Congenital, Infantile, and Juvenile Scoliosis

Congenital spinal anomalies are caused by failures of formation and segmentation that result in scoliosis and kyphosis. The hemivertebra, caused by failure of formation, is the most common anomaly. Fully segmented hemivertebrae contribute to progressive deformity during periods of rapid spinal growth (e.g., first 5 years of life). The most severe deformities are seen in the thoracolumbar spine. Congenital spinal anomalies may be associated with malformations of the ribs, chest wall, and hemifacial microsomia.³¹ Children with congenital scoliosis have a 25% chance of urologic abnormalities and 10% chance of a cardiac abnormality. Bracing or casting techniques are not effective for this form of scoliosis. Surgical options for these patients include fusion in situ, convex hemiepiphysiodesis, hemivertebra excision, growing rods, and vertical expandable prosthetic titanium rib (VEPTR) treatment.³² Although short-term correction is easily achievable, a short thoracic or even thoracic insufficiency syndrome can result.³³ Approximately one half of the children who have extensive thoracic fusions and those whose fusions involve the proximal thoracic spine develop restrictive pulmonary disease (FEV₁ <50%).³⁴ Expansion thoracoplasty and stabilization using a VEPTR may be used.³⁵

Infantile and juvenile scoliosis are part of the spectrum of idiopathic scoliosis but are considered here because they manifest and require treatment at an early age. Infantile scoliosis accounts for less than 1% of idiopathic scoliosis and is defined as scoliosis appearing between birth and 3 years of age.³² It usually occurs in the thoracic spine, and the curve is convex to the left. Bracing and serial casting techniques are used for infantile scoliosis. Improvement and resolution in some cases have been achieved at 9-year follow-up.³⁶

Treatment of infantile scoliosis may begin as early as 4 to 5 months of age or as soon as the diagnosis of scoliosis has been made. Body casting appears useful in selected children, such as those with smaller, flexible spinal curves, but curve progression and the need for secondary treatments affect a significant proportion of these children.³⁷ Bracing is considered when the curve reaches 30 degrees.³² Success has also been reported for more severe curves (60 degrees) when casting was started before 20 months.³⁸ After induction of anesthesia, the child is positioned on the frame (first described by Cottrell and Morel), securing the pelvis to the caudal end of the frame and tethering the head by a chin strap to the rostral end. The spine is mildly distracted, but the main maneuver derotates the spine through the ribs (Fig. 30-3, A). General anesthesia with tracheal intubation is required to facilitate positioning the child, stretching the spine, and molding the body cast. Hemoglobin desaturation frequently occurs when the cast is molded to correct the spinal deformity. An oral airway is also needed to prevent compression of the tracheal tube after the chin strap is applied and tightened. After the cast has hardened, it is cut back and trimmed to maintain the correction to the spine while facilitating breathing, gastrointestinal function, and day-to-day living (see Fig. 30-3, B). Halo traction may be used to stretch and improve the curves. Pin infections occur in almost half of the children.³⁹

FIGURE 30-3 A, Nonoperative correction of scoliosis in infants and toddlers may be achieved with repeated casting. B, Cutting and trimming of the cast allows correction of the spine while facilitating daily living.

Juvenile idiopathic scoliosis comprises 10% to 15% of idiopathic scoliosis and is defined as scoliosis that is first diagnosed between the ages of 4 and 10 years. Approximately 20% of these children and those with infantile scoliosis with a curve greater than 20% have an underlying spinal condition, particularly Arnold-Chiari malformation and syringomyelia.⁴⁰ Although bracing is used to manage these curves, almost all children in this group with curves greater than 30% require surgical intervention.⁴¹

Growing rods may be used for congenital, infantile, or juvenile scoliosis to maintain the correction obtained at initial surgery while allowing spinal growth to continue. Several procedures are required before a definitive fusion.⁴² All the systems (i.e., growing rods and VEPTR) have a moderate complication rate (i.e., rod breakage and hook displacement).VEPTR systems are being used to correct large-magnitude curves in this group of children when conservative treatment is inadequate.⁴³

Idiopathic Scoliosis

Although adolescent idiopathic scoliosis is relatively common, severe morbidity is seen only in children with early-onset (infantile or juvenile) idiopathic scoliosis.⁴⁴ Respiratory deterioration alone is seldom the reason for surgery in those who develop scoliosis after the age of 5 years.²⁵ This is explained by the fact that the respiratory alveoli are mature by this age.^45,46

Scoliosis evolves during growth spurts. The earlier the age of onset and the more immature the bone growth at the time the process begins, the more severe the outcome. The relentless progression of infantile-onset idiopathic scoliosis with rapidly deteriorating curves and lung function is often not amenable to surgery. Treatment involving spinal instrumentation and anterior epiphysiodesis does not prevent the reappearance of the deformity or the decrease in pulmonary function.⁴⁷

Pulmonary impairment correlates directly with the magnitude of the thoracic curve. Severity of the scoliosis is the most accurate predictor of impaired lung function.⁴⁸ The morphology of the thoracic curve, the number of vertebrae in the major curve, and the rigidity of the curve also are associated with deteriorating pulmonary function.⁴⁹ Conventional wisdom has held that there is minimal impact on the vital capacity until the curve exceeds 60 degrees, with clinically relevant decreases in respiratory function occurring only after the thoracic scoliosis has progressed beyond 100 degrees.³⁴ However, it has been shown that children with adolescent idiopathic scoliosis may have pulmonary impairment that is disproportionate to the severity of the scoliosis and that it occurs well before the curve reaches 100 degrees. Forced vital capacity (FVC) may decrease below the normal threshold (<80% of predicted) after the magnitude of the thoracic curve exceeds 70 degrees; FEV₁ decreases to less than the normal threshold after the main thoracic curve exceeds 60 degrees.³⁵ Twenty percent of children with a thoracic curve of 50 to 70 degrees have moderate or severe pulmonary impairment (i.e., less than 65% of predicted) (Fig. 30-4, A).⁵⁰ Those with thoracic hypokyphosis are more likely to have moderate or severe pulmonary impairment; complex curves have a greater prevalence of moderate or severe pulmonary impairment, and the number of vertebrae in the thoracic curve is the most significant predictor of impaired respiratory function (see Fig. 30-4, B).⁵¹ These data indicate that children with a structural cephalad thoracic curve, a major thoracic curve spanning eight or more vertebral levels, or thoracic hypokyphosis are at increased risk for moderate to severe pulmonary impairment.

FIGURE 30-4 A, The bar graph demonstrates increasing pulmonary impairment with increasing curve severity as measured by degrees. B, Pulmonary impairment increases with increasing length of the thoracic curve.

(From Newton PO, Faro FD, Gollogly S, et al. Results of preoperative pulmonary function testing of adolescents with idiopathic scoliosis. A study of six hundred and thirty-one patients. J Bone Joint Surg Am 2005;87:1937-46.)

Neuromuscular Scoliosis

Children with neuromuscular scoliosis have the burden of deteriorating muscle function in addition to mechanical distortion. Crowding of the ribs on the concave side of the curve limits chest wall expansion, and the sitting posture restricts diaphragmatic excursion. This inevitably leads to more rapid deterioration in the curve and respiratory function. These children also have the potential for rapid and unpredictable deterioration of the curve.³⁶ It is important to consider the natural history of the specific neuromuscular disease when trying to balance the risks of surgery against conservative management.

Children with Duchenne muscular dystrophy (DMD) suffer from progressive muscular weakness and increasing disability until death occurs, usually by the beginning of the third decade.⁵² These children tend to become wheelchair bound by 8 to 10 years of age because of increasing motor muscle weakness. Scoliosis then progresses with an acute deterioration during the growth spurt between the ages of 13 and 15 years, such that it becomes difficult or impossible to sit unaided. After the lumbar curve exceeds 35 degrees, further progression becomes inevitable.⁵³ A normal cough requires an inspiratory effort of more than 60% of total lung capacity and effective glottic closure to produce an effective peak flow (more than 160 L/min in adults). Forced expiratory flows are typically reduced proportionally to the decrease in lung volume. As muscle weakness progresses, patients hypoventilate, initially at night. If nocturnal ventilatory support is not provided at this stage, diurnal hypercapnia will result.⁵⁴

There have been two significant changes in the overall management of children with DMD: the use of steroids and the earlier use of nocturnal noninvasive positive-pressure ventilation (NPPV). Steroid treatment in the early phase of the disease appears to slow disease progression for a few years; treatment with prednisone can stabilize strength and function for 6 months to 2 years.^55,56 This may delay the presentation of children for corrective surgery. Earlier adoption of nocturnal NPPV for nocturnal hypoventilation improves survival and quality of life. Clinically unsuspected nocturnal hypoventilation occurs in about 15% of patients with DMD and can be predicted by moderate impairment according to pulmonary function tests (FVC <70% and FEV₁ <65% of predicted) and scoliosis. Those with nocturnal hypoventilation have increased gas trapping, decline of muscle strength, and worse perception of health status despite NPPV.⁵⁷

A 2007 multidisciplinary consensus statement on the “Respiratory and Related Management of Patients with Duchenne Muscular Dystrophy undergoing Anesthesia or Sedation” provided recommendations to standardize the approach to these patients⁵⁸ and others with flaccid neuromuscular diseases undergoing anesthesia; the most important of these are as follows: An FVC less than 50% of predicted indicates an increase in postoperative respiratory complications, and an FVC less than 30% suggests a further increase in that risk. Respiratory function tests should be part of the preoperative evaluation whenever possible and should include FVC, maximal inspiratory pressure, maximal expiratory pressure, peak cough flow, Spo₂ on room air, and Paco₂ if the Spo₂ value is less than 95%. Consider preoperative training and postoperative use of NPPV if FVC is less than 50% of predicted, and strongly consider NPPV if FVC is less than 30%. Consider preoperative training and postoperative use of manual and mechanically assisted cough in those with impaired cough. In older children, this can be predicted by a peak cough flow less than 270 L/min or maximal expiratory pressure less than 60 cm H₂O. Strongly consider planning to extubate the trachea directly to NPPV when the FVC is less than 30%.

