EDX · EP 08 · ELECTRODIAGNOSTICS
Before You Listen
Before You Listen
- Prerequisites: brachial plexus organization (Roots-Trunks-Divisions-Cords-Branches) and the SNAP/paraspinal/distribution rules that distinguish plexopathy from radiculopathy (covered in Episode 7); the dorsal root ganglion (DRG) principle that sensory nerve action potentials (SNAPs) are preserved in radiculopathy and abnormal in plexus or peripheral nerve lesions; nerve conduction study (NCS) basics including compound muscle action potential (CMAP) amplitude, conduction velocity (CV), and distal motor latency (DML); the difference between conduction block (focal demyelination) and reduced distal CMAP amplitude (axonal loss with Wallerian degeneration); needle electromyography (EMG) basics (fibrillation potentials, positive sharp waves, motor unit action potentials).
- Runtime: 1 hour 5 minutes 45 seconds.
- Topic in one line: carpal tunnel syndrome (CTS) with LOAF muscles and the comparison studies (palmdiff, ringdiff, thumbdiff, second lumbrical-interossei) that take sensitivity from 75% to 95%; Bland severity (0-6); pronator and anterior interosseous nerve (AIN) syndromes; ulnar neuropathy at the elbow (UNE) with the across-elbow CV < 50 m/s rule, inching, and the dorsal ulnar cutaneous (DUC) SNAP separating UNE from Guyon canal; the Shea classification at Guyon canal; Saturday night palsy (triceps spared) and posterior interosseous nerve (PIN) syndrome (pure motor, ECRL spared); Wartenberg syndrome (pure superficial radial sensory); musculocutaneous, axillary, suprascapular (notch vs spinoglenoid), and long thoracic neuropathies; demyelinating versus axonal patterns. Memorize them cold.
Vignette. A 47-year-old right-handed woman who works as a packer presents with a four-month history of nocturnal numbness and tingling of the right thumb, index, and middle fingers. She wakes multiple times each night and shakes her hand for relief. Recently, she has dropped jars and noted thinning of the muscle at the base of her thumb. On examination, the abductor pollicis brevis is visibly atrophic, the thumb sits in the plane of the palm, and sensation is reduced over the volar surface of digits 1 through 3 but is normal over the thenar eminence itself. Phalen and Tinel signs at the wrist are positive. Nerve conduction studies show a median sensory distal latency of 5.6 milliseconds with an absent sensory nerve action potential recording from digit 2 at 14 cm. The median motor distal latency is 6.8 milliseconds with a compound muscle action potential of 1.4 millivolts recording from the abductor pollicis brevis. Ulnar sensory and motor responses are normal, and the second lumbrical-interossei latency difference is 1.2 milliseconds favoring the ulnar (interossei) response. Needle electromyography of the abductor pollicis brevis reveals fibrillation potentials, positive sharp waves, and large-amplitude polyphasic motor unit action potentials with reduced recruitment.
Why is sensation preserved over the thenar eminence in this patient when the rest of the median territory is numb, what specifically does the absent median sensory response with a markedly prolonged motor distal latency and reduced compound muscle action potential tell you about severity by the Bland classification, why is the second lumbrical-interossei comparison the single most useful study in this scenario, and what is the most common compression site that explains a similar presentation extending up into the proximal forearm?
(Answer at the end of this chapter)
Section 1: Carpal Tunnel Syndrome — Anatomy, LOAF, and the Comparison Studies
Bottom line: carpal tunnel syndrome (CTS) is the most common entrapment neuropathy and the most heavily tested EDX scenario; the carpal tunnel contains the median nerve and 9 flexor tendons under the transverse carpal ligament; the LOAF mnemonic identifies the four median-innervated hand muscles distal to the tunnel; absolute median latencies have ~75% sensitivity, but comparison studies (palmdiff, ringdiff, thumbdiff, and second lumbrical-interossei) push sensitivity to 85-95%.
The carpal tunnel is an osteofibrous canal at the wrist with the carpal bones as floor and walls and the transverse carpal ligament (flexor retinaculum) as the roof. Ten structures pass through it: the median nerve and 9 flexor tendons (4 FDS, 4 FDP, 1 FPL). The median nerve is the most superficial structure under the ligament, which is why it is the first to suffer when tunnel pressure rises. At the distal edge of the tunnel the median nerve divides into the recurrent motor branch (curving back over the thenar eminence to supply the thenar muscles) and the digital sensory branches (palmar surface of digits 1-3 and the radial half of digit 4).
