Eur Spine J (2015) 24 (Suppl 3):S378–S385
`DOI 10.1007/s00586-015-3871-8
`
`O R I G I N A L A R T I C L E
`
`Can triggered electromyography monitoring throughout
`retraction predict postoperative symptomatic neuropraxia
`after XLIF? Results from a prospective multicenter trial
`
`• Kaveh Khajavi4,5
`• Jim A. Youssef3
`• Robert E. Isaacs2
`Juan S. Uribe1
`Jeffrey R. Balzer6
`• Adam S. Kanter6
`• Fabrice A. Ku¨ elling7
`• Mark D. Peterson8
`SOLAS Degenerative Study Group
`
`•
`
`•
`
`Received: 2 October 2014 / Revised: 24 February 2015 / Accepted: 8 March 2015 / Published online: 15 April 2015
`Ó Springer-Verlag Berlin Heidelberg 2015
`
`Abstract
`Purpose This multicenter study aims to evaluate the uti-
`lity of
`triggered electromyography (t-EMG)
`recorded
`throughout psoas retraction during lateral transpsoas in-
`terbody fusion to predict postoperative changes in motor
`function.
`Methods Three hundred and twenty-three patients un-
`dergoing L4–5 minimally invasive lateral interbody fusion
`from 21 sites were enrolled. Intraoperative data collection
`included initial t-EMG thresholds in response to posterior
`retractor blade stimulation and subsequent t-EMG thresh-
`old values collected every 5 min throughout retraction.
`Additional data collection included dimensions/duration of
`retraction as well as pre-and postoperative lower extremity
`neurologic exams.
`
`& Juan S. Uribe
`juansuribe@gmail.com
`
`1 Department of Neurological Surgery and Brain Repair,
`University of South Florida, 2 Tampa General Circle, Tampa,
`FL 33606, USA
`
`2 Division of Neurosurgery, Duke University Medical Center,
`Durham, NC, USA
`
`3
`
`Spine Colorado, Durango, CO, USA
`
`4 Georgia Spine and Neurosurgery Center, Atlanta, GA, USA
`
`5
`
`INSPIRE Research Foundation, Atlanta, GA, USA
`
`6 Department of Neurological Surgery, University of
`Pittsburgh Medical Center, Pittsburgh, PA, USA
`
`7 Department of Orthopaedic Surgery and Traumatology,
`Kantonsspital St. Gallen, St. Gallen, Switzerland
`
`8
`
`Southern Oregon Spine Care, Medford, OR, USA
`
`123
`
`Results Prior to expanding the retractor, the lowestt-
`EMG threshold was identified posterior to the retractor
`in 94 % of cases. Postoperatively, 13 (4.5 %) patients
`had a new motor weakness that was consistent with
`symptomatic neuropraxia (SN) of lumbar plexus nerves
`on the approach side. There were no significant differ-
`ences between patients with or without a corresponding
`postoperative SN with respect
`to initial posterior
`blade reading (p = 0.600), or
`retraction dimensions
`(p [ 0.05). Retraction time was significantly longer in
`those patients with SN vs. those without (p = 0.031).
`Stepwise logistic regression showed a significant posi-
`tive relationship between the presence of new postop-
`erative SN and total retraction time (p \ 0.001), as well
`as change in t-EMG thresholds over time (p \ 0.001),
`although false positive rates (increased threshold in
`patients with no new SN) remained high regardless of
`the absolute increase in threshold used to define an
`alarm criteria.
`Conclusions Prolonged retraction time and coincident
`increases in t-EMG thresholds are predictors of declining
`nerve integrity. Increasing t-EMG thresholds, while pre-
`dictive of injury, were also observed in a large number of
`patients without iatrogenic injury, with a greater predictive
`value in cases with extended duration. In addition to a
`careful approach with minimal muscle retraction and
`consistent lumbar plexus directional retraction, the inci-
`dence of postoperative motor neuropraxia may be reduced
`by limiting retraction time and utilizing t-EMG throughout
`retraction, while understanding that the specificity of this
`monitoring technique is low during initial retraction and
`increases with longer retraction duration.
`Keywords XLIF DLIF EMG Lumbar plexus
`Neuropraxia Transpsoas approach
`
`NUVASIVE - EXHIBIT 2053
`Alphatec Holdings Inc. et al. v. NuVasive, Inc.
