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Restrictions remain in place across the NHS in relation to the COVID-19 pandemic. The safety measures in place over the 18 months therefore remain in place at The Walton Centre - and in our other clinic settings within the community – until further notice.

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Neurophysiology Department brain mapping and monitoring during an awake craniotomy Leaflet

Your surgeon would like you to have neurophysiology mapping during your surgery. This includes motor mapping while you are asleep and language mapping when you are woken up in theatre. Motor mapping uses motor evoked potentials (MEPs) to map out the regions of your brain that control your muscles and language mapping involves stimulating your brain to identify the areas responsible for speaking.

What are Motor Evoked Potentials (MEPs)?

MEPs are responses recorded from your muscles when your brain is stimulated. Once you are asleep and ready for surgery needle electrodes are placed in different muscles in your face, hand, leg and foot. During brain surgery stimulating electrodes are placed directly onto your brain and a needle electrode is placed in your scalp (forehead).

What is MEP mapping?

During brain surgery the surgeon will stimulate your brain in different locations when they stimulate the region of the brain that controls your hand a response will be recorded from the electrodes placed in your hand muscles. By recording from different muscles the surgeon can identify the parts of your brain that control these muscles.

What is MEP monitoring?

Repeated MEPs allows the surgeon to monitor brain motor function during brain surgery. Changes in MEPs are used to predict the risk of weakness after surgery and if it is likely to be permanent or temporary. Feedback to the surgeon help protect your brain from permanent motor injury, weakness or paralysis. During certain types of brain surgery MEPs can be used to monitor for stroke.

After surgery

You may have small sore areas where the needle electrodes have been placed in your muscles and scalp. All electrodes are removed before you wake up.

Are there any risks to MEPs?

Neurophysiological mapping and monitoring is considered safe; the most serious safety concern is seizure this is considered rare using the stimulus techniques required for brain MEP mapping and monitoring. No reports of seizures resulting from MEP monitoring in anesthetized patients have been published to date. There are unpublished observations by Deletis and MacDonald of rare seizure occurrences. The placement of needle electrodes in a sterile manor for stimulation and recording have never been associated with inflammation or severe bleeding or more serious complications.

Are there any alternative MEP mapping techniques?

The alternative technique to MEP mapping is to perform the surgery with you awake from the start of the operation and you are continually asked to move your arms and legs by the surgeon. Functional MRI mapping and transcranial magnetic stimulation mapping can be undertaken prior to surgery and used to guide surgery, this is not a live image of your brain and is not suitable for monitoring.

What if I don’t have MEP mapping or monitoring?

During brain surgery the Surgeon won’t be able to map or monitor your brain for motor function increasing the risk of permanent motor weakness, dysfunction or paralysis and won’t be able to monitor for stroke.

What is language mapping?

During your surgery you are woken up by the anaesthetist and will be asked to count, say the alphabet, read and name objects by a Neuropsychologist. During this time the surgeon will stimulate you brain using an electrical probe. You won’t feel the stimulation as the brain doesn't feel pain, when the area of your brain responsible for language is stimulated you may be unable to speak, you may briefly pause or misname objects. This will be repeated several times. During this time your brain waves (EEG) will also be monitored using a special electrode to check for seizure activity. Once mapping is complete the Surgeon will decide to either keep you awake or put you back to sleep.

Are there any risks to language mapping?

In addition to the general risks associated with brain surgery which the surgeon will discuss with you the greatest risk associated with language mapping are seizures due to electrical stimulation of the brain. Seizures tend to be short and focal resulting in twitching of a limb or face, generalised tonic-clonic seizures (Grand mal) are less frequent. The incidence of seizure during an awake craniotomy is variable, your brain waves (EEG) are monitored during stimulation to reduce the likelihood of a seizure occurring. If a seizure does occur they are often self terminating and there are strategies that the surgical team can use to stop a seizure. In the majority of cases language mapping and surgery will continue following a seizure.

After surgery

All electrodes are removed before the end of your surgery.

Are there any alternative language mapping techniques?

Functional MRI mapping or transcranial magnetic stimulation can be undertaken prior to surgery and used to guide surgery, this is not a live image of your brain.

What if I don’t have language mapping?

The surgeon won’t be able to map out the language area of your brain increasing the risk of language deficits after surgery.

What type of brain surgery requires an awake monitoring?

  • Epilepsy surgery
  • Brain tumour surgery

If you have any questions (before or after your surgery) please ring 0151 529 5602

  • Last Updated:
    05 April 2021
  • Review Date:
    07 April 2025
  • Author:
    Michael Pridgeon Neurophysiology Department
  • Summary:

    Your surgeon would like you to have neurophysiology mapping during your surgery. This includes motor mapping while you are asleep and language mapping when you are woken up in theatre.

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