Central motor conduction in brachial monomelic amyotrophy

Background: Prevalence of subclinical involvement of motor pathways in clinically diagnosed Brachial Monomelic Amyotrophy (BMMA) is unknown. Aims: To determine the prevalence of subclinical involvement of central motor pathways in BMMA using transcranial magnetic stimulation. Setting and Design: Prospective case-control study. Materials and Methods: Central motor conduction time (CMCT) was determined by ′F′ wave method using figure-of-eight coil attached to Magstim 200 stimulator, in 17 patients with BMMA. Motor evoked potentials were recorded from first dorsal interosseous of the affected (AFF) and unaffected upper limbs (UNAFF) at rest and during partial contraction. Comparison was made with data from 10 healthy controls (CTRL). Statistical Analysis: Descriptive analysis and Analysis of Variance (ANOVA). Results: Compared to controls, the mean CMCT of AFF was significantly prolonged, both at rest and contraction: (a) Rest: AFF-6.68±1.78 ms, UNAFF-6.36±1.16 ms, CTRL-5.71±1.02 ms; Fisher′s PLSD for AFF vs. CTRL: P =0.037, (b) Contraction: AFF-5.78±1.62 ms, UAFF - 4.86±1.38 ms, CTRL-4.06±0.80 ms; Fisher′s PLSD for AFF vs. CTRL; P =0.0002, AFF vs. UNAFF- P =0.044). Prolonged CMCT (>mean+2SD of controls) was observed in 29.4% of AFF and 6.25% of UNAFF at rest, and in 47.1% and 23.5% respectively during contraction. Conclusions: Dysfunction of central motor pathways was observed in both affected and unaffected upper limbs of some patients with BMMA of upper limbs. The dysfunction was more pronounced during voluntary contraction. A larger study is needed to validate the significance of these findings.

Keywords
Central motor conduction time, magnetic stimulation, monomelic amyotrophy.