The split hand in ALS has a cortical basis

Loss of highly fractionated movement involving the thumb and index finger i s an early characteristic of hand dysfunction in many ALS patients. These m ovements are largely subserved by the 'thenar complex' including the first dorsal interosseus muscle (FDI), whereas the 'hypothenar complex', innervat ed by the same myotome, has less ability to fractionate and is relatively s pared. This suggests that in ALS, hand dysfunction and wasting is related t o corticomotoneuronal representation and input. To determine whether cortic omotoneuronal input to the thenar spinal pool is preferentially impaired co mpared to the hypothenar spinal pool in ALS, we studied 18 ALS patients and 11 normal subjects. Compound muscle action potentials (CMAPs) and motor ev oked potentials (MEPs) of the thenar and hypothenar complexes were evoked b y peripheral nerve stimulation and transcranial magnetic stimulation. In he althy control subjects the cortical/peripheral (MEP/CMAP) ratios were signi ficantly larger for the thenar complex suggesting a stronger corticomotoneu ronal input to this muscle complex (P < 0.005). This was not the case in AL S patients. Comparing the ratios between control subjects and patients reve aled a significant reduction for the thenar complex (P < 0.02) in ALS patie nts but not for the hypothenar complex. We conclude that corticomotoneurona l input to the thenar complex is preferentially affected in ALS and that co rticomotoneuronal disease may be the prime determinant of hand dysfunction and wasting. (C) 2000 Elsevier Science B.V. All rights reserved.