Neurosurgery can be unpredictable and potentially dangerous because of the risk of damage to the nervous system. It is therefore important for surgeons to have a technology to monitor nerve tissue during operation. This is called intra operative neurophysiologic monitoring. This monitoring plays a major role in difficult and complex procedures like skull base surgery, curvature correction of spine and cerebrovascular surgery.

WHAT IS INTRAOPERTIVE MONITORING (IONM)?

IONM uses equipment to evaluate the function of the neural tissue during surgery. Its role is to provide the surgeon with immediate feed back and warning before permanent neural injury has occurred. It increases the safety and improves the outcome. INOM has been in vogue since mid 1960s. Most neurosurgical procedures have the risk of permanent neurological injury and in worst cases devastation. In an attempt to reduce such morbidity numerous methods of intra operative monitoring have been developed to guide the neurosurgeon to change operative activity in a way that will prevent or minimize neurological damage.

An ideal intra operative monitoring tool should satisfy several technical criteria

1. Ability to detect neurological damage at an early and reversible stage
2. Any modifications of the operative technique to allow for monitoring must not interfere with the surgeon’s ability to achieve the operative goal
3. Components of the monitoring system should be easy to use and provide consistent and reliable data
4. Information obtained should be resistant to variables of the operative environment
5. The neurological function or region being monitored must be that which is at risk by the operative procedure

TYPES OF MONITORING

• ELECTRO ENCEPHALO GRAPHY (EEG):

  EEG monitoring is being done from 1965. This is a popular mode since it is readily available and is familiar. Other features are simplicity of the pre op set up and well characterized response to various states of neurological function. This modality is commonly used in cerebrovascular surgery like carotid endarterectomy.

• ELECTRO CORTICO GRAPHY (ECoG):

  Here the recordings are obtained from the brain surface unlike EEG, where recordings are obtained from scalp. ECoG is most widely used in epilepsy surgery to mark the boundaries of tissue resection. Chances of seizure free outcome are better if there is no evidence of seizure activity following resection.

• SOMATO SENSORY EVOKED POTENTIALS (SSEP):

  The most widely used technique used in INOM. This measures the conduction of sensation above and below the area of surgery. Electrodes are placed on the limbs that could be affected by surgery and also on the surface of the scalp over the area of brain where the impulses from the limb are received. The activity is recorded as waves. When the limb is stimulated with an electric current there should be a response in the brain.

• ELECTRO MYOGRAPHY (EMG):

  Here the motor portion is monitored. Needles are placed in the muscle groups that correspond to the area where the surgeon will be working. Electrical activity from the muscle can be monitored by a machine and recorded as waves.  Significant change in the waves alerts the surgeon that the neural tissue in the area could be damaged. Surgeon can take action preventing that damage.