`IPR2019-00362
`
`

`

`Eur Spine J (2015) 24 (Suppl 3):S378–S385
`
`S379
`
`Introduction
`
`The minimally invasive lateral retroperitoneal transpsoas
`approach provides minimally disruptive access to inter-
`vertebral discs, while potentially reducing risk and mor-
`bidity that
`is often associated with direct anterior and
`traditional posterior approaches [1–3]. However, the lateral
`approach requires passage of instrumentation through the
`psoas muscle, avoiding the nerves of the lumbar plexus. In
`general, lumbar plexus nerves, which originate posteriorly
`at the foramen, migrate ventral and caudal relative to the
`lumbar disc spaces from L2 to L5 [4–6]. At the upper
`lumbar levels, the psoas is not only smaller, but the nerves
`are typically posterior to the surgical approach, reducing
`the likelihood of encountering a motor nerve during the
`transpsoas approach. At the lower lumbar levels, the plexus
`is denser and the exiting roots conjoin and emanate the
`main motor branches, specifically L4–L5 and to some ex-
`tent L3–L4. Given this anatomy, it is not uncommon for
`nerves to traverse the disc space within a likely approach
`corridor. Directional
`dynamically
`triggered
`elec-
`tromyography (t-EMG) with discrete threshold responses
`mitigates the risk of nerve injury during the approach by
`indicating both the direction and proximity of a nerve in
`relation to the approach. This technique is critical to the
`reproducibility of lateral spine procedures and has been
`previously described [7–9].
`Once successful passage through the psoas muscle has
`been accomplished and blunt injury to the plexus has been
`avoided; the plexus may still be at risk of injury secondary
`to stretch or compression from the posterior retractor blade
`over the course of retraction. Both animal and human
`clinical studies have shown that nerve retraction or com-
`pression can induce microvascular, structural, and elec-
`trophysiological
`changes
`[10–16]
`that
`are
`directly
`correlated with postoperative outcomes such as neurologic
`deficit and pain [13]. Studies suggest that magnitude and
`duration of nerve root manipulation are important factors in
`the incidence and severity of iatrogenic injury and may
`characterize the potential for recovery [13, 14].
`In the current study, it was hypothesized that serial
`t-EMG recorded via stimulation of the posterior blade of
`the retractor may effectively monitor the integrity of nerves
`during the entire course of a minimally invasive lateral
`interbody fusion (MIS LIF) procedure not
`just during
`traversing the psoas muscle. Specifically, that increases in
`the stimulus intensity required to elicit a muscle EMG re-
`sponse (threshold) over time may potentially indicate a
`decline in nerve root integrity. The purpose of this study
`was to evaluate the utility of t-EMG throughout the entirety
`of MI-LIF to better predict postoperative changes in motor
`function.
`
`Materials and methods
`
`Patient sample
`
`Patients from 21 treating surgeons undergoing MIS LIF at
`L4–5, were enrolled in a prospective, institutional review
`board (IRB)-approved, nonrandomized clinical
`study.
`Treatment at spinal levels in addition to L4–5 did not exclude
`patients from study participation. Patients were excluded
`from study participation if they had an underlying neuro-
`logical disease or neurological deficit that was not associated
`with the condition for which the patient was seeking surgical
`intervention (e.g., diabetic peripheral neuropathy).
`
`Surgical technique
`
`Ò
`The MIS LIF (XLIF
`, NuVasive Inc., San Diego, CA)
`approach with integrated directional EMG monitoring and
`subsequent surgical technique was performed as previously
`described [7, 17, 18].
`
`Expandable split-blade retractor
`
`Ò
`Ò
`The MaXcess
`, San Diego, CA)
`4 Retractor (NuVasive
`consists of three blades (posterior, cranial, and caudal)
`which can be manipulated for controlled dilation and in-
`dependent
`retraction in the cranial/caudal and anteri-
`or/posterior directions. Retraction in the cranial/caudal
`direction is performed by compressing the handles of the
`retractor which are connected by a sliding crossbar with
`interlocking teeth. In its closed position, the inside di-
`ameter of the cannulation formed by the three blades is
`12 mm. As the handles are compressed, the retractor blades
`are spread apart in the cranial/caudal direction as the in-
`terlocking teeth pass over each other, ratcheting the re-
`tractor open such that it resists closing with each ratcheting
`step. Each ratcheting step in the cranial/caudal direction
`results in approximately 3 mm of additional retraction.
