By Drayton Perkins, PT, DPT, ECS | EMG Solutions
Let’s walk through a case that came through the clinic recently. It’s a great example of how electrodiagnostic (EDX) testing helps narrow down differentials and gives us clarity when the presentation isn’t totally textbook.
Presenting Symptoms
A 37-year-old female came in with an 8-day history of right foot weakness, mainly dorsiflexion and eversion. She also described a numb, “heavy” sensation over the top of her right foot. No traumatic onset, no recent injury, and interestingly, no back or lateral knee pain at the beginning. Of note, she had only developed lateral leg pain about three days before our visit and stated that it was likely due to her attempting to use kinesiotape to provide dorsiflexion assist.
She has a mild steppage gait on the right, and while her reflexes were absent at the patella and Achilles bilaterally, muscle bulk looked normal—no visible atrophy or fasciculations. Manual muscle testing showed decreased strength (3/5) with right ankle dorsiflexion, eversion, and EHL extension. Sensation to light touch was patchy and diminished across the right lateral and medial lower leg and lateral foot. Provocative testing was negative with clonus and Babinski, but positive with Tinel’s test at the left fibular head.
She denied any worsening of her usual low-level back pain, had no symptoms in the left leg, and had no history of systemic conditions like diabetes, thyroid issues, or prior exposure to chemotherapy.
Differential Diagnosis
At this point, my brain was buzzing with possibilities. Here’s the short list I was working through:
- Fibular (peroneal) neuropathy at the fibular head
- L5 radiculopathy
- Motor neuron disease (unlikely but always on the radar with foot drop)
- Peripheral polyneuropathy
So how do we figure it out?
What was Tested (and Why)
This is where EDX shines. We ran a full nerve conduction study (NCS) and needle EMG of the bilateral lower extremities.
Sensory Clues: Pre-Ganglionic vs. Post-Ganglionic… and Timing Matters
One of the early questions to answer: Is this problem coming from the nerve root (pre-ganglionic) or the peripheral nerve (postganglionic)?
Sensory nerve action potentials (SNAPs) can help. If the lesion is pre-ganglionic, like in an L5 radiculopathy, the dorsal root ganglion remains intact, so the sensory responses are often normal—even if the patient reports numbness or tingling. In contrast, postganglionic lesions (like a mononeuropathy) usually cause abnormal sensory responses because the lesion is distal to the ganglion, and the sensory axon itself is damaged.
In this case:
- The superficial fibular sensory responses were normal and comparable bilaterally.
At first glance, that might seem inconsistent with her symptoms. But here’s the key: it had only been 8 days since symptom onset, and sensory nerve changes—especially in distal recordings—can take time to manifest following a proximal nerve injury. The axonal damage has to travel distally (Wallerian degeneration), and it can take 10–14 days or longer for that change to be reflected in the sensory study. So, the normal response here doesn’t rule out a common fibular neuropathy; it may simply be too early to show up.
Likewise, the normal sensory responses didn’t rule in or rule out an L5 radiculopathy either, because of that same short timeline. So, while sensory studies are helpful, they need to be interpreted in the context of the timing and location of the lesion.
Other Key Findings
- Significant partial conduction block of the right fibular nerve across the fibular head (54% drop in CMAP amplitude). This partial conduction block was present when recording from the deep fibular branch (tibialis anterior, extensor digitorum brevis) and from the superficial fibular branch (fibularis longus), suggesting a compromise of the COMMON FIBULAR nerve at or near the fibular head.
- Slowed conduction velocity of the fibular motor nerve across the fibular head segment when recording from the extensor digitorum brevis muscle.
- Abnormal spontaneous activity in the right tibialis anterior muscle. With the new onset of symptoms (8-days), it is common to only see axonal findings in the muscles closest to the area of compromise. As time progresses, more distal muscles may begin showing abnormal spontaneous activity.
- All other motor and sensory nerves (the bilateral tibial, the bilateral sural, the bilateral superficial fibular sensory, and the left fibular motor nerves) were otherwise normal, ruling out the presence of a polyneuropathy or lumbosacral plexopathy.
- Needle EMG ruled out a radiculopathy or myopathy—the other muscles tested looked clean and within normal limits.
This pattern—focal slowing, conduction block, and spontaneous activity in the right anterior compartment—screamed common fibular neuropathy at the fibular head.
Final Diagnosis
Right common fibular neuropathy at or near the fibular head, with both demyelinating and axonal features.
Clinical Takeaway:
Don’t be fooled if your patient doesn’t present with the “classic” pain at the fibular head or obvious mechanism of compression. Foot drop without back pain doesn’t always mean radiculopathy. In this case, sensory testing hadn’t yet caught up to the motor findings because of how early we saw her—but the conduction block and needle EMG findings sealed the deal.
The timing of testing matters. Understanding how fast (or slow) abnormal findings show up on EDX testing in different parts of the nervous system can help you make sense of what you’re seeing—and avoid jumping to conclusions too soon.
This blog is a feature from Edition 7 of our newsletter, which you can find here.
