4.8 Monitoring in Scoliosis Surgery

Question 1

What is scoliosis

Answer 1

Scoliosis is a spinal deformity associated with lateral curvature of the spine,
rotation of vertebral body, and thoracic cage deformity. The main concerns in
anaesthetising this particular case are as follows:

Question 2

What are the challenges presented to the
anaesthetist by this patient
undergoing surgical correction of scoliosis?

Answer 2

1. Patient factors
   * Paediatric age group

• Coexisting neuromuscular disorders—
  muscular dystrophy,
  cerebral palsy,
  and increased incidence of malignant hyperthermia
• Associated comorbidities—
  respiratory and cardiac compromise
• Difficult airway—
  depending on the level of scoliosis

2.Surgical factors
* Difficult positioning—
prone or lateral

• Potential for excessive blood loss
• Risks of prolonged surgery—
  hypothermia and thromboembolic risks
• Need for intraoperative neurophysiological monitoring (IONM)
• Need for insertion of double lumen tube in certain approaches

Question 3

What are the two main system involvements in patients with scoliosis?

Answer 3

• Respiratory system
• Cardiovascular system

Question 4

• Respiratory system

Answer 4

1. Thoracic curvature decreases the mechanical efficiency

of the chest wall causing a restrictive pulmonary picture

• with decreased lung volumes and compliance,
• but preservation of FEV1/FVC ratio.

2. In severe cases, restricted ventilation
   may lead to alveolar hypoventilation,
   arteriovenous shunting,
   V/Q mismatch.
3. A thorough assessment of functional impairment
   and optimisation of any reversible cause of pulmonary dysfunction

such as chest infection with
antibiotics, bronchodilators, and physiotherapy.

Question 5

• Cardiovascular system

Answer 5

Patients with high-degree spinal curvature are at risk of developing
cor-pulmonale.

Hypoxic pulmonary vasoconstriction develops
in the face of arterial hypoxaemia,

resulting pulmonary hypertension leads to right heart failure.

Question 6

What investigations would you consider necessary in this patient?

Answer 6

1. • Full blood count,
     urea and electrolytes,
     clotting screen, cross-matching of
     blood and blood products
2. • Plain CXR for respiratory and cardiac assessment
3. • 12-lead ECG to assess cardiac function
4. • Echocardiogram in patients with long-standing and severe scoliosis
5. • Lung function tests:
     Preoperative vital capacity of < 35% is associated
     with increased postoperative respiratory morbidity
     and is considered a relative contraindication for surgery
6. • In cases where difficult airway is suspected,
     flexion and extension radiographs and CT/MRI of
     cervical spines are recommended

Question 7

Describe the blood supply of the spinal cord. Which part of the spinal cord is most at risk of ischaemia?

Answer 7

1. The anterior spinal artery

supplies the anterior two-thirds of the spinal cord,

2. The posterior spinal arteries supply posterior third.
3. In addition, radicular branches from local arteries
   feed into the spinal arteries,

• including the artery of Adamkiewicz at the
  lower thoracic/upper lumbar level.

4.This segmental blood supply results in the formation of watershed
areas.

5. The areas of the spinal cord,
   which are the most at risk of ischaemia, are
   T3–5 and T12–L1.

Question 8

How can you monitor the neurological function during scoliosis surgery?

Answer 8

Given the risk of spinal cord ischaemia during surgery,

methods of detection of spinal cord compromise
are essential for preservation of function.

IoNM of evoked potentials provides information
about the functional integrity of neural
pathways in anaesthetised patients.

The most commonly used techniques include:

1. transcranial Motor evoked Potentials (tc-MePs):
2. somato sensory evoked Potentials (ssePs):
3. spontaneous and triggered electromyographic (eMG) responses:
4. Stagnara wake-up test:

Question 9

1. MEPs

Answer 9

1. Significant changes in muscle MEP
   during scoliosis surgery bears a strong
   correlation to cord injury.
2. This involves monitoring of the

descending anterior and lateral corticospinal tracts by
transcranial electrical
stimulation of excitable regions in the

cortex producing segmental
muscle contraction.