`Similarly, retraction in the anterior/posterior direction is
`controlled by a cross bar cylindrical gear that is attached to
`the handles of the retractor. Turning a knob on the cross bar
`causes the cylindrical gear to roll along a track, ratcheting
`the retractor open and increasing the distance between the
`posterior blade and the cranial/caudal blades by shifting the
`cranial/caudal blades in the anterior direction. Each ratch-
`eting step in the anterior/posterior direction is equal to
`approximately 1.5 mm of additional retraction. It should be
`noted that as retraction increases, pressure is placed against
`the retractor blades from the surrounding tissue resulting in
`inward deflection of the retractor blades, such that the ac-
`tual amount of retraction may be smaller than estimated by
`the values stated above.
`
`123
`
`

`

`S380
`
`Triggered EMG
`
`The posterior blade, which includes an exposed electrode at
`Ò
`its distal tip, is designed to integrate with the NVM5
`(NuVasive Inc., San Diego, CA) neuromonitoring platform
`to provide directional t-EMG monitoring throughout re-
`traction. At any time during retraction, the surgeon can
`choose to deliver a measured, constant-current stimulus to
`the tissue contacting the retractor. The intensity of
`stimulation required to elicit a measurable response
`(threshold) and the resulting muscle potential recorded in
`the corresponding myotome is recorded and displayed by
`the neuromonitoring software.
`
`Study design
`
`institutional review board-
`A prospective, multicenter,
`(IRB) approved study was undertaken in evaluation of
`the hypothesis. Preoperative data collection included
`patient demographics and diagnosis. Pre- and postop-
`erative data collection included 0–5 motor and 0–2
`sensory function using the modified ASIA exam to
`evaluate changes in motor and sensory function, and
`0–10 patient-reported visual analog scale (VAS) for legs
`and back. In addition to evaluating motor strength, sur-
`geons also indicated whether motor weakness was a re-
`sult of neuropraxia during the lateral spine surgery, or
`more likely to be related to postoperative pain. Intraop-
`erative data collection consisted of procedure details in-
`cluding description, duration of procedure, and blood
`loss. Additional
`level-specific data were collected for
`levels L4–L5 and L3–L4 (if treated) which included the
`lowest
`threshold reading and direction (i.e., anterior,
`posterior, cranial, or caudal)
`from each of
`the three
`dilators during the approach, and the initial
`t-EMG
`threshold and direction of the lowest threshold from the
`retractor’s
`posterior
`blade
`after
`initial
`expansion.
`Throughout retraction, posterior blade t-EMG threshold
`values were recorded every 5 min. Duration of retraction
`and retraction size, collected in a number of ratcheting
`steps of the retractor in the anterior/posterior and cra-
`nial/caudal directions, were also collected for L4–L5 and
`L3–L4 levels. Postoperative follow-up visits were con-
`ducted 0–2 and 6 weeks after surgery. Patients with new
`postoperative decreases in motor or sensory function
`were followed beyond 6 weeks as per each investigator’s
`standard of care. Additional follow-up was scheduled to
`monitor motor and sensory function until all dermatomes
`and myotomes had returned to preoperative function, or
`until the decrease was deemed permanent by the treating
`surgeon. Any complications that occurred during surgery
`or within
`the
`designated
`follow-up
`period were
`documented.
`
`123
`
`Eur Spine J (2015) 24 (Suppl 3):S378–S385
`
`Statistical analysis
`
`Univariate analysis was performed using Chi squared test
`and Fishers’ exact test for categorical variables and inde-
`pendent samples student t test for continuous variables. A
`binary multivariate logistic regression was used to identify
`independent risk factors for new postoperative neuropraxia.
`Model selection criteria were set as stepwise and variables
`with p \ 0.10 were included in the final model. Adjusted
`odds ratios (aOR) and 95 % confidence intervals were
`calculated for all variables in the model.
`All statistical analyses were performed using JMP
`software (version 11.1.1 for Windows, SAS Institute Inc.,
`Cary, NC). Statistical significance was defined as p \ 0.05.
`
`Results
`
`Three hundred and twenty-three patients were enrolled.
`The mean patient age was 63.2 years (range 30–90) and
`67 % were female. The mean Charlson comorbidity index
`score was 2.4 (range 0–12), and mean BMI was 30.0 kg/m2
`(range 17–50). The majority of procedures (70 %) were
`performed through a left-sided approach. In addition to
`treating L4–5, L3–L4 was also treated in 57 % of patients.
