What is the diagnostic approach to lumbosacral plexopathy?

Diagnostic Approach to Lumbosacral Plexopathy

MRI of the lumbosacral plexus is the gold standard imaging modality for diagnosing lumbosacral plexopathy due to its superior soft-tissue contrast and ability to visualize intraneural anatomy when clinical and electrodiagnostic findings are nonspecific. 1

Initial Diagnostic Evaluation

• Electrodiagnostic studies (EMG/NCS) should be performed to confirm plexopathy and distinguish it from radiculopathy or peripheral neuropathy 2, 3, 4
• Clinical features that suggest plexopathy rather than radiculopathy include:
  • Pain distribution crossing multiple dermatomes 2
  • Weakness in regions innervated by multiple nerves from the affected plexus 2
  • Sensory loss across multiple nerve distributions 2
  • Flaccid loss of tendon reflexes in regions innervated by the affected plexus 2

Imaging Algorithm

First-Line Imaging

• MRI lumbosacral plexus (with and without IV contrast) is the preferred initial imaging study 1
  • Provides superior soft-tissue contrast and spatial resolution 1
  • Can detect abnormal intraneural signal and localize pathologic lesions 1, 5
  • IV contrast helps characterize masses, tumors, and inflammatory conditions 1

When Initial Diagnosis is Uncertain

• MRI lumbar spine may be performed first when clinical distinction between radiculopathy and plexopathy is unclear 1, 6
  • Higher prevalence of radiculopathy-related degenerative spine disease makes this a practical first step 1
  • Note that lumbar spine MRI does not evaluate the plexus lateral to the neural foramina 1

Alternative Imaging When MRI is Contraindicated

• CT with IV contrast offers the next highest level of anatomic visualization 1, 7
  • Useful for detecting soft-tissue masses and tumors 1
  • Limited in visualizing intraneural anatomy compared to MRI 1

Specific Etiologies to Consider

• Entrapment neuropathies (spinal or extraspinal compression) 7
  • Piriformis syndrome (sciatic nerve compression by piriformis muscle) 7
• Neoplastic causes:
  • Benign peripheral nerve sheath tumors (schwannomas, neurofibromas) 7
  • Malignant peripheral nerve sheath tumors (more common in neurofibromatosis) 7
  • Primary or metastatic tumors 7
• Non-neoplastic masses:
  • Hematoma, abscess, aneurysm, amyloidosis, endometriosis 7
• Inflammatory/immune-mediated processes:
  • Diabetic amyotrophy 7, 8
  • Acute inflammatory demyelinating polyneuropathy (Guillain-Barré syndrome) 7
  • Chronic inflammatory demyelinating polyneuropathy 7
  • Autoimmune plexopathy 3
• Other causes:
  • Ischemic nerve injury 7
  • Hereditary neuropathies (e.g., Charcot-Marie-Tooth disease) 7
  • Sarcoidosis 7
  • Infection (e.g., zoster-associated limb paresis) 7

Common Pitfalls in Diagnosis

• Failure to distinguish between plexopathies and radiculopathies 7, 6
  • Radiculopathies typically follow a single dermatome distribution 7
  • Plexopathies affect multiple nerve distributions 2
• Not recognizing that lateral disc herniations may cause plexopathy not detected on standard lumbar spine MRI 7
• Delaying specialized imaging when clinical suspicion for plexopathy is high 4
• Inadequate EMG sampling of muscles innervated by different nerve branches of the plexus 4

Clinical Impact of Proper Imaging

• Studies show that MRI lumbosacral plexus leads to changes in diagnosis in 78% of patients and changes in management in 71-81% of patients 1
• Combining MRI and electrodiagnostic studies significantly improves diagnostic accuracy 1, 4
• Early and accurate diagnosis can guide appropriate treatment and improve outcomes 9