Dilated cardiomyopathy occurs in up to 90% of DMD individuals older than 18 years of age. The severity of the physical disability in boys with DMD in their late teens often masks the clinical symptoms of cardiac failure. Cardiomyopathy has been considered responsible for death of up to 20% of individuals with DMD, but this proportion may increase in the future for individuals in whom NPPV prevents respiratory-related mortality.⁵⁶ (See also Chapter 22.)

Idiopathic Scoliosis

Improvements in pulmonary function are not impressive after correction of idiopathic scoliosis. Early studies suggested that spinal fusion stabilized the respiratory dysfunction that existed preoperatively, but failed to offer any improvement.⁷⁷ Improvements may be possible in certain subgroups of patients with some surgical techniques, but it takes months to years for pulmonary function to improve. Children with a preoperative curve less than 90 degrees undergoing a posterior procedure only had a greater than 10% increase in vital capacity, maximum voluntary ventilation, and maximum respiratory mid-flow rate after 2 years; this improvement did not occur in those who underwent anterior surgery.⁷⁸ Harrington rod instrumentation in children with idiopathic scoliosis resulted in only a small improvement in vital capacity.⁷⁹

The newer instrumentation systems, such as the Cotrel-Dubousset instrumentation that allows segmental realignment and approximation, result in further improvements in pulmonary volumes.⁸⁰ Pulmonary function returns to preoperative values within 3 months after the posterior approach using the newer instrumentation systems, with additional improvements occurring and being sustained for 2 years.⁸¹ Pedicle screws provide greater curve correction in adolescent idiopathic scoliosis, with a trend toward improved pulmonary function after 2 years compared with other instrumentation techniques.²³ A 10-year follow-up analysis demonstrated an absolute increase in the FVC (3.25 versus 3.66 L) and FEV₁ (2.77 versus 3.10 L) but no changes in percent of predicted values in children who underwent a posterior fusion and instrumentation only. In the same analysis, those with chest wall disruption experienced no change in FVC and FEV₁ over 10 years, but the assessment demonstrated a significant decrease in percent of predicted FVC (85% versus 79%,) and FEV₁ values (80% versus 76%).⁸²

Chest cage disruption (i.e., thoracoplasty or anterior thoracotomy) is associated with reduced pulmonary function at 3 months and a 10% to 20% decrease in total lung capacity and FVC. These values do not return to baseline until 1 to 2 years after surgery. Improvements in lung function with this approach are seldom seen.^44,45 Video-assisted thoracoscopic surgery (VATS) for anterior release and instrumentation appears to result in less pulmonary morbidity and a smaller decrease in pulmonary function at 3 months. One year after surgery, values for children treated thoracoscopically had returned to baseline, but this did not occur for those undergoing open thoracotomy (Fig. 30-6).^83,84 Two- and 5-year follow-up evaluations of those undergoing VATS showed no significant changes with regard to the correction of the major Cobb angle (56% ± 11% and 52% ± 14%, respectively) or average total lung capacity as a percent of the predicted value (95% ± 14% and 91% ± 10%) over the study period.⁸⁵

FIGURE 30-6 Changes in percent of forced vital capacity (FVC) for thoracoscopic versus open anterior instrumentation during the first year after surgery.

(From Faro FD, Marks MC, Newton PO, Blanke K, Lenke LG. Perioperative changes in pulmonary function after anterior scoliosis instrumentation: thoracoscopic versus open approaches. Spine 2005;30:1058-63.)

Changing surgical techniques may challenge these findings in the future because some surgeons think that anterior fusions with modern systems offer short-term benefits of reduced blood loss and transfusion and long-term benefits due to shorter fusions, better maintenance of thoracic kyphosis, and improved spontaneous lumbar curve correction. A 2-year postoperative study concluded that VATS for thoracic curves and open procedures for thoracolumbar curves resulted in minimal to no permanent pulmonary impairment 6 months after the procedure compared with posterior spinal fusion, despite a short-term decrease observed after VATS.⁸⁶

Neuromuscular Scoliosis

Improvements in the scoliosis angle and the degree of pelvic obliquity are achieved after spinal instrumentation in children with neuromuscular disease. Significant improvement can usually be shown in the ability to sit unaided, particularly if children are unable to do so beforehand.^87–91 Some investigators have reported an increase in the quality of life perceived by the child or caregiver.^89,91

There is little evidence for any improvement in respiratory function in this group of children. There may be a period of delay or even stabilization of the inevitable deterioration of respiratory function.^52,88,92 Other investigators have shown no difference in respiratory function after 5 years compared with patients managed conservatively^90,93 or an early loss in vital capacity after surgery with a progressive decrease of 25% over 4 years, with 66% of patients requiring mechanical respiratory assistance by that time.⁸⁹ A Cochrane review was unable to find any data evaluating the effectiveness of scoliosis surgery in patients with DMD and even suggested, “Patients should also be informed about the uncertainty of benefits on long-term survival and respiratory function after scoliosis surgery.”⁹⁴

Some retrospective analyses, however, deserve consideration. A review of the long-term survival of children with DMD after spinal surgery and nocturnal ventilation demonstrated that those having spinal surgery and ventilation had a median survival of 30 years, whereas those receiving nocturnal ventilation only survived to 22.2 years. This result occurred despite a decrease in mean vital capacity from 1.4 to 1.13 L in the first postoperative year.⁹⁵ Posterior spinal fusion for scoliosis in DMD was associated with a significant slowing in the rate of decrease in respiratory function; the rate of 4% per year before surgery decreased to 1.75% per year (over 8 years) after surgery.⁹⁶ In a study of 14 children with DMD and an FVC of less than 30%, the mean rate of decrease in percent of FVC after surgery was 3.6% per year. Most children and parents thought scoliosis surgery improved their function, sitting balance, and quality of life and gave it high satisfaction scores.⁹⁷

Less outcome information is available for children with cerebral palsy. Surgery is perceived as having a positive impact on patients’ quality of life, overall function, and ease of care by parents and other caregivers,⁹⁸ despite the high complication rates described earlier. A 3-year follow-up after a pedicle screw construct for scoliosis, which reduced the mean Cobb angle to 31%, demonstrated improved functional ability in 42% of children. Most children had improved sitting balance and nursing care requirements. A 32% complication rate occurred; most were pulmonary in origin but ultimately reversible. There were two perioperative deaths and one transient neurologic deficit due to screw impingement among 56 patients.⁶⁸

Less morbidity has been claimed for the same-day (one-stage) surgery compared with the two-staged approach in children with neuromuscular disease requiring anterior and posterior spinal surgery.^99,100 However, it seems reasonable to avoid anterior thoracotomy in neuromuscular patients in view of the poor respiratory function after chest cage disruption.^81,83 Currently, pedicle screw systems in children with neuromuscular scoliosis produce outcomes similar to those of earlier systems but with quicker operating times and less blood loss.⁶³

Wake-up Test

The wake-up test measures gross motor function of the upper and lower extremities. It has been used widely since it was first described.¹¹¹ The test consists of decreasing the depth of anesthesia almost to the point of wakefulness and asking the patient to respond to verbal commands. Failure to move the feet and toes while being able to squeeze a hand suggests a problem with the spinal cord. The test requires limiting or reversing muscle relaxation and reducing the depth of anesthesia sufficiently to enable the patient to follow commands. When the test was initially described, 3 of 124 patients had a positive result (i.e., no movement) and were saved from paraplegia.¹¹¹ A major concern is that the test is conducted after maximal spinal correction, which may occur after any neurologic insult has occurred; however, removal or modification of the spinal instrumentation within 3 hours of the onset of the neurologic deficit has been reported to prevent the risk of permanent neurologic sequelae.¹¹² The wake-up test is unlikely to detect isolated nerve root injury or sensory changes. It is limited to neurologically normal patients with an appropriate developmental age who can follow instructions.

With the clinical application of SSEP and MEP monitoring (Fig. 30-7) well established and in the absence of intraoperative changes, there is no justification to perform the wake-up test.¹¹³ Nonetheless, some surgeons still regard the wake-up test to be the gold standard, and it may be used to confirm changes demonstrated by SSEP or MEP monitoring.¹¹⁴ Risks include lack of nerve root and sensory information, accidental extubation, dislodgement of the instrumentation, intraoperative recall with subsequent psychological trauma, air embolism, and cardiac ischemia. If a wake-up test is planned, it is prudent to fill the wound with saline to reduce the risk of an air embolism.

