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초록
Background
Automated nerve excitability testing is used to assess various peripheral neuropathies and motor neuron diseases. Comparing these excitability parameters with normal data provides information regarding the axonal excitability properties and ion biophysics in diseased axons. This study measured and compared normal values of axonal excitability parameters in both the distal motor and sensory axons of normal Koreans.
Methods
The axonal excitability properties of 50 distal median motor axons and 30 distal median sensory axons were measured. An automated nerve excitability test was performed using the QTRACW threshold-tracking software (Institute of Neurology, University College London, London, UK) with the TRONDF multiple excitability recording protocol. Each param-eter of stimulus–response curves, threshold electrotonus, current–voltage relationship, and recovery cycle was measured and calculated.
Results
Our Korean normal data on axonal excitability showed ranges of values and characteristics similar to previous reports from other countries. We also reaffirmed that there exist characteristic differences in excitability properties between motor and sensory axons: compared to motor axons, sensory axons showed an increased strength–duration time constant, more prominent changes in threshold to hyperpolarizing threshold electrotonus (TE) and less prominent changes in threshold to depolarizing TE, and more prominent refractoriness and less prominent subexcitability and superexcitability.
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Fig. 1.
Responses to excitability testing in the motor and sensory axons of the median nerve. After stimulating the wrist, the excitability of the motor axon was measured at the abductor pollicis brevis (n = 50, black circles) and that of the sensory axon was measured at the index finger (n = 30, red circles). The graphs show the strength–duration properties (A), threshold electrotonus (B), recovery cycle (C), and current–voltage relationship (D) for each group. The actual data are given in Table 1. Data are mean and SEM values.
Table 1.
Normal values of excitability parameters in distal median motor and sensory axons
| Median motor axons a (n = 50) | Median sensory axons b (n = 30) | |
|---|---|---|
| Age | 46.2 ± 8.9 | 39.6 ± 6.7 |
| Sex (number of males) | 27 | 14 |
| Stimulus–response and strength–duration relationships | ||
| Stimulus for 50% CMAP (mA) | 2.47 ± 1.05 | 1.78 ± 1.06 |
| Strength–duration time constant (ms) | 0.49 ± 0.01 | 0.52 ± 0.02 |
| Rheobase current (mA) | 1.60 ± 1.05 | 0.80 ± 1.06 |
| Stimulus–response slope | 4.36 ± 1.04 | 2.90 ± 1.10 |
| Maximal CMAP (mV) / peak SNAP (µV) | 13.6 ± 1.5 | 19.3 ± 5.2 |
| Threshold electrotonus | ||
| c (%) TEd(10-20) | 71.9 ± 0.6 | 61.8 ± 1.6 |
| d (%) TEd(40-60) | 54.4 ± 0.6 | 52.0 ± 1.8 |
| e (%) TEd(90-100) | 50.9 ± 0.7 | 54.7 ± 1.9 |
| TEd undershoot f (%) | –17.2 ± 0.5 | –24.1 ± 1.8 |
| TEd peak (%) | 71.7 ± 0.6 | 62.0 ± 1.2 |
| g (%) TEh(10-20) | –77.4 ± 0.8 | –89.3 ± 3.3 |
| h (%) TEh(20-40) | –96.6 ± 1.2 | –114.0 ± 4.5 |
| i (%) TEh(90-100) | –127.7 ± 2.5 | –150.4 ± 10.1 |
| TEh overshoot j (%) | 14.1 ± 0.7 | 22.5 ± 1.7 |
| TEh slope 101-140 | 2.06 ± 0.04 | 2.70 ± 0.15 |
| Current–threshold relationship | ||
| Resting I/V slope | 0.50 ± 0.01 | 0.48 ± 0.03 |
| Minimum I/V slope | 0.25 ± 0.01 | 0.22 ± 0.01 |
| Hyperpolarizing I/V slope | 0.43 ± 0.02 | 0.38 ± 0.04 |
| Recovery cycle | ||
| Superexcitability (%) | –27.6 ± 1.0 | –24.3 ± 1.8 |
| Subexcitability (%) | 13.1 ± 0.5 | 11.9 ± 1.0 |
| Relative refractory period (ms) | 3.09 ± 1.01 | 3.69 ± 1.03 |
CMAP, compound muscle action potential; SNAP, sensory nerve action potential; TEd, depolarizing threshold electronus; TEh, hyperpolarizing threshold electrotonus; I/V, current–voltage relationship.
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