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REVIEW ARTICLE
Year : 2023 | Volume
: 24
| Issue : 1 | Page : 4-10
The relevance of the wake-up test in scoliosis surgery: A narrative review
Sapna Annaji Nikhar1, Abinash Patro2, Raju Iyengar3, Ramachandran Gopinath4
1 Additional Professor, Department of Anaesthesia and Intensive Care, Nizam's Institute of Medical Sciences, Hyderabad, Telangana, India
2 Associate Professor, Department of Anaesthesia and Intensive Care, Nizam's Institute of Medical Sciences, Hyderabad, Telangana, India
3 Department of Orthopaedics, Nizam's Institute of Medical Sciences, Hyderabad, Telangana, India
4 Department of Anaesthesia and Intensive Care, ESIC Medical College, Hospital and Super Specialty Hospital, Sanath Nagar, Hyderabad, Telangana, India
Correspondence Address:
Dr. Sapna Annaji Nikhar
Department of Anaesthesiology and Intensive Care, Nizam's Institute of Medical Sciences, Hyderabad - 500 082, Telangana
India
Source of Support: None, Conflict of Interest: None
CheckDOI: 10.4103/TheIAForum.TheIAForum_25_23
The corrective scoliosis surgery is associated with many complications; the neurologic complication is one of them. It is very important to assess spinal cord integrity intraoperatively to avoid postoperative damage and have a good outcome. Wake-up test was the standard of care in old generations but now with the availability of advanced neurophysiologic monitoring somatosensory and motor-evoked potentials; the relevance of wake-up test is questionable. This review aimed to assess the relevance of wake-up test in scoliosis surgery with availability of advanced technology. Comprehensive literature search was performed in PubMed, Google Search and Scopus, EndNote X6 version, and the keywords used for the search were scoliosis, neurophysiologic monitoring, and wake-up test. The different study results were interpreted to come to conclusion. This review gives us a detailed idea of the different tests available and pitfalls with each and finally what should be our stand. The review highlighted the good collaboration between surgeon, anesthesiologist, and neurologist to conclude the results of the tests of neurophysiologic monitoring. Intraoperative neurologic monitoring improves postoperative outcome. Wake-up test is only and strongly recommended, in case of unavailability of advanced monitoring, confusion on test results, and persistent signal reduction.
Keywords: Neurophysiologic monitoring, scoliosis, wake up test
Scoliosis is a complex deformity of the spine that increases with age.[1] The complications increase if the corrections are not done at the proper time.[2] The surgery done for its correction is posterior Moss-Miami instrumentation and posterior spine fixation along with rib dissections and/or hemivertebrectomy sometimes. Out of all complications observed, a neurologic complication is the most dreaded one. The most interesting point is that these complications are preventable if assessed properly and given notice at the proper time with necessary management. Hence, it is very important to assess fixation intraoperative to avoid postoperative complications. The tests used are wake-up test, clonus test, somatosensory-evoked potentials (SSEPs), and motor-evoked potentials (MEP) monitoring.[3] In the recent era, as we have all newer monitoring modalities, the use of SSEP with MEPs is standard for assessment. However, most institutes do not have neurophysiologic monitoring and surgeons still prefer to do wake up the test for neurologic monitoring. This text will deal with neurophysiologic monitoring for scoliosis surgery and the use of wake-up test: Its relevance.
Spine AnatomyThe spine is made up of vertebrae supported by muscles, ligaments, flexible tendons, and strong bones. The spinal cord runs from the brain stem to the first lumbar vertebra protected within the spinal canal. It serves the role of relaying messages from higher centres to body and body to higher centres through spinal reflexes helping in movement and also protecting our body from harm. The bony projections on the back of each vertebra form a vertebral arch, which is supported by two pedicles and laminae [Figure 1]a and [Figure 1]b.[4] A pair of spinal nerves exist under each pedicle and pass through the intervertebral foramen to branch out to our body. The nerves are at risk at many places of their course. The risk can be because of surgery itself, instrumentation, traction, direct injury, or pressure effect. Anatomy makes us to understand the course where nerves at risk. Even understanding of normal anatomy makes our thought clear that in rotated or curved spine, the spinal cord is already at risk. The relationship of spinal nerves to surrounding structures and different pathways involved in carrying signals to higher centers have been shown in [Figure 2] and [Figure 3], as a guide to interpret the neurological deficit or to predict the risk prone zone.[5],[6]
Figure 1: (a) The four natural curvatures of the spine in the sagittal plane: Cervical and lumbar lordoses and thoracic and pelvic kyphosis. (b) Abnormal curvature of spine with rotation
Figure 2: Normal anatomy of spinal nerves and relation to adjacent structures. This figure describes the closeness of nerves to pedicles and nearby structures likely possibility of getting damage
