Chang Ming-Hong, Lee Yi-Chung, Hsieh Peiyuan F
Section of Neurology, Taichung Veterans General Hospital, No. 160, Chung-Kang Road, Section 3, Taichung 40705, Taiwan.
Clin Neurophysiol. 2008 Dec;119(12):2800-3. doi: 10.1016/j.clinph.2008.09.014. Epub 2008 Oct 30.
A decrease of forearm median motor conduction velocity (CV) is a common electrophysiological finding in carpal tunnel syndrome (CTS), ascribed to two possible mechanisms: either conduction block or slowing of the fastest myelinating fibers in the carpal tunnel, or retrograde axonal atrophy (RAA) with retrograde conduction slowing (RCS). We hope to utilize both direct and derived forearm median mixed nerve conduction studies to clarify the mechanism of the decrease of forearm median motor CV in CTS.
Seventy-five CTS patients and 75 age-matched control subjects received conventional motor and sensory nerve conduction studies of median and ulnar nerves and forearm median mixed nerve conduction techniques. First, direct measurement of forearm median mixed conduction velocity (Forearm mixed CV) and nerve action potential amplitude (Forearm mixed amplitude) was determined with recording at elbow and stimulation at wrist. Then, stimulating electrode was placed over palm and recording at elbow and then at wrist to calculate the derived Forearm mixed CV. Electrophysiological parameters, including direct Forearm mixed CV and amplitude and derived Forearm mixed CV, were compared between CTS patients and controls.
CTS patients had significantly prolonged wrist-palm sensory and motor conduction, significantly decreased forearm median motor CV, and normal ulnar nerve conduction. The direct Forearm mixed amplitude was significantly decreased in CTS patients. The direct Forearm mixed CV was similar in CTS patients and controls, but there was a significant decrease in derived Forearm mixed CV in CTS group. The difference between direct and derived Forearm mixed CV was significantly greater in the CTS, suggesting that direct and derived Forearm mixed CV represent CV from different nerve fibers, one passing outside carpal tunnel without undergoing RAA or the other through the carpal tunnel with occurrence of RAA.
A decrease of direct Forearm mixed amplitude really occurs in CTS, implying that RAA and RCS will develop over proximal median nerve at distal nerve injury and the decreased forearm median motor CV is best ascribed to RAA and RCS. Furthermore, in CTS, the direct Forearm mixed CV measures the CV from undamaged nerve fibers without passing through carpal tunnel, resulting in the misinterpretation of the cause of proximal conduction slowing secondary to conduction block or slowing over the wrist.
We provide a direct evidence of the occurrence of RAA and RCS that would explain the cause of proximal median nerve conduction slowing. However, the clinical significance of RAA and RCS is uncertain.
在前臂正中运动传导速度（CV）降低是腕管综合征（CTS）常见的电生理表现，其原因可能有两种：一是传导阻滞，或是腕管内最快的有髓纤维传导减慢，二是逆行性轴索萎缩（RAA）伴逆行性传导减慢（RCS）。我们希望通过直接和推导的前臂正中混合神经传导研究来阐明CTS患者前臂正中运动CV降低的机制。
75例CTS患者和75例年龄匹配的对照者接受了正中神经和尺神经的常规运动和感觉神经传导研究以及前臂正中混合神经传导技术检测。首先，通过在肘部记录、腕部刺激来直接测量前臂正中混合神经传导速度（前臂混合CV）和神经动作电位幅度（前臂混合幅度）。然后，将刺激电极置于手掌上，先在肘部记录，然后在腕部记录，以计算推导的前臂混合CV。比较CTS患者和对照者的电生理参数，包括直接前臂混合CV和幅度以及推导的前臂混合CV。
CTS患者的腕-掌感觉和运动传导明显延长，前臂正中运动CV明显降低，尺神经传导正常。CTS患者的直接前臂混合幅度明显降低。CTS患者和对照者的直接前臂混合CV相似，但CTS组推导的前臂混合CV明显降低。CTS患者直接和推导的前臂混合CV之间的差异明显更大，这表明直接和推导的前臂混合CV代表来自不同神经纤维的CV，一种通过腕管外未发生RAA的神经纤维，另一种通过发生RAA的腕管。
CTS患者确实出现直接前臂混合幅度降低，这意味着在远端神经损伤时近端正中神经会发生RAA和RCS，前臂正中运动CV降低最好归因于RAA和RCS。此外，在CTS中，直接前臂混合CV测量的是未受损神经纤维的CV，这些神经纤维不通过腕管，导致对继发于传导阻滞或腕部传导减慢的近端传导减慢原因的误解。
我们提供了RAA和RCS发生的直接证据，这可以解释近端正中神经传导减慢的原因。然而，RAA和RCS的临床意义尚不确定。
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