Ischaemic sensitivity of axons in carpal tunnel syndrome.

Although carpal tunnel syndrome (CTS) is the most common human entrapment neuropathy characterized by paraesthesiae and numbness with nocturnal exacerbation, the mechanisms underlying the generation of these symptoms remain unclear. Consequently, the aim of the present study was to investigate the relationship between changes in axonal excitability and the development of neurological symptoms in response to an ischaemic insult in CTS patients. Sensory and motor excitability were measured in 10 CTS patients and compared with 10 healthy controls, with participants asked to report symptom generation and intensity during the development of limb ischaemia. To induce ischaemia, a sphygmomanometer was inflated above the elbow and maintained at 200 mmHg for 10 min. During ischaemia there were decreases in axonal threshold, with less overall reduction in CTS patients when compared with controls. Associated with these differences in threshold, both sensory (p < 0.001) and motor (p < 0.05) refractoriness increased dramatically in CTS patients. This prominent increase in refractoriness was accompanied by a significant reduction in compound sensory action potentials and compound motor action potentials amplitudes for CTS patients when compared with controls (p < 0.05). These changes in axonal excitability resulted in a higher intensity of numbness and paraesthesiae reported by CTS patients during ischaemia. The present study has established differences in the nerve excitability and symptom development during ischaemia for patients with mild and moderate CTS, and may suggest that axons in the median nerve of CTS patients have an altered functional capacity to respond to an ischaemic insult, further contributing to nocturnal exacerbation of their symptoms.