John D. Lloyd, PhD, CPE
Board Certified Ergonomist
32824 Michigan Avenue
San Antonio, Florida 33576
Tel: 813-624-8986
Email: DrJohnLloyd@Tampabay.RR.com
Neurovascular Considerations of Median Nerve Neuropathy with Implications for Clinical Diagnosis
Background - A peripheral nerve, such as the median, is a complex structure consisting of three major tissues; nerve fibers, connective tissue, and a vascular plexus. Since the median nerve is fragile compared to surrounding structures, it is compressed as the available volume of the carpal tunnel is compromised.
Individual nerve fibers are able to maintain normal function, even under very high pressures, as long as an adequate supply of oxygen is present. However, when pressures on the microvascular vessels within the median nerve rise above a critical threshold, venous obstruction causes capillary blood flow to be reduced below the level required for nerve viability. The resulting metabolic consequences of lack of nutrients and oxygen cause axon degeneration, which produces nerve conduction disturbances and symptoms of carpal tunnel syndrome.
Key Objectives- The two primary objectives of this study were:
Methodology -In a laboratory environment, 27 clinically-confirmed asymptomatic female participants were required to perform continuous repetitive motions of the hand/wrist in the flexion-extension plane (neutral forearm) for a period of two hours, during which an angle of 120 degrees was subtended about the neutral position. Three levels of dynamic wrist motion were prescribed, corresponding with 22 repetitions per minute (RPM), 38 RPM and 49 RPM. A static task (0 RPM) was also included for comparative evaluation. Task assignment was randomized, where participants were asked to complete all tasks. Environmental conditions were strictly controlled.
Wrist workload was recorded using a state-of-the-art 3D electromagnetic human motion tracking system (HumanTRAC), from which mathematical descriptors of angular wrist motion were calculated. Sensory median nerve response to imposed biomechanical stressors was monitored antidromically using a clinical electroneurometer every ten minutes throughout the work-session. Near-nerve skin temperature was recorded every twenty minutes at three sites along the distal digital sensory median nerve using Sharin, Inc.'s DermaTherm™ perfusion monitors.
Results - After correcting for physiological effects of aging, a highly significant change in median nerve conduction was detected across participants and conditions, attributable to task demands (R2=0.569, p<0.001). A mean waveform shift in median nerve function across participants was observed consistent with electroneurological evidence of carpal tunnel syndrome.
Using linear regression analysis, the mean linear effect of near-nerve skin temperature gradient across participants and workload conditions was Ro=0.140 (p=0.005). Sensory median nerve conduction velocity decreased by 0.794 ms-1 for each 1oC decrease in skin temperature distal to the transverse ligament.
Using 40.2 ms-1 as the empirical lower limit of normal nerve conduction, it was deduced that a temperature gradient greater than 3.6oC across the median distribution was associated with clinical evidence of carpal tunnel syndrome, where near-nerve skin temperature at the wrist should be maintained above 30.9oC.
Discussion- A highly significant effect of near-nerve skin temperature has been demonstrated, where nerve conduction decreased as near-nerve skin temperature decreased. Since nerve temperature is primarily regulated by blood flow, any occupational or environmental factor that reduces circulation to the hand may affect nerve function. Findings present considerations for etiologies of median neuropathy and its clinical diagnosis.
The effect of wrist workload on median nerve function may not be direct. Instead, work performed on and by the wrist might compromise microcirculation within the nerve fibers, thereby indirectly affecting nerve conduction. A significant association between workload and temperature differential across the median distribution of the hand (R2= 0.125, p=0.022) offers support for this explanation.
For this reason, it is suggested that nerve temperature correction of electrodiagnostic findings might obscure true neurological function. It is therefore proposed that, instead of nerve temperature correction, adequate opportunity be afforded for nerve adaptation to environmental elements, resulting in potentially more reliable findings.
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