CNIM Electromyography (EMG) and Nerve Monitoring 1 — Questions and Answers
Question 1: Free-running (spontaneous) EMG monitoring during spine surgery is used primarily to detect:
- Cortical spreading depression
- Nerve root irritation or injury from surgical manipulation (Correct answer)
- Spinal cord ischemia
- Motor cortex activation
Correct answer: Nerve root irritation or injury from surgical manipulation
Free-running EMG detects spontaneous electrical activity (trains of potentials) generated when nerve roots are mechanically irritated, stretched, or injured during surgery.
Question 2: Neurotonic discharges in free-running EMG during lumbar surgery are characterized by:
- Isolated single motor unit potentials
- Sustained high-frequency trains of motor unit potentials indicating nerve irritation (Correct answer)
- Electrical silence with no activity
- Fibrillation potentials only
Correct answer: Sustained high-frequency trains of motor unit potentials indicating nerve irritation
Neurotonic discharges are sustained, high-frequency trains of motor unit activity that are highly significant for nerve root mechanical irritation and require surgical team notification.
Question 3: Triggered EMG (t-EMG) is used to assess pedicle screw placement by:
- Continuously recording muscle activity
- Stimulating the screw and measuring the threshold current needed to elicit a muscle response (Correct answer)
- Recording brainstem potentials
- Measuring intradiscal pressure
Correct answer: Stimulating the screw and measuring the threshold current needed to elicit a muscle response
Triggered EMG assesses pedicle screw integrity by stimulating the screw and determining the minimum current required to elicit a CMAP response; low thresholds suggest cortical wall breach.
Question 4: A triggered EMG threshold of less than 6–7 mA for a pedicle screw suggests:
- Correct screw placement within bone
- Possible medial wall breach with proximity to the nerve root (Correct answer)
- Lateral screw malposition
- Equipment malfunction
Correct answer: Possible medial wall breach with proximity to the nerve root
Threshold values below 6–7 mA for pedicle screws are considered concerning for a medial cortical wall breach, placing the screw close to the adjacent nerve root.
Question 5: EMG monitoring of which muscle confirms the integrity of the facial nerve (CN VII) during parotid or posterior fossa surgery?
- Masseter
- Orbicularis oculi and orbicularis oris (Correct answer)
- Sternocleidomastoid
- Trapezius
Correct answer: Orbicularis oculi and orbicularis oris
The orbicularis oculi (upper face) and orbicularis oris (lower face) are the primary muscles monitored for CN VII integrity, representing the upper and lower branches of the facial nerve.
Question 6: During spinal deformity surgery, the L5 nerve root is best monitored using EMG from which muscle?
- Gastrocnemius
- Tibialis anterior (Correct answer)
- Adductor magnus
- Vastus medialis
Correct answer: Tibialis anterior
The tibialis anterior is primarily innervated by the L4-L5 nerve roots via the deep peroneal nerve and provides the most sensitive EMG monitoring of L5 root function.
CNIM Electromyography (EMG) and Nerve Monitoring 1
Free-running (spontaneous) EMG monitoring during spine surgery is used primarily to detect: