What are the clinical differences between L5 radiculopathy and common peroneal (fibular) nerve palsy causing foot drop?

Distinguishing L5 Radiculopathy from Peroneal Nerve Palsy in Foot Drop

The key clinical discriminator is hip abductor weakness: present in 86% of L5 radiculopathy but only 4% of peroneal nerve palsy, making it the single most useful bedside test to differentiate these conditions 1.

Critical Clinical Examination Findings

Hip Abductor Strength (Most Important)

• L5 radiculopathy: Hip abductor weakness present in 85.7% of cases
• Peroneal nerve palsy: Hip abductor weakness present in only 3.6% of cases
• This test has 85.7% sensitivity and 96.4% specificity for L5 radiculopathy 1
• Positive predictive value of 94.7% when hip abductor weakness is present 1

Tibialis Posterior Muscle

• L5 radiculopathy: Tibialis posterior weakness is present (this muscle is L5-innervated but NOT supplied by the peroneal nerve)
• Peroneal nerve palsy: Tibialis posterior strength is NORMAL (preserved)
• This muscle is electrically accessible and provides discriminatory value 2

Distribution of Weakness

L5 radiculopathy affects:

• Tibialis anterior (foot dorsiflexion)
• Extensor hallucis longus (great toe extension)
• Peroneus longus (foot eversion)
• Hip abductors (gluteus medius/minimus)
• Tibialis posterior (foot inversion)

Peroneal nerve palsy affects:

• Tibialis anterior (foot dorsiflexion)
• Extensor hallucis longus (great toe extension)
• Peroneus longus (foot eversion)
• SPARES: Hip abductors and tibialis posterior

Sensory Examination Differences

L5 radiculopathy:

• Sensory loss over lateral leg AND dorsal foot
• May extend to medial foot (L5 dermatome)

Peroneal nerve palsy:

• Sensory loss limited to lateral leg and dorsal foot (superficial peroneal nerve distribution)
• First web space involvement (deep peroneal nerve)

Electrodiagnostic Patterns

Nerve Conduction Studies

Superficial peroneal nerve sensory responses:

• L5 radiculopathy: Abnormal in only 21.2% of cases 3
• Peroneal nerve palsy: Abnormal in 26.4% of cases 3
• When superficial peroneal sensory responses are abnormal in L5 radiculopathy, they are typically accompanied by peroneal motor abnormalities (unlike isolated peroneal neuropathy) 3

Needle EMG Findings

L5 radiculopathy:

• Abnormalities in tibialis anterior (75.8%) AND peroneus longus (92.4%)
• Crucially: Abnormalities also present in paraspinal muscles and tibialis posterior 3

Peroneal nerve palsy:

• Abnormalities in tibialis anterior (100%) and peroneus longus (64.2%)
• Normal paraspinal muscles and tibialis posterior 3

Associated Clinical Features

Pain Characteristics

L5 radiculopathy:

• Back pain radiating down lateral leg
• Positive straight leg raise test
• Pain worse with Valsalva maneuvers

Peroneal nerve palsy:

• Localized pain at fibular head (if present)
• No back pain or radicular pattern
• History of leg crossing, trauma, or compression at fibular neck 4

Tinel's Sign

• Present at fibular head in 60% of peroneal nerve palsy cases 5
• Absent in L5 radiculopathy

MRI Findings

MRI of lower leg shows distinct muscle denervation patterns:

• L5 pattern: Signal changes in anterior compartment muscles PLUS tibialis posterior 6
• Peroneal pattern: Signal changes limited to anterior and lateral compartment muscles, sparing tibialis posterior 6
• MRI has 92% agreement with EMG and can detect denervation before EMG becomes positive 6

Critical Pitfall: Double Crush Syndrome

Be aware that 10% of patients may have BOTH conditions simultaneously 5, 7. Consider this when:

• Clinical findings are mixed or inconsistent
• EMG shows both active L5 radiculopathy AND active peroneal mononeuropathy
• Symptoms persist despite addressing one lesion

In double crush cases, surgical decompression of the peroneal nerve can still provide significant benefit even with coexisting L5 radiculopathy, with 88% improvement in motor weakness 7.

Algorithmic Approach to Diagnosis

1. Test hip abductor strength: If weak → strongly suggests L5 radiculopathy
2. Test tibialis posterior strength (foot inversion): If weak → L5 radiculopathy
3. Check for Tinel's sign at fibular head: If positive → peroneal nerve palsy
4. Assess pain pattern: Radicular back pain → L5 radiculopathy; localized fibular head pain → peroneal palsy
5. Order EMG/NCS: Look for paraspinal and tibialis posterior involvement (L5) vs. isolated peroneal territory (peroneal palsy)
6. Consider MRI of lumbar spine (if L5 suspected) or lower leg MRI (if diagnostic uncertainty remains) 6