The LOAF mnemonic identifies the four median-innervated muscles distal to the tunnel: Lumbricals 1-2; Opponens pollicis; Abductor pollicis brevis (APB); Flexor pollicis brevis (superficial head). The APB is the standard motor recording site because it is innervated exclusively by the recurrent branch and is technically easy to examine. Severe CTS with thenar atrophy produces the ape hand deformity with the thumb falling into the plane of the palm. One critical anatomic detail separates CTS from proximal median lesions: the palmar cutaneous branch arises proximal to the tunnel and runs superficial to the transverse carpal ligament rather than through the tunnel. Sensation over the thenar eminence is therefore spared in CTS; thenar-eminence numbness localizes the lesion proximal to the tunnel (pronator syndrome).
Clinical Pearl — Thenar-eminence sensation is the proximal-vs-distal median tell
A patient with median digit-1 to digit-3 numbness who also has numbness over the thenar eminence does not have CTS. The palmar cutaneous branch leaves the median nerve proximal to the tunnel and runs over the top of the transverse carpal ligament; CTS spares it. Numbness over the thenar eminence localizes the median lesion proximal to the tunnel, most commonly to pronator syndrome. Conversely, classic CTS gives palmar digit-1-to-3 numbness with a spared thenar pad.
The AANEM CTS protocol uses median sensory NCS (recording digit 2 or 3), median motor NCS (recording APB), at least one comparison study, ulnar or radial NCS to exclude polyneuropathy, and needle EMG of APB and selected C5-T1 muscles. The earliest abnormality is prolonged median sensory distal latency because sensory fibers are smaller-diameter myelinated fibers more susceptible to compression. Absolute thresholds: median sensory distal latency > 3.5 ms (digit 2, 14 cm), median motor distal latency > 4.2 ms (APB, 8 cm), median sensory CV < 50 m/s. Absolute values alone reach approximately 75% sensitivity.
Comparison studies push sensitivity to 85-95% by using the median nerve’s neighbors as internal controls. The palmdiff (median-ulnar palmar mixed) stimulates at the palm and records both at the wrist; abnormal > 0.3-0.4 ms favoring median, sensitivity ~95% (the highest single test). The ringdiff records from digit 4 and stimulates median and ulnar at the wrist; abnormal > 0.4-0.5 ms, ~90%. The thumbdiff records from digit 1 comparing median to radial; abnormal > 0.4-0.5 ms, ~85%. The second lumbrical-interossei (2L-INT) is the most useful in severe CTS with absent median SNAP: both muscles are recorded from the same hand location, eliminating distance as a variable; abnormal > 0.4 ms. When a stem describes an absent median SNAP, the 2L-INT is the answer.
CTS severity is graded by two systems. The Combined Sensory Index (Robinson CSI) sums palmdiff + ringdiff + thumbdiff; a CSI ≥ 0.9 ms is abnormal. The Bland scale runs grade 0 (normal) to grade 6 (extremely severe). Grade 1 = very mild (only comparison or inching abnormal). Grade 2 = mild (sensory CV < 40 m/s, normal DML < 4.5 ms). Grade 3 = moderate (DML 4.5-6.5 ms, sensory CV < 40 m/s). Grade 4 = severe (DML 4.5-6.5 ms with absent SNAP). Grade 5 = very severe (DML > 6.5 ms). Grade 6 = extremely severe (CMAP < 0.2 mV), with poor prognosis after release.
Needle EMG of APB in moderate-to-severe CTS assesses motor axon loss: fibrillations and positive sharp waves (active denervation), long-duration polyphasic MUAPs (chronic reinnervation), reduced recruitment (axon loss). The needle exam also excludes proximal lesions. Temperature is critical: AANEM requires hand temperature ≥ 32°C, because cooling slows conduction across all nerves and can mask the median-vs-ulnar difference. A vignette of “normal” CTS studies with a 28°C hand and classic symptoms requires warming and repeat testing.
High Yield — CTS
- Anatomy: 10 structures (median + 9 tendons); transverse carpal ligament roof; median nerve most superficial.
- LOAF: Lumbricals 1-2, Opponens pollicis, APB, FPB superficial head (recurrent branch territory).
- Palmar cutaneous branch spares thenar-eminence sensation in CTS; thenar numbness localizes proximal (pronator).
- Absolute thresholds: median sensory DL > 3.5 ms; median motor DML > 4.2 ms; sensory CV < 50 m/s. Sensitivity ~75%.
- Comparison studies: palmdiff (>0.3-0.4 ms, ~95% sensitivity); ringdiff (>0.4-0.5 ms, ~90%); thumbdiff (>0.4-0.5 ms, ~85%); 2L-INT (>0.4 ms) is the study of choice when median SNAP is absent.
- Bland grade 6 = CMAP < 0.2 mV → poor prognosis after release.
- Temperature ≥ 32°C required; cooling can falsely normalize.
Instead of diving deep into the tunnel with the rest of the nerve, it travels superficial to the transverse carpal ligament to supply sensation to the skin over the thenar eminence. So it takes the bypass route over the tunnel. Sensation over the thenar eminence—that fleshy base of the thumb—is preserved in true carpal tunnel syndrome.
— EDX-08 podcast, ~09:53