`Mean retraction time at L4–5 was 23 min (range
`6–100). Mean retraction size at L4–5 was 2.9 ratcheting
`steps (approximately 20.7 mm) in the cranial/caudal di-
`rection and 5.5 ratcheting steps (approximately 20.3 mm)
`in the anterior/posterior direction.
`Total blood loss for the lateral procedure, which was
`inclusive of adjacent levels was estimated to be under
`100 cc in 85 % of patients. Mean hospital stay was 3.6 days
`(range 0–31) inclusive of staged procedures. Eighty-nine
`percent of patients completed at least one postoperative
`evaluation. Postoperative changes in motor/sensory func-
`tion on the approach side included 91 (31 %) patients with
`new postoperative hip flexion weakness, 38 (13 %) with a
`new decrease in sensory function, and 13 (4.5 %) with a
`new motor weakness that was identified by the treating
`surgeon as symptomatic neuropraxia (SN) on the approach
`side. Of the 13 patients identified as having symptomatic
`neuropraxia, weakness often occurred in more than one
`myotome. In this group, new postoperative weakness was
`identified in knee extension (n = 11), ankle dorsiflexion
`(n = 3), great toe dorsiflexion (n = 3), and ankle plantar
`flexion (n = 2). Twelve of
`the thirteen patients with
`symptomatic neuropraxia also presented with correspond-
`ing hip flexion weakness.
`As per the protocol, prior to advancing the retractor over
`the final dilator, the dilator was rotated through the psoas to
`identify the direction of the lowest threshold to indicate the
`direction of the closest nerve with respect to where the
`
`

`

`Eur Spine J (2015) 24 (Suppl 3):S378–S385
`
`S381
`
`retractor will be placed. In 70 % of L4–L5 levels treated,
`the location of the lowest threshold was posterior to the
`retractor. In the remaining 30 % of patients, the lowest
`threshold was either equal in all directions (8 %), anterior
`to the dilator (4 %), or cranial or caudal to the retractor
`(18 %). Symptomatic neuropraxia occurred in 5 % of pa-
`tients where the lowest threshold was posterior to the re-
`tractor, 0 % of cases where the threshold was equal in all
`directions, 8 % of cases where the lowest threshold was
`anterior to the dilator, and 4 % of cases where the lowest
`threshold was in the cranial or caudal direction.
`Retraction time was significantly longer in those patients
`with SN versus
`those without
`(32.3 vs. 22.6 min,
`p = 0.031). There were no significant differences between
`patients with or without postoperative corresponding SN
`with respect
`to the initial posterior blade threshold
`stimulation (with SN: 14 mA, without SN: 12.8 mA,
`p = 0.600), retraction size in the cranial/caudal direction
`(with SN: 3.1 ratcheting steps, without SN: 2.8 ratcheting
`steps, p = 0.551), or retraction size in the anterior/poste-
`rior direction (with SN: 6.4 ratcheting steps, without SN:
`5.5 ratcheting steps, p = 0.419) (Table 1).
`For the purpose of this analysis, it was assumed that a
`threshold increase of at least 1 mA may be indicative of a
`change in the patient’s nerve function. Of the 13 patients
`with SN on the approach side during the postoperative
`period, 10 had a stimulation threshold increase of at least
`1 mA compared to the initial
`stimulation threshold
`throughout L4–L5 retraction (true positive). The remaining
`three patients had no increase in stimulation threshold
`(false negative). Of the 252 patients who did not experi-
`ence SN on the approach side during the postoperative
`period, the stimulation threshold did not increase above the
`initial threshold throughout retraction in 119 (true nega-
`tive). In the remaining 133 patients with threshold readings
`who did not experience postoperative SN on the approach
`side, the stimulation threshold increased at least one 1 mA
`above the initial stimulation threshold during L4–L5 re-
`traction (false positive). Using the following sensitivity
`
`equation: true positive/(true positive ? false negative), the
`sensitivity of this method of nerve monitoring is 77 %.
`Using the following specificity equation: true negative/
`(true negative ? false positive),
`the specificity of this
`method of nerve monitoring is 47 %. When the same
`analysis was repeated assuming a 2 mA increase was
`indicative of a change in nerve function, the sensitivity and
`specificity of this technique were 77 and 56 %, respec-
`tively. When the same analysis was repeated assuming a
`3 mA increase was indicative of a change in nerve func-
`tion, the sensitivity and specificity of this technique were
`62 and 64 % respectively (Table 2).
