15.3 Plexus Lesions: Erb’s Palsy, Neurogenic TOS, and Parsonage-Turner

Why plexus patterns are SNAP patterns

Plexus lesions are where sensory NCS earn their keep. A postganglionic plexus lesion drops the relevant sensory nerve action potential (SNAP) because the lesion sits distal to the dorsal-root ganglion. A preganglionic root avulsion can devastate motor axons while preserving the SNAP, because the ganglion and the peripheral sensory process remain in continuity. Independent OpenExamPrep teaching for this outline cluster—Erb’s palsy, true neurogenic thoracic outlet syndrome (nTOS), and Parsonage-Turner syndrome—is about matching which SNAPs fall to which trunk, then adding the motor CMAPs that should travel with that trunk. Needle EMG of paraspinals can support preganglionic localization in a physician examination; it is not an R.NCS.T. conduction step, but you still need to know why preserved SNAPs matter.

Erb’s palsy: upper trunk C5–C6

Erb’s palsy is an upper-trunk (C5–C6) lesion. In newborns it follows shoulder dystocia or downward traction on the head. In older patients it follows stretch, motorcycle trauma, or a severe burner/stinger. The classic posture is waiter's tip: shoulder adducted and internally rotated, elbow extended, forearm pronated, wrist often flexed. That posture is combined loss of abductors and external rotators (suprascapular, axillary), elbow flexors (musculocutaneous), and sometimes C6-weighted wrist extensors.

SNAPs that should fall (postganglionic upper trunk)

Motor NCS: low CMAPs from biceps, deltoid, and supraspinatus/infraspinatus; relatively spared abductor pollicis brevis (APB) and abductor digiti minimi (ADM). Accessory trapezius CMAP should be spared (CN XI, section 15.1). Phrenic may be involved if the stretch reached C4, but that is not required for the Erb label.

Root avulsion versus trunk. Flail C5–C6 muscles with preserved LABC, median-thumb, and radial SNAPs push you toward preganglionic avulsion. Abnormal SNAPs push you toward postganglionic upper trunk (true plexopathy). Early after trauma, remember Wallerian timing from the facial-nerve lesson: CMAPs can still look deceptively large in the first several days. Sensory potentials also take days to fade. Do not close localization on a day-1 SNAP panel after a high-energy stretch.

A median SNAP to the middle finger is a poor sole “upper trunk” sensory test. That digit is largely C7. If you skip LABC and digit 1, you can call a real upper-trunk lesion “normal SNAPs.”

True neurogenic thoracic outlet syndrome

True neurogenic thoracic outlet syndrome is uncommon and anatomically specific. It is a lower-trunk lesion, with T1 fibers often more affected than C8. A fibrous band from an incomplete cervical rib or an elongated C7 transverse process tethers the lower trunk. It is a chronic axonal process, not a weekend of arm numbness from a backpack, and not the disputed pain-only “TOS” that has normal SNAPs.

The SNAP and thenar signature

• Ulnar SNAP is chronically low (C8 sensory fibers traveling in the lower trunk).
• MABC SNAP is especially low or absent (T1). A very small MABC with a relatively less affected ulnar SNAP is the fingerprint many EDX discussions emphasize.
• Median SNAPs (especially middle finger, C7) are typically preserved. Thumb SNAP is not your nTOS marker.
• Thenar median motor CMAP to APB (T1-weighted) is often more reduced than the hypothenar ulnar CMAP to ADM (more C8). That thenar-greater-than-hypothenar motor pattern is the opposite of a garden ulnar neuropathy at the elbow (UNE), which should spare APB and spare MABC.

The wasted hand of classic nTOS (Gilliatt-Sumner pattern) emphasizes APB and first dorsal interosseous, with ADM relatively better. Do not force that picture onto every patient with a cervical rib on an x-ray and a normal MABC.

If MABC is normal, a low ulnar SNAP is not nTOS. If APB is normal and there is focal ulnar slowing at the elbow, stay at the elbow.

Parsonage-Turner syndrome (neuralgic amyotrophy)

Parsonage-Turner syndrome (neuralgic amyotrophy, acute brachial neuritis) is an acute painful patchy plexopathy or extra-plexus mononeuritis multiplex. Severe pain lasts days to a couple of weeks; weakness becomes obvious as pain recedes. Favorite targets: anterior interosseous nerve (AIN), long thoracic, suprascapular, also axillary, musculocutaneous, and phrenic. The patchiness is the point: it does not respect a single dermatome or a clean upper-versus-lower trunk map.

SNAPs may be patchy—LABC falls if musculocutaneous sensory fibers are hit; radial or median SNAPs may be normal if those fascicles were spared. Many attacks are motor-predominant, so you do not require nTOS-style dual SNAP loss. Long thoracic and AIN lesions may have no abnormal SNAP at all, because those nerves are essentially motor. After the pain, look for serratus winging, spinati wasting, or AIN signs (weak OK sign, unopposed flexion posture of thumb and index) rather than forcing every case into Erb or nTOS templates. Recurrent or familial attacks exist; the NCS pattern is still patchy, not a mandatory lower-trunk signature.

SNAP-pattern table: upper trunk vs lower trunk vs patchy neuralgic amyotrophy

Handbook case: which studies support upper-trunk plexopathy

A 24-year-old rider is thrown onto the right shoulder. The arm hangs in waiter's-tip posture. Sensation is reduced on the lateral forearm and thumb. You are asked which NCS support upper-trunk plexopathy rather than isolated axillary neuropathy, lower-trunk nTOS, or complete preganglionic avulsion.

Supportive set:

1. Low or absent LABC SNAP
2. Low median SNAP to digit 1
3. Low superficial radial SNAP
4. Normal ulnar SNAP and normal MABC SNAP
5. Low CMAPs from biceps, deltoid, and spinati
6. Relatively preserved APB and ADM CMAPs
7. Preserved accessory trapezius CMAP (do not treat trapezius as an upper-trunk muscle)

Not supportive:

• Isolated low deltoid CMAP with normal LABC and normal biceps CMAP → axillary mononeuropathy (section 15.2).
• Low MABC plus low ulnar SNAP plus low APB, spared spinati → lower trunk / nTOS.
• Normal SNAPs with a flail C5–C6 arm after high-energy trauma → think preganglionic avulsion, not postganglionic trunk axonal loss.
• Acute fire-like shoulder pain for a week, then serratus winging and a poor OK sign, with a messy SNAP map → shift pretest toward Parsonage-Turner and add long thoracic and AIN-related motor studies rather than stopping at an Erb SNAP panel.

Exam traps

• Using median SNAP to the middle finger as your only upper-trunk sensory test.
• Diagnosing nTOS without MABC, or equating UNE with nTOS because both can drop an ulnar SNAP.
• Calling every painful plexopathy nTOS, or every winged scapula Erb’s palsy.
• Ignoring Wallerian timing in acute trauma.
• Attributing trapezius loss to Erb’s palsy.
• Treating needle EMG of cervical paraspinals as the technologist’s required NCS maneuver for avulsion.