Cortical function and corticomotoneuronal adaptation in monomelic amyotrophy

Objective: To evaluate corticomotoneuronal integrity in monomelic amyotrophy using threshold tracking transcranial magnetic stimulation (TT-TMS).

Methods: Cortical 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.

Results: Maximal 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 1-7ms) 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.

Conclusions: TT-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.

Significance: Corticomotoneuronal dysfunction does not drive the lower motor neurone loss presented in monomelic amyotrophy.