Background
Hand weakness is a common symptom in motor neurone disease (MND). The split hand phenomenon involves preferential wasting of the lateral hand muscles relative to medial muscles and is commonly encountered in MND. It can be quantified with standard nerve conduction tests through the 'split hand index', which uses compound muscle action potential amplitudes from abductor pollicis brevis, first dorsal interosseous and abductor digiti mini.
It has been postulated that hyperexcitability, both centrally and peripherally, precedes motor neurone degeneration in MND. In this study we asked if peripheral nerve hyperexcitability manifested before an abnormal split hand index.
Methods
50 patients under investigation for suspected MND were prospectively recruited and informed consent obtained. Patients underwent nerve excitability testing with the TROND protocol in abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti mini (ADM). Excitability studies were performed on either the most symptomatic hand, or the dominant hand, so long as CMAP amplitudes were all 0.5mV or greater, in which cause the other hand was studied. Standard neurophysiological testing was also performed and the split hand index calculated. Standard statistical tests (e.g. t-tests, receiver operating characteristic (ROC) curves) were undertaken. The study was performed with NHS research ethics permission and good clinical practice guidelines were followed.
Results
23 patients went on to a diagnosis of MND with the remaining diagnosed with alternative conditions. There were no significant differences in the age or gender of the two groups. The standard split hand index calculated with CMAP amplitudes was significantly different between the two groups (p<0.05). Significant differences in axon excitability between MND and non-MND patients were evident in all three muscles, with the most prominent changes seen in superexcitability (APB p<0.0001, FDI, ADM p<0.001). Nerve excitability measures demonstrated better diagnostic potential than the split hand index.
Conclusions
Peripheral nerve hyperexcitability is evident across the hand at the time of investigation into possible MND. Axonal hyperexcitability measurements were more evident than split hand index abnormalities, in keeping with hyperexcitability occurring early in disease. Axonal excitability studies may have greater diagnostic potential than routine nerve conduction assessments.