14.1 Fibular Neuropathy at the Fibular Head versus L5 Radiculopathy
Key Takeaways
- Across-fibular-head conduction block or focal slowing localizes common fibular neuropathy at the fibular neck; L5 radiculopathy does not produce that segmental block.
- Superficial fibular SNAP amplitude falls in axonal (postganglionic) fibular neuropathy and is typically spared in an L5 root lesion (preganglionic to the dorsal root ganglion).
- Foot inversion (tibialis posterior, tibial nerve) is spared in fibular neuropathy at the fibular head and is often weak in L5 radiculopathy.
- Short-head biceps femoris is innervated by the common fibular division above the fibular head; needle EMG of that muscle is a physician study used only as an anatomic pearl.
- Rapid weight loss, habitual leg crossing, and short-leg casts are classic mechanical risks for fibular-head compression and foot drop.
Lower-extremity foot drop is a high-yield localization problem on nerve conduction study (NCS) examinations. The same bedside picture — weak ankle dorsiflexion, a steppage gait, and sensory change on the dorsum of the foot — can come from common fibular (peroneal) neuropathy at the fibular head, from an L5 radiculopathy, or from a more proximal sciatic lesion that preferentially injures the fibular division. Independent OpenExamPrep teaching in this section trains you to use NCS patterns, not the wording on the referral, to separate a focal lesion at the fibular neck from a root-level process. This chapter covers outline topics VI.B.5.g (fibular nerve at the fibular head) together with the L5 comparison that makes those studies interpretable.
Why the fibular head is vulnerable
The common fibular nerve winds around the fibular neck just distal to the fibular head. At that point the nerve is superficial, relatively fixed against bone, and exposed to external compression. Classic risk settings include:
- Habitual leg crossing, in which the free knee compresses the opposite fibular neck
- Marked weight loss, which reduces the subcutaneous padding over the nerve
- Short-leg casts, intraoperative positioning straps, or prolonged bed rest with the fibular neck against a rail
- Squatting or repetitive stretch where the nerve dives under the origin of peroneus longus
After the fibular head, the nerve divides into the deep fibular nerve (tibialis anterior, extensor hallucis longus, extensor digitorum brevis, and sensation in the first dorsal web space) and the superficial fibular nerve (peroneus longus and brevis; sensory to the dorsum of the foot and the lateral distal leg).
Motor NCS: across-fibular-head conduction block versus axonal loss
A technologist-ready fibular motor study records the extensor digitorum brevis (EDB) compound muscle action potential (CMAP) and stimulates at the ankle, below the fibular head, and above the fibular head (lateral popliteal fossa). Some laboratories also record tibialis anterior (TA) when the EDB response is tiny or when the clinical question is proximal conduction block into a more proximal muscle.
Two electrophysiologic patterns matter:
- Focal demyelination at the fibular head. Look for conduction block (CB) or focal slowing across the fibular-head segment: a clear drop in CMAP amplitude or negative-peak area comparing below-head stimulation with above-head stimulation, and/or a segment velocity that is disproportionately slow compared with the ankle-to-fibular-head segment. Distal stimulation can still produce a relatively robust EDB CMAP if axons remain in continuity distal to the block. That combination is the NCS signature of fibular neuropathy at the fibular head.
- Axonal loss to EDB and TA. Low CMAPs from all stimulation sites, including distal sites, mean that motor axons have degenerated. That pattern is less localizing by itself. It can be a severe fibular-head lesion that has gone axonal, an L5-predominant radiculopathy, or a sciatic (fibular-division) injury. You then lean on sensory NCS, comparison with tibial studies, and the inversion finding discussed below.
Do not call every small EDB CMAP a fibular-head lesion. EDB can be small from local foot trauma, from an accessory deep fibular nerve that is not being stimulated at the ankle, or from age-related atrophy. Side-to-side comparison and a TA recording reduce that trap. Measure distances carefully across the fibular head; a short, inaccurately measured segment manufactures fake “slowing.”
Superficial fibular SNAP: postganglionic versus preganglionic
The superficial fibular sensory nerve action potential (SNAP) is recorded over the dorsum of the ankle or foot while stimulating the nerve in the lateral calf. Interpretation hinges on the dorsal root ganglion (DRG):
- In fibular neuropathy, if the lesion is axonal, Wallerian degeneration proceeds distal to the injury. The SNAP is low or absent because those sensory axons are postganglionic. A purely neurapraxic (demyelinating) fibular-head lesion can leave the SNAP intact; a low SNAP therefore supports axonal involvement, not merely a blocking injury.
- In an L5 root lesion, compression is typically proximal to the DRG (preganglionic). The peripheral sensory axon remains connected to its cell body, so the superficial fibular SNAP is typically spared, even when the patient has marked sensory loss in an L5 distribution and a very small TA CMAP.