Figure 3: The tracts and division. The pathways need to be functionally intact to assess MEP, SSEP, and wake-up set. MEP: Motor-evoked potentials, SSEPs: Somatosensory-evoked potentials Definitions Kyphoscoliosis is an abnormal curvature of the spine. Kyphosis is an abnormal curvature of the spine resulting in bowing of the spine while scoliosis is a complex deformity of the spine due to various causes resulting in lateral curvature and rotation of vertebrae with deformity of the rib cage. Scoliosis is common with the prevalence may vary between 0.3% and 15.3%[1] in the general population and most of the cases are of idiopathic origin (75%–90%).[7],[8] Scoliosis is a deformity of the spine that tends to be evident during growth spurts and usually, that is the time many approaches the surgeon for management. The severity of curvature increases with the growth and can lead to respiratory and cardiovascular complications.[2] Hence, surgical correction of scoliosis is needed to stop the progression of the disease and avoid cardiorespiratory complications.[4],[9] Few patients surgery is done as cosmetic surgery.[10] Basically, target is early fusion to avoid progression of disease with growth and avoid complications. The treatment of scoliosis can be surgical or nonsurgical. There are three approaches for the surgical correction of scoliosis. The first and most commonly done is posterior spinal fusion with contoured or adjustable rods, which are commonly used for younger children and allow for lengthening over time.[11] The second approach is anterior, through a thoracoabdominal incision and mainly preferred to remove intervertebral discs or hemivertebrae contributing to abnormal curvature. The last or third one is combined posterior fusion and anterior release.[10],[12] There are many stages during this correction when the spinal cord is at risk and any injury can lead to neurological damage. This if goes unrecognized can lead to permanent neurological consequences. Hence, this warrants the monitoring of spinal cord function for a better outcome. Multiple factors can lead to the spinal cord damage and if we are attentive to take care of all those factors at the proper time, we can identify and prevent permanent neurological damage.
Risk factors for more neurological damage and are directly related to it:[9],[13]
Patient characteristics, age >60 yearsPreoperative neurologic deficit and stage of disease measured with Cobb's angle. Higher the angle, more complications we can expectLarge coronal plane deformitiesSurgical approachLength and type of surgical procedure, multilevel surgeryUse of osteotomiesThoracic hyperkyphosisRevision surgery statusSpinal cord perfusion pressure (SCPP)Underlying spinal pathologyPressure on neural tissue during surgery.Scoliosis needs proper preoperative assessment, understanding the deformity, levels of vertebral surgery, any plan of rib dissection, and approach of the surgery. Hence, discussion with surgeon about their plan of surgery is very important. Similarly, evaluation of all systems with Cobb's angle estimation is very important. Intraoperative anesthesia management needs to be very meticulous as from positioning to hemodynamics any of the factors can affect our interpretation of surgical outcome. The neurologic monitoring plays a very pivotal role in scoliosis surgery. The balanced anesthesia, maintenance of all norms of hemodynamics with normothermia, and proper neurologic monitoring can lead to good outcome.[9],[14] There is still practice of steroid use if neurologic monitoring is suspicious and too much manipulation of neural tissue is suspected to have least or no neurologic complications.[15]
The incidence of neurologic complications can be as high as 3.7% and 6.9% in the absence of neurologic monitoring.[16] This can be reduced to 0.5% with the advance monitoring.[17] The currently available methods to monitor the integrity of the spinal cord include the Stagnara wake-up test, SSEPs, and MEPs.[3] There are preventive strategies and therapeutic strategies for better outcome of scoliosis surgery. The neurologic monitoring and balanced anesthesia practice is a part of preventive strategy.
The commonly used methods areWake-up test
Vauzelle et al. introduced the concept of waking the patient up during surgery to determine the functional integrity of the spinal cord.[18] Abnormal (positive) wake-up test is interpreted as the difference in upper versus lower limb movements, the difference in right and left lower limb movements, and the absence of clonus. This test is limited to gross motor movements of the lower extremities and can be influenced by anesthetics and the cognitive integrity of the patient. Furthermore, complications related to the test include inadvertent extubation of the patient during movement in the prone position, air embolism during deep inspiration, and dislodgment of the instrumentation during violent movements. The Stagnara wake-up test when properly administered is 100% accurate in detecting gross motor movements; however, the patient has to be adequately awake to follow the commands. However, it does not give any impression of sensory pathways and also done once or twice. Hence, spinal cord damage may occur in between tests which can be easily missed.
Multimodal intraoperative monitoring has been added to the standard of care for complex reconstructive spinal surgery.[19] The monitoring includes SSEP, MEP, and electromyogram.
