Pulmonary Considerations

The pulmonary system is the most obvious preoperative concern, and can be significantly affected by the structural changes brought about by a scoliotic spine. In cases of extreme scoliosis, exercise tolerance testing is the best screening tool for pulmonary performance. Patients should undergo preoperative pulmonary function tests if they have:

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  A history of poor exercise tolerance

  
  
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  A curve >60 degrees associated with a history of reactive airway disease,

  
  
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  A curve >80 degrees

  
  
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  Neuromuscular scoliosis

Patients for whom an anterior approach is planned should receive additional consideration for pulmonary evaluation. The most common respiratory defect in scoliosis is restrictive, with a decrease in vital capacity (VC) and forced expiratory volume in 1 second (FEV1) as the scoliotic angle increases. The respiratory compromise may take the form of chronic alveolar hypoventilation, atrial hypoxia, ventilation-perfusion (V-Q) mismatch, and pulmonary hypertension with progression to cor pulmonale.2 Patients with preoperative Cobb angles >100 degrees can have a significantly diminished VC, nearing 45% of normal. A VC of 45% or less or a forced vital capacity (FVC) of 30% less than predicted is an indicator of the possible need for postoperative ventilation.3 Patients with severe Cobb angles can have difficulty with clearing their airways through coughing, particularly with the coupling of postoperative pain. This can result postoperatively in atelectasis, pneumonia, and possible aspiration.

Cardiac Considerations

Depending on the magnitude of a patient’s scoliosis and the patient’s coexisting disease state, preoperative cardiac testing may be required. This includes an electrocardiogram (ECG), echocardiogram, or stress testing. The cardiac system can be secondarily affected by severe deformities, possibly leading to cor pulmonale. Patients with scoliosis associated with genetic deformities have a significantly higher rate of cardiac deformities and merit preoperative investigation. Mitral valve prolapse, coarctation, and cyanotic heart disease are the most commonly found pathologies in patients with scoliosis.2 Duchene muscular dystrophy can present as septal hypertrophy, which can lead to cardiomyopathy and manifest as arrhythmias or heart blocks.4

Hemotological and Nutritional Considerations

Patients with scoliosis should have blood work done to evaluate their initial hematocrit and platelet count. Because major blood loss (>50% blood volume) may occur during scoliosis surgery, a blood type and crossmatch analysis should be obtained preoperatively. Nutritional status, particularly in patients with neuromuscular scoliosis, should be evaluated with blood testing, including assays for albumin and vitamin K, and a basic metabolic panel. Clotting abnormalities are associated with patients with poor nutrition and vitamin K deficiency.5 These concerns need to be corrected before surgery to optimize the patient’s status for surgery. Discussions should be initiated about blood replacement during surgery and the possibility of autologous donation. Murray et al reported that 90% of adolescent patients with scoliosis who had autologous predonation of blood avoided allogenic red-cell transfusions.6 Postoperative facial swelling should be discussed with the patient’s family because it may result from necessary fluid replacement as well as from placement of the patient in the prone position for an extended period.

Neurological Considerations

A neurological evaluation should be done before surgery to monitor for deficits and identify whether any changes have occurred in the patient’s neurological status. To this end, a basic discussion of neurophysiological monitoring and the possibility of a wake-up test should be discussed with the patient and the patient’s family.

Fasting Guidelines

Guidelines for adolescents and adults undergoing scoliosis surgery require that nothing be taken by mouth after midnight of the night before surgery, with the exception of a sip of water with morning medications. Younger patients may be given clear liquids until 2 hours before surgery, breast milk until 4 hours before surgery, and a light meal or cow’s milk until 6 hours before surgery.7

Induction

Anesthesia in pediatric patients can be induced either through an inhalational or intravenous technique. Patients with airways that are difficult to intubate should have an intravenous catheter placed before the induction of anesthesia whenever possible. Sevoflurane is currently the most commonly used volatile agent for induction of anesthesia via a face mask. Its advantages include a nonpungent odor, low incidence of respiratory irritation, and little myocardial depression and arrhythmia in normal clinical use. Sevoflurane may be combined with nitrous oxide to hasten the onset of induction. Although the choice of intravenous induction for IV induction may vary depending on the patient’s comorbidities, propofol is the most commonly used intravenous induction agent.

