14.2 Tarsal Tunnel, Tibial Neuropathy, and S1 Localization
Key Takeaways
- Isolated tarsal tunnel (tibial nerve at the ankle) produces plantar mixed or sensory slowing or amplitude loss and may prolong the abductor hallucis CMAP distal latency.
- The sural SNAP is typically spared in isolated tarsal tunnel because the sural nerve is formed in the calf and does not pass under the flexor retinaculum.
- The tibial H-reflex tests the S1 and proximal tibial pathway and does not travel through the tarsal tunnel, so it helps separate S1/proximal lesions from ankle entrapment.
- Length-dependent polyneuropathy usually drops sural and plantar potentials together, often bilaterally, unlike a unilateral retinacular lesion.
- Cool temperature, edematous feet, and high electrode impedance from callus or lotion can mimic plantar slowing or an absent mixed potential.
Tarsal tunnel syndrome is tibial-nerve compression at the medial ankle, under the flexor retinaculum. Outline topic VI.B.5.f groups this with tibial neuropathy at the ankle and asks you to separate it from S1 radiculopathy and from length-dependent polyneuropathy. Independent OpenExamPrep material here focuses on what a technologist can record: plantar mixed and sensory studies, the abductor hallucis (AH) CMAP, the sural SNAP, and the tibial H-reflex. Needle EMG of S1 muscles remains a physician study; your job is to produce NCS geography the physician can trust.
Anatomy the protocol actually uses
The tibial nerve enters the tarsal tunnel behind the medial malleolus with the posterior tibial artery and the flexor tendons. Inside or just distal to the tunnel it divides into the medial plantar, lateral plantar, and calcaneal branches. Those plantar nerves supply the sole of the foot and intrinsic muscles such as AH (medial plantar) and abductor digiti minimi pedis (lateral plantar).
The sural nerve does not travel through the tarsal tunnel. It is formed in the calf from the medial sural cutaneous nerve (tibial) and, in most people, the sural communicating branch from the lateral sural/fibular complex. Standard sural NCS stimulates in the calf and records behind the lateral malleolus — proximal to the flexor retinaculum. Isolated tarsal tunnel therefore has no anatomic reason to abolish the sural SNAP.
The H-reflex is a late response that tests Ia afferents and the S1/proximal tibial pathway. Typical technique uses submaximal stimulation of the tibial nerve in the popliteal fossa and recording over soleus. That circuit never enters the tarsal tunnel. An abnormal H-reflex with normal plantar distal studies points proximal (S1 root, sacral plexus, sciatic, or proximal tibial), not to the ankle retinaculum.
Plantar mixed and sensory NCS
Medial and lateral plantar mixed or sensory studies are the most direct NCS window on the tarsal tunnel. Mixed studies often stimulate in the sole (or on the toes for orthodromic sensory) and record posterior to the medial malleolus, capturing the segment that crosses the tunnel.
Expect one or both of:
- Slowing across the ankle — prolonged latency or reduced mixed-nerve velocity compared with the contralateral side or the laboratory reference
- Amplitude loss of the plantar mixed or sensory potential, which implies axonal loss or conduction block in plantar fibers
Side-to-side comparison matters more than a single absolute latency. Many laboratories treat a clear unilateral latency shift of several tenths of a millisecond, or a clear unilateral amplitude drop, as more persuasive than a borderline bilateral value — especially in older patients, in whom plantar responses can be small even without entrapment. Report distance, temperature, and whether the potential was truly absent after impedance was corrected.
AH CMAP
Tibial motor NCS to AH (and sometimes to abductor digiti minimi pedis) adds a motor dimension. Isolated tarsal tunnel may show a prolonged distal latency to AH if demyelination is distal, with a relatively preserved CMAP amplitude. Axonal tarsal tunnel or a more proximal tibial/S1 lesion drops the AH amplitude. A prolonged AH distal latency with slowing of plantar mixed studies supports an ankle-level lesion. A low AH CMAP with a normal plantar mixed study and an abnormal H-reflex is the wrong pattern for isolated tarsal tunnel; think S1 or proximal tibial/sciatic.
Sural SNAP as a geographic marker
Use the sural SNAP as a geographic test, not as a tarsal-tunnel test:
- Isolated tarsal tunnel: sural SNAP typically spared
- Length-dependent polyneuropathy: sural SNAP is often the first lower-limb sensory response to fall, and plantar potentials fall with it; both feet are usually involved
- S1 radiculopathy: sural SNAP is typically spared because the lesion is preganglionic (proximal to the DRG), analogous to the spared superficial fibular SNAP in L5 radiculopathy
- Sciatic neuropathy or sacral plexopathy: sural SNAP may be low because those lesions are postganglionic
A unilaterally absent sural SNAP plus plantar abnormalities should make you think beyond an isolated retinacular entrapment — sciatic, plexus, ganglionopathy, or polyneuropathy, depending on the opposite limb and the fibular studies.