FIGURE 30-7 Comparison of pathways involved in somatosensory evoked potential (SSEP) and motor evoked potential (MEP) monitoring.

(Modified from de Haan P, Kalkman CJ. Spinal cord monitoring: somatosensory- and motor-evoked potentials. Anesthesiol Clin North Am 2001;19:923-45.)

Ankle Clonus Test

The ankle clonus test uses the clonus that occurs just before consciousness is regained during wakening from anesthesia. Rhythmic muscle contractions are thought to result from spinal reflexes returning while the higher neurologic centers remain inhibited by anesthesia, and the oscillations demonstrate an intact spinal cord. Inability to demonstrate clonus suggests spinal cord injury.¹¹⁵ Like the wake-up test, it is a post hoc test rather than real-time monitoring. However, in a review of more than 1000 patients undergoing spinal procedures in which six postoperative neurologic deficits occurred, this test identified all the deficits but produced three false-positive findings, giving a sensitivity of 100% and a specificity of 99.7%. In comparison, the wake-up test produced false-negative results for four of the five patients who developed deficits.¹¹⁵

Somatosensory Evoked Potentials

SSEPs involve stimulating a peripheral nerve and measuring the response to that stimulation using scalp electrodes (i.e., cortical SSEPs).^116,117 Alternatively, the response can be measured subcortically near the spinal cord by electrodes placed in the epidural space, interspinous ligament, or spinous processes of the vertebrae.¹¹⁸ An intranasally placed pharyngeal electrode can act as a surrogate for these. The advantage of the subcortical evoked potential is that the responses are more stable, reproducible, and resistant to the effects of anesthesia.

The signal produced with SSEP monitoring travels from the peripheral nerve through the nerve root and up the ipsilateral dorsal column. The impulses then cross over at the level of the brainstem and progress rostrally through the thalamus to the primary sensory cortex. The rationale for using SSEP to monitor motor deficits is based on the fact that the sensory tracts are in proximity to the motor tracts of the spinal cord. Injury to the motor tracts indirectly affects the sensory tracts and causes changes in the SSEP. When spinal cord function is significantly impaired, there is usually an increase in latency and a decrease in amplitude in the SSEP, with eventual loss of signal. A 10% increase in latency of the first cortical peak (P1) or 50% decreases in the peak-to-peak amplitude (P1N1) constitute an indication for intervention.^119,120 Although SSEP signals primarily monitor transmission through the sensory dorsal columns, they are effective,¹²⁰ and SSEP monitoring is associated with a 50% decrease in the incidence of neurologic deficits.

It is unusual for motor tract injury to occur when SSEPs remain unchanged, but false-positive and false-negative results have been reported.^120,121 Seventy percent of the postoperative complications were detected by the monitor, but 30% (false negatives) were not detected. Pedicle screw misplacement leading to radiculopathy may not be detected by SSEP monitoring.¹²² Several case reports of paraparesis also attest to the limitations of SSEP monitoring. These problems are caused by injury occurring outside the monitored domain of SSEP rather than a failure of this modality. These concerns encouraged development of methods to monitor the motor tracts of the spinal cord. SSEP monitoring is possible in patients with cerebral palsy, whereas MEPs may not be.¹²³

Motor Evoked Potentials

The motor pathways can be activated by transcranial stimulation of the motor cortex or by spinal cord stimulation. Transcranial stimulation is achieved using electrical or magnetic stimulation applied to the scalp. Electrical stimulators are most commonly used in spinal surgery and operate by applying high-voltage pulses to the scalp using corkscrew, needle, or surface electrodes. The stimulation pulses can be applied as single stimuli or brief pulse trains with intervals between the pulse trains. Multiple stimuli result in a stronger signal with less variability due to temporal summation of the excitatory postsynaptic potential.¹²⁴ Epilepsy and proconvulsant medicines are considered relative contraindications to MEP monitoring because of concerns about brain injury from prolonged seizure activity due to the electrical current required for stimulation.^125,126 Not surprisingly, MEPs may be difficult to record and interpret in patients with cerebral palsy and should not be attempted if the child has seizures.

MEP monitoring may be a problem in younger children, particularly those younger than 6 or 7 years of age.^125,127 Use of a spatial summation technique in addition to temporal summation increased the success rate from 78% to 98% over all ages. Using this technique along with ketamine anesthesia in children younger than 6 years of age, reliable MEPs were documented in 98% (111 of 113) of children older than 6 years of age and in 86% (18 of 21) in children younger than 6 years of age.¹²⁷ There is also evidence that younger children require a greater stimulating voltage and pulse train frequency for MEP monitoring, probably because of immaturity of the central nervous system, specifically the descending corticospinal tracts.¹²⁸

Spinal cord stimulation is achieved electrically and can be applied using electrodes placed outside or inside the spinal cord rostral to the area of interest. Single stimuli rather than brief pulse trains typically are used for spinal cord stimulation.¹²⁹ This approach is not commonly used in scoliosis surgery.

Responses can be recorded anywhere distal to the area of interest. They have included the lower lumbar epidural space (i.e., epidural MEP), peripheral nerve (i.e., neurogenic MEP), and peripheral muscles using compound muscle action potential (CMAP) (see Fig. 30-7).¹³⁰ Each recording site has its limitations regarding the accuracy of the information displayed and the susceptibility to anesthetic drug interference. Epidural MEPs are the least affected by neuromuscular blocking drugs (NMBDs), but they only monitor conduction in the corticospinal tract and provide no information about the anterior horn gray matter.¹³¹ They have a much slower response to acute spinal cord ischemia when compared with myogenic responses (i.e., CMAPs).¹³² Neurogenic MEPs are also resistant to anesthetic interference but appear not to accurately measure motor conduction. Most of the spinally elicited peripheral nerve responses seen with neurogenic MEPs occur through the dorsal columns in a retrograde fashion and are sensory rather than motor.¹³³ Anterior spinal cord injury has been demonstrated with normal neurogenic MEPs.¹³⁴ CMAPs after transcranial stimulation are thought to be exclusively generated by motor tract conduction, and unlike epidural MEPs, they include the ischemia-sensitive anterior horn alpha motor neurons.¹³⁰ These responses are very sensitive to anesthetic agents. The responses obtained with CMAPs after spinal cord stimulation also appear to contain signals that include transmission through the dorsal columns and may represent a mixed response.¹³⁵

One outstanding problem with MEP monitoring is deciding when and how much change in the signal is significant and indicative of spinal cord ischemia. Some centers use the same criteria they have adopted for SSEP monitoring, whereas others require a greater degree of change, such as a 75% decrease in amplitude.¹³⁶ An amplitude decrease of 80% at one of six sites using transcranial myogenic MEP monitoring was demonstrated to have a sensitivity of 1.0 and a specificity of 0.91 when used as the sole monitor during spinal surgery.¹³⁷ A 65% decrease in amplitude identified all postoperative motor deficits (SSEP changes identified only 43%) in children with idiopathic scoliosis.¹³⁸ An alternative technique of measuring MEP has been described in which a minimum threshold for producing a response is established, and a significant increase in that threshold is used to signal a problem.¹³⁹ Supportive data for this technique are lacking.

The dorsal columns may be injured without involvement of the motor tract.¹⁴⁰ Occasionally, adverse changes in SSEPs occur without changes in MEPs.^129,141 Because of these reports, MEP monitoring should be used in addition to SSEP monitoring rather than as a replacement.^107–110142

Triggered Electromyographic Techniques

The increasing use of pedicle screws allows greater curve and rotational correction than earlier techniques but has an additional risk of direct nerve root trauma. Triggered EMGs using a monopolar needle or bipolar handheld stimulator have been described, with a threshold stimulation level of more than 8 mA considered to be normal, 5 to 8 mA to be critical, and less than 5 mA to be pathologic, indicating that there was not enough distance between the screws and the neural tissue.¹⁴² This technique requires monitoring rectus abdominis or intercostal muscles when used for thoracic curves.^143,144

Intraoperative Intrathecal and Intravenous Opioids

Intraoperative intrathecal morphine (2 to 5 µg/kg) has provided potent analgesia during the first 24 hours after spinal fusion in children.^155,156 Intrathecal morphine also decreases the amount of remifentanil required intraoperatively, contributing to less pain on cessation of remifentanil.^157,158 However, perioperative administration of intravenous morphine, when using remifentanil as part of the anesthesia technique, does not result in any measurable benefit.¹⁵⁹

Methadone (0.2 mg/kg) decreases pain scores and opioid requirements for 36 hours in children undergoing surgery, but it seems to have been ignored in modern practice.¹⁶⁰ This drug is used in adult spinal surgery, but reports of its use in children are few.¹⁶¹

Nonsteroidal Antiinflammatory Drugs

NSAIDs, but not acetaminophen, impair fracture healing in animal models.¹⁶² Cyclooxygenase-2 (COX-2) activity plays an important role in bone healing, and the use of NSAIDs decreases osteogenic activity that may increase the incidence of nonunion after spinal fusion.^8,9 The effect on osteogenic activity is dose dependent and reversible.¹⁶³ Similar effects have not been demonstrated in humans. Nonetheless, based on animal evidence, NSAIDs should be used with caution and in consultation with the surgeon during the first 3 to 5 days after scoliosis surgery.¹⁶⁴

