How to differentiate common peroneal neuropathy from L5 (lumbar 5) radiculopathy using nerve conduction studies (NCS)?

Differentiating Common Peroneal Neuropathy from L5 Radiculopathy Using Nerve Conduction Studies

The key to distinguishing common peroneal neuropathy from L5 radiculopathy on nerve conduction studies is that the superficial peroneal sensory nerve action potential (SNAP) is preserved in L5 radiculopathy but reduced or absent in peroneal neuropathy, while motor studies show focal slowing or conduction block at the fibular head in peroneal neuropathy versus normal distal latencies with F-wave abnormalities in radiculopathy. 1

Critical Sensory Nerve Findings

Superficial Peroneal Sensory Response

• The superficial peroneal sensory nerve response is the single most discriminating test because this nerve is formed from L4, L5, and S1 roots after they exit the spinal canal, meaning it remains intact in pure radiculopathy 1
• In L5 radiculopathy patients under 60 years old, the superficial peroneal SNAP should be normal in approximately 79% of cases, whereas it is typically abnormal or absent in peroneal neuropathy 2
• A reduced or absent superficial peroneal SNAP strongly suggests peroneal neuropathy rather than radiculopathy in patients under 60 years 2
• In patients over 60 years, this distinction becomes less reliable as 27-32% of both healthy subjects and L5 radiculopathy patients may have abnormal responses 2

Sural Nerve Response

• The sural nerve (S1 distribution) should be normal in both L5 radiculopathy and isolated peroneal neuropathy, serving as an internal control 1
• An abnormal sural response suggests polyneuropathy or a more extensive process 3

Motor Nerve Conduction Patterns

Peroneal Neuropathy Findings

• Focal slowing or conduction block at the fibular head is the hallmark finding, with velocity drop across this segment compared to the distal leg 3
• Reduced compound muscle action potential (CMAP) amplitude when stimulating above versus below the fibular head indicates focal demyelination or axonal loss 3
• Prolonged distal motor latency to the extensor digitorum brevis is common 4
• F-waves may be absent or prolonged, but this is less specific 4

L5 Radiculopathy Findings

• CMAP amplitudes and distal latencies are typically normal because the lesion is proximal to the dorsal root ganglion 4
• The most characteristic finding is increased F-wave maximum-minimum latency range rather than prolonged minimal F-wave latency 4
• F-wave persistence may be reduced, but complete absence is less common than in peripheral neuropathy 4
• Side-to-side amplitude asymmetry may be present but is less pronounced than in focal neuropathy 4

Algorithmic Approach to Differentiation

Step 1: Obtain Superficial Peroneal Sensory Study

• If absent or reduced amplitude (<5 μV) in a patient under 60 years → strongly favors peroneal neuropathy 2, 1
• If normal (>5 μV, latency <4.1 msec) → favors L5 radiculopathy 1
• If patient is over 60 years, proceed to Step 2 as this finding is less discriminating 2

Step 2: Examine Peroneal Motor Studies

• Stimulate at ankle, below fibular head, and above fibular head 3
• Calculate conduction velocity across the fibular head segment
• If focal slowing >10 m/s compared to distal segment or >50% amplitude drop → confirms peroneal neuropathy 3
• If velocities are uniform and distal latency is normal → favors radiculopathy 4

Step 3: Analyze F-Wave Pattern

• If F-waves show increased maximum-minimum range with relatively preserved minimal latency → favors L5 radiculopathy 4
• If F-waves are absent or show prolonged minimal latency → suggests more distal pathology 4
• This pattern correctly classifies disease state in 76% of cases with 74% sensitivity and 80% specificity 4

Step 4: Check Tibial Nerve Studies

• Normal tibial motor and sensory studies support isolated peroneal neuropathy or L5 radiculopathy 3
• Abnormal tibial studies suggest S1 radiculopathy, sciatic neuropathy, lumbosacral plexopathy, or polyneuropathy 3, 1

Common Pitfalls and Caveats

Age-Related Considerations

• In patients over 60 years, up to 27% may have absent superficial peroneal responses even without pathology, reducing the specificity of this test 2
• Consider using the contralateral limb as a control in older patients 2

Technical Factors

• Approximately 6-8.6% of normal subjects have unelicitable superficial peroneal responses even with proper technique, so always compare side-to-side 1
• The superficial peroneal nerve may be anatomically variant or difficult to stimulate in some individuals 1

Coexisting Pathology

• Patients may have both conditions simultaneously, particularly those with diabetes or other risk factors for polyneuropathy 3
• If findings are mixed or atypical, consider needle EMG of paraspinal muscles (abnormal in radiculopathy, normal in peroneal neuropathy) and proximal L5-innervated muscles like tibialis posterior 3

Limitations of Nerve Conduction Studies

• Early in disease course, all electrodiagnostic studies may be normal, requiring repeat testing in 3-4 weeks if clinical suspicion remains high 5
• Purely demyelinating lesions without axonal loss may show conduction abnormalities but preserved amplitudes 3

Additional Diagnostic Considerations

While nerve conduction studies are essential, needle EMG examination of the short head of biceps femoris, tibialis posterior, and paraspinal muscles provides complementary information that nerve conduction studies cannot 3. Paraspinal denervation indicates radiculopathy, while its absence with distal denervation suggests peripheral nerve pathology 3.

Logistic regression models using these motor conduction patterns can correctly classify radiculopathy versus axonal neuropathy in 76% of cases, demonstrating the utility of systematic analysis of these parameters 4.