Flail Arm After a Highway Crash: Reading the SNAPs
Ms. K is a 24-year-old right-handed motorcycle courier who crashed at highway speed 3 weeks ago. Her right arm is flail: deltoid, biceps, triceps, and wrist extensors are 0/5, finger flexors are 3/5, with sensory loss across the C5-T1 dermatomes and a right Horner syndrome. Reflexes are absent from biceps through triceps. Imaging shows pseudomeningoceles at C6 and C7 and no fracture. Nerve conduction studies show normal right median, ulnar, and radial SNAPs (42, 30, and 26 µV), median and ulnar CMAPs of 1.2 and 0.8 mV, and an absent musculocutaneous CMAP. Needle EMG shows fibrillations in the C6 and C7 paraspinals and no motor unit potentials in deltoid or biceps. The orthopedic surgeon proposes observation for 3 months before deciding about exploration.
1. Are these injuries preganglionic or postganglionic? Justify the answer from the specific electrodiagnostic and imaging findings, and explain the paraspinal abnormality.
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Preganglionic - root avulsion.
The decisive finding is the combination of normal SNAPs with absent or markedly reduced CMAPs. The sensory neuron cell body sits in the dorsal root ganglion within the intervertebral foramen. If the lesion is proximal to the ganglion (avulsion), the peripheral sensory axon remains connected to its cell body and the SNAP stays normal even though the patient is insensate. If the lesion is distal to the ganglion (rupture), Wallerian degeneration reaches the peripheral nerve and the SNAP disappears within roughly 7-11 days. At 3 weeks, a postganglionic lesion would already show absent SNAPs.
Supporting features:
- Paraspinal fibrillations at C6 and C7: the paraspinal muscles are innervated by the dorsal rami, which arise at the root level. Denervation there places the lesion at or proximal to the root.
- Horner syndrome: preganglionic sympathetic fibers travel with the T1 root, implicating the lower trunk/root levels.
- Pseudomeningoceles at C6 and C7: these are radiographic markers of dural sleeve avulsion and correlate with, though do not prove, root avulsion.
Note that CMAPs are still present in median and ulnar distributions despite 0/5 distal strength, consistent with the early stage and with residual motor axons before full degeneration.
2. Is the surgeon's plan to observe for 3 months appropriate? Outline the timing and priorities of surgical referral and reconstruction.
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Observation for 3 months without referral is not appropriate. This is a pan-plexus injury with multiple root avulsions; the reconstructive clock starts at the time of injury, not at the time of a delayed decision.
What to recommend now:
- Refer to a brachial plexus/peripheral nerve surgeon immediately for co-management rather than after 3 months.
- Repeat the electrodiagnostic study at approximately 6-8 weeks and again at 3-4 months. The 3-week study can understate denervation; later studies look for nascent motor unit potentials, which are the first sign of reinnervation and change the operative plan.
- Obtain high-resolution MRI or CT myelography of the cervical roots if not already definitive.
Timing: nerve transfers are generally best performed within 6 months of injury, and some surgeons operate at 3-6 months once the pattern of root vs trunk injury is clear. Motor endplates degenerate with prolonged denervation, so transfers late in the second year yield far less.
Priorities:
- Elbow flexion (musculocutaneous), often via ulnar or median fascicular transfer when a donor is available, or intercostal transfer.
- Shoulder stability and abduction (spinal accessory to suprascapular).
- Triceps and wrist extension.
- Hand function last, and with the poorest prognosis when the lower trunk is avulsed.
Avoid phrenic-to-musculocutaneous transfer if diaphragmatic function is uncertain. Meanwhile: shoulder sling and range-of-motion to prevent contracture and subluxation, neuropathic pain management, and hand therapy.
3. The 3/5 finger flexion is unexpected given the C5-T1 sensory loss. What are the possible explanations, and how does this alter prognosis and planning?
Read the worked answer
Partial finger flexion does not overturn the avulsion diagnosis, but it must be explained before committing to reconstruction.
The normal median and ulnar SNAPs at 3 weeks make a postganglionic lower-trunk rupture unlikely for the bulk of the lower trunk, because Wallerian degeneration would have eliminated those responses. That leaves several possibilities:
- Incomplete avulsion with spared rootlets. Avulsion is often partial; a few surviving anterior horn cells can supply 3/5 force to the long finger flexors. This is the most likely explanation and carries a meaningfully better prognosis for the hand than complete lower-trunk loss.
- Contribution from C7 or from an intact neighboring root. Cross-innervation and rootlet variability exist; do not assume that visible finger flexion equals functional hand use.
- A false localizing contraction, such as tenodesis effect or proximal substitution, which should be excluded by careful isolated testing.
How this changes the plan:
- Preserve and protect whatever lower-trunk function exists: no exploration that risks sacrificing intact fascicles, and no tendon transfers that burn bridges.
- Track it serially. If finger flexion strengthens over 6-12 weeks with nascent units on EMG, you have evidence of continuity and can defer hand reconstruction.
- Prioritize elbow flexion and shoulder control, which remain absent and are the rate-limiting functions for independence, and counsel that useful grasp is the least likely recovery.