The Real Role of Forearm Mixed Nerve Conduction Velocity in the Assessment of Proximal Forearm Conduction Slowing in Carpal Tunnel Syndrome

Abstract

The decrease of forearm median motor conduction velocity (CV) in carpal tunnel syndrome (CTS) is a common electrodiagnostic finding in clinical practice and is possibly secondary to either conduction block at wrist or retrograde conduction slowing (RCS). This study is attempted to confirm the existence of RCS and to explore why this controversy occurs for a long time. Eighty CTS patients and controls were recruited. In addition to conventional electrodiagnosis, subjects received further electrodiagnostic protocol. First, a recording electrode was placed over the wrist and then at elbow with palm stimulation to calculate indirect forearm mixed nerve CV (forearm-mix CV) that represented real measurement of nerve fibers through the carpal tunnel. Then, direct measurement of forearm-mix CV was performed with recording at the elbow and stimulation at the wrist. CTS patients had markedly prolonged distal motor and sensory latencies and significantly prolonged wrist-palm sensory and motor conduction. There was a significant decrease in forearm median motor CV; however, there was no difference in ulnar distal motor latency and forearm motor CV. The mild decrease of forearm median motor CV was not proportional to the marked reduction of W-P MCV and there was no demonstrated conduction block at wrist, implying the reduction of forearm median motor CV is unlikely due to conduction blockage or slowing of the large myelinating fibers at the wrist and RCS really occurs over the forearm median nerve. In addition, the direct Forearm-mix CV was similar in CTS and controls; however, there was a significant decrease in indirect forearm-mix CV only in the CTS. Moreover, the difference between direct and indirect forearm-mix CV was significantly greater and poor consistency of direct and indirect forearm-mix CV in CTS, suggesting that direct and indirect forearm-mix CV represent CV from quite different nerve fibers. Therefore, we conclude that RCS really does occur in CTS and the direct forearm-mix CV reflects the CV of nerve fibers without damage in CTS. The misinterpretation and measurement of different components of forearm-mix CV results in the existence of this controversy till now.

Clinical Diagnosis and Wide-Awake Surgical Treatment of Proximal Median Nerve Entrapment at the Elbow: A Prospective Study

“…In this age of insurance-driven medicine, the dependency on so-called objective measurements, i.e., electromyography (EMG) and nerve conduction studies (NCS), is brought to the forefront of medicine whereas clinical diagnosis and examination is often considered subjective, biased, and unreliable [8]. In the case of PMNE, however, there is a conundrum in that proximal median EMG/NCS often is normal or inconclusive, may simulate CTS [5], and has an unreliable specificity of 30-70 % [3]. The shortcomings of electrodiagnostic studies lies in the fact that a nerve may be compressed and cause symptoms distal to the level of compression due to changes in axonal transport and intraneural circulation [17], but the pressure levels of compression are too low to actually cause axonal injury [7] and thus no visible changes in EMG/NCS.…”

Proximal Segment Conduction Slowing in Carpal Tunnel Syndrome

“…Daha önce de KTS'de ön kol median motor iletim hızlarında yavaş-lamayı gösteren birkaç çalışma mevcuttur (3)(4)(5). Chang ve ark.…”

Proximal axonal changes after peripheral nerve injury in man

“…The number of cases with injury and no possible recovery was relatively small 12–14. Other studies used distal entrapment neuropathy as the model of distal nerve injury to assess the proximal nerve effects 15–24. Reduction of forearm motor nerve conduction velocity is produced by entrapment of the median nerve in carpal tunnel syndrome15–23 and by entrapment of the ulnar nerve at the wrist 24.…”

“…Other studies used distal entrapment neuropathy as the model of distal nerve injury to assess the proximal nerve effects 15–24. Reduction of forearm motor nerve conduction velocity is produced by entrapment of the median nerve in carpal tunnel syndrome15–23 and by entrapment of the ulnar nerve at the wrist 24. Findings from median nerve forearm mixed nerve conduction studies in carpal tunnel syndrome have been marred by the presence of unaffected fibers that do not pass through the carpal tunnel 18–23.…”

“…Reduction of forearm motor nerve conduction velocity is produced by entrapment of the median nerve in carpal tunnel syndrome15–23 and by entrapment of the ulnar nerve at the wrist 24. Findings from median nerve forearm mixed nerve conduction studies in carpal tunnel syndrome have been marred by the presence of unaffected fibers that do not pass through the carpal tunnel 18–23. Measurements derived from the compressed fibers indicate reduction of forearm mixed nerve conduction velocity and mixed nerve action potential amplitude 22, 23…”