`Multivariate analysis showed a significant positive re-
`lationship between the presence of SN and total retraction
`time (p \ 0.001), change in posterior blade t-EMG
`threshold over time (p \ 0.001), and smoking (p \ 0.003).
`There was a significant negative relationship between the
`presence of SN and age (p \ 0.001), and BMI (p \ 0.001)
`(Tables 3, 4). Figure 1 depicts the relationship between the
`change in initial posterior blade t-EMG threshold over time
`between patients with and without a corresponding post-
`operative motor deficit.
`initial L4–L5 posterior blade
`Age, BMI, smoking,
`stimulation threshold, L4–L5 retraction size, and L4–L5
`retraction time, were not significantly different between
`patients who experienced a postoperative decrease in sen-
`sory function and those who did not (p [ 0.05). Postop-
`erative decreases in sensory function occurred more
`commonly in females than males (16 vs. 7 %, p = 0.023).
`
`Discussion
`
`The reported rate of postoperative lower extremity motor
`weakness, exclusive of hip flexion, after lateral interbody
`fusion ranges from 0 to 9.3 % [19–21], with approach-
`related dysesthesia ranging from 1 to 75 % [22–24]. It is
`important to note that the wide range of motor and sensory
`outcomes has been derived from multiple MI-LIF
`
`Table 1 Univariate analysis of intraoperative risk factors for postoperative symptomatic neuropraxia
`
`Risk factor
`
`SN (n = 13)
`
`No SN (n = 310)
`
`Unadjusted OR
`
`95 % Confidence interval
`
`P value
`
`Age
`
`Sex
`
`BMI
`
`Tobacco use
`
`Retraction time (min)
`
`Initial threshold stimulation (mA)
`
`Retraction size (ratcheting steps)
`
`Cranial/caudal
`
`Anterior/posterior
`
`55.3
`
`63.5
`
`76 % female
`
`66 % female
`
`29.5
`
`32.3
`
`14
`
`3.1
`
`6.4
`
`30.0
`
`22.6
`
`12.8
`
`2.8
`
`5.5
`
`1.069
`
`1.722
`
`1.012
`
`0.962
`
`0.976
`
`1.112
`
`0.978
`
`1.017–1.124
`
`0.512–7.817
`
`0.932–1.110
`
`0.926–0.999
`
`1.037–1.024
`
`0.742–2.034
`
`0.818–1.204
`
`0.398
`
`0.784
`
`0.044
`
`0.427
`
`0.628
`
`0.823
`
`123
`
`

`

`S382
`
`Eur Spine J (2015) 24 (Suppl 3):S378–S385
`
`Table 2 Sensitivity (Sen), specificity (Spec), and true and false positive and negative results for various changes in t-EMG thresholds as an
`‘alarm criteria’ for symptomatic neuropraxia
`
`‘‘Alarm criteria’’
`threshold change (mA)
`
`False positive
`
`True negative
`
`True positive
`
`False
`negative
`
`Sen
`(%)
`
`Spec
`(%)
`
`1
`
`2
`
`3
`
`4
`
`n
`
`133
`
`111
`
`92
`
`84
`
`% of
`positives
`
`n
`
`% of negatives (negative
`predictive value)
`
`n % of positives (positive
`predictive value)
`
`n % of
`negatives
`
`93.0
`
`91.7
`
`92.0
`
`91.3
`
`119
`
`140
`
`160
`
`168
`
`97.5
`
`97.9
`
`97.0
`
`97.1
`
`10
`
`10
`
`8
`
`8
`
`7
`
`7.0
`
`8.3
`
`8.0
`
`8.7
`
`8.3
`
`3 2.5
`
`3 2.1
`
`5 3.0
`
`5 2.9
`
`6 3.3
`
`77
`
`77
`
`62
`
`62
`
`54
`
`47
`
`56
`
`63
`
`67
`
`69
`
`5
`
`6
`
`77
`
`69
`
`91.7
`
`90.8
`
`175
`
`183
`
`96.7
`
`96.8
`
`7
`
`9.2
`
`6 3.2
`
`54
`
`73
`
`As the change in threshold for the alarm criteria increases from 1 to 6 mA, the number of false positives is reduced by nearly half; however, over
`the same period the false negative rate increases, meaning that patients with symptomatic neuropraxia would go unidentified if higher thresholds
`were used
`
`Table 3 Nominal logistic regression parameter estimates
`
`Term
`
`Estimate
`
`P
`
`Parameter estimates
`
`Intercept
`
`Time elapsed
`
`Threshold change from baseline
`
`BMI
`
`Age
`
`Smoker (no)
`
`-1.611
`
`-0.040
`
`-0.049
`
`0.061
`
`0.050
`
`0.370
`
`0.047
`\0.001
`\0.001
`\0.001
`\0.001
`0.003
`
`techniques. The results of this study specifically evaluate
`the results of the XLIF procedure with integrated, advanced
`neuromonitoring. Lumbar plexus nerve complications are
`generally caused by direct interaction of the nerve with
`instrumentation or indirect ischemic injury, caused by ei-
`ther stretching or compressing the nerve over time. In this
`study, once properly positioned, retractor time within the
`psoas was the most predictive factor for determining neu-
`rologic injury. Increasing t-EMG thresholds during retrac-
`tion indicating declining function is a highly compelling
`and traceable finding with a sensitivity of almost 80 %.
