Cortical function and corticomotoneuronal adaptation in monomelic amyotrophy

•Motor cortical excitability as tested with threshold tracking transcranial magnetic stimulation is normal in monomelic amyotrophy.•Corticomotoneuronal dysfunction does not drive lower motor neurone loss in monomelic amyotrophy.•Corticomotoneuronal plasticity may represent an adaptive change in monomelic amyotrophy.

ObjectiveTo evaluate corticomotoneuronal integrity in monomelic amyotrophy using threshold tracking transcranial magnetic stimulation (TT-TMS).MethodsCortical excitability studies were prospectively performed in 8 monomelic amyotrophy patients and compared to 21 early-onset amyotrophic lateral sclerosis (ALS) patients and 40 healthy controls. Motor evoked potentials responses were recorded over abductor pollicis brevis.ResultsMaximal motor evoked potential (MEP/CMAP ratio) was significantly increased in monomelic amyotrophy compared with controls (monomelic amyotrophy 51.2 ± 12.4%; control 22.7 ± 2.1%, p = 0.04). Averaged short-interval intracortical inhibition (SICI, ISI 1-7 ms) in monomelic amyotrophy patients was similar to controls (monomelic amyotrophy 9.6 ± 2.1%; control 10.0 ± 0.9%, p = 0.98). However, it was significantly reduced in early-onset ALS in comparison with monomelic amyotrophy patients (monomelic amyotrophy 9.6 ± 2.1%; ALS 2.3 ± 1.7%, p < 0.001). Averaged SICI is a good parameter (area under the curve 0.79, p = 0.02) to discriminate between monomelic amyotrophy and early-onset ALS patients.ConclusionsTT-TMS technique has identified normal cortical function in monomelic amyotrophy, a feature that distinguishes it from early-onset ALS. The greater corticomotoneuronal projections to spinal motoneurons may represent central nervous system adaptive change in monomelic amyotrophy.SignificanceCorticomotoneuronal dysfunction does not drive the lower motor neurone loss presented in monomelic amyotrophy.