Year : 2007 | Volume
: 51 | Issue : 6 | Page : 486--495
Scoliosis and anaesthetic considerations
Anand H Kulkarni1, M Ambareesha2,
1 MD, DNB, Assistant Professor, Department of Anesthesia, Kasturba Medical College, Mangalore, India
2 MD, DA, Professor and Head, Department of Anesthesia, Kasturba Medical College, Mangalore, India
Anand H Kulkarni
C 3-20, KMC Staff Quarters, Lighthouse Hill Road, Mangalore -575003. Karnataka
Scoliosis may be of varied etiology and tends to cause a restrictive ventilatory defect, along with ventilation-perfusion mismatch and hypoxemia. There is also cardiovascular involvement in the form of raised right heart pressures, mitral valve prolapse or congenital heart disease. Thus a careful pre-anaesthetic evaluation and optimization should be done. Intraoperatively temperature and fluid balance, positioning, spinal cord integrity testing and blood conservation techniques are to be kept in mind. Postoperatively, intensive respiratory therapy and pain management are prime concerns.
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Kulkarni AH, Ambareesha M. Scoliosis and anaesthetic considerations.Indian J Anaesth 2007;51:486-495
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Kulkarni AH, Ambareesha M. Scoliosis and anaesthetic considerations. Indian J Anaesth [serial online] 2007 [cited 2020 Jan 25 ];51:486-495
Available from: http://www.ijaweb.org/text.asp?2007/51/6/486/61185
Scoliosis is a complex deformity of the spine and anaesthesia for scoliosis surgery can be challenging, with several aspects to be kept in mind simultaneously. Abrief review is presented to highlight important aspects of the pre-anaesthesia evaluation and anaesthesia management.
Scoliosis is a complex deformity of the spine resulting in lateral curvature and rotation of the vertebrae as well as a deformity of the rib cage  . There is usually secondary involvement of the respiratory, cardiovascular and neurologic systems.
The initial classification was given by Schulthess W  . He classified scoliosis by the region involved.
Cervico thoracicThoracicThoracolumbarLumbarCombined double primaryThe etiologic classification was introduced by the Terminology Committee of the Scoliosis Research Society in 1969  , that is shown in [Table 1].
Scoliosis can develop at any age, but tends to become clinically evident during periods of rapid somatic growth. It's reported prevalence in the general population varies from 0.3 - 15.3% ,, . However the prevalence is less than 3% for curves more than 10 o and less than 0.3% for curves more than 30 o. It is more common in adolescents and has a female to male ratio of about 3:1  .
75 -90% of cases of scoliosis are of the idiopathic type, out of which the adolescent type is most common. Remaining 10 - 25% cases belong to various other etiologies  .
Measurement of severity
The Cobb's method of measurement, recommended by the Terminology Committee of the Scoliosis Research Society, consists of three steps.
Locating the superior end vertebra.Locating the inferior end vertebra.Drawing intersecting perpendicular lines from the superior surface of the superior end vertebra and from the inferior surface of the inferior end vertebra.The angle of deviation of these perpendicular lines from straight line is the angle of the curve [Figure 1]. If the end plates are obscured pedicles can be used instead for quantification  . The draw back of the Cobb's method is that it measures a complex deformity in only two dimensions. Nevertheless it maintains a uniform method of measurement.
Surgery is performed when the Cobb's angle exceeds 50 o in the thoracic spine and 40 o in the lumbar spine. The goal of surgery is to stop the progression of cardiopulmonary disease. If untreated, idiopathic scoliosis is often fatal in the fourth or fifth decades of life as a result of pulmonary hypertension or respiratory failure  . The severity of scoliosis and clinical implications are as in [Table 2].
Non surgical - braces, traction, plaster applicationsSurgical- posterior approaches, anterior approaches, combined/staged procedures
Pre anaesthetic assessment
I. Airway assessment: Airway difficulties may be anticipated when the scoliosis involves the upper thoracic or cervical spine. Also devices like halo traction may interfere with securing the airway. Some disorders like Duchenne muscular dystrophy may lead to tongue hypertrophy.
