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Neurosign Spinal

Neurosign Spinal Nerve Monitoring System

A major challenge in spinal surgery is to maintain the function of the spinal cord. Traditionally, this has involved the use of Somatosensory Evoked Potentials (SEPs), where a peripheral nerve, often the tibial, is stimulated, and responses are collected from the scalp or epidural space. Because the signals are extremely small, this usually involves many hundreds or even thousands of stimuli which need to be averaged.  Clearly, this technique involves the use of expensive equipment, and the talents of a neurophysiologist to interpret the signals.

One drawback of this method is that it utilises sensory rather than motor potentials. Sensory information from the tibial nerve may enter the cord through several different nerve root levels, and so it is possible that an associated nerve root may be compromised, but that no change is seen in the SEP. Motor potentials may also exit the spinal cord via more than 1 nerve root, but each nerve root from L2 to S5 can be monitored using specific muscles so that any change in motor spinal cord function is detected, in procedures below cervical levels.

It is now possible to stimulate the motor cortex electrically and evoke a motor potential (MEP) down the spinal cord, and to measure this from the leg muscles. Because these signals are large, there is no need to average them, and so the answer following a stimulation is immediate. This method provides a PASS/FAIL test in a range of specific applications and does not require technical assistance.

It should be stressed that the ideal situation is that both motor and sensory stimulation is performed; but where no such monitoring is carried out, then motor evoked potentials are simple to obtain, reliable in nature, and an excellent tool in preventing spinal cord injury.

Method:

After the patient is anaesthetised, 2 corkscrew electrodes are inserted sub-dermally into the scalp over the motor cortex. The electrodes are inserted at C1 and C2, or 2cm in front of the central vertex and about 6cm apart. These electrodes are connected to a cortical electrical stimulator which forms part of the Neurosign Spinal package.

EMG recordings are detected using the Neurosign 800 motor nerve monitor. This is an 8 channel machine, although not all surgeries require the use of all channels. Needle electrodes are inserted into particular leg muscles which monitor the spinal nerve roots from L2 to S1, with variations to allow levels S2 to S5 to be included. The choice of nerve root to be monitored will depend on the exact nature of the procedure. This montage will also detect interference with the motor tracts higher in the spinal cord.

When the surgeon is not operating, a stimulus is applied either by pressing a control button or by operating a foot switch, the latter making it possible for the stimulation to be under the surgeon’s control. The response is immediately displayed on the Neurosign 800. Experience has shown that in the majority of cases the signal is either present or absent, so it is important that the stimulations are carried out at appropriate times. During scoliosis operations, this is likely to be immediately prior to distraction, and during and after distraction.

It is possible that spinal cord compromise may lead to a signal being present but a with a diminution of amplitude. Generally, a loss of 50% of baseline is considered to be significant, and the surgeon should visually check the amplitude of each stimulation so that appropriate corrective action can be taken. It is therefore important that noticeable changes in amplitude should be considered by the surgeon. However, for the intended applications listed below, the All or Nothing response is more probable.

Anaesthesia:

It is important that no neuromuscular blocking agents are used, as these will impair EMG collection. It is also important that no inhalational gases are used for anaesthesia other than during intubation, as these operate on the mid-brain and either depress or block the signals from the motor cortex. Only intravenous anaesthesia should be used.

A bite guard must be fitted to ensure the patient's safety during stimulation.

Intended applications:

Scoliosis
Anterior spinal fixation
Spinal tumour removal, from the cauda equina to C5
Major instrumentation procedures

Contraindications:

This technique is not suitable for trauma cases unless appropriate setup time can be given, or for procedures where the cervical area of the neck is unstable, as the patient is likely to twitch during the stimulation.

Other contraindications are previous brain surgery or surgical implants such as pacemakers, vascular clips or shunts. A history of epileptic seizure may be a contraindication, although it is not an absolute bar to cortical stimulation. Other considerations include previous craniotomy or convexity skull fracture, history of cortical lesion, or current course of pro-convulsive medication.

The patient must have a bite guard in place to prevent injury to the tongue. This is because the jaw may close due to the stimulation.

For further information, queries or feedback, please contact neurosign.info@magstim.com