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Open Access

Clinical and electrophysiological evaluation of carpal tunnel syndrome: approach and pitfalls

Mohammed H. Alanazy
Neurosciences Journal July 2017, 22 (3) 169-180; DOI: https://doi.org/10.17712/nsj.2017.3.20160638

Article Figures & Data

Figures

  • Figure 1
    Figure 1

    Median and ulnar motor nerve conduction studies recording abductor pollicis brevis and abductor digiti minimi, respectively. A) Electrode placement (black: active, red: reference, and green: ground) and stimulation sites. B) Compound muscle action potential waveform of the median (top trace) and ulnar (middle trace) nerves. Notice the change in the median waveform when ulnar co-stimulation occurs (bottom trance).

  • Figure 2
    Figure 2

    Median-ulnar antidromic sensory comparison study. Electrodes placement (black: active, red: reference, and green: ground) and stimulation sites are shown. A) Median digit 2 versus ulnar digit 5. B) Median digit 4 vs ulnar digit 4 sensory comparison studies. C) Sensory nerve action potential waveform morphology.

  • Figure 3
    Figure 3

    Comparison studies A) Median-radial antidromic sensory comparison study. B) Segmental median sensory conduction velocity of wrist-to-palm compared to palm-to-digit segments. Electrodes placement (black: active, red: reference, and green: ground) and stimulation sites are shown.

  • Figure 4
    Figure 4

    Palmar orthodromic mixed nerve study. A) Electrodes placement (black: active, red: reference, and green: ground) and stimulation sites. B) The recorded median and ulnar mixed nerve action potential (MNAP) waveforms and peak latency comparison.

  • Figure 5
    Figure 5

    Lumbrical-interossei comparison study. A) Electrodes placement (black: active, red: reference, and green: ground) and stimulation sites. B) In CTS, the median distal motor latency is prolonged (6.1 ms, arrow) in comparison to the ulnar latency (2.8 ms). The dashed-line in the top trace shows the change in median waveform morphology when ulnar co-stimulation occurs.

  • Figure 6
    Figure 6

    Stimulus lead. Median compound muscle action potential (CMAP) waveform is shown. Note the shortening of distal motor latency without increment in the amplitude (dashed-line) indicating a stimulus lead.

  • Figure 7
    Figure 7

    Combination of Martin-Gruber Anastomosis (MGA) and Carpal Tunnel Syndrome (CTS). An MGA with cross-over fibers innervating the thenar muscles. Routine median motor study reveals higher compound muscle action potential (CMAP) amplitude with antecubital-fossa stimulation than that obtained with wrist stimulation. Ulnar nerve stimulation recording thenar muscles shows a drop in CMAP amplitude with proximal stimulation. With a coexistent CTS, there is a positive deflection (arrow) with antecubital- fossa stimulation, and a factitiously fast conduction velocity of the median nerve in the forearm.

  • Figure 8
    Figure 8

    Nerve conduction study algorithm for patients with suspected CTS.

  • Figure 9
    Figure 9

    An NCS grading scale for the severity of CTS. The severity of involvement of NCS parameters generally follows the pattern depicted in the chart above. An unequivocal deviation from this pattern indicates a coexistent or a different diagnosis.

Tables

  • Table 1

    Sensory motor NCS and needle EMG findings in disorders that may mimic Carpal Tunnel Syndrome.

    DisordersSensory NCSMotor NCSMuscles involved by EMG
    Motor neuron disease e.g. ALSNormal38Median 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 radiculopathyNormal39Median 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 lesionMedian 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 radiculopathyNormal39Median CMAP: normal
    Ulnar CMAP: normal39    		C5 muscles: supraspinatus, infraspinatus, deltoid, brachioradialis, biceps, and C5 paraspinals40
    C6 radiculopathyNormal39Median CMAP: normal
    Ulnar CMAP: normal39    		C6 muscles: as in C5 + PT, FCR, triceps, anconeus, EDC and C6 paraspinals40
    C7 radiculopathyNormal39Median CMAP: normal
    Ulnar CMAP: normal39    		C7 muscles: triceps, anconeus, PT, FCR, EDC, and C7 paraspinals40
    Upper trunkMedian-D1& 2 SNAP: low amplitude
    Radial SNAP: low amplitude
    Lateral antebrachial: low amplitude
    Ulnar SNAP: normal41    		Median CMAP: normal Ulnar CMAP: normal41All or some of C5/6 muscles (listed above) sparing paraspinals, serratus anterior and rhomboids41
    Lateral cordMedian-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 elbowMedian SNAP: decreased
    Ulnar SNAP normal43    		Median CMAP: low amplitude
    Ulnar CMAP: normal43    		APB, FPL, FDP-D2 & 3, FDS, PQ, FCR and PT43
    AIN neuropathyNormalNormalFPL, FDP-D2 & 3, PQ43
    Length dependent axonal polyneuropathySural SNAPs are affected earlier and more severely than upper limb SNAPs44Lower limb CMAPs are affected earlier and more severely than upper limb CMAPs44Denervation 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

  • Table 2

    Reference values for the nerve conduction studies used in the evaluation of Carpal Tunnel Syndrome.

    StudyOnset-to-peak amplitude: LLN (3rd percentile)Peak latency: ULN (97th percentile) (ms)Onset latency: ULN (97th percentile) (ms)Conduction velocity: LLN (3rd percentile) (m/s)
    Digit 2 median antidromic sensory11 mv*4.03.3NA
    Digit 5 ulnar antidromic sensory10 mv*4.03.1NA
    Digit 2 median vs digit 5 ulnar peak latency differenceNA<0.5NANA
    Digit 4 median vs digit 4 ulnar peak latency differenceNA<0.5NANA
    Palmar orthodromic peak latency difference.NA<0.3NANA
    Median motor4.1 mv*NA4.549
    Ulnar mot7.9 mv*NA3.752 (below elbow)
    43 (across elbow)
    50 (above elbow)

    • LLN - lower limit normal, ULN - upper limit normal, NA - not applicable.

    • ↵* Values represent all ages. For stratification by age and body mass index, we refer the reader to the paper of Chen et al11

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