Systemic Analgesics

Morphine remains the mainstay of systemic analgesic regimens. Morphine infusions of 20 to 40 µg/kg/hr are required during the first 48 hours after surgery. Achieving a balance of effective analgesia while avoiding sedation can be difficult in children with neurodevelopmental delay. Regular evaluation of these children is important if complications are to be avoided. Patient-controlled analgesia (PCA) is appropriate for children older than 6 to 7 years of age. It can be used with a typical bolus dose of 20 µg/kg and a lockout interval of 5 to 10 minutes. The use of a background morphine infusion may be effective in some patients, although its inclusion is controversial.^165,166 Our preference is to use a nighttime background infusion at 5 to 10 µg/kg/hr but to use PCA alone during the day (see Chapter 43). Nurse- and parent-controlled analgesia are effective if the child is too young or unable to use PCA.¹⁶⁷ Intrathecal morphine plus PCA appears to offer the optimal combination of effective analgesia and minimal adverse effects in idiopathic scoliosis patients compared with PCA morphine alone or epidural morphine.¹⁶⁸

Low-dose ketamine infusion (0.05 to 0.2 mg/kg/hr) has been used as an adjunct to morphine infusions or PCA, although its role is debated.^169–174 Ketamine may be initiated intraoperatively (initial infusion of 5 µg/kg/min, decreasing to 2 µg/kg/min at the end of surgery) as part of the anesthetic technique to minimize the hyperalgesia reported after high-dose remifentanil infusions.^175,176 The literature continues to be confused with conflicting reports showing no effect¹⁷⁷ or a modest decrease in pain scores and morphine consumption.¹⁷⁸ Ketamine added to morphine PCA has produced mixed results, with no clear beneficial effect in orthopedic surgery, despite such evidence being apparent for thoracic surgery.¹⁷⁹ If added to PCA, the optimal combination of morphine: ketamine is a 1 : 1 ratio.¹⁷² Despite scoliosis causing severe pain, it is probably best to reserve the use of ketamine for those with significant preoperative pain or morphine-resistant pain.

Gabapentin and pregabalin provide some benefit with an opioid-sparing effect,¹⁸⁰ although postoperative nausea and vomiting benefits are limited.¹⁸¹ Gabapentin (15 mg/kg followed by 5 mg/kg three times daily for 3 days) reduced morphine consumption by about 30% over the study period but without any improvement in morphine’s adverse effects. Improved pain relief was observed only until the morning after surgery.¹⁸²

Epidural Analgesia

Continuous epidural analgesia using single- and double-catheter techniques may provide effective analgesia after spinal surgery. The single-catheter technique using bupivacaine-fentanyl and sited at T6-7 for patients undergoing a mean 12-level scoliosis surgery resulted in analgesia similar to that of PCA but with more postoperative nausea and vomiting and pruritus. Bowel sounds returned earlier in the epidural group, but liquid intake and hospitalization were similar.¹⁸³ Similar results were reported with a bupivacaine-morphine combination in patients undergoing 10-level spinal fusions. Full diet and discharge from hospital were achieved one-half day earlier with the epidural technique than with PCA.¹⁸⁴ A retrospective review of more than 600 patients treated with an epidural or PCA for analgesia after scoliosis surgery, in which the average number of segments fused was 8.5, confirmed the effectiveness of epidural analgesia.¹⁸⁵ In that study, a bupivacaine-hydromorphone epidural combination was used, and although pain management was effective, more complications occurred in the epidural group. Respiratory depression and transient neurologic changes were the most common complications observed. Thirteen percent of patients with an epidural catheter required discontinuation of the epidural, most commonly for inadequate pain relief.¹⁸⁵ Effective analgesia and a large incidence of postoperative nausea and vomiting and pruritus have been features of studies that combined bupivacaine and morphine.^186,187

Patient-controlled epidural analgesia (PCEA) has been successfully used in children older than 5 years of age for orthopedic surgery and thoracotomies,¹⁸⁸ In scoliosis surgery, PCEA with bupivacaine and hydromorphone compared with PCA resulted in a slightly improved pain score but a 37% failure rate.¹⁸⁹ PCEA with a single- or double-catheter technique, depending on the number of spinal segments involved, and using a bupivacaine, fentanyl, and clonidine solution has been successful and is associated with a relatively small incidence of complications.¹⁹⁰

Improved pain control and bowel function with decreased adverse effects may be possible by using a double-epidural technique using moderate amounts of fentanyl and clonidine with local anaesthetics.¹⁹¹ Double-epidural techniques comprise an upper catheter positioned in the upper to middle thoracic segments and a lower catheter at the upper to middle lumbar level.^192,193 This modality resulted in improved pain control compared with morphine infusion and had fewer gastrointestinal adverse effects.

Hypotensive Techniques

Controlled hypotension has been used to minimize blood loss during scoliosis surgery since it was first described more than 30 years ago. A greater than 50% decrease in blood loss with a decreased need for blood replacement and a reduced operating time was demonstrated in early studies. Ganglion-blocking agents (i.e., pentolinium and trimethaphan) have been superseded by β-blockers, direct arterial vasodilators, calcium channel blockers, and α₂-agonists. It remains uncertain whether reduced blood loss results from lowered blood pressure²¹² or lower cardiac output.²¹³ A target mean arterial pressure (MAP) of 50 to 65 mm Hg has been recommended. Although this appears to be safe, concerns that the margin of safety for cerebral and spinal cord ischemia is reduced by controlled hypotension apply to operations of prolonged duration, and there is the potential for periods of hypovolemic hypotension in addition to drug-induced (controlled) hypotension.^214,215 The incidence of these feared complications is fortunately very small with or without hypotension. Renal function appears well preserved even when hypotensive anesthesia is used during scoliosis surgery.^216,217 Because of these concerns and the concomitant use of hemodilution, less extreme degrees of hypotension are usually employed. Robust data supporting the beneficial effects of controlled hypotension in scoliosis surgery is minimal, although clear benefits have been demonstrated for orthognathic and orthopedic surgery.^218,219

In modern practice, moderate hypotension with good control of the heart rate can often be achieved without the use of specific vasoactive drugs by using a remifentanil infusion titrated to the desired blood pressure.²²⁰ Although not considered a hypotensive agent, intrathecal morphine decreases blood loss and may facilitate blood pressure control, particularly with a remifentanil infusion. At an analgesic dose of 5 µg/kg, a decrease in EBL from 41 to 14 mL/kg occurred.¹⁵⁶ Using this technique, blood pressure control can frequently be achieved without any additional agents. We have found that using an anesthetic combination of less than 1 minimal alveolar concentration (MAC) of inhalational agent plus remifentanil with clonidine (2 µg/kg)²²¹ provides controlled hypotension in most patients without the need for additional agents. Dexmedetomidine may be used as part of a technique for controlling blood pressure.^222,223

Short-acting calcium channel blockers have been used for controlled hypotension for scoliosis surgery, and although they are effective, experience is limited. Nicardipine is associated with less blood loss at the same MAP compared with sodium nitroprusside, although a slower return to baseline blood pressure (27 versus 7 minutes) is seen.^224,225 Clevidipine has been used without evidence of clear benefit, but it may be associated with an increase in heart rate.²²⁶ If a hypotensive technique is to be used, invasive arterial monitoring is essential, and central venous pressure catheters are useful for the safe conduct of anesthesia (see Chapter 10).

Hemodilution

Decreasing the hemoglobin concentration by removing red cells and replacing the volume with a combination of crystalloid and colloid means that for a given volume loss, there is less red cell loss (see Chapter 10). The decreased metabolic rate during anesthesia suggests that oxygen delivery can be maintained with a reduced hemoglobin concentration if normovolemia is maintained.

It has been estimated that more than 2 to 3 units of blood must removed during hemodilution to significantly reduce transfusion requirements. Hemodilution modeling in adult patients has suggested that as many as 5 units of blood must be removed before there is a decrease in transfusion requirements.²²⁷ Deciding on the degree of hemodilution and establishing a threshold for transfusion may be difficult. In scoliosis surgery, reduction to an initial hematocrit of 30% has been effective for reducing and minimizing transfusion requirements.²²⁸ Some posit that only modest benefits are gained from the technique.²²⁹ Hypotensive anesthesia, hemodilution, and a cell saver used as part of a “bloodless surgery” program with erythropoietin and supplemental iron resulted in an average EBL of 855 mL (blood returned by the cell saver averaged 341 mL), with an average drop in hemoglobin after surgery of 3.1 g/dL.²³⁰

Tachycardia and hemodynamic instability are common at hemoglobin concentrations less than 7 g/dL. Myocardial ischemia becomes a risk as the hemoglobin concentration decreases below 5 g/dL.²³¹ At this level of anemia, cyanosis cannot develop because 5 g/dL of desaturated hemoglobin are required for cyanosis to be detected. Extreme hemodilution techniques such as these are reserved for patients who oppose blood transfusion. One report detailed patients who had hemodilution during scoliosis surgery that produced a hemoglobin concentration of 3 g/dL in the absence of preexisting cardiac disease.²³² Cardiac output increased by more than 30%, with only a modest increase in heart rate and decrease in blood pressure.²³² Although no cerebral sequelae were reported, this degree of extreme hemodilution is not recommended.

