What is the significance of an abnormal unilateral tibial nerve Somatosensory Evoked Potential (SSEP) in the leg?

Significance of Abnormal Unilateral Tibial Nerve SSEP

Abnormal unilateral tibial nerve Somatosensory Evoked Potential (SSEP) in the leg most likely indicates a pathology in the sensory pathway from the peripheral nerve to the cortex, which may suggest spinal cord compression, nerve root impingement, or peripheral neuropathy requiring further clinical correlation and imaging studies.

Anatomical and Physiological Significance

Tibial nerve SSEPs assess the integrity of the somatosensory pathway from the peripheral nerve through the spinal cord to the cerebral cortex:

• Normal tibial nerve SSEP includes several key components:

  • Peripheral components (N8)
  • Spinal components (N22)
  • Subcortical components (P28, P30)
  • Cortical components (P38/N38) 1

• An abnormal unilateral tibial SSEP suggests pathology somewhere along this pathway:

  • May localize to peripheral nerve, nerve root, spinal cord, brainstem, or cortex
  • Unilateral abnormality typically indicates a lesion affecting only one side of the body

Clinical Significance by Location

1. Spinal Cord Pathology

• Highly correlated with cervical myelopathy when abnormal

  • In patients with cervical myelopathy, 56% showed abnormal tibial nerve SSEPs 2
  • Significant reduction in SSEP abnormalities after surgical decompression 2

• Prognostic value:

  • Abnormal preoperative SSEPs correlate with severity of myelopathy 3
  • Improvement in SSEPs following surgery correlates strongly with clinical improvement 2
  • Patients with normal SSEPs have better recovery rates (73%) compared to those with abnormal SSEPs (43%) 3

2. Peripheral Neuropathy

• In acute inflammatory demyelinating polyradiculoneuropathy (AIDP):
  • 17 of 19 patients had abnormal tibial nerve SSEPs 4
  • Most common findings were absent or prolonged latency of N8 or N22 4

3. Diabetic Neuropathy

• Abnormally prolonged tibial SSEPs statistically correlate with lower urinary tract dysfunction in diabetic patients 3
• Serves as an early marker of autonomic neuropathy affecting bladder function 3

4. Position-Related Nerve Injury

• While less common for tibial nerve, SSEP monitoring can detect position-related neuropathies during surgery
  • Similar monitoring for ulnar nerve detected potential neuropathy in 5.2% of patients undergoing lumbosacral spine surgery 5

Diagnostic and Monitoring Implications

Surgical Monitoring

• Tibial nerve SSEPs are recommended as an adjunct during instrumented lumbar spinal fusion procedures 3

  • High correlation between postoperative neurological deficit and intraoperative SSEP changes
  • However, abnormal intraoperative SSEPs often do not correlate with postoperative injury due to high false-positive rate 3

• Combined multimodal monitoring using SSEPs and Motor Evoked Potentials (MEPs) increases sensitivity and specificity for detecting impending neural injury 6

Localization Value

• P30 potential abnormalities help differentiate lesion location:
  • P30 remains normal with supramedullary lesions (above decussation)
  • P30 becomes abnormal with lesions caudal to the cervicomedullary junction (below decussation) 7
  • Helps distinguish between high cervical/brainstem vs. lower spinal cord pathology

Management Implications

1. Further diagnostic evaluation is warranted:

   • MRI of the suspected region (lumbar, thoracic, or cervical spine depending on clinical presentation)
   • Nerve conduction studies and EMG if peripheral neuropathy is suspected
   • Consider additional neurophysiological testing (F-waves, H-reflexes)

2. Treatment considerations:

   • Surgical decompression may be indicated if evidence of spinal cord or nerve root compression
   • Medical management for peripheral neuropathies
   • Monitoring of disease progression and treatment response

Pitfalls and Limitations

• False positives can occur due to technical factors:

  • Poor electrode contact
  • Patient positioning
  • Anesthetic effects
  • Temperature changes

• SSEPs assess only the dorsal column-medial lemniscus pathway:

  • May miss pathology in motor pathways or spinothalamic tracts
  • Normal SSEPs do not rule out all neurological pathology

• Interpretation requires correlation with clinical findings:

  • Abnormal SSEPs without symptoms may represent subclinical pathology
  • Normal SSEPs with symptoms may indicate pathology in tracts not assessed by SSEPs

In conclusion, an abnormal unilateral tibial nerve SSEP is a significant finding that warrants thorough clinical correlation and appropriate imaging studies to identify the underlying pathology, which could range from peripheral neuropathy to spinal cord compression.