H-reflex: S1 and the proximal tibial pathway
The tibial H-reflex is one of the few routine NCS tools that interrogates the proximal S1 pathway. Practical points:
- Compare side-to-side latency (many laboratories treat about a 1.5 ms difference as significant) and presence versus absence
- An absent or delayed H-reflex with preserved distal tibial CMAPs supports S1 radiculopathy or another proximal lesion
- A normal H-reflex does not exclude every S1 lesion (a very chronic or motor-predominant root lesion can trick you), but a normal H-reflex plus focal plantar slowing is the pattern you want for tarsal tunnel
- Bilateral absent H-reflexes in a patient over 60 years old are less specific; interpret them with the rest of the study and the clinical S1 signs (plantar flexion, Achilles reflex, lateral-foot sensation)
Use a true submaximal H stimulus first, then a supramaximal M-wave. Mixing those technical steps produces fake “absent H-reflexes.”
Tarsal tunnel versus S1 versus polyneuropathy
| Feature | Isolated tarsal tunnel (tibial at ankle) | S1 radiculopathy | Length-dependent polyneuropathy |
|---|---|---|---|
| Plantar mixed/sensory | Focal slowing and/or low amplitude across the ankle | Typically spared (preganglionic sensory) | Low or absent, usually bilateral |
| AH CMAP | Prolonged distal latency; amplitude may fall if axonal | May be low if axonal S1 | Often low when intrinsic-foot muscles are wasted |
| Sural SNAP | Spared | Typically spared | Low or absent |
| Tibial H-reflex | Typically normal | Often delayed or absent | Often abnormal, usually bilaterally |
| Distribution | Plantar sole, medial-ankle Tinel, night burning | S1 dermatome, calf, lateral foot; Achilles reflex | Stocking sensory loss, both feet |
| Opposite limb | Usually normal | May be abnormal if radiculopathy is bilateral | Usually abnormal |
Technical traps: swollen feet, temperature, and impedance
Plantar studies fail for technical reasons more often than fibular motor studies. Before you report “absent plantar mixed potentials”:
- Temperature. Cool feet prolong latencies and reduce velocities. A “slow” plantar mixed study on a 28°C sole is not tarsal tunnel. Warm the limb to the laboratory range (often near 32°C at the recording site) and re-measure.
- Swollen feet. Edema increases the distance from electrode to nerve, dampens SNAP and mixed amplitudes, and raises stimulus requirements. Document pitting edema. A side-to-side comparison on an equally swollen opposite foot is more honest than a comparison with a thin laboratory control.
- High impedance. Callus, lotion, dry cracked skin, and edema all raise electrode impedance. Abrade, clean, and confirm impedance before concluding axonal loss. High impedance also distorts the stimulus artifact so that a mixed-nerve potential hides in the noise.
- Stimulus site and sweep. Plantar mixed responses are small. Use adequate gain, a sweep that captures the expected latency, and a stimulus that is supramaximal without flooding the whole foot through volume conduction.
- Patient tolerance. Plantar stimulation is uncomfortable. A submaximal shock can mimic conduction block or a low amplitude. Explain the procedure and obtain a true supramaximal response when the patient can tolerate it.
Case: plantar burning versus an S1 story
A 48-year-old cashier reports burning in the sole of the right foot, worse at night, with a Tinel sign behind the medial malleolus. The Achilles reflex is symmetric. NCS: right medial plantar mixed latency is 0.8 ms longer than the left with a 50 percent amplitude drop; AH distal latency is prolonged; sural SNAPs are symmetric and normal; tibial H-reflex latencies are symmetric. Left plantar studies are normal. This pattern is tibial neuropathy at the ankle, not S1: the lesion is distal, the sural and H-reflex pathways are intact, and the findings stop at the retinaculum.
Contrast a second patient with posterolateral calf pain, a reduced Achilles reflex, and lateral-foot numbness. Plantar mixed studies are normal, AH amplitude is mildly low, the sural SNAP is intact, and the H-reflex is absent on that side only. That is an S1 localization pattern. A third patient has bilateral burning feet, absent sural and plantar potentials, and delayed H-reflexes on both sides — polyneuropathy, not a unilateral tarsal tunnel.
Key Takeaways
- Plantar mixed or sensory slowing or loss is the direct NCS window on the tarsal tunnel.
- Sural sparing argues against polyneuropathy and against a postganglionic sciatic lesion as the whole story.
- The H-reflex belongs to S1 and the proximal tibial pathway, not to the flexor retinaculum.
- Warm the foot, lower impedance, and document edema before you call a plantar study absent.
In isolated tibial neuropathy at the tarsal tunnel, the sural SNAP is typically which of the following?
Which study is most useful for assessing the S1 and proximal tibial pathway when distinguishing S1 radiculopathy from isolated tarsal tunnel syndrome?
A plantar mixed study looks spuriously slow and small. Which technical factor must be corrected before interpreting the tracing as tarsal-tunnel slowing?