Basic MEP techniques and understanding for Intraoperative neuromonitoring of the motors tracts during Brain and Spinal surgeries to prevent postoperative complications.

1. S. S. SIMON JEBA KUMAR
IONM Tech

2. CONTENTS :
MEP DEFINITION
MOTOR CORTEX
MUSCLES INVOLVED IN MEP
PRE OPERATIVE ASSESSMENT
ANESTHESIA & MEP
MEP MONITORING
SAFETY CONSIDERATION
POST OPERATIVE ASSESSMENT

3. DEFINITION :
Motor evoked potentials (MEPs) are electrical signals recorded
from neural tissue or muscle following activation of central
motor cortex.

4. MOTOR CORTEX

5. MOTOR CORTEX:
The motor cortex is the region of the cerebral cortex involved in
the execution of voluntary movements.
Classically the motor cortex is an area of the frontal lobe located
immediately anterior to the central sulcus.

6. The primary motor cortex is the main contributor to generating
neural impulses that pass down to the spinal cord and control the
execution of movement.
However, some of the other motor areas in the brain also play a
role in this function.
Electrical stimuli over area 4 produce activation of contra-lateral
muscles; the face, mouth and hand muscles occupy 2/3rd of
primary motor area.

7. MEP applications:
Based on the regions –
Cortical and sub-cortical – Brain surgeries (Eg- tumor removal)
Spinal cord –
Intramedullary & extramedullary intradural spinal
tumor removal
•Deformity corrections (Eg-scoliosis)
Peripheral nervous system – Repair of Limb ischemia
- Cervical and lumbar surgeries

8. Muscles involved in the MEP:

9. Stimulating electrode:
At C3 C4 ; CZ FZ; C1 C2
CORK SCREW ELECTRODE

10. PRE OPERATIVE ASSESMENT:
Patient History
Diagnosis
Surgery planned
Anaesthesia
Clinical examinations

11. Anesthesia For MEP:
MEPs are sensitive to anesthetics, especially inhalational agents
can be avoided during recording of myogenic MEPs.
Benzodiazepines, barbiturates depress MEPs,

12. Adequate recordings can be obtained during propofol anesthesia
by controlling serum levels and increasing stimuli rates.
Muscle relaxants can affect the recorded EMG response by
depressing myoneural transmission.
Adequate MEP recordings can be achieved as long as one or two
twitches on TOF can be maintained.

13. MEP MONITORING:
Amplifier settings
TIME BASE 100 ms
SENSITIVITY 7 – 35uv
LFF 5 Hz
HFF 500 Hz
NOTCH ON

14. Stimulator settings:
CURRENT INTENSITY 100 – 400 V
DURATION 50 u s
FREQUENCY RATE 0.7 Hz
TRAIN COUNT 5
TRAIN RATE 500 Hz
INTER PULSE INTERVAL 2 ms

15. Safety considerations:
During direct brain stimulation, the high extracranial current
densities can cause contraction of the temporalis muscles and
forceful jaw closure, which in turn
can cause
- Mouth injury
- Tongue or lip injuries
- Also mandible fracture
- Endo-tracheal tube rupture
Padding or soft bite blocks should be used to prevent mouth
injury or Endo-tracheal tube damage during TCS – MEP.

16. Communication with the rest of the surgical team:
Significant changes in the IONM data should be communicated
rapidly to the rest of the surgical team.
If the MEPs are not obtainable (due to preexisting neurologic
compromise in the patient, anesthesia, or technical factors), this
should also be communicated to the surgeons, lest they proceed
with surgical procedures in the mistaken belief that the MEP data
is demonstrating that the potentials are intact.

17. Post Operative Role:
 Check the number of needle electrodes.
 Clean the areas where electrodes placed.
 Clean the electrodes
 Send electrodes for sterilization and for double
sterilization too if it is used for Bio-hazard
patients.
 Clean the whole machine setup.

18. S. S. SIMON JEBA KUMAR
IONM Tech
(Fb/simon jeba Kumar)