1. Anaesthesia in Scoliosis Surgery
Dr WMHW Hisham, Dr MR Mamat et al

2. Layout Introduction Definition Classification Epidemiology
Measurement of severity
Pre Anaesthetic Assesment
Anaesthetic Technicque

3. Introduction
Scoliosis is a complex deformity of the spine and anaesthesia for scoliosis surgery can be challenging.
Unfamiliarity in the nature of the surgery can spell trouble

4. Definition
Scoliosis is a complex deformity of the spine resulting in lateral curtvature and rotation of the vertebrae as well as a deformity of the rib cage.
There is usually a secondary involvement of the respiratory, cardiovascular and neurologic systems.

5. Classifications

6. Epidemiology
Scoliosis can developed at any age, but tends to become clinicallly evident during periods of somatic growth.
Reported in general populationvaries from %.
<3% for curves > 10o
<0.3% for curves > 30o
More common in adolescent and has female to male ratio of 3:1

7. 75-90% of cases of scoliosis are of idiopathic type where adolescent is the most common
Remaining 10-25% of cases belongs to various etiologies.

8. Measurement of Severity
Cobb’s Method
The angle between intersecting lines drawn perpendicular to the top most affected vertebrae and the bottom of the lowermost affected vertebrae
Patient with AIS who have Cobb angles in the thoracic spine measuring less than 30o at skeletal maturity will have minimal disease progression in adulthood.

10. Cobb angles measuring more than 30o in the thoracic spine are likely to progress an average of 19o during a life span.
Thoracic curvatures measuring more than 50o progress approximately 1o to 2o per year.
Surgery is performed when the Cobb's angle exceeds 50 o in the thoracic spine and 40 o in the lumbar spine.

12. 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.

13. Pre Anaesthetic Assesment

14. Airway Assesment
Airway difficulties may be anticipated when the scoliosis involves the upper thoracic or cervical spine.
devices like halo traction may interfere with securing the airway.
Some disorders like Duchenne muscular dystrophy may lead to tongue hypertrophy.

16. 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 post operative mechanical ventilation requirements include preexisting neuromuscular disease, severe restrictive pulmonary dysfunction with a vital capacity(VC) of <35% predicted, congenital heart defects, right ventricular failure,obesity, anterior thoracic spine surgery and blood loss of >30ml.kg-1

17. 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.

19. 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.

20. Prolonged periods of hypoxemia result in pulmonary hypertension, hypercapnia and eventual respiratory failure.
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.

21. 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.

22. 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.

23. 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.
Significant displacement or rotation of the trachea or main stem bronchi may cause mechanical airway obstruction.

24. 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.

25. 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.
Preoperatively respiratory function should be assessed by a thorough history, focusing on functional impairment and effort tolerance, physical examination and appropriate investigations.

26. 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.

27. 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.

28. The effect is a restrictive pericarditis with a possible secondary pericardial effusion.
Limited cardiac filling decreases any potential increases in cardiac output.
the displacement or compression of the heart due to thoracic deformity may not allow an increase in stroke volume necessary during exercise.
At this point cardiac reserves are limited and may not able to withstand the increased haemodynamic demand of major surgery.

29. 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.

30. 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.

31. Patients with idiopathic scoliosis also have been found to have a high incidence of mitral valve prolapse (up to 25%).
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 .

32. Neurologic system
A detailed neurologic evaluation and documentation is important because of medicolegal issues.
patients who have preexisting neurologic deficits are at an increased risk of developing spinal cord injury during scoliosis surgery.

33. Anaesthesia Technique

34. 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.

35. 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.
Therefore it may be prudent to avoid succinylcholine in these cases and use nondepolarising neuromuscular blocking agents for intubation.

36. 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 intraoperative lung retraction, after discussion with the surgeon.
In posterior approaches a single lumen tube is used.

38. 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-depolarizing neuromuscular 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.

39. 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.

40. Also serial blood gas measurements may be done where required.
CVP values are not reliable in the prone position or with an open chest

41. 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.

42. 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.

43. Spinal cord Monitoring
Wake up test
SSEP
MEP

44. 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 autoregulation range, spinal cord blood flow depends on perfusion pressure.

45. 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

47. The incidence of post operative neurologic injury is estimated at 1
Somatosensory Evoked Potential (SSEP), Motor Evoked Potential (MEP) and the "wake-up" tests are commonly used to help safeguard spinal cord and nerve root function during surgery.

48. 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.
Anaesthetic 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.
A wakeup 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.

49. 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.

50. 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.

51. 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 microsec duration, strength mA, stimulation rate 1­6 Hz is commonly used.

53. 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.

54. Etomidate has been reported to increase amplitude.
The use of inhaled agents up to 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.

56. Physiologic factors influencing SSEPs include blood pressure, temperature and blood gas tensions.
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.

57. 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.

58. Reducing Blood Loss Autologous Blood Transfusion
Blood Conservation
Reducing Blood Loss
Autologous Blood Transfusion

59. In extensive spine surgeries blood losses are typically 10 to 30 ml
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.

60. This is accomplished by techniques to reduce blood loss and by autologous blood transfusion.

61. 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.

62. Hypotensive Anaesthesia
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 60-65mmHg.
Hypotensive anaesthesia can be achieved by the use of inhalational agents , sodiumnitroprusside, trimethaphan, nicardipine , propranolol, esmolol, labetalol, nitroglycerin.

63. Antifibrinolytic agents e. g
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.

64. Autologous Blood Transfusion
Autologous blood can be made available to the patient by 3 methods
Preoperative autologous blood donation (PABD)
Acute normovolemic hemodilution (ANH)
Intraoperative cell salvage

65. 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)

66. 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.

67. 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.

68. Post operative care

69. 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.

70. 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.

71. 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

72. Post operative analgesia
Pain management can be challenging and pain is of a severe degree in more extensive procedures.
A multimodal 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.

73. The side effects like respiratory depression, nausea vomiting, sedation and ileus tend to limit their use.
Nonsteroidal antiinflammatory drugs may be used as adjuncts, but the side effects are increased bleeding, gastritis and renal dysfunction.