II. Respiratory system: Assessment of the pulmonary system must focus on evidence of pre existing lung injury or pulmonary disease, pneumonia and severity of scoliosis  . Factors associated with postoperative mechanical ventilation requirements include pre-existing neuromuscular disease, severe restrictive pulmonary dysfunction with a vital capacity(VC) of 30ml.kg-1 9 Scoliosis results in reduced VC, reduced functional residual capacity (FRC), and restrictive pulmonary disease pattern characterized by increased respiratory rate and decreased tidal volume. The severity of pulmonary impairment is influenced by the scoliosis angle (>70 o ), number of vertebra involved (7 or more), cephalad location of the curvature and degree of loss seen in the thoracic kyphosis. Pulmonary impairment is manifested by a decreased arterial oxygen tensiondue to pulmonary shunting.  There is significant controversy regarding the degree of improvement in pulmonary function after scoliosis surgery. One study found that patients with an anterior component to correction had worse pulmonary function testing variables at 3 months but improved function at 2 years. Patients who had posterior correction only had a trend for improved function at 3 months but no significant difference from the anterior or combined group at 2 years  . Scoliosis surgery is more likely to have immediate pulmonary complications if the curvature is >60 o . Reduced VC is the first manifestation of restrictive lung disease. As the disease progresses gas exchange is affected by ventilation-perfusion mismatch, alveolar hypoventilation, an increased dead space and an increased alveolar- arterial gradient. Prolonged periods of hypoxemia result in pulmonary hypertension, hypercapnia and eventual respiratory failure. Surgery for scoliosis is performed to slow disease progression and prevent complications 10 . Scoliosis may limit the function of the respiratory muscles i.e., intercostals may be overstretched or unable to stretch due to intercostal space changes, putting them at a mechanical disadvantage. Moreover, the effectiveness of the muscles may be hampered by limiting the ability of the thorax to expand. The distortion of the thoracic cage makes the respiratory system much less compliant, thus increasing the work of breathing even when the lungs themselves are healthy  . Scoliosis has generally been associated with the development of restrictive lung defect manifested by a decrease in total lung capacity (TLC) on pulmonary function testing. Infantile and juvenile scoliosis are more likely to be associated with true lung hypoplasia because the thoracic deformity is present during the period of rapid lung growth and development  . In adolescent scoliosis, in contrast, the decrease in TLC is more likely to reflect the impaired chest wall mechanics that prevent normal inflation of the lungs  . Long standing hypoinflation and atelectasis leads to further reduction of lung volume. The decreased TLC is often associated with increased residual volume (RV), resulting in very high RV /TLC ratio reflecting the dysfunction of expiratory muscles, which do not allow full exhalation  . In severe cases of scoliosis, flow-volume loops may show evidence of lower airway obstruction, which may be a result of chronic airway inflammation secondary to poor clearance of secretions  . Significant displacement or rotation of the trachea or main stem bronchi may cause mechanical airway obstruction  . In severe restrictive defects, there is decrease in inspiratory capacity and also ineffective ventilatory patterns which rely on increase in frequency of respiration rather than increase in tidal volume, which increases the work of breathing and promotes respiratory muscle fatigue in response to exercise. They also have a decreased response to carbon dioxide  . When the Cobb's angle is 100 o patients are at an increased risk of developing chronic respiratory failure and pulmonary hypertension. The pulmonary hypertension is a product of chronic atelectasis, chronic hypoxemia and chronic hypercapnia  . Scoliosis of neuromuscular dysfunction etiology usually starts from early infancy, when the chest wall is very compliant, and the distortion of the thorax is severe. Also the lung growth is severely impaired. Moreover the potential for complications is higher because of problems like chronic recurrent aspiration and pneumonias due to impaired secretion clearance  . Preoperatively respiratory function should be assessed by a thorough history, focusing on functional impairment and effort tolerance, physical examination and appropriate investigations. Respiratory function should be optimized by treating any reversible cause of pulmonary dysfunction like infection by physiotherapy and bronchodilator therapy as indicated. Preoperative incentive spirometry is advisable before thoracotomy for anterior approach corrections.
III. Cardiovascular system: The cardiovascular changes associated with scoliosis are less common but more serious than the changes in the respiratory system and share a common etiology. The alteration in the cardiovascular system is related primarily to the changes in the structure of the mediastinum and secondarily to the effects of chronic respiratory insufficiency on the function of the cardiac system. The primary changes are related to structure of the mediastinum following scoliotic curves. The effect is a restrictive pericarditis with a possible secondary pericardial effusion. Limited cardiac filling decreases any potential increases in cardiac output  . In response to exercise, the already elevated pulmonary artery pressure increases. Moreover the displacement or compression of the heart due to thoracic deformity may not allow an increase in stroke volume necessary during exercise 7 . Eventually even normal filling can be impaired and the cardiac output at rest can be impaired. At this point cardiac reserves are limited and may not able to withstand the increased haemodynamic demand of major surgery. Echocardiography and stress testing, either physical or pharmacologic, can be done to determine the performance of the myocardium. In addition to mechanical impairment of myocardium, there can be cardiovascular pathology secondary to the chronic insufficiency of the respiratory system. Pulmonary hypertension is the natural evolution from chronic hypoxemia  . Other factors contributing to pulmonary hypertension are that the number of vascular units per unit volume of lung is lesser than in normal lungs  .Also in the compressed lung regions, the alveoli become smaller than at residual volume, leading to blood flow in extra alveolar vessels which have a higher resistance  . Eventually right ventricular strain and failure will evolve from increased work of right sided cardiac output. Detection of any right ventricular dysfunction should be a stronger indication for complete cardiac evaluation  . Patients with idiopathic scoliosis also have been found to have a high incidence of mitral valve prolapse (up to 25%). It may indicate a common basis for both the entities, namely a collagen disorder  . Moreover the incidence of scoliosis is higher in patients with congenital heart disease than in normal subjects. Hence the patients should be evaluated for the presence of congenital heart disease like ventricular or atrial septal defects, patent ductus arteriosus, tetralogy of Fallot  . Assessment of the cardiovascular system should be done keeping all above in mind. Minimum investigations include an electrocardiogram and echocardiography to assess left ventricular function and pulmonary artery pressures. Dobutamine stress echo may be used to assess cardiac function in those with limited effort tolerance  . The preoperative investigations suggested are as in [Table 3].
IV. Neurologic system: A detailed neurologic evaluation and documentation isimportant because of medicolegal issues. Moreover, patients who have preexisting neurologic deficits are at an increased risk of developingspinal cordinjuryduring scoliosissurgery. Prepoerative considerations for patients undergoing major reconstructive spinal surgery are summarized in [Table 4].
I. Premedication: It is advisable to avoid use of narcotics or heavy sedation as premedication in presence of pulmonary function impairment. Bronchodilators may be used as part of optimization of lung function preoperatively.Antisialogogues may be of value in procedures where a fibre- optic intubation is planned or when prone or lateral position is required to minimize secretions and avoid wetting of the tape securing the endotracheal tube. In those at risk of aspiration H 2 blocking agents or proton pump inhibitors may be administered with or without sodium citrate.
II. Induction: Routine induction by the intravenous route is common. Alternatively an inhalational induction may be used guided by the patient's condition. Use of succinylcholine may be associated with a hyperkalemic response in presence of myopathies or denervation. It may also cause malignant hyperthermia in certain syndromes like King-Denborough, central core disease, adenylate kinase deficiency etc  . Therefore it may be prudent to avoid succinylcholine in these cases and use nondepolarising neuromuscular blocking agents for intubation.
III. Intubation: Anterior approaches to spine may necessitate the use of a double lumen tube for lung isolation to enable access to the anterior spine. This may be difficult in cases where there is involvement of upper thoracic or cervical spine by the scoliosis since distortion of the tracheobronchial tree is a common accompaniment. On the other hand a single lumen tube may be used, allowing more limited intraopertive lung retraction, after discussion with the surgeon. In posterior approaches a single lumen tube is used.