`The resiliency of neural elements, though, does not man-
`date that the person will develop a postoperative deficit.
`Interestingly, smokers, whose microvascular supply is
`
`compromised, were found to be more at risk for developing
`deficits. Based on the results of this study and prior expe-
`rience [25–27], the authors believe that neurologic injury
`can be minimized if specific attention is given to the neural
`anatomy of the psoas in relationship to the approach tra-
`jectory, monitoring t-EMG thresholds throughout retrac-
`tion, while limiting retraction time and magnitude.
`With the exception of the genitofemoral nerve, which is
`typically on the far anterior aspect of the psoas, plexus
`nerves from L2–L5 are generally located in the posterior
`50 % of the psoas or outside of the muscle [5]. In most
`cases, the preferred technique is to target the disc between
`the middle and posterior third of the lateral disc space with
`the direction of the lowest EMG stimulation threshold
`during transpsoas passage posterior to the approach. In
`70 % of L4–L5 levels treated in this series, the retractor
`was positioned such that the lowest threshold readings were
`located posterior to the retractor. This trajectory is advan-
`tageous because it positions the retractor anterior to the
`majority of neural structures and maximizes coverage of
`the implant–endplate interface across the load-bearing
`column of the anterior spine, on the border between the
`posterior thirds of the disc space. Once the posterior blade
`is in position and secure using an intradiscal shim, it re-
`mains stationary and the other two blades retract away
`from the posterior blade, protecting neural structures from
`
`Table 4 Multivariate analysis
`of risk factors for postoperative
`symptomatic neuropraxia
`
`Risk factor
`
`Retraction time
`
`Change in posterior blade reading
`
`Tobacco use
`
`Protective factors
`
`Age
`
`BMI
`
`Adjusted OR
`
`95 % Confidence interval
`
`P value
`
`0.9667
`
`0.972
`
`0.407
`
`1.050
`
`1.048
`
`0.952–0.982
`
`0.949–0.995
`
`0.275–0.609
`
`1.032–1.067
`
`1.018–1.079
`
`\0.001
`0.018
`\0.001
`
`\0.001
`0.001
`
`123
`
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`

`Eur Spine J (2015) 24 (Suppl 3):S378–S385
`
`S383
`
`Fig. 1 Change from initial
`posterior blade electrode
`stimulation required to elicit a
`response over time for patients
`with (black) and without (gray)
`symptomatic neuropraxia
`
`additional compression or stretching by the retractor. This
`positioning also places the posterior blade electrode in
`close proximity to neural structures allowing for continued
`and frequent t-EMG monitoring during retraction. In gen-
`eral,
`investigators in this study limited retraction to
`relatively small apertures; in 90 % of cases, in patients
`with and without neuropraxia, L4–L5 anterior retraction
`was less than 27 mm, and cranial caudal retraction was less
`than 24 mm. Although comparison of the retraction di-
`mensions in patients with and without symptomatic neu-
`ropraxia in this study does not
`imply that retraction
`aperture is directly related to postoperative neuropraxia,
`the authors caution that wide retractor openings are not
`recommended and may result in neurologic injury and
`unnecessary trauma to the psoas.
`One of the most valuable reasons to monitor changes in
`t-EMG thresholds would be to allow for early intervention
`by the surgical team to prevent impending nerve injury.
