12.2 Ulnar Neuropathy at the Elbow and Guyon’s Canal
Key Takeaways
- Ulnar neuropathy at the elbow shows across-elbow slowing, conduction block, and/or axonal loss; measure that segment with the elbow flexed about 70–90° so skin distance does not underestimate nerve length.
- Inching localizes a latency or amplitude step to the retrocondylar groove versus the cubital tunnel a few centimeters distal to the medial epicondyle.
- First dorsal interosseous and abductor digiti minimi can be affected unequally because of fascicular anatomy; a normal ADM study does not exclude an elbow lesion.
- Dorsal ulnar cutaneous SNAP is spared in Guyon’s canal and is often low in axonal elbow lesions when those sensory fibers are involved.
- Guyon zone 1 is mixed, zone 2 is deep motor (FDI plus or minus ADM sparing), and zone 3 is superficial sensory; FCU and FDP to digits 4–5 remain normal in isolated Guyon lesions.
Ulnar neuropathy is the second localization the outline expects you to separate: VI.B.5.b Ulnar at the elbow versus VI.B.5.c Ulnar of Guyon’s canal. Both can numb digit 5 and weaken ulnar-innervated hand muscles. Electrode recipes were already covered; the NCS job now is to decide whether the lesion is across the elbow, at the wrist, or axonal without a focal demyelinating marker — and to use muscles and cutaneous branches that leave the nerve at known sites. Independent OpenExamPrep teaching for this pair of outline items is localization, not a claim of unpublished official AAET cutoffs.
Across-elbow slowing, conduction block, and axonal loss
Record ulnar motor responses from ADM and, when the question is fascicular or severe, from first dorsal interosseous (FDI). Stimulate at the wrist, below the elbow, and above the elbow. The across-elbow segment is where cubital-tunnel and retrocondylar compression live.
Three electrophysiologic faces of ulnar neuropathy at the elbow (UNE):
- Focal demyelinating slowing — across-elbow conduction velocity drops relative to the forearm segment, without a large amplitude step.
- Conduction block — a drop in CMAP amplitude and area across a short elbow segment that is not explained by a matching increase in duration (temporal dispersion). Distal CMAP may still look robust because stimulation at the wrist is distal to the block. That is why a patient can have a floppy, clumsy hand and a “normal” distal ADM CMAP.
- Axonal loss — distal ADM and/or FDI CMAP amplitudes fall, ulnar SNAP to digit 5 falls, dorsal ulnar cutaneous (DUC) SNAP often falls if those sensory axons are involved, and EMG shows denervation in ulnar-innervated hand muscles with or without flexor carpi ulnaris (FCU) and flexor digitorum profundus to digits 4–5 (FDP IV/V).
Typical lab teaching (not official unpublished AAET cutoffs) often flags an across-elbow velocity drop of about 10 m/s compared with the forearm, or an absolute across-elbow velocity below about 50 m/s, as suspicious when the segment is measured correctly. Amplitude-drop rules vary by lab; the concept to carry into the exam is a focal step at the elbow, not a slightly low number on a long, sloppy segment.
Elbow flexed for measurement
Measure the across-elbow distance with the elbow flexed about 70–90°. In full extension the ulnar nerve is redundant in the groove; skin distance underestimates true nerve length, so calculated velocity is falsely slow and you will over-call UNE. Flexion straightens the nerve path so the tape more closely matches anatomy. Keep the across-elbow segment long enough to reduce measurement error (commonly about 10 cm in teaching labs) but short enough that a focal lesion is not diluted by a long stretch of normal nerve. Below-elbow stimulation should sit distal to the cubital tunnel / humeroulnar arcade so the “forearm” velocity is not contaminated by the lesion you are trying to isolate.
Inching
Short-segment incremental stimulation (inching) moves the stimulator in 1–2 cm steps around the medial epicondyle. A sudden latency jump and/or amplitude step-off localizes the lesion to the retrocondylar groove (at or just proximal to the epicondyle) versus the cubital tunnel / Osborne’s ligament (typically 1–3 cm distal to the epicondyle). Inching is how you turn “slow across a 10 cm elbow segment” into a site you can name. It is also a check against co-stimulation or an anastomosis artifact that appears at only one stimulus site. Typical teaching labs look for a latency increase of several tenths of a millisecond over a 1 cm step plus an amplitude drop at the same site; use your lab’s published inching reference, and on the exam defend the step location, not a secret AAET millisecond.
FDI versus ADM (fascicular selectivity)
Ulnar motor fascicles are not uniformly compressed. FDI is often weaker and more abnormal on NCS and EMG than ADM in elbow lesions (the FDI fascicle may sit more superficially in the groove), but the reverse occurs. A normal ADM CMAP does not exclude UNE. If the clinical deficit is first-dorsal wasting, record FDI CMAP across the elbow and needle FDI. Distal latency to FDI is usually normal in an elbow lesion (the entrapment is proximal); a prolonged FDI distal latency with a normal across-elbow study pushes you toward Guyon / deep motor branch.
FCU and FDP IV/V may be spared even in genuine elbow lesions because of fascicular anatomy — so normal FCU EMG does not prove Guyon. Abnormal FCU or FDP IV/V EMG, however, places the lesion at or proximal to those branches in the elbow or proximal forearm and excludes isolated Guyon.
Dorsal ulnar cutaneous: the Guyon discriminator
The DUC leaves the ulnar nerve in the distal forearm, about 5–8 cm proximal to the ulnar styloid, and supplies the dorsal ulnar hand. It does not pass through Guyon’s canal.
- Abnormal DUC SNAP (with a low digit-5 SNAP) supports a lesion at or proximal to the DUC takeoff — elbow or forearm — if those sensory fibers are among the ones damaged.