IV. Maintenance: A stable anaesthetic depth is required to enable proper interpretation of somato sensory evoked potentials (SSEPs) or motor evoked potentials (MEPs). Either a nitrous oxide-narcoticinhalation agent technique may be employed or an intravenous technique using propofol may be used. Non-depolarizingneuromuscular blocking agents are used to maintain relaxation. When MEPs are to be recorded it is advisable to use atracurium by continuous infusion and maintain a constant depth of block by neuromuscular monitoring. Intravenous fluids should be warmed and a warming mattress device is preferable.
V. Intraoperative monitoring: Minimum monitoring should include ECG, NIBP, pulse oximetry, capnography, esophageal stethoscope and a temperature probe. Also a urinary catheter should be placed and urine output measured. The prolonged anaesthesia in unusual positions, combined with significant blood loss, haemodynamic effects of thoracic surgery and possible need for deliberate hypotension mandate an invasive arterial line. Also serial blood gas measurements may be done where required. CVP values are not reliable in the prone position or with an open chest  .
VI. Positioning: Patient positioning for surgery varies depending on the level of spine to be operated upon and nature of proposed surgery. Repositioning may be required intraoperatively. Peripheral nerves, eyes, genitals and bony points should be padded and protected. Intraoperative imaging is often required, thus the surgical site should be placed away from the table's central support area. Prone positioning requires an uncompressed abdomen. Anterior approaches to thoracic spine are via a thoracotomy with the patient supported in the lateral position. Anterior approach to the lumbar spine necessitates laparotomy.
VII.Malignant hyperthermia: Malignant hyperthermia is a rare pharmacogenetic myopathy affecting humans  Affected patients are susceptible to acute hyperthermia which may be triggered by potent inhalational anaesthetics or succinyl choline  .There are several published reports of myopathies associated with malignant hyperthermia and several of these syndro 2 mes have skeletal abnormalities including scoliosis. It is critically important to be alert for early evidence of malignant hyperthermia like rise in body temperature, elevated heart rate, ventricular arrhythmias or hypercapnia. The key to successful management of malignant hyperthermia is immediate cessation of triggering agents, 100% oxygen, cooling, supportive respiratory, cardiovascular and acid-base procedures; and drugs like dantrolene which lower free ionized intracellular calcium  .
VIII.Spinal cord monitoring: The cervical and lumbar ganglionic areas of the spinal cord are metabolically more active and the number and size of the cervical and lumbar feeders are greater than those in the thoracic cord and thus the thoracic circulation is described as"water shed". This critical zone extends from T4 to T9 where the vascular supply is least generous and special care should be taken during surgery  . Distraction of the spine, placement of pedicle screws and bony decompression are intraoperative events in which the spinal cord or nerves may suffer injury  . Above and below the auto-regulation range, spinal cord blood flow depends on perfusion pressure. Spinal cord injury due to above reasons leads to loss of auto regulation. In this situation hypotension may further compromise spinal cord blood flow and compound the injury. Spinal cord blood flow is also highly sensitive to PaCO2 alterations during induced hypotension  . The risks of spinal cord damage and methods to minimize the risks are as given in [Table 5]. The incidence of post operative neurologic injury is estimated at 1.84%  .SSEPs, MEPs and the "wake-up" tests are commonly used to help safeguard spinal cord and nerve root function during surgery.
A. Wake-up test: It was first described by Vauzelle, Stagnara et al in 1973  . It is a gross test of spinal motor function. It remains the most reliable assessment of the intact spine for several reasons. An aesthetic agents may suppress SSEP signals, certain patient conditions like neuromuscular degeneration may make SSEPs impossible to obtain; and anterior cord injury may go completely undetected in spite of SSEP monitoring. Awake-up test should be planned for well in advance and discussed with the patient in the pre-anaesthesia visit. Because of neuromonitoring concerns a predominantly nitrous oxide and narcotic technique is typically used. Small doses of volatile anaesthetics, if used, should be discontinued an hour before wake-up is anticipated. Two or three twitches on a train-of-four are sufficient to allow the patient to move his or her toes. After discontinuation of nitrous oxide and ventilation with 100% oxygen, the patient should be able to follow commands to move their toes within ten minutes. It is not advisable to reverse neuro muscular blockade or narcotics to speed a wake-up test because this may result in violent movements that can damage instrumentation or hurt the patient. Also the sympathetic discharge accompanying narcotic reversal may further compromise spinal cord blood flow.As soon as satisfactory movement is observed, anaesthesia is reestablished. A successful wake-up test suggests an intact cortex and spinal cord.
