Distinctive abnormalities of motor axonal strength-duration properties in multifocal motor neuropathy and in motor neurone disease

The strength-duration function is a classic measure of neural excitability. When studied on peripheral motor axons it reflects the intrinsic nodal membrane properties, and its time-constant (τ_SD or chronaxie) predominantly depends on non-voltage-gated, rest Na⁺ inward conductances. We assessed the strength-duration curve of ulnar motor axons in 22 nerves of healthy controls, in 18 nerves of patients with multifocal motor neuropathy with conduction blocks (MMN), and in 19 nerves of patients with motor neurone disease (MND). The compound muscle action potential (CMAP) was smaller in nerves of both groups of patients than in controls (P <0.05). The rheobasic current (rh_50%) [mean ± standard deviation (SD)] was higher in patients with MMN than in controls (13.3 ± 16.3 mA; controls 4.7 ± 1.7 mA, P <0.05). The τ_SD was differentially abnormal in the nerves of the two groups of patients: it was prolonged in the nerves of patients with MND for ≥40 years (227.2 ± 34.5 μs; controls 190.9 ± 51.0 μs, P <0.05), but it was shortened in the nerves of patients with MMN (146.5 ± 55.4 μs; controls 208.6 ± 51.2 μs, P <0.05) who had not been treated recently with high-dose intravenous immunoglobulin (IVIg). Nerves of patients with recently treated MMN (SD. Our results suggest that, probably due to an immuno-mediated rest Na⁺ channel dysfunction, Na⁺ conductances are reduced in MMN. This abnormality is a function of the time after the last IVIg treatment and involves also the axonal membrane outside the conduction block. Conversely, in MND, possibly owing to the ionic leakage of degenerating membrane, rest Na⁺ conductances are increased. Measuring the strength-duration curve of the ulnar motor axons might be useful in the differential diagnosis between de novo MMN and MND.