Somatosensory-evoked potentials
They are a type of sensory-evoked response. It provides the ability to monitor the functional integrity of sensory pathways in the anesthetized patient undergoing surgical procedures which place the spinal cord at risk. It is recorded after the electrical stimulation of a peripheral nerve. In the patient with intact SSEPs, it is still possible for the significant motor deficit to develop postoperatively as blood supply to motor tracts may get damaged.[20] This test is affected by almost all anesthetic agents, mean arterial pressure, temperature changes, and hypoxia.[9] Hence, wake-up test and further anatomic manipulation then can be performed based on the surgeon's discretion.[21]
Motor-evoked potential
Just to overcome the limitations of the wake-up test, MEPs came into the picture. However, its assessment is markedly affected by all anesthetic agents.[22] This is also less reliable, and reports of false-positive results suggest that this method of spinal cord monitoring also have a sensitivity of < 100%, strict anesthetic protocol needs to be followed during MEP monitoring.[9],[23]
Ankle clonus
It gives us a good sign of spinal cord integrity, but it provides a very brief period (time window) between anesthesia and wakefulness when it is possible to elicit clonus, making it difficult to time it with the light plane of anesthesia.[24]
There are different studies to give us an idea of the feasibility of different tests. As we have advanced neurologic monitoring, we can have a continuous assessment of cord function but still, all tests are not having 100% specificity and sensitivity. The coordination between the surgeon, anesthesiologist, and neurophysiologist is necessary to get the best outcome in spine surgery.[25]
All the tests are going to be affected by volatile agents over 1 minimum alveolar concentration. MEPs are very much sensitive to even lower alveolar concentration of inhaled anaesthetics. Opioids and lower doses are propofol do not suppress MEP and SSEP, but MEPs are suppressed by barbiturates. Benzodiazepines at premedication doses do not affect recordings. Hence, propofol and opioids can be used safely for monitoring but better to avoid boluses as higher concentration may affect MEPs. Dexmedetomidine is an evolving drug for the use of neurophysiologic monitoring, and the recordings are unchanged by the infusion doses of 0.3–0.5 μg/kg/h.[26] Even hypoxia, anemia, hypocapnia, and decrease in systemic or regional blood pressure affect the latency and amplitude of MEPs and SSEPs. Hypothermia increases SSEP latency and alters pH and also has an effect on the metabolism of muscle relaxant and hence affecting all SSEP and MEPs[9][Table 1]. At the time of the wake-up test, we make sure that we optimized all parameters and then only we go ahead with an assessment, so the effect of the above parameters is less as compared to continuous monitoring techniques. Different studies showing the effectiveness of advanced neurophysiologic monitoring in scoliosis surgery are represented in [Table 2].[27],[28],[29],[30],[31],[32],[33],[34],[35],[36]
Risk related to anesthesia and surgery can be minimized as preventive strategy:[13]
Good interaction with the surgeon and understanding the procedure and need of osteotomiesCareful positioningMaintaining good SCPP-keep mean arterial pressure at least 90 mmHg during manipulation[14]Methylprednisolone was given < 8 h after damage though the effectiveness is not proven[15]Early recognition of the problemPrevention of hematoma formation and careful hemostasisMaintenance of norms of anesthesia means maintaining mean arterial pressures, normothermia, normal oxygenation and pH, normal sugars.[9]As we could understand, intraoperative wake-up test, although does not provide continuous monitoring but is least affected by factors if anesthesia is managed in a balanced way with proper monitoring and also provides visual confirmation of the integrity of spinal cord function. A well-experienced surgeon can assess change in tone and difference in movements in two limbs, giving more clinical judgment. Intraoperative neurologic monitoring is a sensitive and valid diagnostic tool to detect neurologic injury during spinal cord and spinal column surgical procedures but cannot be considered standard of care because the use of MIOM during spinal surgery has neither proved neurological outcome, not cost-effectiveness.[37] The effectiveness of MEPs and SSEP as the only diagnostic tool has been explained in [Table 3].[38]
Table 3: Recommendations for the use of electrophysiological monitoring for spinal cord functionLike the above recommendations, in scoliosis surgery, we can use the above tests as diagnostic tools to detect early and take measures as described to avoid permanent damage and false-positive results. We need to correct a few aspects after getting wrong signals as part of therapeutic strategy: [9],[13],[14],[27],[29]
Administer 100% oxygenIncrease the SCPP using a mean arterial pressure of at least 90 mmHg, which should be above 75 mmHgInitiate infusion of dopamine if required to do soOptimize hemoglobin/hematocrit and coagulation statusAvoidance of neuromuscular blockers, use only to facilitate intubationTurn off volatile anesthetics and nitrous oxide and convert to total intravenous anesthesiaIf signals do not improve, discuss with the surgeons, and be cognizant that the anesthetic and physiologic parameters have been optimizedAdvice wake-up test for all patients with a persistent signal reduction[24] and with no availability of neurophysiologic monitoring or equivocal results of these tests[6]Administration of methylprednisolone 30 mg/kg intravenous (limited data to support efficacy)[38]Imaging of the spine under the same general anesthetic to rule out spinal cord compression. ConclusionIn the recent era with the availability of neurophysiologic monitoring, it is better to use them but supplement with wake-up test if needed to get the best possible results. In centers with no advanced monitoring, yes it is still a relevant clinical diagnostic tool and should be recommended for all patients with a persistent reduction in signals of SSEP and MEPs.
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