B. SSEP: They are a type of sensory evoked response. It provides the ability to monitor functional integrity of sensory pathways in the anaesthetized patient undergoing surgical procedures which place the spinal cord at risk. It is recorded after electrical stimulation of a peripheral mixed nerve. Stimulation is by surface electrodes placed on the skin above the nerve. A square wave stimulus of 50-250 microsec duration, strength 20-50 mA, stimulation rate 16 Hz is commonly used. Sites of stimulation are common peroneal nerve at knee or posterior tibial nerve at ankle. For best results an anaesthetic technique that does not markedly depress the SSEP should be chosen and the physiologic status of the patient should remain constant during periods of potential surgical injury.  The blood supply to the motor tracts is derived from the anterior spinal artery. It is therefore possible for significant motor deficit to develop post-operatively in patients with intact SSEPs throughout surgery  . All anaesthetic drugs affect SSEPs. Generally they tend to increase latency and decrease amplitude. Exceptions are nitrous oxide, ketamine and midazolam which do not affect latency. Etomidate has been reported to increase amplitude. The use of inhaled agents upto 1 MAC may not significantly affect SSEP monitoring. Bolus doses of opioids or sedatives or sudden increase in concentration of anaesthetic agents alter SSEPs. Therefore the best anaesthetic technique is one that provides smooth and continuous anaesthetic effect avoiding bolus dosing  . Physiologic factors influencing SSEPs include blood pressure, temperature and blood gas tensions. When mean arterial pressure falls to below the lower limit of auto- regulation there is progressive decrease of 33 amplitude with no change in latency 3 . Hypothermia causes increase in latency and decrease in amplitude  .Hyperthermia decreases amplitude and causes loss of wave at 42 0 C  . Hypoxia decreases amplitude  . An amplitude decrease of 50% or latency increase of 10% may suggest a correctable problem. It is to be confirmed that capnography; pulse oximetry and temperature readings are all constant for the patient. The blood pressure is to be raised in attempt to improve spinal cord perfusion. If hemodilution had been performed it should be reversed. Awake-up test or anatomic manipulation may then be performed based on the surgeon's discretion  .
C. MEP: The limitations of the wake- up test led investigators to explore the possibility of monitoring MEPs  . Compared to SSEPs, MEPs are markedly depressed by almost all anaesthetic agents  .The marked influence of anaesthetic drugs on MEPs demands a rigid anaesthetic protocol. During the MEP recording anaesthesia is maintained by minimum dose of ketamine or etomidate infusion.An alternative is to use a titrable infusion of droperidol-fentanyl  .
IX. Blood conservation: In extensive spine surgeries blood losses are typically 10 to 30 ml.kg-1 . It is desirable to keep allogenic blood transfusion to a minimum considering the risks of allogenic transfusion i.e., hypothermia, impaired coagulation, hyperkalemia, hypocalcaemia, transfusion reactions, acute lung injury, transmitted infections etc. This is accomplished by techniques to reduce blood loss and by autologous blood transfusion.
A. Reducing blood loss
When patients are placed prone intraabdominal pressure should be minimized. This leads to a reduced epidural venous pressure and thus the venous surgical bleed. Hypotensive anaesthesia is considered a reasonably safe and effective method for reducing blood loss by up to 58% during spine surgery , Mean arterial pressure is typically maintained at 60-65mm of Hg. Hypotensive anaesthesia can be achieved by the use of inhalational agents  , sodiumnitroprusside  , ganglion blocking drugs e.g.trimethaphan  , calcium channel blockers e.g. nicardipine  , beta blockers e.g. propranolol, esmolol, labetalol  , nitroglycerin  , fenoldopametc  . Antifibrinolytic agents e.g. aprotinin inhibits plasmin and kallikrein and preserves platelet function . Urban et al found significantly reduced blood loss in major spine surgeries where aprotinin infusion was used intraoperatively  .