Intubation of the trachea may be facilitated by use of a muscle relaxant. The choice of muscle relaxant is based on the required onset and duration of paralysis, with consideration also given to the side effects and comorbidities of the individual patient. Muscle relaxation with a nondepolarizing neuromuscular blocking agent such as rocuronium, vecuronium, cisatracurium, or atracurium produce paralysis for ~20 to 30 minutes, which typically coincides with the period needed to obtain vascular access, place monitors, and position the patient. Succinylcholine is the only depolarizing neuromuscular blocking agent available, and can have adverse side effects including malignant hyperthermia in susceptible patients, severe hyperkalemia leading to cardiac arrest, myalgias, bradycardia, and flushing. Typically, succinylcholine is held in reserve by pediatric anesthesiologists as an emergency drug.

Airway Management

After the induction of anesthesia, maintaining and managing the patient’s airway is of utmost importance to the anesthesiologist. Typically, the patient is mask ventilated until adequate muscle relaxation is obtained. This is accomplished with a face mask and bag technique with the patient’s head tilted and jaw lifted anteriorly. An oral or nasal airway device can be inserted as necessary to maintain a patent upper airway.

Children with adolescent idiopathic scoliosis (AIS) rarely present difficulty in airway management and intubation. However, patients with coexisting syndromes may present a more difficult situation, and the anesthesiologist should make preoperative airway management plans for them. Klippel-Feil syndrome, spondyloepithelial dysplasia, any of the mucopolysaccharidoses, arthrogryposis multiplex, mandibulofacial dystosis, or Goldenhar syndrome can be associated with particularly difficult airway anatomy. These patients may require a fiberoptically guided intubation under sedation and use of additional airway equipment, such as a laryngeal mask airway or the GlideScope ( Fig. 11.1 ).

A wire-reinforced endotracheal tube may be considered for avoiding tube kinking and occlusion when turning the patient from the supine to the prone position. After the airway is secured, particular attention must be given to securing the endotracheal tube, because the patient will remain in the prone position and may have secretions that pool around the mouth.

Maintenance

The maintenance of anesthesia for patients undergoing surgical correction of scoliosis largely depends on the necessity of monitoring the spinal cord and on surgical preference. Monitoring of somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) is accepted as the standard of care for neurophysiological monitoring during scoliosis surgery.9 The impact of anesthetic agents on spinal cord monitoring increases as more synapses in the neurological pathways are monitored.10 Because inhalational anesthetic agents considerably depress the amplitude of transcranial electrical MEPs (TceMEPs) in a dose-dependent manner, total intravenous anesthesia (TIVA) has been increasingly used during spine surgery to provide adequate anesthesia with minimal interference of monitored neurophysiological signals. TIVA techniques with propofol and narcotic infusion as a central component have been advocated for optimizing the monitoring of TceMEPs. Because of its sedative, analgesic, and neuroprotective properties, dexmedetomidine has recently been added to TIVA regimens to reduce infusion rates of propofol and to facilitate emergence from anesthesia for the intraoperative wake-up test and at the completion of surgery.11 The other consideration of the anesthesiologist in determining the maintenance of anesthesia is minimizing blood loss through specific fluid management, drug therapy, and careful positioning of the patient to minimize venous congestion and abdominal compression.12

Respiratory Monitoring

Monitoring of the respiratory system should always include the measurement of end-tidal carbon dioxide concentration and peak airway pressure. Patients with severe respiratory dysfunction as a result of scoliosis may have an increased alveolar-arterial oxygen gradient, which may be further increased during prolonged anesthesia because of regional hypoventilation.14

Temperature Monitoring

Body temperature may be difficult to maintain because of the duration of surgery and environmental factors. Because hypothermia has been shown to increase infection rates and blood loss, the use of temperature monitoring, warming of all intravenous fluids, and a warm air mattress device is recommended for the duration of the procedure.15 Kurz et al found a 3-fold increase in wound infection when patients’ temperatures were decreased by 1.9°C, with a 20% longer duration in hospitalization.15 Room temperature should be maintained at 29°C from the time of a patient’s entry into the operating room until the patient is draped.