Autologous Predonation

Preoperative strategies, including predonation of blood and preoperative red cell augmentation, may be used alone or in addition to intraoperative techniques to minimize exposure to blood (see Chapter 10). In idiopathic scoliosis surgery, preautologous blood donation plus intraoperative red cell salvage and controlled hypotension resulted in an average EBL of 1055 mL, avoided transfusion, and the hematocrit only decreased by 10% (35.6 to 32.4) at discharge.²³³ The advantages of preoperative erythropoietin in addition to preautologous blood donation include a greater preoperative hemoglobin concentration and fewer donated units. Its effect on blood use depends on the total blood loss.^234,235 In children with neuromuscular scoliosis, erythropoietin alone did not affect the amount of blood transfused, although the preoperative and discharge hematocrits were greater in treated patients.²³⁶

Antifibrinolytic Agents

The use of synthetic antifibrinolytic agents to decrease perioperative blood loss after scoliosis surgery has produced mixed results. To be most effective, an effective plasma concentration of the antifibrinolytics should be established before skin incision. ε-Aminocaproic acid (Amicar) decreases the EBL by 25% during the perioperative period,²³⁷ mainly attributable to decreasing the postoperative suction drainage.²³⁸ In contrast, an initial dose of tranexamic acid (10 mg/kg) followed by an infusion of 1 mg/kg/hr failed to significantly decrease blood loss in a small sample.²³⁹ High-dose tranexamic acid (100 mg/kg loading dose, followed by an infusion of 10 mg/kg/hr) did decrease blood loss by 40% but did not affect transfusion requirements. Post hoc analysis in patients with secondary (neuromuscular) scoliosis showed significant reduction in blood loss and transfusion requirements.²⁴⁰ The correct dose of tranexamic acid remains elusive, but it may be one half of the high dose reported previously.²⁴¹

A meta-analysis of aprotinin, tranexamic acid, and ε-aminocaproic acid on blood loss and use of blood products in children undergoing scoliosis surgery showed that all antifibrinolytic drugs decreased the amount of blood transfused and that aprotinin, tranexamic acid, and ε-aminocaproic acid were equally effective.²⁴² A similar meta-analysis of major pediatric surgery showed that in the scoliosis studies, aprotinin and tranexamic acid reduced blood loss compared with placebo (385 mL, 95% confidence interval [CI] 42 to 727 mL, versus 682 mL, 95% CI 214 to 1149 mL).²⁴³ In all operations, both drugs also decreased red cell transfusion. Demonstration that tranexamic acid is as effective as aprotinin is fortunate, because aprotinin has been withdrawn in many countries after reports of increased morbidity and mortality in adults after cardiac surgery.²⁴⁴ However, after a review of the evidence, aprotinin has been reintroduced in Canada, citing off-label use of the drug as the cause of complications. Studies are being conducted in the United States. Use of ε-aminocaproic acid reduces mean intraoperative blood loss (1125 mL versus 2194 mL), total perioperative blood loss (1805 mL versus 3055 mL), and transfusion requirements (660 mL versus 1548 mL) compared with placebo.²⁴⁵ Reduced blood loss has significant cost savings from decreased operating room time and blood products use.²⁴⁶ Desmopressin is ineffective in decreasing blood loss associated with spinal surgery. Initial beneficial results with desmopressin²⁴⁷ have not been reproduced in patients with idiopathic scoliosis^248,249 or in those with neuromuscular scoliosis.^250,251

Intraoperative Salvage of Shed Blood

Decisions concerning the use of intraoperative salvage of shed blood (e.g., cell saver) depend on the anticipated blood loss, size of the patient, and use of other methods to minimize blood transfusion, such as predonation and hemodilution (see Chapter 10). It is important to have some idea of the blood loss associated with idiopathic scoliosis in your institution when deciding to use cell saver techniques. For example, the cell saver was found to be beneficial in less than 5% of adolescents with idiopathic scoliosis involved with an autologous predonation program or modest intraoperative hemodilution in one institution.²²⁸ At another institution, however, allogeneic transfusion rates were reduced from 55% to 18% when the cell saver was used. The allogeneic transfusion relative risk was 2.04 for patients undergoing surgery lasting more than 6 hours and 5.87 for patients not receiving cell saver blood.²⁵²

Pediatric systems are available with small spinning bowls (55 to 75 mL). These systems benefit children with smaller body weights and greater than anticipated blood loss such as patients with neuromuscular scoliosis undergoing extensive spinal fusion.^253,254 Among children undergoing anterior instrumentation for thoracolumbar curves, cell saver use decreased the number requiring allogenic blood transfusion from 39.4% to 6.7% and with similar mean postoperative hemoglobin values (10.2 versus 9.6 g/dL).²⁵⁵

Inhalational Anesthetics

Inhalational anesthetics cause dose-dependant depression of the SSEP and myogenic MEP. At equipotent concentrations, the MEP is affected to a greater degree than the SSEP. This means that while inhalation agents can be used during SSEP monitoring, they often need to be administered in subanesthetic doses during MEP monitoring. Adequate cortical SSEPs and subcortical SSEPs can be measured with up to 1 MAC of isoflurane, sevoflurane, and desflurane, although some increase in latency and decrease in amplitude may be detected.^272,273 It is important to maintain constant end-tidal concentrations throughout anesthesia after baseline measurements have been established. Concentrations of these anesthetics that allow adequate monitoring are significantly less than was possible with halothane.²⁷⁴

Myogenic MEPs (i.e., CMAPs) are recordable only at low concentrations of inhalational anesthetics. The exact concentration depends on the system being used and is greatly influenced by the number of pulses in the stimulus. Single-pulse transcranial stimuli may be inhibited by end-tidal concentrations as small as 0.2 MAC and abolished by end-tidal concentrations as small as 0.5 MAC.^275–277 This suppression can be partially overcome by using greater intensity stimuli with multipulse stimulation of up to 6 pulses per stimulus. An increasing number of patients lose recordable myogenic MEPs, even when multipulse stimuli are used, as the concentration of inhalational anesthetic exceeds 0.5 MAC. At end-tidal concentrations in excess of 0.75% isoflurane, monitoring conditions become unacceptable.^278–282 Stimulus intensity and pulse train frequency probably are factors in determining successful myogenic MEPs with inhalational anesthetics. Using direct stimulation of the cortex during craniotomy, CMAP was easily recordable at 1 MAC of isoflurane and sevoflurane.²⁸³ Similar results have been demonstrated with sevoflurane using transcranial stimulation.²⁸⁴ Information regarding desflurane is limited, and although it causes a dose-dependent depression, myogenic MEPs have been successfully recorded at 0.5 MAC.^282,285 Using a multipulse stimulation technique, intraoperative recording of MEPs was equally successful during desflurane or propofol anesthesia.²⁸⁶ In contrast to its effects on SSEPs, halothane depresses myogenic MEPs to a lesser extent than the newer inhalational anesthetics.²⁸⁴

Nitrous Oxide

Nitrous oxide reduces the amplitude of the cortical SSEP, but comparisons with other inhalational anesthetics are limited. Nitrous oxide (0.5 MAC) depresses SSEPs to a greater extent than isoflurane at a similar MAC.²⁸⁷ Similarly, 66% nitrous oxide depressed SSEPs to a greater extent than propofol (6 mg/kg/hr; 100 µg/kg/min).²⁸⁸ Nitrous oxide depresses myogenic MEPs.²⁷³ The effect relative to other inhalational anesthetics is difficult to determine. Nitrous oxide appears to affect CMAP amplitude to a lesser extent than isoflurane.²⁸⁹ Multipulse stimulus techniques can partially reverse nitrous oxide–induced depression of amplitude. Compared with a propofol infusion designed to maintain a target concentration of 3 µg/mL, 50% nitrous oxide decreases CMAPs with single or paired stimuli to a lesser extent.²⁹⁰ When 60% nitrous was added to low-dose propofol infusion at a target concentration of 1 µg/mL, adequate CMAPs were obtained using multipulse transcranial stimulation.²⁹¹ Conversely, the addition of nitrous oxide to a variety of different total intravenous techniques significantly depressed the CMAP such that some were not recordable.²⁹² With the widespread availability of remifentanil and the variable but mostly negative effects of nitrous oxide on SSEP and MEP signals, it seems that nitrous oxide is best avoided when spinal cord monitoring is used.

