Brain, Vol. 122, No. 5, 933-941,
May 1999
© 1999 Oxford University Press
Conduction block in carpal tunnel syndrome
Prince of Wales Medical Research Institute and Department of Neurology, Prince of Wales Hospital, Randwick, Sydney, Australia
Correspondence to:
Dr Matthew Kiernan, Prince of Wales Medical Research Institute, High Street, Randwick, NSW 2031, Australia E-mail: Matthew.Kiernan{at}unsw.edu.au
Wrist extension was performed in six healthy subjects to establish, first, whether it would be sufficient to produce conduction block and, secondly, whether the excitability changes associated with this manoeuvre are similar to those produced by focal nerve compression. During maintained wrist extension to 90°, all subjects developed conduction block in cutaneous afferents distal to the wrist, with a marked reduction in amplitude of the maximal potential by >50%. This was associated with changes in axonal excitability at the wrist: a prolongation in latency, a decrease in supernormality and an increase in refractoriness. These changes indicate axonal depolarization. Similar studies were then performed in seven patients with carpal tunnel syndrome. The patients developed conduction block, again with evidence of axonal depolarization prior to block. Mild paraesthesiae were reported by all subjects (normals and patients) during wrist extension, and more intense paraesthesiae were reported following the release of wrist extension. In separate experiments, conduction block was produced by ischaemic compression, but its development could not be altered by hyperpolarizing currents. It is concluded that wrist extension produces a `depolarization' block in both normal subjects and patients with carpal tunnel syndrome, much as occurs with ischaemic compression, but that this block cannot be altered merely by compensating for the axonal depolarization. It is argued that conduction slowing need not always be attributed to disturbed myelination, and that ischaemic compression may be sufficient to explain some of the intermittent symptoms and electrodiagnostic findings in patients with carpal tunnel syndrome, particularly when it is of mild or moderate severity.
conduction block; cutaneous afferents; paraesthesiae; carpal tunnel syndrome
CMAP = compound muscle action potential; CNAP = compound nerve action potential; CSAP = compound sensory action potential
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