What is the role of the snap test in neurosurgery?

The Snap Test in Neurosurgery: Role and Applications

The snap test is not a recognized or standardized neurophysiological monitoring technique in neurosurgery. Instead, neurosurgeons rely on established electrophysiological monitoring methods such as somatosensory evoked potentials (SSEP), motor evoked potentials (MEP), and electromyography (EMG) during surgical procedures 1, 2.

Established Neurophysiological Monitoring in Neurosurgery

Spinal Surgery Applications

• Intraoperative evoked EMG recording is recommended as an option during lumbar spinal fusion surgery when surgeons need immediate information about pedicle wall integrity 1
• SSEP or dermatomal sensory evoked potential (DSEP) monitoring is recommended as an adjunct during instrumented lumbar spinal fusion procedures when immediate information about potential neurological injury is desired 1
• These monitoring techniques have high sensitivity but limited specificity, with high false-positive rates that don't necessarily correlate with postoperative neurological deficits 1, 2

Limitations of Current Monitoring Techniques

• The American Association of Neurological Surgeons guidelines indicate that reliance on changes in EP monitoring to alter surgical plans has not been observed to reduce neurological injury incidence during routine surgery for cervical spondylotic myelopathy or radiculopathy (Class III evidence) 1, 2
• Intraoperative EP worsening is not specific—it may not represent clinical worsening and its recognition does not necessarily prevent neurological injury or improve outcomes (Class II evidence) 1, 3
• The sensitivity of SSEP monitoring for detecting new deficits in lumbar procedures is only about 50%, with high false-positive rates 1, 3

Peripheral Nerve Applications

Peripheral Nerve Injury Assessment

• Electrophysiological studies including nerve conduction studies and electromyography are considered extensions of clinical examination in patients with peripheral nerve injuries 4
• These studies help in localizing lesion sites, determining lesion type and severity, and providing prognostic information 4
• Intraoperative monitoring helps identify injured nerves, determine nerve continuity, localize lesion sites, and prevent surgical damage to nerves 4

Sensory Nerve Action Potential (SNAP) Testing

• SNAP amplitude is rarely reduced in lumbosacral radiculopathy due to herniated disc, except in cases with foraminal herniation (approximately 7% of cases) 5, 6
• Intrafascicular neurography has been shown to be more sensitive than conventional nerve conduction studies in evaluating axonal degeneration 7
• Quantitative sensory testing correlates with SNAP amplitudes and can provide more objective information than qualitative testing 8

Current Recommendations for Neurophysiological Monitoring

Evidence-Based Guidelines

• Current guidelines provide Class II evidence that monitoring modalities have not been convincingly demonstrated to favorably influence patient outcomes in degenerative lumbar disease 1, 3
• Guidelines recommend intraoperative monitoring as an "adjunctive option" during instrumented lumbosacral fusion procedures, not as a standard of care 1
• While routine use is not recommended, there may be specific circumstances where surgeons desire additional intraoperative diagnostic information that EP monitoring may provide 1, 2

Monitoring Modality Selection

• EMG monitoring has shown greater utility than SSEP monitoring for detecting nerve injuries during lumbosacral procedures 1
• EEG monitoring during spinal surgery lacks sufficient evidence to support its routine use 2, 3
• The correlation between intraoperative monitoring changes and postoperative neurological deficits is inconsistent, with both false positives and false negatives reported 1, 3

Conclusion

The term "snap test" does not refer to a recognized neurophysiological monitoring technique in neurosurgery. Established monitoring methods like SSEP, MEP, and EMG have specific but limited roles in neurosurgical procedures, primarily serving as adjunctive options rather than standard practice. Current evidence does not strongly support that these monitoring techniques significantly improve patient outcomes in terms of morbidity, mortality, or quality of life during routine neurosurgical procedures for degenerative spinal conditions.