Altered motor axonal excitability in patients with cervical spondylotic amyotrophy.

OBJECTIVE: To investigate the changes in motor axonal excitability properties in cervical spondylotic amyotrophy (CSA).

METHODS: Threshold tracking was used to measure the median motor axons in 21 patients with CSA, 10 patients with cervical spondylotic radiculopathy (CSR) and 16 normal controls.

RESULTS: Compared with normal controls, patients with distal-type CSA showed increased threshold electrotonus hyperpolarization (TEh [90-100]) and increased superexcitability on the symptomatic side (P < 0.05), which are suggestive of distal motor axonal hyperpolarization, presumably due to motor axonal regeneration. More importantly, compared with normal controls and CSR cases, both distal- and proximal-type CSA cases showed lower accommodation during depolarising currents (reduced S2 accommodation, decreased TEd [undershoot] and/or lower subexcitability) (P < 0.05), indicating that slow K+ conductance may be less active in motor axons in patients with CSA.

CONCLUSIONS: The present study demonstrated changes in motor axonal excitability in patients with CSA compared with both normal controls and patients with CSR.

SIGNIFICANCE: Less expression of slow K+ conductance may confer greater instability in membrane potential in CSA, thereby presumably contributing to the increased vulnerability of motor axons in patients with CSA.