would have made identification of a small anterior or posterior spinal artery by angio impossible.
Replicable IOM data for SSEPs and MEPs were acquired at the beginning of the case (Figures 3, 4) as described in the Methods section of this paper. IOM data remained unchanged throughout the entire procedure (Figure 3, 4). The surgeon proceeded with embolization with no neurological sequelae.
Here we presented two illustrative cases demonstrating the utility of IOM in provocative testing during endovascular embolization procedures. Employment of both SSEPs and MEPs to assess the neurological function of the (anterolateral) corticospinal tracts and the posterolateral tracts (dorsal columns) of the spinal cord are essential for safety and decision making during these procedures. In addition, while SSEPs are averaged responses, MEPs are instantaneous recordings that provide immediate feedback regarding the integrity of the spinal columns in the event of ischemic insult.
Spinal vascular supply is highly variable in individuals with critical vessels able to come off any of approximately 70 named vessels. Additionally, the spinal cord supply can come from as few as three to as many as ten vessels. Between these contributors to the spinal cord and the vessels below, above, and across, there are multiple significant anastomoses. Inadvertent or unrecognized infusion into these vessels or reflux / overflowing into any of these vessels can lead to catastrophic damage to the spinal cord. Recoveries from such infarcts are slow, if at all. Infarcts to the anterior spinal artery territory essentially transect the cord, leaving only dorsal column functionality intact below that level. Infarcts to the posterior spinal column disconnect the dorsal columns (proprioception), resulting in a loss of joint position sense and making use of the limb impossible without visualization.
Also, relying only on angiographic confirmation of the vascular territory is extremely limited in the spine and therefore IOM will be beneficial for several reasons:
The loss of MEPs described in Case 1 indicated an impending neurological deficit due to possible ischemic insult to the anterior- lateral columns. These IOM data allowed the surgeon to abort embolization and preserve neurological function. The absence of IOM data changes in Case 2 allowed the surgeon to confidently proceed with the tumor embolization. Data from these cases demonstrate that IOM can predict the absence or presence of the functional impact of the provocative test and sequential embolization.
Thus, both the absence and presence of SSEPs and /or MEPs data change, IOM is invaluable in endovascular procedures: