Introduction
Although spine surgeons attempt to maximize surgical success, such surgeries have inherent risks of iatrogenic neurological deficits due to mechanical injury and altered vascular supply to the spinal cord. Current studies have reported various rates of neurological complications following spine surgery, ranging from 0.8% to 32% of cases, with a mean rate of 1.4% [1–3]. Researchers have attributed the discrepancies in these rates to specific disorders and surgical procedures involved. Intraoperative neurophysiological monitoring (IONM) has emerged as a crucial tool for minimizing iatrogenic neurological complications [4,5], leading to a global increase in its utilization for spine surgery [6–8].
IONM can be performed using several modalities, such as transcranial electrical stimulation-muscle evoked potentials (Tc-MEP), somatosensory evoked potentials (SEP), transcranial electrical stimulation-spinal cord evoked potentials (Tc-SCEP [D-wave]), spinal cord-evoked potentials after spinal cord stimulation (Sp-SCEP), spontaneous electromyography (EMG), triggered EMG, and muscle-evoked potentials after electrical stimulation of the spinal cord (Sp-MEP) [9]. Notably, multimodal IONM has demonstrated superior results compared with single-modality approaches, resulting in its increased utility in spine surgery [10,11].
Despite the inherent appeal of IONM for maximizing safety and reducing the risk of complications, its availability and type of monitoring likely vary in Asia–Pacific countries compared with other regions. Surgeon experience, training background, and resource availability may influence IONM usage. Therefore, the current IONM trends in the Asia–Pacific countries require investigation.
The primary aims of this study were (1) to clarify the current trends in IONM usage in Asia–Pacific countries and (2) to explore deeper into the specifics of its application. Given the predominance of Tc-MEP in Japan [8], targeted questions were prepared to further understand this modality. The secondary aim was to clarify the perceived necessity of IONM and identify the recommended modalities for several representative spinal disorders and surgical procedures.
Materials and Methods
Ethics statement
Institutional ethical approval has been received from Nara Medical University (approval no., 3625). The need for informed consent was waived because the collected data did not include patient information.
Study design
A web-based survey was conducted using Google Forms, which was sent to all members of the Asia–Pacific Spine Society (APSS) via email between October 1 and November 30, 2023. The APSS consisted of 626 spine surgeons across Asia–Pacific countries as of October 2023. The specifications of the questionnaire are available online in Google Forms (Google LLC, Mountain View, CA, USA) (Supplement 1). The researchers briefed the participating respondents about their choice to opt out of the study.
Details of the questionnaire
First, the respondents’ background information, including nationality, years of experience in spine surgery and IONM use, number of spine surgeons in the respondent’s hospital, hospital type, and number of spine surgeries performed annually were collected. Then, data on the commonly used IONM modalities, such as Tc-MEP, SEP, D-wave, Sp-SCEP, and spontaneous electromyography, were collected. Focusing on Tc-MEP monitoring, the following details were gathered: (1) alarm point; (2) management protocols (e.g., preparation, intraoperative management, and judgment of wave amplitudes); (3) channel configuration; (4) complication experience and prevention; and (5) strategies for managing inadequate initial wave amplitudes. In addition, the survey explored respondents’ experiences with the wake-up test.
Finally, the respondents’ views on IONM standardization were probed, and modalities for various spinal disorders were recommended, including adult spinal deformity (ASD), scoliosis (idiopathic, congenital, or neuromuscular in pediatric patients), cervical ossification of the posterior longitudinal ligament (OPLL), cervical spondylotic myelopathy (CSM), thoracic OPLL, thoracic ossification of the yellow ligament (OYL), intramedullary spinal cord tumor (IMSCT), extramedullary spinal cord tumor (EMSCT), cauda equina tumor, lumbar spinal canal stenosis (LSS), and low- or high-grade spondylolisthesis (Supplement 1). Similarly, the same questions were asked regarding spinal surgical procedures, including kyphosis correction, growing rod insertion, and cervical or thoracic instrumentation (Supplement 1). The respondents were also invited to share their surgical experiences associated with each spinal disorder and surgical procedure. The requirement ratio of IONM was calculated as follows: requirement ratio of IONM (%)=necessity of IONM (number)/[IONM user (number)-no treatment experience (number)]×100.
Results
Survey respondents
Overall, 193 APSS members responded to the questionnaire (response rate, 30.8%). The Philippines contributed the highest number of respondents (22.8%), followed by Malaysia (19.7%), India (18.1%), China and Hong Kong (14%), and Japan (8.3%) (Table 1). Among the 193 respondents, 177 used IONM routinely, whereas 16 (8.3%) did not use IONM (Fig. 1).
Status of IONM usage among routine users
Regarding the years of experience in spine surgery, among the 177 IONM users, 38 reported ≥20 years, 25 reported 16–19, 48 reported 11–15, 48 reported 6–10, and 18 reported 0–5. Regarding the years of experience with IONM use, 16 reported ≥20 years, 11 reported 16–19, 47 reported 11–15, 57 reported 6–10, and 46 reported 0–5. The number of spine surgeons, hospital types, and number of spine surgeries are shown in Table 2.
Several IONM modalities are available at present. This survey showed that the most preferred modality was Tc-MEP (n=160, 94.1%), followed by SEP (n=144, 81.4%), spontaneous EMG (n=78, 44.1%), triggered EMG (n=67, 37.9%), Sp-SCEP (n=15, 8.5%), and D-wave (n=13, 7.3%) (Fig. 2). Notably, 27 respondents were single-modality users (Tc-MEP only [n=21] and SEP only [n=6]), whereas the remaining majority employed multimodal IONM, with Tc-MEP with SEP (n=130) as the most common multimodality. Regarding the frequency of IONM usage, 101 (57.1%) reported its use for less than half of spine surgeries, 60 (33.9%) used it more than half of spine surgeries, 12 (6.8%) used it for all spine surgeries, and 4 (2.3%) reported otherwise.
Tc-MEP monitoring among Tc-MEP users
Operation
Various medical personnel with distinct roles are involved in the IONM processes, including preparation, management, and judgment (Table 3). Medical technologists mainly handle the technical aspects, whereas orthopedic surgeons play a crucial role in interpreting the alarm points (n=69). In contrast, 165 respondents (40.6%) answered that only nonphysician medical personnel were involved in IONM operations. For the remaining respondents, orthopedic surgeons, neurologists, and anesthesiologists played active roles in IONM operations.
Alarm point
Among the Tc-MEP users, 103 (64.4%) considered a >50% wave amplitude reduction as the alarm point. Meanwhile, 22 respondents (13.8%) considered a >70% reduction as the alarm point, and 24 (15%) had different alarm points based on specific spinal diagnoses. One response each was noted for >90% reduction and increased stimulation intensity as alarm points, whereas the alarm point for nine respondents was unknown.
Number of channels
Among the Tc-MEP users, 101 respondents (63.1%) used 8–14 channels, 28 (17.5%) used 16 channels, and 7 (4.4%) used four channels. Meanwhile, the channel configuration of the 24 respondents was unknown.
Experiences of complications
Although 82 respondents (51.3%) reported no complications with Tc-MEP monitoring, 78 (48.8%) encountered at least one. Among these complications, bite injuries were the most common (tongue injuries, 43 respondents; buccal mucosa injuries, 26 respondents; tooth injuries, 11 respondents). Notable complications included four reports of hair loss, two of burns, and two of epileptic seizures.
Regarding methods of preventing complications, respondents primarily minimized the intensity and frequency of transcranial stimulation (n=82). Other significant methods included filling the oral cavity with gauze (n=81) or using a hard bite block (n=45), whereas 36 respondents avoided Tc-MEP monitoring for patients with a history of epilepsy. Notably, these answers may have overlapped because the respondents mentioned multiple prevention methods.
Response to an alarm point
Among Tc-MEP users, 83 (51.9%) had planned responses to a potential alarm point, whereas 62 (38.8%) had no planned response. In addition, the response plans for the 15 respondents were unknown.
Experience of the wake-up test
Regarding the use of the wake-up test, 82 respondents (51.3%) had experience with such tests, 54 (33.8%) reported no experience, and 24 reported no experience with situations requiring the wake-up test.
Management of insufficient wave amplitudes
In cases involving insufficient initial wave amplitudes, 69 respondents (43.1%) persisted in identifying at least one muscle for monitoring. In contrast, 51 respondents (31.9%) opted to introduce SEP/other modalities and proceed with surgery. Furthermore, 17 respondents (10.6%) decided to start surgery without Tc-MEP, 9 (5.6%) preferred surgical postponement, and 14 (8.8%) provided unknown and diverse responses.
Recommended modalities for spinal disorders and surgical procedures
Respondents identified the following spinal conditions with the perceived necessity of IONM in descending order of importance: (1) scoliosis (idiopathic, congenital, or neuromuscular in pediatric cases), (2) ASD, (3) IMSCT, (4) thoracic OPLL, (5) EMSCT, (6) cervical OPLL, (7) thoracic OYL, (8) CSM, (9) cauda equina tumors, and (10) LSS (Table 4). Owing to data-loading difficulties in Google Forms (Google LLC), the requirement ratio could not be calculated for spondylolisthesis surgery.
The perceived necessity of IONM in surgical procedures, in descending order of importance, includes the following: (1) kyphosis correction, (2) growing rod insertion, (3) cervical instrumentation, and (4) thoracic instrumentation (Table 4).
Across all spinal disorders and surgical procedures presented in the questionnaire, Tc-MEP was the most frequently employed modality, followed by SEP. Multimodal IONM was also commonly used by respondents, with an average utilization rate of 84.7% (range, 66.7% [LSS] to 92.2% [thoracic OPLL and thoracic OYL]) in spinal disorders and 88.3 % for surgical procedures (Table 4).
Discussion
To the best of our knowledge, this is the first study that comprehensively investigated current trends in IONM usage in the Asia–Pacific. Although 16 respondents (8.3%) did not use IONM, 177 (91.7%) routinely used IONM in spine surgeries. As expected, Tc-MEP was the most commonly used IONM modality (n=160, 90.4%), followed by SEP (83.1%). Interestingly, SCEP monitoring, such as Sp-SCEP and D-wave, was a less prevalent modality in Asia–Pacific countries (n=27, 15.3%) compared with its use in other regions.
Our findings differed from those in surveys from other geographic areas. For instance, the most recent survey in German-speaking countries, including Germany, Austria, and Switzerland, revealed that Tc-MEP and SEP were the most used modalities (93.7% and 94.3%, respectively), followed by Sp-SCEP (66.5%) and spontaneous EMG (48.1%) [12]. Similarly, a survey from Canada conducted more than a decade ago reported SEP as the most commonly used modality (65.3%), followed by spontaneous EMG (44.2%) and Tc-MEP (28.4%) [13]. These studies have suggested that SEP might play a more prominent role in IONM utilization in other countries. Nevertheless, SEP utilization in Asia–Pacific countries remains quite substantial.
The historical development of IONM modalities played a role in shaping regional trends. Tamaki et al. [9] were the first to report the use of SEP, the first available evoked potential for monitoring [14]. Advancements in processing technology enabled a clearer recording of evoked potentials, leading to the pioneering development of Sp-SCEP by Japanese orthopedic surgeons in 1972. At that time, the predominant IONM modality in Japan primarily focused on the sensory-related tract [15]. However, in the present study, only a few respondents used Sp-SCEP. Regarding motor pathway monitoring, Merton and Morton [16] reported the first observation of evoked potentials from conscious patients without anesthesia. However, this new modality was challenging to implement because recording action potentials needed more time after stimulating the motor cortex under anesthesia [17]. The discovery of the multiple-train electrical stimulation method contributed significantly to its development [18], leading to the rise of Tc-MEP as the primary IONM modality in Japan [8]. Although we could not clarify the specific trajectory of IONM trends in other Asia–Pacific countries, the advancements of each modality have significantly influenced the present usage patterns. Notably, 150 respondents (84.7%) reported the prevalence of multimodality approaches, confirming our study results.
Tc-MEP monitoring among Tc-MEP users
A multidisciplinary team approach that involves anesthesiologists, surgeons, and monitoring specialists is crucial for IONM implementation. This study revealed that medical technologists primarily handle the majority of IONM operations, whereas orthopedic surgeons interpret the alarm points and make clinical decisions accordingly.
Regarding alarm points, approximately two-thirds of the respondents used the 50% decrease in Tc-MEP amplitude as the threshold, whereas some had different alarm points according to the surgery type (n=24, 15%). At present, several alarm points on Tc-MEP have been suggested, such as the use of the >50% decrease in baseline amplitude as an indicator of significant change [19] or the use of the 70% decrease in amplitude, given its high sensitivity of 95% and specificity of 91% for spinal deformity, OPLL, and spinal cord tumors [20]. However, the latter criterion was less prevalent in Asia–Pacific countries.
Regarding monitoring channels, Ito et al. [21] recommended that multichannel monitoring of at least eight channels should be performed to minimize false-negative cases and maximize the detection of muscular motor deficits. Our data are consistent with this finding because multichannel monitoring (>8 channels) was common in the Asia–Pacific.
Half of the respondents reported complications associated with Tc-MEP monitoring, with bite injuries being the most common. The existing literature has shown similar incidence rates of 0.13%–6.5% for bite injuries [22,23]. To prevent these complications, respondents emphasized on minimizing the intensity and frequency of stimulation and using soft or hard bite blocks in the oral cavity. Although epileptic seizures were a rare complication, respondents avoided IONM for patients with a history of epilepsy. However, this decision should be carefully considered on a case-by-case basis (seizure risk versus IONM need) because current research does not support a causative link between Tc-MEP and increased seizure activity [24].
Regarding the management of the alarm points, half of the respondents had planned responses for such situations. Although no single approach is universally ideal, the multidisciplinary team approach remains crucial. In particular, the use of checklists for managing these situations may further improve patient care [25,26].
In cases of inadequate initial wave amplitudes, most respondents were reluctant to postpone surgery. Although a minority proceeded with surgery without Tc-MEP monitoring, approximately three-fourths attempted to maintain IONM, either by identifying another muscle or incorporating another modality, such as SEP. These decisions likely reflect the diverse nature and differing risk profiles of spinal disorders.
Recommended modalities for spinal disorders and surgical procedures
Regarding spinal disorders where IONM was perceived to be necessary, scoliosis (pediatric cases) was the most commonly indicated disease (98.8%), followed by ASD, IMSCT, and thoracic OPLL (Table 4). Conversely, despite the emerging utility of IONM as a standardized procedure for all spinal deformity cases, LSS had the least indication for IONM. These trends were consistent with previous reports [8,13].
Regardless of the surgery type, >80% of the respondents preferred multimodal IONM, except for LSS and spondylolisthesis surgeries (Table 4), because of its high sensitivity and specificity for detecting neurologic injuries [11]. Although further research is needed to determine the optimal multimodality combinations for specific scenarios, this approach is deemed practical and effective. However, no consensus has been established regarding the use of IONM during low-risk spine surgeries. In this study, only 27.7% of respondents considered IONM as necessary in LSS surgery, showing similar findings in Japan [8]. Meanwhile, another study reported decreased neurological complications in lumbar surgeries with IONM [27]. In addition to exploring the effectiveness of IONM, future studies should investigate the cost–benefit analyses for its use in each spine procedure.
Despite the valuable insights offered by this study, it has some limitations. First, the response ratio was relatively low and could not cover all Asia–Pacific regions. Second, some respondents belonged to the same institution, which may have introduced some bias into the results. Nevertheless, this study is the first and largest survey analysis on IONM use among spine surgeons in Asia–Pacific countries. Third, although we addressed current trends in IONM use in the Asia–Pacific countries, we could not delve into the regional economics of utilizing monitoring. Further studies are needed to clarify this issue.
Conclusions
This study revealed the widespread use of IONM as a standard of care for spine surgeries in Asia–Pacific countries, with Tc-MEP as the predominant IONM modality, followed by SEP. Overall, the findings support the routine use of multimodal IONM for scoliosis (pediatric) surgeries, ASD, and IMSCT. Although we investigated spine surgeries where consistent IONM usage is well supported, future cost–benefit analyses may be warranted.
Key Points
A prospective survey was performed to examine the trend of intraoperative neurophysiological monitoring (IONM) usage in Asia–Pacific countries.
Among respondents (n=160) from Asia–Pacific countries, transcranial electrical stimulation-muscle evoked potential was the most utilized IONM modality (94.1%), followed by somatosensory evoked potentials (81.4%).
Although most respondents strongly recommended IONM in spinal deformity surgeries for adult and pediatric populations and tumor resection surgery for intramedullary spinal cord tumors, IONM was least recommended in lumbar spinal canal stenosis surgery.
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