Question 10

2. SSEPs

Answer 10

1. Electrical impulses are delivered

to a peripheral nerve via surface electrodes,

2. which reach the primary sensory cortex
   through the dorsal column;

this electrical activity is recorded via scalp electrodes.

3. Changes in the SSEP waveforms reflect loss of
   integrity of the dorsal column sensory pathways.

Question 11

3. Spontaneous and triggered electromyographic EMG

Answer 11

Detects peripheral nerve injury quickly and easily.

Question 12

Stagnara wake up test

Answer 12

Despite limitations,

this method still remains ‘gold standard’ for assessment of motor function.

The test involves lessening the level of anaesthesia

until the patient is able to follow commands,

allowing for a gross assessment of motor function.

Question 13

How do anaesthetic drugs affect these techniques?

Answer 13

1. • iV induction agents:

The use of bolus doses of i.v. induction agents
reduces the amplitude of evoked potential responses,

and in particular, cortical responses,
but these effects do not prevent useful intraoperative
recording of SSEPs and MEPs.

2. inhalational agents:
   There is a dose-dependent reduction in SSEP and MEP amplitude.
3. Muscle relaxants do not affect SSEPs,
   but MEPs are affected with moderate doses.

When myogenic motor evoked responses
are to be recorded, stable level of muscle relaxation as reflected by one
or two twitches on train of four (ToF) should be maintained.

4.* Opioids: Small effect on waveform amplitude and latency

5. • Hypothermia: Decreases nerve conduction and decreases the
     amplitude of SSEP waveform.
6. • Hypotension: SSEPs are lost and ischaemic injury can occur when
     MAP < 60 mm Hg.

Question 14

What problems exist with patient positioning for this procedure?

Answer 14

Different approaches call for different modes of positioning:

knee-to-chest, prone, or lateral.

Ensure optimal position to aid free excursion of chest wall
to promote adequate ventilation;

in the presence of restrictive pattern,

this could otherwise be detrimental to the respiratory function.

Also prevent increased intra-abdominal pressure
to avoid engorgement of epidural venous
plexus and increased surgical site bleeding.

Question 15

What can you do to decrease blood loss during surgery?

Answer 15

Typical blood loss may exceed 50% of patient’s blood volume

and is related to the duration and extent of surgery,

anaesthetic factors such as
induced hypotension,
optimal positioning,
use of antifibrinolytic agents.

Question 16

What determines the need for postoperative ventilation?

Answer 16

Patient factors
1. * Presence of pre-existing neuromuscular disorder

2. • Severe restrictive pulmonary disease (< 35% vital capacity)
3. • Associated cardiac involvement and right heart failure
4. • Obesity

Surgical factors
1. * Duration and extent of surgery

2. • Invasion of thoracic cavity
3. • Blood loss > 30 mL/kg
4. • Presence of complications such as pneumothorax and haemothorax

Question 17

What are the analgesic options?

Answer 17

1. The surgery involves a large incision over several dermatomes,
   and significant postoperative pain can be expected.
2. A multimodal analgesic technique
   involving the combination of
   simple analgesics,
   opioids, and
   regional blocks is chosen.
3. Nonsteroidal antiinflammatory
   drugs are generally avoided for the
   fear of increased bleeding
   and renal failure because of the
   high incidence of intraoperative
   hypotension and hypovolemia.
4. Various regional techniques:
   spinal and epidural catheters
   inserted intraoperatively by the surgeon;

paravertebral and intrapleural infusion of
local anaesthetics are used variedly in the UK.

Question 18

Key points

Answer 18

1. • Scoliosis surgery poses significant challenges
2. • Preoperative lung function determines postoperative respiratory morbidity
3. • IONM has shown to effectively predict
     the adverse outcomes of nerve injury.
4. • Anaesthetic technique is tailored to suit the
     use of IONM and to prevent blood loss