How do you interpret a nerve conduction study showing no response in the peroneal (peroneal nerve) motor recording at the Extensor Digitorum Brevis (EDB) muscle, but a normal response at the Tibialis Anterior (TA) muscle, along with an absent superficial peroneal (superficial peroneal nerve) sensory response?

Interpretation of Nerve Conduction Study Findings

Primary Interpretation

This nerve conduction study pattern indicates a distal peroneal nerve lesion localized below the fibular head, most likely at the level of the anterior tarsal tunnel or distal deep peroneal nerve, with concurrent superficial peroneal nerve involvement. 1

Anatomical Localization

The key to interpreting this pattern lies in understanding the peroneal nerve anatomy and its branching pattern:

• The common peroneal nerve divides at the fibular head into the deep peroneal nerve (which innervates the tibialis anterior proximally and extensor digitorum brevis distally) and the superficial peroneal nerve (which is purely sensory). 2

• A normal peroneal motor response at the tibialis anterior (TA) indicates that the deep peroneal nerve is intact from the fibular head down to the TA muscle level. 1

• The absent response at the extensor digitorum brevis (EDB) with preserved TA response localizes the lesion to the distal deep peroneal nerve, below the level where the branch to TA originates. 3, 4

• The absent superficial peroneal sensory response indicates concurrent involvement of the superficial peroneal nerve branch. 3

Differential Diagnosis Pattern Recognition

This specific electrodiagnostic pattern suggests several possible etiologies:

• Anterior tarsal tunnel syndrome is the most likely diagnosis, where both the deep peroneal nerve (distal portion) and superficial peroneal nerve can be compressed beneath the extensor retinaculum. 4

• Distal peroneal neuropathy from local trauma, compression, or entrapment at the ankle level should be considered, as the pattern excludes proximal lesions at the fibular head. 2, 5

• Length-dependent polyneuropathy is less likely given the normal TA response, as polyneuropathy would typically show diffuse abnormalities with relatively uniform involvement of tested nerves rather than this selective distal pattern. 1

• L5 radiculopathy is effectively ruled out by this pattern, since radiculopathy would affect both TA and EDB equally, and the superficial peroneal sensory response helps distinguish radiculopathy from peripheral nerve lesions. 3

Technical Considerations and Pitfalls

Several technical factors must be considered when interpreting these findings:

• Absent EDB responses can occur in 6-8.6% of normal subjects, so clinical correlation is essential before attributing pathological significance. 3

• The superficial peroneal nerve response may be absent in up to 6% of normal limbs tested, which represents a potential false-positive finding. 3

• Temperature effects on nerve conduction should be excluded, as cooler limb temperature can affect conduction properties and amplitude measurements. 6

• EDB muscle atrophy from any cause (including age-related changes or chronic foot deformities) can result in low or absent motor responses independent of nerve pathology. 5

Clinical Correlation Requirements

To confirm the diagnosis and guide management, specific clinical findings should be sought:

• Examine for sensory loss in the first web space (deep peroneal sensory distribution) and dorsum of the foot (superficial peroneal distribution). 4

• Assess for weakness of toe extension (EDB function) while confirming normal ankle dorsiflexion strength (TA function). 1

• Palpate for tenderness or masses at the anterior ankle beneath the extensor retinaculum, which would support anterior tarsal tunnel syndrome. 4

• Evaluate for foot positioning abnormalities or history of trauma, tight footwear, or repetitive ankle movements that could cause distal nerve compression. 2, 5

• Check for signs of more widespread neuropathy by examining other nerves, reflexes, and sensory modalities to exclude polyneuropathy masquerading as focal findings. 1

Recommended Next Steps

Based on this electrodiagnostic pattern, the following diagnostic approach is warranted:

• Perform a detailed neurological examination focusing on the distribution described above to clinically confirm the localization. 1

• Consider imaging of the ankle and distal leg with MRI to evaluate for structural causes of nerve compression, masses, or anatomical variants. 7

• Evaluate for systemic causes if bilateral involvement or other neuropathic features are present, including diabetes screening, vitamin B12, thyroid function, and inflammatory markers. 1

• Repeat nerve conduction studies are not recommended for routine monitoring unless there is clinical uncertainty about new or worsening neurological processes. 7, 1