Propofol

Propofol decreases the amplitude of the cortical SSEP, but adequate signals can be recorded, even in the presence of nitrous oxide, at doses used for anesthesia (6 mg/kg/hr; 100 µg/kg/min).²⁹³ Propofol better preserves cortical SSEP amplitude and provides a deeper level of hypnosis as measured by processed electroencephalographic values than combinations of low-dose isoflurane and nitrous oxide or low-dose isoflurane or sevoflurane alone.^294–296

Propofol depresses the amplitude of myogenic MEPs. In addition to its cortical effect, it suppresses activation of the alpha motor neuron at the level of the spinal gray matter.^297,298 Low-dose propofol infusions have become popular as part of the anesthetic technique used with MEP monitoring due to the rapid improvement of signals when the drug is terminated and because multipulse stimulation techniques can improve the response amplitude.^279,299 Propofol, even in combination with nitrous oxide, depresses multipulse transcranial CMAPs less than isoflurane.²⁷⁹ Propofol (5 mg/kg/hr; 83 µg/kg/min) combined with 66% nitrous oxide produced satisfactory CMAP recordings in 75% of patients when a four-pulse stimulation sequence was used. In contrast, no recordings were possible with 1 MAC of isoflurane.²⁸⁰ The infusion rates or target concentrations that allow acceptable myogenic MEP recordings vary considerably and reflect different adjuvants (e.g., opioids, ketamine, nitrous oxide), degrees of neuromuscular blockade, and transcranial pulse rates. Propofol at a target of 4 µg/mL or at an infusion rate of 6 mg/kg/hr (100 µg/kg/min) produces acceptable signals with multipulse stimuli.^299–301

α₂-Adrenoreceptor Agonists: Clonidine and Dexmedetomidine

The cerebral effects of the α₂-agonists appear to act mainly at the locus ceruleus, rather than by the more generalized inhibition of synaptic pathways, as in the case of general anesthetics.³⁰² Clonidine at doses of 2 to 5 µg/kg given intravenously had minimal effects on cortical SSEPs when added to isoflurane.^303–305 In view of its lack of effect on SSEPs and its anesthetic-sparing properties with inhalational agents and propofol,^305–307 it seems reasonable to consider clonidine at a dose of 2 to 4 µg/kg as part of an anesthetic technique. Dexmedetomidine has similar beneficial properties on SSEPs.^308,309

There are no published studies on the effects of clonidine on MEPs, but a few publications have examined dexmedetomidine and MEPs with variable results.^310–313 The effects of dexmedetomidine, like other anesthetic agents, produce a dose-dependent depression of MEPs, and the ability to interpret these signals may depend on the depth of anesthesia, which suggests that depth of anesthesia monitoring should be used when recording MEPs to maintain a plane of anesthesia that is adequate to prevent recall but still enable good MEP signals to be recorded.

We have observed similar effects with clonidine as an adjunct to desflurane anesthesia. A temporary decrease in MEPs sometimes occurs if clonidine is administered too rapidly, but signals are improved if the bispectral index (BIS) is maintained at 50 to 60.

Opioids

Alfentanil, fentanyl, sufentanil, and remifentanil minimally depress SSEP and MEP signals.^314,315 Dose-dependent depression of the CMAP does occur at doses of opioids that far exceed those used in clinical anesthesia.^316,317 Comparison of alfentanil, fentanyl, and sufentanil at doses sufficient to suppress noxious stimuli suggested that sufentanil exerted the least effect.³¹⁶ A similar study that included remifentanil showed that this drug had the least depressive effects, with CMAPs measurable at infusion rates of 0.6 µg/kg/min.³¹⁷ It is likely that greater doses can be used if clinically indicated.

Ketamine and Etomidate

Ketamine enhances the cortical SSEP amplitude and has a minimal effect on subcortical and peripheral SSEP responses.³¹⁸ It also produces minimal effects on the myogenic MEP responses as a bolus of 0.5 mg/kg ³¹⁹ or when used in moderate doses (1 to 4 mg/kg/hr; 17 to 83 µg/kg/min) as a supplement to a nitrous oxide–opioid anesthesia.^319,320 Experimental evidence suggests S(+)-ketamine modulates the CMAP by a peripheral mechanism at or distal to the spinal alpha motor neuron.³²¹ Ketamine (4 µg/kg/min) has been successfully used with MEP monitoring during propofol-remifentanil anesthesia for scoliosis correction.^127,322

Etomidate, although capable of inducing general anesthesia, behaves more like ketamine in its effect on evoked potentials. It improves the quality of SSEPs and enhances the amplitude of MEPs.³²³ It produces minimal changes in MEPs compared with barbiturates or propofol.²⁹⁷ Etomidate infusions (10 to 35 µg/kg/min) produce adequate MEP monitoring signals.^319,324 Concerns regarding adrenocortical depression with etomidate infusions remain and limit its widespread use.³²⁵ Bolus doses of etomidate, however, can transiently depress MEPs.³¹⁹

Midazolam

Intravenous midazolam (0.2 mg/kg) decreases the SSEP amplitude by 60%.³²⁶ This does not occur with subcortical SSEPs, for which a slight increase in latency but no change in amplitude has been demonstrated.³²⁷ Although midazolam (0.5 mg/kg) caused marked depression of MEPs in monkeys that persisted during awakening,³²⁸ this finding does not hold true in human studies. MEP amplitude was unaffected by a midazolam-ketamine infusion technique compared with propofol-ketamine or propofol-alfentanil techniques.²⁹² Midazolam did not suppress myogenic MEPs, even at doses sufficient to produce anesthesia.³¹⁷ Effects were similar to those with etomidate.³¹⁷

Neuromuscular Blockade

NMBDs exert little or no effect on the SSEP. They prevent or limit recording of CMAPs during myogenic MEP recording because of their effects on the neuromuscular junction. Partial neuromuscular blockade, however, is commonly used during MEP monitoring because it improves conditions for surgery by providing adequate muscle relaxation when retraction of the tissues is required and limits any patient movement during the stimulus generation. Partial muscle relaxation may also reduce noise caused by spontaneous muscle movement. Constant neuromuscular blockade must be maintained during the procedure. Many centers avoid neuromuscular blockade after intubation, the initial incision, and muscle dissection.

Two methods have been used to assess the degree of neuromuscular blockade for MEP monitoring. One is measurement of the amplitude of the CMAP produced by single supramaximal stimulation (T1) before an NMBD is administered. When T1 is maintained between 20% and 50% of the baseline level, reproducible CMAP responses can be obtained with a degree of muscular blockade that allows surgery.^324,329 The other technique is adjustment of the neuromuscular blockade based on the train-of-four responses. Comparison of the fourth twitch (T4) with that of first twitch (T1) suggests acceptable CMAP monitoring is possible when two of the four twitches remain.^329–331 Neuromuscular blockade should be evaluated in the specific muscle groups that are used for electrophysiologic monitoring because different muscle groups have different sensitivities to the NMBDs. Patients with preoperative neuromuscular dysfunction tend to demonstrate greater effect after partial neuromuscular blockade than those with normal preoperative motor function. It is appropriate to avoid neuromuscular blockade in most of these patients.³²⁴

Ischemia

Ischemia leads to tissue hypoxia and acidosis. The severity and consequences of the associated changes (e.g., increased capillary permeability, coagulation alteration, cell membrane sodium pump activity) depend on the tissue type, duration of ischemia, and collateral circulation. Muscle is more susceptible to ischemic damage than nerves. Histologic changes are more pronounced in muscle beneath the tourniquet compared with muscle distal to the tourniquet.

Reperfusion

Reperfusion removes toxic metabolites and restores energy supplies. There is a sudden release of lactic acid, creatinine phosphokinase (i.e., creatine kinase), potassium (peak increase of 0.32 mEq/L), and CO₂ (peak increase of 0.8 to 18 mm Hg) when the cuff is deflated suddenly. Metabolic changes increase after longer periods of ischemia but return to baseline within 30 minutes. Muscle damage may release myoglobin, which can collect in the collecting tubules of the kidney, precipitating renal failure.

Systemic effects after deflation of the tourniquet include a shift of blood volume back into the limb with a transient decrease in blood pressure that is exacerbated by a postischemic reactive hyperemia in the limb. CO₂ release transiently increases the minute volume. The rapid increase in CO₂ is also associated with a transient (8 to 10 minutes) increase in cerebral blood volume that may affect patients with raised intracranial pressure.³³⁵

Increased microvascular permeability of muscle and nerve tissue occurs with tourniquet release after 2 to 4 hours of ischemia. Interstitial and intracellular edema and capillary occlusion due to endothelial edema and leukocyte aggregation may take months to resolve.

Ischemic Conditioning

Short periods of ischemia followed by reperfusion render muscle more resistant to subsequent ischemia. Ischemic preconditioning improves skeletal muscle force, contractility, and performance and decreases fatigue of skeletal muscle. This preconditioning may enable prolongation of orthopedic and reconstructive procedures.³⁴⁰

Local Complications

Systemic Complications

Clinical Features

Cerebral palsy is an umbrella term that describes a group of nonprogressive, but often changing motor impairment syndromes caused by lesions or anomalies in the brain that occur during the early stages of its development.³⁷⁵ It is the leading cause of motor disability during childhood, with a prevalence of approximately 2 cases per 1000 live births in developed countries.³⁷⁶

Disorders include cognitive impairment, sensory loss (i.e., vision and hearing), seizures, and communication and behavioral disturbances. Systemic disorders resulting from cerebral palsy affect the gastrointestinal, respiratory, urinary tract, and orthopedic systems. Cerebral palsy is divided into three broad categories: spastic (70%), dyskinetic (10%), and ataxic (10%). Children with spastic cerebral palsy commonly present for orthopedic procedures because of contractures at major peripheral joints.^377,378 Functional improvement after surgery in children with spastic diplegia and spastic hemiplegia is better than in those suffering spastic quadriplegia.³⁷⁷

Orthopedic Considerations

Orthopedic manipulations form only part of the treatments designed to improve performance or improve the ease of care. Management, including orthopedic surgery, physical and occupational therapy, recreational therapy, orthotics, and assistive devices, improve functional outcomes. Medical modalities such as intramuscular injections of botulinum toxin and intrathecal administration of baclofen by means of an implanted pump may also be of benefit.³⁷⁹ Selective dorsal rhizotomy has been used to control spasticity.^380,381

The indications for and timing of surgical interventions vary. Gait analysis increases the age of the patient at the first orthopedic surgical procedure, and botulinum toxin type A treatment delays and reduces the frequency of surgical procedures on the lower extremities.^382,383 Bone and soft tissue surgical procedures are designed to lengthen or weaken spastic muscles to give opposing muscles a chance to attain muscle balance.