- Normal DUC with a low palmar digit-5 SNAP is the classic Guyon sensory pattern (zones 1 or 3).
- A normal DUC does not fully exclude UNE: purely demyelinating elbow block can spare distal SNAPs entirely, and fascicular elbow lesions can spare DUC fibers.
So DUC is powerful when it is abnormal, and still useful when it is normal if the rest of the study shows a wrist-ulnar picture without across-elbow slowing.
Guyon zones
Guyon’s canal (ulnar tunnel) runs from the proximal pisiform to the hook of the hamate. Teaching zones:
| Zone | Anatomy | NCS / clinical pattern |
|---|---|---|
| Zone 1 | Proximal canal, before the nerve splits | Mixed: low digit-5 SNAP, low ADM and FDI CMAP possible, DUC spared, no across-elbow slowing |
| Zone 2 | Deep motor branch (pisohamate hiatus / hook of hamate) | Motor only: SNAP and DUC normal; FDI and interossei affected; ADM may be spared if the lesion is distal to the ADM branch; distal latency to FDI may be prolonged |
| Zone 3 | Superficial sensory branch | Sensory only: low digit-5 SNAP, normal motor, DUC spared |
Handlebar palsy in cyclists, a hook-of-hamate ganglion or fracture, and hypothenar hammer syndrome (ulnar-artery trauma) are common teaching causes. Wrist ulnar stimulation still activates the nerve proximal to some zone 2 lesions; palm or hamate stimulation and FDI distal latency are the motor clues.
Elbow versus Guyon localization table
| Feature | Ulnar neuropathy at the elbow | Ulnar neuropathy at Guyon’s canal |
|---|---|---|
| Across-elbow CV / conduction block | Slowing and/or block | Normal |
| Elbow inching | Latency or amplitude step at groove or cubital tunnel | No elbow step |
| DUC SNAP | Often low if axonal sensory fibers are involved; may be normal in pure block or fascicular sparing | Spared |
| Digit-5 (palmar) SNAP | Low if axonal; may be normal in pure demyelinating block | Low in zones 1 and 3; normal in zone 2 |
| FDI vs ADM | Fascicular; either may dominate | Zone 2: FDI plus or minus interossei; ADM may be spared |
| Distal latency to FDI | Usually normal | May be prolonged (deep branch) |
| FCU / FDP IV/V EMG | May be abnormal (or spared fascicularly) | Normal |
| Forearm ulnar CV distal to the elbow | Normal once you are below the lesion | Normal |
Case: conduction block at the elbow
A 41-year-old wakes with a weak, clumsy right hand after a long flight sleeping on a flexed elbow. Digit-5 SNAP and DUC SNAP are normal. Wrist ADM CMAP is 9.8 mV; below-elbow CMAP is 9.5 mV; above-elbow CMAP is 3.1 mV with a clear area drop and little duration increase. Across-elbow velocity is 38 m/s versus forearm 58 m/s, elbow flexed 90°, 10 cm segment. FDI shows the same proximal drop. Inching steps off just proximal to the medial epicondyle (retrocondylar groove). FCU EMG is normal; FDI shows reduced recruitment without fibrillations on day 5.
Interpretation: demyelinating ulnar neuropathy at the elbow with conduction block, axons still in continuity (distal SNAPs and CMAPs preserved). Do not call this Guyon: a wrist lesion does not drop the above-elbow CMAP, and DUC status is not the whole story when the elbow segment itself is blocked.
Case: Guyon zone 2 after a hamate hook injury
A 28-year-old cyclist has FDI wasting and weak finger abduction without numbness. Digit-5 SNAP and DUC are normal. ADM CMAP and ADM distal latency are normal. FDI CMAP is low with a prolonged distal latency. Across-elbow velocities from ADM and FDI are normal. Needle FDI shows fibrillations; ADM, FCU, and APB are normal.
Interpretation: deep motor-branch (zone 2) ulnar neuropathy at Guyon’s canal. Sparing of DUC and palmar SNAP is expected. Sparing of ADM is the distal-to-ADM-branch clue. This is not an elbow fascicular motor lesion, because the across-elbow study is clean and the FDI distal latency is long.
Case: zone 1 mixed Guyon versus axonal elbow
A carpenter has palmar numbness of digit 5 and hypothenar weakness. Digit-5 SNAP is low; DUC is normal. ADM and FDI CMAPs are mildly low; across-elbow velocities are 55 m/s with no block; inching is flat. FCU is normal. That is zone 1 Guyon (mixed motor and palmar sensory, DUC already gone). Contrast a similar hand in which DUC is also low and across-elbow velocity is 41 m/s: that is an axonal elbow lesion involving those sensory fibers, not Guyon.
Case: the “ulnar hand” that is not a mononeuropathy
If digit-5 SNAP, DUC, and medial antebrachial cutaneous SNAPs are all low, and ADM, FDI, and APB are denervated, think lower trunk / medial cord (low SNAPs) or consider C8–T1 root disease (SNAPs should be spared in a root lesion). Elbow-versus-Guyon tables assume the rest of the brachial plexus is intact. Independent OpenExamPrep localization names the site of conduction failure, whether block or axon loss dominates, and which spared structures (DUC, FCU, ADM, digit-5 SNAP) make Guyon or elbow the only fit.
Across-elbow ulnar motor velocity should be calculated with the elbow flexed about 70–90° primarily because:
A patient has a low ulnar SNAP to digit 5 and a low FDI CMAP. Dorsal ulnar cutaneous SNAP is normal, across-elbow conduction is normal without block, and flexor carpi ulnaris EMG is normal. The best localization is:
Isolated FDI and interosseous weakness, a normal digit-5 SNAP, a normal dorsal ulnar cutaneous SNAP, a prolonged distal latency to FDI, and a relatively spared ADM CMAP most likely represent: