PROBLEM SOLVING
Intraoperative neurophysiological monitoring (IONM) provides many important benefits, chief
among them the ability of the neurotechnologist (NT) to determine what has caused a change in the information being recorded or a shift from baseline as it relates to the surgical procedure. We call this the “Red Flag Warning.” Many factors under the control of the anesthesiologist, the surgeon and the NT can influence the information obtained from monitoring. In each case, the problem must be corrected or the surgeon notified so that he or she does not continue the surgery with a false sense of security.

Technical Factors
Dislodged stimulating or recording electrodes:

• Broken wires
• Failure of the computers amplifier
• Failure of the stimulating pod
• Excessive 60 cycle interference
• Salt bridge between stimulating electrodes causing a stimulus impedance error

Anesthetic Factors

• High concentrations of inhaled agents such as nitrous oxide can minimize the ability to obtain somatosensory and motor evoked potentials.
• Inadequate time allowed for the muscle relaxants to wear off will inhibit the ability to monitor spontaneous and electrically elicited EMG activity.

Anesthesthetic - related factors can be minimized by a close working relationship between the anesthesiologist and the NT, along with an understanding of how specific anesthetic agents affect the various electrodiagnostics.

Physiological Factors

• Hypotension causes prolongation and reduced amplitude in somatosensory and motor evoked potentials. Increasing blood pressure will reduce the chance of an ischemic event.
• Hypothermia can be detected by the degradation of somatosensory evoked potentials prolonging recovery.
• Low hemoglobin and hematocrit can degrade the somatosensory and motor evoked potential.
• Inadequate ventilation causing hypoxia will cause somatosensory wave forms to prolong as a result of oxygen deficiency.
• Inadequate blood flow to the spinal cord will cause wave forms to prolong.

Patient Positioning

• Having the patient’s arms extended or providing insufficient padding under the shoulder can cause stretching of the brachial plexus. Detection can be accomplished by obtaining an ulnar nerve somatosensory evoked potential.
• Cervical traction and hyperextension may cause a rapid change in the central conduction of the somatosensory evoked potential and an abrupt loss of the motor evoked potential.
• Spinal cord ischemia may occur as a result of positioning and can be detected by both evoked potentials.

Neurological conditions such as ulnar nerve palsy, paralysis, muscle weakness, and sensory deficits can be avoided with early detection followed by corrective action such as repositioning the patient.

Surgically Related Factors
Bone Graft Impaction or Migration: A bone graft impacted incorrectly in the cervical and thoracic spine can cause loss of the sensory and motor evoked potentials in addition to spontaneous EMG activity.

• Potential neurological deficits include (level dependent) quadriplegia, paraplegia, arm or leg pain with or without sensory deficits and muscle weakness.
• Corrective action: Remove and reinsert the graft.

Excessive Retraction: May cause sensory changes consisting of prolonged latencies and loss of amplitude due to the delicate composition of the cord and nerve roots. A loss of motor evoked potentials, especially in surgeries involving the cervical through the thoracic spine and first lumbar vertebrae, may also be evident due to excessive retraction. Spontaneous EMG activity is specifically observed when retracting the nerve root.

• Potential neurological deficits include (level dependent) quadriplegia, paraplegia, arm or leg pain, muscle weakness, bladder or bowel dysfunction, and sexual dysfunction.
• Corrective action: Reduce retraction.

Misplaced Pedicle Screw: Can affect the dermatome evoked potential and cause spontaneous EMG activity as a result of irritation to the motor component of the nerve root. Electrical stimulation is used to confirm placement.

• Potential neurological deficits include leg or arm pain with or without sensory deficits.
• Corrective action: When the screw causes spontaneous activity and/or the electrical stimulation causes a triggered response, the screw can be removed and redirected or, if necessary, removed completely. If the problem is left unresolved, a second surgery may be required.

Cord compression: May be caused by an accumulation of blood in a confined space and could result in the gradual degradation of the sensory and/or motor evoked potential. Additional causes of sensory and motor degradation include misplaced bone grafts, excessive retraction, and tumors around or within the spinal cord.

• Potential neurological deficits include (level dependent) quadriplegia, paraplegia, and arm or leg pain with or without sensory deficits, muscle weakness, bladder or bowel dysfunction, and sexual dysfunction.
• Corrective action: Hemostasis (control bleeding): removal and reinsertion of the bone graft: reduce retraction: remove tumor.

Ischemia: Vascular occlusion (lack of blood flow) to the spinal cord may have significant effect on the sensory evoked and motor evoked potential. In the thoracic spine, the cord receives blood in part, through the radicular arteries following the course of the nerve root. If the neuro-foramina are compromised, ischemia will cause a prolongation of the latency and loss of amplitude. The motor evoked response may also be affected.

• Potential neurological deficits include (level dependent) quadriplegia, paraplegia, arm or leg pain with or without sensory deficits, and muscle weakness.
• Corrective action: Increase blood pressure; change the patient’s position.

Excessive correction of a spinal deformity: Overcorrection of a curvature (e.g., as in scoliosis) may cause a loss of sensory and motor evoked responses.

• Potential neurological deficits include paraplegia, arm or leg pain with muscle weakness and/or sensory deficits, bladder or bowel dysfunction, and sexual dysfunction.
• Corrective action: reduce the distraction, compression and or rotation of the spine.

Tumors: May be intramedullary (within the spinal cord) or extramedullary (outside the spinal cord, but intradural, meaning under the spinal cord covering). Tumors may also be epidural (outside the dura) or paraspinal, generally involving the boney structures of the spinal column.

• Potential neurological deficits include sensory disturbances, balance and coordination difficulties affecting the ability to walk, weakness, paralysis, and bladder and bowel dysfunction.
• Corrective Action: Remove as much of the offending pathology as possible.

Cervical traction: Used during surgery on the anterior cervical spine, the amount of traction varies depending on the size of the patient and muscular development, but is generally around 15 pounds. The purpose is to open the intervertebral disc space, which in turn allows better access to the offending pathology such as a disc herniation. Potential neurological deficits include paralysis, muscle weakness and sensory deficits.

It is advisable to have a medical doctor trained in IONM for consultation if routine trouble-shooting does not lead to a logical conclusion. Through remote monitoring or direct supervision, the supervising physician can interact with the NT, providing an additional level of expertise.

Frequently, causes of neurological changes will go undetected without a way to measure them. Using the IONM biomedical computer allows these changes to be detected.

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