Intraoperative Neurophysiological Monitoring

Knowledge of the influence of anesthesia on neuromonitor-ing is essential. A close working relationship among the members of the neuromonitoring team, the anesthesiologist, and the surgeon is mandatory for the successful conduct and interpretation of neuromonitoring. The effect of anesthetic agents on neurophysiological monitoring increases with the number of synapses in the pathway being monitored, because all anesthetic agents produce their effects by altering neuroexcitability through changes in synaptic function or axonal conduction.16

Somatosensory Evoked Potentials

The subcortical SSEP can be very useful intraoperatively because it is not very susceptible to anesthetic effects ( Table 11.1 ).17 Most studies consider a decrease in amplitude of 50% or more, an increase in latency of 10% or more, or both to be significant changes in SSEP reflecting loss of integrity of a neural pathway, provided these changes are not caused by anesthetic agents or temperature.18,20 All volatile anesthetic agents produce a dose-dependent increase in SSEP latency and a decrease in SSEP amplitude.21,23 Sevoflurane and desflurane are associated with less amplitude reduction than isoflurane in the range of minimum alveolar anesthetic concentration (MAC) of 0.7 to 1.3%.24 In contrast to their effects on the cortical SSEP, all volatile anesthetic agents, even at concentrations above 1.0 MAC, only minimally affect the subcortical waveform, resulting in a high recordability and reliability of the SSEP.25 Nitrous oxide (60 to 70%) generally diminishes cortical SSEP amplitude by ~50% while leaving cortical SSEP latency and subcortical waves unaffected.26,27 Intravenous anesthetic agents generally affect SSEPs less than do inhaled anesthetic agents. Human SSEPs are preserved even at high doses of narcotics and barbiturates, but are abolished at high concentrations of volatile anesthetic agents. Neuromuscular blocking drugs do not directly influence SSEPs. However, they may improve the waveform quality of SSEPs by favorably reducing myogenic noise, allowing quicker and more reliable SSEP information.28

Motor Evoked Potentials

Despite reports of improved outcomes obtained with SSEP monitoring, there have been case reports of isolated motor injury with normal sensory function during anesthesia, making it clear that monitoring of motor-function is needed. All currently used inhalational anesthetic agents have been found to markedly attenuate transcranial motor-induced compound muscle action potentials (CAMPs).29,32 This includes sevoflurane, isoflurane, desflurane, and nitrous oxide in concentrations >50%.33 Therefore, numerous studies have determined that TIVA techniques optimize the monitoring of TceMEP.33,35

Abbreviations: IV: intravenous; MAC: monitored anesthesia care; SSEP: somatosensory evoked potential.

The newer synthetic opioids sufentanil, alfentanil, and remifentanil moderately decrease the amplitude (peak to trough) of motor-evoked potential waveforms.36 Fentanyl and morphine have shown a strong effect after bolus administration as compared with continuous infusion.37 Propofol seems to be the most popular agent used in TIVA because of its easy titratability, although it has also been shown to depress motor evoked potentials (MEPs).38 Use of dexmedetomidine as an anesthetic adjunct at target plasma concentrations up to 0.6 ng/mL does not change somatosensory or motor evoked potential responses during complex spine surgery by any clinically significant amount.39

Wake-Up Test

Before the mid-1970s, the only method for detecting spinal cord injury during corrective scoliosis surgery was the Stagnara wake-up test, which consisted of waking the patient intraoperatively and observing voluntary lower-extremity movement. It is occasionally done to verify the clinical alarm triggered by changes in SSEP and MEP. The performance of a wake-up test requires use of anesthetic technique that allows rapid awakening of the patient to a level of consciousness at which a response to commands can be effected. Ultrarapid-acting opioids such as remifentanil can have an important role in rapid recovery to the point of the ability to follow commands. Short-acting hypnotics (such as propofol) are also of great value. In a recent study, however, the new volatile anesthetic desflurane had a shorter wake-up time than did propofol.40