Anesthetic Considerations

Children with cerebral palsy who present for orthopedic surgery often have previous experience of operating rooms. They should be handled with sensitivity because communication disorders and sensory deficits may mask mildly impaired or normal intellect. They may be accompanied by a parent or caregiver or be premedicated for induction of anesthesia or in the recovery room. If there is a communication problem, the parent or caregiver should be present before and after anesthesia.^377,384

Medical conditions (e.g., seizure control, respiratory function, gastroesophageal reflux) should be optimized preoperatively. Contracture deformities, spinal deformities, decubitus ulceration, and skin infection must be considered when positioning the child for anesthesia and surgery. Poor nutritional status affects postoperative wound healing and the risk of infection. Concurrent medications may influence anesthesia; cisapride for gastroesophageal reflux is associated with prolonged QT interval, sodium valproate can cause platelet dysfunction and affect drug metabolism, and anticonvulsant use increases resistance to NMBDs.³⁸⁵ A history of latex allergy should be sought because of exposure to latex allergens from an early age.³⁸⁶

Intravenous access may be difficult. Drooling, a decreased ability to swallow secretions, and gastroesophageal reflux may dissuade some against an inhalational induction, but there is no evidence that rapid-sequence induction is safer. Succinylcholine may be used because it does not cause hyperkalemia in these children, whose muscles have never become denervated. Noncommunicative or nonverbal children with cerebral palsy require less propofol to obtain the same BIS values (i.e., 35 to 45) than do otherwise healthy children.³⁸⁷ The minimum alveolar concentration of halothane is 20% less in children with cerebral palsy, whether they took anticonvulsant drugs or not (MAC of 0.62 and 0.71, respectively).³⁸⁸

Intraoperative hypothermia is common in children with disordered temperature regulation due to hypothalamic dysfunction, reduced muscle bulk, and fat deposits. Thermal homeostasis should be managed aggressively from the moment the child enters the operating room.

Extensive plaster casting is an important component of bone and soft tissue surgical procedures. These casts may conceal blood loss, and limb swelling within the cast may contribute to compartment syndromes. Plaster jackets and hip spicas have been associated with mesenteric occlusion and acute gastric dilatation.

Pain and spasm are regular features postoperatively. Epidural analgesia is particularly valuable when major orthopedic procedures are performed. Occasionally, two epidurals at different spinal sites may be required for multilevel surgery. Systemic benzodiazepines, baclofen, dantrolene, and clonidine have been used to reduce muscle spasms. Selective dorsal rhizotomy is associated with severe pain, muscle spasms, and dysesthesia. Epidural and intrathecal forms of morphine have been used to control this pain. Intravenous morphine and midazolam also have been successfully used.³⁸⁹ Oral benzodiazepines may be required to reduce the incidence and severity of muscle spasms but should be used with caution if combined with opioid analgesia.

Pain assessment is difficult in these children, but several scoring systems are available.^390,391 (See also Chapter 43.) The opinions of parents and caregivers are extremely valuable in the assessment of pain and discrimination from other factors such as irritability on anesthetic emergence, poor positioning, a full bladder, or nausea.

Clinical Features

Nerve root dysfunction results in muscle paralysis and a neurogenic bowel and bladder. Eighty percent of children develop hydrocephalus because of aqueductal stenosis (Arnold-Chiari [type II] malformation). Skeletal abnormalities such as clubfoot and congenital dislocation of the hip are common. Scoliosis may result from congenital vertebral abnormalities or, more commonly, abnormal neuromuscular control. Epilepsy and learning disorders can also occur, but most children have normal intelligence.

Orthopedic Considerations

Denervation causes muscle imbalance that results in abnormalities at the hip, knee, and foot. The aims of surgery are to reduce flexor posture at the hip and knee and plantigrade feet. Children with clubfeet, hip subluxation, or scoliosis commonly present for orthopedic correction.

Anesthetic Considerations

The potential for infection of the central nervous system dictates closure of the sac within the first few days of life. Subsequent surgical procedures and urinary catheterizations set the stage for a sensitivity to latex.³⁹² Primary prophylaxis (i.e., avoiding all latex materials and using a latex-free operating room) is recommended for prevention of latex allergy and anaphylaxis.³⁹³

Preoperative assessment should include motor and sensory deficits, respiratory and renal function, and functioning of a ventriculoperitoneal shunt. Positioning on the operating table may require additional pillows for support of limbs with contractures. As a result of hypesthesia in the lower extremities, intravenous cannulae can be inserted painlessly. However, venous access is usually poor in the lower extremities because of limited use. The risk of endobronchial intubation is increased because of a short trachea (36% of patients).³⁹⁴ Kyphoscoliosis may distort tracheal anatomy. Renal dysfunction may dictate NMBD choice and avoidance of NSAIDs. Succinylcholine may be used because it does not cause hyperkalemia in these children.³⁹⁵ A reduced hypercapnic ventilation response means that these children should be closely observed in the recovery period.

Clinical Features

OI is a genetically determined disorder of connective tissue that is characterized by bone fragility. The disease state encompasses a phenotypically and genotypically heterogeneous group of inherited disorders that result from mutations in the genes that code for type I collagen.³⁹⁶ The disorder manifests in tissues in which the principal matrix protein is type I collagen—mainly bone, dentin, sclerae, and ligaments. Musculoskeletal manifestations vary in severity along a continuum ranging from perinatal lethal forms with crumpled bones to moderate forms with deformity and propensity for fracture to clinically silent forms with subtle osteopenia and no deformity.³⁹⁶

Classification (types I through IV) is based on the timing of fractures or on multiple clinical, genetic, and radiologic features. Type I is the most common (1 case per 30,000 live births), and type I and type IV have autosomal dominant inheritance patterns. These children have the classic triad of blue sclera, multiple fractures, and conductive hearing loss in adolescence. Bowing of the lower limbs, genu valgum, flat feet, and scoliosis develop with age. Type IV is characterized by osteoporosis, leading to bone fragility without many of the other features of type I. Types II and III are more severe forms of OI and have autosomal recessive inheritance patterns. Molecular genetic studies have identified more than 150 mutations of the COL1A1 and COL1A2 genes, which encode for type I procollagen.³⁹⁶

Orthopedic Considerations

The goals of treatment of OI are to maximize function, minimize deformity and disability, maintain comfort, achieve relative independence in activities of daily living, and enhance social integration. Physiotherapy, rehabilitation, and orthopedic surgery are the mainstays of treatment for moderate and severe forms of OI. Medical treatment with the antiresorptive bisphosphonates (e.g., pamidronate) can decrease pain, lower the fracture incidence, and improve mobility. Initial investigations have demonstrated an acceptable safety profile for pamidronate. Long-term follow-up data are lacking and will be necessary for the development of responsible therapeutic guidelines.³⁹⁷ Medical therapies other than bisphosphonates, such as growth hormone and parathyroid hormone, have only minor roles. Gene-based therapy remains in the early stages of preclinical research.^398,399

Operative intervention is indicated for recurrent fractures or deformity that impairs function.³⁹⁶ Fractures in mild to moderately severe cases of OI type I are treated using the same methods as for patients without OI. Deformed bones that are fracturing are realigned, frequently followed by providing external or internal support. In selecting various modes of treatment, it is important to consider the natural history of the particular type of OI and to set realistic goals.^400–404

Anesthetic Considerations

In common with other children suffering chronic disabilities, children with OI are veterans of the operating room. Chronic pain from frequent fractures complicates handling. Deafness may hinder communication. Preoperative assessment centers on the chest wall deformity because it determines the severity of restrictive lung disease and subsequent cardiovascular compromise. Neck mobility, mouth opening, and dentition should also be assessed.

There is a risk of further fractures with positioning, tourniquet application, airway handing, and use of a blood pressure cuff. Invasive pressure monitoring may be less traumatic than a blood pressure cuff for some patients. If noninvasive blood pressure monitoring is used, less frequent monitoring of the blood pressure is recommended if possible. A laryngeal mask airway may avoid pressure from facemasks. An individual history may help determine the risk/benefit ratio for each child. Succinylcholine has the potential to cause fasciculation-induced fractures.

Abnormal temperature homeostasis may result in intraoperative hyperthermia that may be severe and accompanied by tachycardia and metabolic acidosis. This response is different from that of malignant hyperthermia in that there is an absence of respiratory acidosis and muscle rigidity.⁴⁰⁵ Surface cooling is usually effective in restoring thermal homeostasis.

Clinical Features

DMD is the most common of the progressive muscular dystrophies. It is an X-linked recessive disorder with an incidence of 3 cases per 10,000 births (see Chapter 22). The DMD gene (located at Xp21.2) codes for a large sarcolemmal membrane protein, dystrophin, which is associated muscle cell membrane integrity and signal transduction. Dystrophin is missing or nonfunctional in DMD patients. Both sexes can carry the DMD mutation, but girls rarely exhibit signs of the disease.

