Forms Spinal cord monitoring is a method to detect injury to the spinal cord during operative procedures most common forms are EMG (electromyography) SEP (somatosensory evoked potentials) 25% sensitive, 100% specific MEP (motor evoked potentials) 100% sensitive, 100% specific Anatomy Spinal cord pathways sensory (afferent) dorsal column spinothalamic tract motor (efferent) lateral corticospinal tract ventral corticospinal tract Blood supply anterior spinal artery primary blood supply to anterior 2/3 of spinal cord, including both the lateral corticospinal tract and ventral corticospinal tract posterior spinal artery (right and left) primary blood supply to the dorsal sensory columns Sensory evoked potentials (SEPs) Function monitor integrity of dorsal column sensory pathways of the spinal cord Technique signal initiation lower extremity usually involves stimulation of posterior tibial nerve behind ankle upper extremity usually involves stimulation of ulnar nerve signal recording transcranial recording of somatosensory cortex Advantages reliable and unaffected by anesthetics Disadvantages not reliable for monitoring the integrity of the anterior spinal cord pathways reports exist of an ischemic injury leading to paralysis despite normal SEP monitoring during surgery Intraoperative considerations loss of signals during distraction mandates immediate removal of device and repeated assessment of signals decrease in amplitude of 50% and/or 10% prolongation in latency is considered a significant change Motor Evoked Potentials (MEP) Function monitor integrity of lateral and ventral corticospinal tracts of the spinal cord Technique signal initiation transcranial stimulation of motor cortex signal recording muscle contraction in extremity (gastroc, soleus, EHL of lower extremity) Advantages effective at detecting a ischemic injury (loss of anterior spinal artery) in anterior 2/3 of spinal cord Disadvantages often unreliable due to effects of anesthesia Intraoperative considerations loss of signals during distraction mandates immediate removal of device and repeated assessment of monitoring signals >100 V increase in threshold is suggestive of an early injury >50% decrease in MEP amplitude is considered significant Mechanical Electromyography (spontaneous) Introduction monitor integrity of specific spinal nerve roots Technique concept microtrauma to nerve root during surgery causes depolarization and a resulting action potential in the muscle that can be recorded contact of a surgical instrument with nerve root will lead to "burst activity" and has no clinical significance significant injury or traction to a nerve root will lead to "sustained train" activity, which may be clinically significant signal initiation mechanical stimulation (surgical manipulation) of nerve root signal recording muscle contraction in extremity Advantages allows monitoring of specific nerve roots Disadvantages may be overly sensitive (i.e. sustained train activity does not necessarily reflect a nerve root injury) Electrical Electromyography (triggered) Introduction allows detection of a breached pedicle screw Technique concept bone conducts electricity poorly an electrically stimulated pedicle screw that is confined to bone will not stimulate a nerve root if there is a breach in a pedicle, stimulation of the screw will lead to activity of that specific nerve root signal initiation electrical stimulation of placed pedicle screw signal recording muscle contraction in extremity thresholds <8 mA may be indicative of a breach Advantages allows monitoring of specific nerve roots Disadvantages may be overly sensitive (i.e. sustained train activity does not necessarily reflect a nerve root injury)