`Justification for intervention is predicated on the sensi-
`tivity and specificity of the neurophysiological measure
`being utilized. This being said, despite calculating sensi-
`tivity and specificity with increasingly higher threshold
`change as our assumed ‘alarm criteria’ for nerve com-
`promise (1–6 mA), these values produced a positive rate
`that was too high to justify surgical intervention based on
`any increase in threshold at least in the early stages of
`retraction (e.g.,
`in the first 20 min). In an attempt
`to
`identify whether or not the changes in threshold became
`more meaningful as the total retraction time increased, we
`repeated the analysis of looking for an ‘alarm criteria’
`only for readings occurring after 20, 25, 30, and 35 min of
`retraction. We found that when change in t-EMG threshold
`
`was evaluated later in the retraction period (at the 30 min
`mark), there was an obvious trend for decreasing false
`positives. While encouraging, the false positive rate re-
`mained above 50 % for alarm criteria between 1 and
`6 mA. Taken together, these analyses reveal that using
`change in t-EMG threshold alone as a means to modify
`surgical technique has very low specificity, i.e., a high
`false positive rate. Length of retraction time is a clear
`indicator of nerve injury in the XLIF procedure and in-
`terestingly, increases in t-EMG thresholds later during the
`retraction period may be the only reasonable alarm for
`surgical intervention. The results of this study illustrate the
`importance of mixed multimodality neurophysiological
`monitoring. The majority of nerves within the lumbar
`plexus are mixed nerves containing both motor and sen-
`sory fibers. As such one may have expected to see a re-
`lationship between the t-EMG monitoring of motor nerves
`and the outcomes of sensory function, even in the absence
`of a motor injury. However, the results of this study do not
`indicate that monitoring motor nerves can predict
`the
`outcome of sensory function.
`It should be noted that one of the limitations of the
`results presented is that if a response was not observed at
`30 mA, stimulation was not increased; therefore, discrete
`threshold measures were only measured up to 30 mA at
`most institutions. As a result, the stepwise logistic model
`created to describe the relationship between change in
`t-EMG threshold and likelihood of a postoperative symp-
`tomatic neuropraxia is a limited model, and the increase in
`threshold in those patients with symptomatic neuropraxia
`may be greater than what is described by this model.
`Limitations of the monitoring technique used include an
`
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`S384
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`Eur Spine J (2015) 24 (Suppl 3):S378–S385
`
`inability to monitor the nerves of the psoas muscle because
`of the inability to place surface electrodes on the corre-
`sponding muscle group as well as the inability to directly
`monitor a specific nerve within the lumbar plexus. Rather
`than monitoring each nerve individually, stimulation re-
`sponses are monitored across all myotomes of the lower
`extremities. Using this technique, if multiple myotomes are
`stimulated by the electrode, it is possible that a healthy
`myotome could continue to respond to the t-EMG stimulus
`while a compromised nerve was failing to respond. Future
`studies on this topic must aim to directly monitor changes
`in response to the stimulated thresholds at each myotome,
`rather than the entire lower extremity. The low specificity
`of the t-EMG measures in this study may be explained
`secondary to precisely where the stimulus is being deliv-
`ered with reference to where the nerve is potentially being
`compromised. Using this technique, the stimulus is deliv-
`ered at the site of suspected perturbation potentially cre-
`ating a degree of variability in threshold recordings. Future
`studies evaluating the response to stimulation delivered
`above the surgical site may eliminate the variability of
`these results.
`The results of this study provide evidence that prolonged
`retraction time is a predictor of declining nerve integrity.
`While increasing t-EMG thresholds can indicate nerve root
`compromise, its low specificity raises question concerning
`its routine utility with regards to surgical
`intervention
`during the early stages of retraction. In addition to a careful
`approach using directional discrete-threshold t-EMG, lim-
`iting retraction time and monitoring t-EMG for increasing
`thresholds, particularly during extended retraction times,
`may prove effective for reducing the incidence of postop-
`erative motor neuropraxia.
`
`Acknowledgments This study was funded by NuVasive, Inc.
`
`Conflict of interest Authors JSU, REI, JAY, KK, ASK, FAK, and
`MDP are consultants to NuVasive. JSU, REI, JAY, KK, and MDP
`receive research support from NuVasive. JSU and MDP hold shares
`of NuVasive stock, JSU, REI, JAY, and MDP receive royalties from
`NuVasive, FAK has been reimbursed for travel on behalf of NuVa-
`sive, and MDP is a member of a surgeon advisory board for NuVa-
`sive. Author JRB has no conflicts to report.
`
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