B. Autologous blood transfusion
Autologous blood can be made available to the patient by 3 methods.
Preoperative autologous blood donation (PABD): The patient donates blood 3 -5 weeks before surgery for use intraoperatively. Recombinant erythropoietin has been used before major surgery to rise hemoglobin levels, to reduce allogenic blood requirements and facilitate PABD and acute normovolemic hemodilution (ANH).Acute normovolemic hemodilution (ANH): This is performed immediately before surgery. The removed blood is replaced by the infusion of colloids or crystalloids to achieve normovolemia with reduced hematocrit. During surgery blood of a lower hematocrit is lost. The donated blood may be retransfused once hemostasis is achieved.Intraoperative cell salvage: Blood lost during surgery is collected using commercially available equipment and is then anticoagulated, filtered for clots and debris, centrifuged, resuspended in saline and reinfused to the patient. Clotting factors need to be replaced using fresh frozen plasma. The technique is unsuitable in the presence of malignancy or infection.
X. Post operative care: The patients undergoing scoliosis surgery frequently have preexisting morbidity, and surgery imposes several further stresses like significant blood loss and fluid shifts, prolonged anaesthesia, hypothermia etc. After scoliosis correction preferably all patients should be cared for in an intensive care setting. This is particularly important in those with pre existing myelopathy, pulmonary dysfunction, cardio vascular disease, extensive spine surgery, airway edema or those who have had massive transfusion  . Oxygen by mask is given for the first few hours after extubation and may be required for longer periods in those with pre existing pulmonary dysfunction. Pulmonary complications (ARDS, pneumonia, atelectasis, pulmonary embolism) are the most common post operative complications, and vigilant monitoring, incentive spirometry and aggressive pulmonary toilet are essential for reducing morbidity particularly in those with pre existing pulmonary disease. Certain other complications which could occur after scoliosis surgery are neurologic injury, ileus, pneumothorax, dural tears, urinary complications and syndrome of inappropriate ADH secretion ,.
XI. Post operative analgesia: Pain management can be challenging and pain is of a severe degree in more extensive procedures.Amultimodal approach to analgesia is recommended using a combination of primary analgesics, opioids and regional techniques where appropriate. Intravenous opioids by infusion or patient controlled analgesia devices is the mainstay of analgesia. The side effects like respiratory depression, nausea-vomiting, sedation and ileus tend to limit their use. Nonsteroidal anti-inflammatory drugs may be used as adjuncts, but the side effects are increased bleeding, gastritis and renal dysfunction. Local anaesthetic agents or opioids or both have been used by the epidural route, the epidural catheter being placed intraoperatively by the surgeon  . However epidural anaesthesia with local anaesthetic agents makes neurologic assessment difficult. Also concerns over risk of epidural hematoma and infection have hindered its widespread use. Intrathecal opioids can be injected with technical ease before wound closure. Studies suggest the optimum dose of morphine to be 2-5 mcg.kg-1 which provides analgesia for 24 hours with few side effects. The use of opioids would not interfere with neurologic assessment. However the effects of a single intrathecal opioid dose would have a limited duration of effect. Other techniques like intrapleural infusions of local anaesthetics or opioids or both have been used  .The use of low dose intravenous ketamine has demonstrated efficacy with an initial dose of 0.25 mg.kg-1, followed by an infusion of 22.5 mcg.kg-1.min-1 improves pain scores, decreases nausea, reduces narcotic requirements and is not associated with hallucinations.
Scoliosis, which may be of varied etiology, leads to respiratory involvement characterized by restrictive lung disease, ventilation-perfusion maldistribution and hypoxemia. Cardiovascular involvement is usually in the form of raised right heart pressures, mitral valve prolapse or congenital heart disease. Anaesthesia is often needed for corrective orthopaedic surgery, which is very challenging.Adetailed pre-anaesthetic assessment and optimization of the respiratory and cardiovascular systems is imperative. Important intraoperative considerations are monitoring, temperature and fluid balance maintenance, positioning, spinal cord integrity monitoring and blood conservation. Post operative intensive care, respiratory care and pain therapy deserve special mention.
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