Children usually present before school age with a waddling gait and later develop a lumbar lordosis and difficulty climbing stairs. Children use their arms to assist standing up (i.e., Growers sign) due to proximal weakness of the hip girdle. Distal muscles, such as the calves, appear hypertrophied. The disease process is progressive, with increasing muscle weakness occurring with age. These boys often become wheelchair bound by 10 to 11 years of age. Respiratory weakness, often exacerbated by scoliosis and by difficulty swallowing secretions due to pharyngeal involvement, can progress to a terminal pneumonia in late teenage years.⁴⁰⁶ By late adolescence, most children with DMD have cardiac disease, whether it is an abnormal electrocardiogram, arrhythmias, or a cardiomyopathy. Death from cardiorespiratory failure usually occurs before age 30, although respiratory support is extending life expectancy.

DMD is not a static disease. In early childhood, skeletal muscle is constantly catabolized and becomes unstable. The use of membrane-destabilizing medications such as succinylcholine and potent inhalational anesthetics (halothane in particular) in these young children can result in hyperkalemia, rhabdomyolysis, and cardiac arrest. However, after the children reach adolescence, the bulk of the skeletal muscle catabolism has arrested, and membrane-destabilizing medications are left with no substrate. Succinylcholine and potent inhalational anesthetics may be used without sequelae in adolescents with DMD who were undergoing scoliosis surgery and instrumentation. In contrast, the cardiac and smooth muscle consequences of DMD are minor and insignificant in childhood but become life-threatening during adolescence and adulthood. The anesthesiologist must appreciate the developmental nature of DMD and recognize the risks that may be associated with the use of succinylcholine and inhalational anesthetics in this group of children.

DMD can affect cardiac, smooth, and skeletal muscle. Right ventricular function may be compromised by nocturnal oxygen desaturation and sleep apnea contributing to pulmonary hypertension. Sinus tachycardia and arrhythmias may occur at an early age, but clinically apparent cardiomyopathy usually does not develop before 10 years of age. One third of children have some degree of intellectual impairment.

DMD should be suspected in male preschool children with delayed walking ability, and measurement of high serum creatinine phosphokinase concentrations provides a screening tool. Steroids are increasingly used for the management of DMD because they appear to increase muscle mass by decreasing protein breakdown.⁴⁰⁷

Becker muscular dystrophy (BMD) is a milder form of DMD with an onset at puberty or later in adolescence. Clinical expression varies, but even adolescents presenting with mild or subclinical weakness can develop cardiomyopathy with age progression. Death from cardiac or respiratory failure does not usually occur until the fourth or fifth decade. Because of improvements in respiratory care, dilated cardiomyopathy has become the major cause of death.⁴⁰⁸

BMD is an autosomal recessive myopathy that also results from mutations of dystrophin caused by a deletion of the exons 11 to 13 in the DMD gene (located at Xp21.2).⁴⁰⁹ Dystrophin exerts its effect at the voltage-gated chloride channel 1 (CLCN1). Genetic analysis is an essential step in confirming the diagnosis. Additional EMG procedures may be of diagnostic value even when muscle biopsy may reveal no evidence of dystrophy.

Of importance to anesthesiologists, two thirds of patients with mild or subclinical BMD have evidence of right ventricular dilation, and one third have evidence of left ventricular dysfunction.⁴¹⁰ A thorough cardiac evaluation (similar to that for DMD) is recommended before scoliosis surgery.⁴⁰⁸ Hyperthermia and heart failure, mimicking malignant hyperthermia and hyperkalemia with rhabdomyolysis after inhalational agents, have been reported in patients with BMD.^411,412 Despite these reports, the relationship between BMD and malignant hyperthermia remains unclear.

Orthopedic Considerations

Orthopedic surgery is indicated to improve or maintain ambulation and standing. Early treatment of contractures of the hips and the lower limbs prevents severe contractures and delays the progression of scoliosis.⁴¹³ Techniques designed to improve deformities and permit early postoperative mobilization include subcutaneous release of contracted tendons and percutaneous removal of cancellous bone with corrective manipulation of the feet. Maintenance of upright posture extends the ability of patients to attend to the tasks of daily living.⁴¹⁴ Spinal deformities attributable to muscle imbalance or a collapsing spine are corrected to improve or maintain sitting posture. Spinal fusion may also decrease the rate of deterioration of respiratory function, although this has been questioned.⁴¹⁵

Anesthetic Considerations

Respiratory and cardiovascular compromise dominates preoperative assessment. Deformities and contractures of limb joints hinder vascular access, regional anesthetic techniques, and positioning on the operating table. Hypertrophy of the tongue may cause difficulty during intubation. Gastric motility is delayed, and gastric emptying times are prolonged.⁴¹⁶ Tracheobronchial tree compression has been described in a child positioned prone for spinal instrumentation.⁴¹⁷ These children tend to have greater blood loss during surgery. The precise cause remains unclear, but it may be because fat and connective tissue have replaced muscle or because of abnormalities in the blood vessels.²¹⁰

Nondepolarizing NMBDs have a slow onset and prolonged duration of action.^418–420 All NMBDs should be monitored with a peripheral nerve stimulator.⁴²¹ Succinylcholine is contraindicated in these children because of the risk of hyperkalemia, muscle rigidity, rhabdomyolysis, myoglobinuria, arrhythmias, and cardiac arrest. There is no clear link between DMD and malignant hyperthermia.^422,423 The predominant candidate gene (RYR1) for malignant hyperthermia is located on the long arm of chromosome 19, whereas the DMD gene is located on the short arm of the X chromosome.⁴²⁴ Although volatile anesthetic agents continue to be used in young children with DMD, rhabdomyolysis and hyperkalemia have been reported in the recovery room after halothane, isoflurane, desflurane, and sevoflurane anesthesia.^425–429 Potent inhalational anesthetics are best avoided in young children with DMD; instead, alternative anesthetics that do not trigger rhabdomyolysis and hyperkalemia, such as propofol, ketamine, opioids, and benzodiazepines, are preferred.⁴³⁰

Regional techniques such as epidurals may be technically more difficult due to kyphoscoliosis and obesity. The use of ultrasound-guided peripheral nerve blockade can improve the quality and reduce complications of neuronal blockade.⁶ Opioids are not contraindicated in the postoperative period but should be used with caution in children with respiratory compromise. Tramadol is an effective alternative. Noninvasive ventilation support using BiPAP or continuous positive airway pressure is sometimes required after major surgery or in those already receiving this treatment overnight.

Clinical Features

Arthrogryposis multiplex congenita is a spectrum syndrome of multiple, persistent limb contractures often accompanied by associated anomalies, including cleft palate, genitourinary defects, gastroschisis, and cardiac defects.⁴³¹ The incidence is 1 case per 3000 births. Joint contractures are present at birth and are a result of immobility in utero, commonly related to a neurogenic abnormality or myopathy.⁴³² These children have been likened to a “thin, wooden doll,” because muscles connected to affected joints are atrophic and replaced by fibrous tissue and fat.⁴³³ The temporomandibular joint may also be involved, causing restricted jaw opening and micrognathia. Scoliosis commonly develops. Restrictive lung disease, rib cage deformities, and pulmonary hypoplasia predispose to recurrent chest infections.

Orthopedic Considerations

The aim of surgery is to improve function. Most operations involve the soft tissues, tendons, and osteotomies of the lower limbs and hips.⁴³⁴ Upper limb surgery is less common. Extension contracture of the elbow joint makes it impossible to reach the mouth or to perform hygienic necessities. Improvement in passive elbow flexion by capsulotomy or in active flexion by triceps transfer can increase independence and personal hygiene. When both arms are involved, consideration may be given to maintaining one arm in flexion for reaching the head and mouth passively or even actively and one arm in extension for basic hygiene cares.⁴³⁵

Anesthetic Considerations

Arthrogryposis multiplex congenita is commonly associated with other syndromes that may complicate anesthesia.^431,436 Venous cannulation is difficult because veins tend to be small and fragile. The concavity of joints is difficult to access. Care must be taken in positioning the patient on the operating table and protecting skin overlying bony joints to prevent pathologic fractures.

These children should be evaluated for a difficult airway because of temporomandibular joint limitation and micrognathia.^437–439 Fusion or underdevelopment of the first and second cervical vertebrae may further complicate laryngoscopy and tracheal intubation with a severe reduction in neck mobility. Tracheal intubation may become progressively more difficult with age. During infancy, however, evaluating mouth opening may be difficult; it may be necessary to insert a tongue blade into the mouth to determine whether the mandible can be distracted from the maxilla.

Succinylcholine has been used without incident in these children, although teleologically, the use of a depolarizing muscle relaxant in the presence of anterior horn cell disease is contentious. The response to nondepolarizing NMBDs should be monitored.

Hyperthermia and persistent tachycardia have been reported during general anesthesia.^{431,440–442} These signs occur irrespective of the anesthetic agent and are not associated with malignant hyperthermia. In this case, hyperthermia responds to simple cooling techniques.

Pulmonary dysfunction and an increased sensitivity to opioids dictate suitable monitoring postoperatively. Regional techniques may be difficult in the presence of contractures, but if successful, they offer intraoperative and postoperative analgesia.⁴³³ Success can be improved by using ultrasound-guided techniques.