Intraoperative monitoring
Options are:
- Stagnara wake up test
- Clonus test
- Somatosensory evoked potentials
- Motor evoked potentials
Stagnara Wake Up Test
Once the deformity has been corrected, the anaesthesia is lightened to the point where the patient can comply with instructions. The patient is asked to move his or her hands and feet. The patient's ability to comply is evidence of structural continuity of the motor system. The test is much easier to do if rehearsed with the patient prior to surgery. Note that the anaesthetist needs to be informed about 30 minutes before the test is to take place.
Drawbacks of the Stagnara wake up test include the following:
- It is only done once, at the end of surgery, so it may fail to reveal the onset of neurologic dysfunction.
- It does not test sensory functions.
- It can be dangerous as the patient may flop around if too light.
The Clonus Test
At the end of surgery, as the patient is waking up, clonus can be elicited during a brief window period where the anaesthetic is able to remove cortical inhibition.
During this period, if there is no clonus present, this is evidence of spinal shock. Drawbacks of the clonus test include the following:
- It is done at the end of surgery, so it may miss an acute onset of deficit.
- There is a narrow window period where clonus can be obtained.
Thus, the presence of clonus is helpful in ruling out spinal injury, but its absence isn’t very useful in confirming a spinal injury.
Somatosensory Evoked Potentials (SSEPs)
An electrode is used to stimulate the tibial nerve posterior to the medial malleolus; evoked potentials are read off from needle electrodes over the cerebral cortex. The amplitude and latency are compared with baseline values, obtained after the skin incision. A significant change is considered to be a decrease of more than 50% in amplitude and or an increase in latency of more than 10%. With mechanical damage to the cord, changes in SSEPs may occur within 2 minutes; after vascular damage, changes may not occur for 20 minutes.
These SSEPs are very effective at detecting problems with the sensory tracts, but less so at detecting motor problems. In particular, the anterior spinal arteries can be occluded, leading to a complete motor paralysis without affecting the SSEPs. Another problem with SSEPs is their lack of reliability in patients who are not neurologically normal. They are very effective in idiopathic scoliosis but less so in neuromuscular scoliosis.
Motor Evoked Potentials
Motor evoked potentials can be produced by an electrical stimulus from a subdermal needle or a magnetic stimulus. They can be recorded from an EMG electrode or from the motor nerve. These techniques are slightly less reliable than SSEPs, but directly monitor the motor tracts.
