Intraoperative Neuromonitoring for Spinal Surgery in a Pregnant Patient: Case Report and Literature Review
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.360931
Source of Support: None, Conflict of Interest: None
Keywords: Anesthesia, fetal monitoring, intraoperative neuromonitoring, pregnancy, spine surgery
Intraoperative neurophysiological monitoring (IONM) is considered standard of care during spinal surgeries which are at risk of neuronal insult to the motor tracts. There is limited literature on the use and safety of IONM and its anesthetic concerns in pregnant women undergoing spinal surgery., This case report describes anesthesia strategy and IONM in a pregnant patient posted for surgery for aggressive vertebral body hemangioma.,
A 29-year-old female, 22 weeks pregnant presented with backache, tingling numbness in legs, and progressive difficulty in walking. The magnetic resonance imaging was suggestive of hemangioma of the 7th dorsal vertebra. [Figure 1]. Neurological examination revealed 3/5 power in the lower limbs and decreased pain sensation below the lesion. Transabdominal ultrasound (TA-USG) revealed diamniotic monochorionic twin pregnancy with one alive fetus. The patient was scheduled for laminectomy and posterior fixation under general anesthesia. Considering a high risk of injury to the motor tracts, transcranial motor-evoked potential (TcMEP) and somatosensory-evoked potential (SSEP) were planned.
The patient was assessed by a multidisciplinary team of neurosurgeons, neuro-anesthesiologists, neurophysiologists, and obstetricians. Informed risk consent was obtained for preterm labor and fetal loss. Preoperativefetal heart rate (FHR) monitoring was done. Maternal optimization and resuscitation were considered the primary goals.
The patient was placed in a 15-degree left lateral tilt; standard monitors were applied. Anesthesia was induced with propofol 70 mg, fentanyl 100 ug, and tracheal intubation was facilitated with rocuronium 50 mg using a C-MAC video laryngoscope using rapid sequence intubation. The total intravenous anesthesia with a titrated dose of propofol and fentanyl infusions with 50% O2 in air targeted to bispectral values between 40 and 60 was used for maintenance. Thereafter, a muscle relaxant was avoided. The left radial arterial was cannulated for hemodynamic monitoring and arterial blood gas analysis. The patient was placed in a left lateral decubitus position with a lead shield wrapped around the abdomen to avoid radiation exposure.
Corkscrew electrodes were placed at C3′ and C4′ for stimulation (train of 8 pulses, 75 μs duration each, 250–500 Hz, and in each instance, a single of double stimulation was given to elicit MEP response), and a dual twisted needle electrode was placed bilaterally in the abductor pollicis brevis (control), rectus abdominus, rectus femoris, tibialis anterior, extensor halluces longus, and abductor halluces to record the evoked potentials (100 ms window, 30–3,000 Hz bandpass). A lesser train count was not used because of patient preoperative neurological deficits and to avoid the use of higher stimulation strength which may precipitate fetal distress or labor. Therefore, the stimulation strength was gradually increased from 50 V with continuous monitoring of the fetal heart rate and was limited to 500 V to reduce risk. Consistent TcMEPs could be obtained only in control muscles at 150 V and left abductor halluces at 500 V owing to the pre-existing motor deficit.
The posterior tibial nerve was stimulated with patch electrodes (30 mA, 200–400 μs, 3–5 Hz), and the evoked potential was recorded and observed best SSEP traces with corkscrew electrodes at C3, C4, Cz, and FPz (100 ms window, 30–500 Hz bandpass, 200–300 traces averaged). Direct nerve monitoring was not used during pedicle screw implantation. The NIM-Eclipse system, Medtronic was used for all IONM procedures.
D7 laminectomy and posterior spine fixation (pedicle screw rod fixation done one level above and below D7) with intralesional 3 mL absolute alcohol (<1% hydrated ethyl alcohol) injection at D7 pedicle (bilateral) was performed. The surgical procedure lasted for 6 h. The estimated blood loss was 800 mL. Intraoperative hemodynamics remained stable. There were no intraoperative changes in TcMEP and SSEP responses as compared to the baseline after decompression and this correlated with the postoperative motor and sensory status. Tracheal extubation was facilitated at the end of the surgery. Normal fetal heart rate and viability were confirmed by postoperative cardiotocography. She was discharged on the 5th postoperative day. A healthy baby was delivered at 37 weeks gestation.
Pregnancy is a well-recognized state during which asymptomatic vertebral hemangiomas can become symptomatic.,, Surgery is suggested in case of severe pain or progressive neurological deficit. There is little literature regarding intraoperative anesthetic management and IONM in pregnant women undergoing spine surgery.
Besides maintaining normal hemodynamic, neurophysiological, and uteroplacental variables, a special anesthetic requirement is choosing an appropriate anesthetic agent which does not interfere with the IONM recordings and is safe for the mother and fetus. Maintenance of an adequate and constant depth of anesthesia, and body temperature is vital. Total intravenous anesthesia (a combination of propofol and opioids) is the most commonly used anesthetic regimen for IONM in pregnant women [Table 1]., Bolus injections of intravenous agents should be avoided as they can temporarily disrupt MEPs and cause maternal hypotension impairing the uteroplacental perfusion., Halogenated anesthetics elevate muscle TcMEP stimulus thresholds, abolish MEPs, and have a greater sedative effect on the neonates. Muscle relaxants are not recommended as they can decrease or abolish TcMEPs and affect signal transmission across the neuromuscular junction.
TcMEP provides invaluable intraoperative information to monitor motor tracts.,,, However, it requires electrical stimulation of the motor cortex to elicit evoked potential in all muscles. This is accompanied by a muscular contraction in the whole body, and the amplitude of this contraction increases with the strength of stimulation, and thus, can precipitate fetal distress or preterm labor. Hence, the risk can be minimized by titrating the IONM protocols to elicit the best response with minimal stimulation and adequate monitoring and readiness in the face of precipitation of adverse events. SSEP, on the other hand, can be safely used in pregnant patients as it involves localized stimulation of the peripheral nerves with no risk of generalized body movement or great electrical spread. However, SSEP can only indicate the status of the sensory tract, whereas it is the motor deficit that can reduce the duration for the persistence of injury and has a greater impact on the quality of life.
[Table 1] describes the IONM protocols and anesthesia techniques used during IONM in pregnant patients. It includes using the lowest possible currents and limiting the TcMEP stimulations to get evoked potentials. Local stimulation at a distance from the abdominal cavity with 2 cm inter-electrode distance for SSEP recordings was used by one author to minimize the effect of electrical field on myometrium. No substantial changes were observed either in the uterus muscle tone or FHR with a voltage of 340 V. Reducing the number of TcMEP stimulation trains, limiting the voltage strength to 500 V, using a multimodality approach to IOMN, along with continuous monitoring of the fetal heart activity have been described in the literature. In our case, the stimulation strength was gradually increased from 50 V with continuous monitoring of fetal heart rate and was limited to 500 V to reduce risk.
There are no guidelines for fetal monitoring in cases with IONM. If the fetus is considered pre-viable (<24-28 weeks), it is sufficient to ascertain fetal heart monitoring using the Doppler pre- and post-procedure. Intraoperative electronic fetal monitoring is recommended in a viable fetus and when there are provisions and consent for emergency cesarean section for fetal indications.
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