A low superficial fibular SNAP with foot drop therefore argues against an isolated L5 radiculopathy and toward a postganglionic lesion (fibular nerve, lumbosacral plexus, or sciatic fibular division). A normal SNAP does not exclude a demyelinating fibular-head block; it does make a severe axonal L5 lesion more plausible if motor studies lack across-fibular-head CB.
Foot inversion: the tibial and L5 discriminator
Foot inversion is produced primarily by tibialis posterior, a tibial-innervated muscle that still belongs to the L5 myotome. That single fact splits the differential:
- Fibular neuropathy at the fibular head: ankle dorsiflexion and eversion are weak; inversion is spared because the tibial nerve never travels around the fibular neck.
- L5 radiculopathy: dorsiflexion, eversion, and inversion can all be weak, because L5 motor fibers supply TA, the fibularis muscles, and tibialis posterior.
Eversion weakness is not a discriminator: both fibular neuropathy (superficial fibular) and L5 radiculopathy can impair eversion. Sensory loss confined to the first web space (deep fibular) plus the dorsal foot (superficial fibular), with a spared plantar sole, fits a common fibular lesion. An L5 dermatome can involve the dorsum of the foot but often extends in a root pattern and may be accompanied by hip abduction weakness (gluteus medius) that a fibular-head lesion cannot produce.
Anatomic pearl: short-head biceps femoris
The short head of biceps femoris is the only hamstring muscle innervated by the common fibular division of the sciatic nerve, and that innervation occurs above the fibular head. Needle electromyography (EMG) of this muscle is a physician study; the technologist does not needle it. The pearl is anatomic, not a request to expand the NCS protocol: if a physician later finds abnormal short-head biceps femoris activity in a patient with fibular-distribution weakness, the lesion cannot be confined to the fibular head — it must involve the sciatic nerve (fibular division) or a more proximal L5/S1 process. Conversely, a fibular-head lesion should leave that muscle unaffected.
Localization table
| Finding | Fibular neuropathy at fibular head | L5 radiculopathy |
|---|---|---|
| Across-fibular-head CB or focal slowing | Present in demyelinating lesions | Absent |
| EDB/TA CMAP axonal loss | May occur if the lesion is severe | Common if the root lesion is axonal |
| Superficial fibular SNAP (axonal injury) | Low (postganglionic) | Typically spared (preganglionic) |
| Foot inversion (tibialis posterior) | Spared | Often weak |
| Foot eversion and dorsiflexion | Weak | Weak |
| Short-head biceps femoris (physician needle EMG) | Normal | May be abnormal if L5/S1 axon loss reaches that myotome; also abnormal in sciatic fibular-division lesions |
| Hip abduction | Normal | May be weak (gluteus medius) |
Case: foot drop after weight loss, leg crossing, and casting
A 34-year-old man completes a rapid 40-pound weight-loss program. He studies with the right leg crossed over the left for long sessions. After a distal fibular fracture is treated in a short-leg cast, he notices a right foot drop and numbness on the dorsum of the foot. Plantar flexion and inversion remain strong. NCS shows a large EDB CMAP with ankle and below-fibular-head stimulation, a greater than 50 percent amplitude drop stimulating above the fibular head, and focal slowing across that segment. Superficial fibular SNAP amplitude is mildly reduced compared with the left. Tibial motor studies and the sural SNAP are normal.
This is the textbook fibular neuropathy at the fibular head: mechanical compression against bone, a demyelinating across-fibular-head conduction block, and enough axonal sensory involvement to drop the SNAP. An L5 radiculopathy would not produce a focal block at the fibular neck, would more often spare the SNAP completely if purely radicular, and would threaten inversion. Document stimulation distances, skin temperature, and side-to-side amplitudes so the physician can interpret true block versus a technical drop-off from submaximal proximal stimulation.
Key Takeaways
- Across-fibular-head CB or slowing is the localizing NCS finding for fibular-head neuropathy.
- A low superficial fibular SNAP supports a postganglionic axonal lesion, not an isolated L5 root lesion.
- Spared inversion keeps the lesion on the fibular nerve; weak inversion pulls the localization toward L5.
- Short-head biceps femoris innervation above the fibular head is a physician EMG pearl, not a technologist needle site.
A patient has unilateral foot drop. Superficial fibular SNAP amplitude is low on the weak side, and the deep fibular CMAP shows conduction block across the fibular head. Which localization is most consistent with these findings?
Which clinical finding favors L5 radiculopathy over common fibular neuropathy at the fibular head?
The short head of biceps femoris is useful as an anatomic pearl in foot-drop localization because it is innervated by which nerve segment?