Sensory motor NCS and needle EMG findings in disorders that may mimic Carpal Tunnel Syndrome.
| Disorders | Sensory NCS | Motor NCS | Muscles involved by EMG |
|---|---|---|---|
| Motor neuron disease e.g. ALS | Normal38 | Median CMAP: low amplitude Ulnar CMAP: low amplitude38 | EMG signs of LMN dysfunction in at least 2 of the 4 CNS regions: bulbar, cervical, thoracic, or lumbosacral spinal segments38 |
| C8/T1 radiculopathy | Normal39 | Median CMAP: normal or low amplitude Ulnar CMAP: normal or low amplitude39 | All or some of C8/T1 supplied muscles (APB, FDI, ADM, FPL, EIP, and paraspinals)40 |
| Thoracic outlet syndrome (lower trunk) | Median SNAP: normal Ulnar and medial antebrachial SNAP: low amplitude41 | Median CMAP: low amplitude Ulnar CMAP: less severe involvement than median CMAP42 | All or some of C8/T1 supplied muscles (T1 worse than C8), sparing paraspinals41 |
| Medial cord lesion | Median SNAP: normal Ulnar and medial antebrachial SNAPs: low amplitude41 | Median CMAP: low amplitude Ulnar CMAP: low amplitude41 | C8/T1 muscles; spares fibers traveling through posterior cord (e.g., EIP) and Paraspinals41 |
| C5 radiculopathy | Normal39 | Median CMAP: normal Ulnar CMAP: normal39 | C5 muscles: supraspinatus, infraspinatus, deltoid, brachioradialis, biceps, and C5 paraspinals40 |
| C6 radiculopathy | Normal39 | Median CMAP: normal Ulnar CMAP: normal39 | C6 muscles: as in C5 + PT, FCR, triceps, anconeus, EDC and C6 paraspinals40 |
| C7 radiculopathy | Normal39 | Median CMAP: normal Ulnar CMAP: normal39 | C7 muscles: triceps, anconeus, PT, FCR, EDC, and C7 paraspinals40 |
| Upper trunk | Median-D1& 2 SNAP: low amplitude Radial SNAP: low amplitude Lateral antebrachial: low amplitude Ulnar SNAP: normal41 | Median CMAP: normal Ulnar CMAP: normal41 | All or some of C5/6 muscles (listed above) sparing paraspinals, serratus anterior and rhomboids41 |
| Lateral cord | Median-D1, 2 & 3 SNAP: low amplitude Lateral antebrachial: low amplitude Ulnar SNAP: normal41 Radial SNAP: normal41 | Median CMAP: normal Ulnar CMAP: normal41 | Biceps, brachialis, PT, and FCR41 |
| Median nerve at or proximal to the elbow | Median SNAP: decreased Ulnar SNAP normal43 | Median CMAP: low amplitude Ulnar CMAP: normal43 | APB, FPL, FDP-D2 & 3, FDS, PQ, FCR and PT43 |
| AIN neuropathy | Normal | Normal | FPL, FDP-D2 & 3, PQ43 |
| Length dependent axonal polyneuropathy | Sural SNAPs are affected earlier and more severely than upper limb SNAPs44 | Lower limb CMAPs are affected earlier and more severely than upper limb CMAPs44 | Denervation is worse in distal compared to proximal muscles, and in lower more than upper limbs44 |
ADM - abductor digiti minimi, AIN - anterior interosseous nerve, ALS - amyotrophic lateral sclerosis, APB - abductor pollicis brevis, CMAP - compound muscle action potential, EDC - extensor digitorum communis, EIP - extensor indicis proprius, EMG - electromyography, FCR - flexor carpi radialis,
FDI - first dorsal interosseous, FDP D2 & 3 - flexor digitorum profundus digits 2 and 3, FDS - flexor digitorum superficialis, FPL - flexor pollicis longus, LMN - lower motor neuron, NCS - nerve conduction study, PQ - pronator quadratus, PT - pronator teres, SNAP - sensory nerve action potential