Corticomotoneuronal connections in primary lateral sclerosis (PLS).

BACKGROUND: The relationship between primary lateral sclerosis (PLS) and amyotrophic lateral sclerosis (ALS) is uncertain. The slow progression and dominant upper motor neuron features of PLS are associated with a high threshold to cortical magnetic stimulation and sometimes slow central motor conduction. In ALS the cortical threshold may be reduced early in the disease and central conduction is usually normal. Corticomotoneuronal function appears to be impaired differently in PLS and ALS. SUBJECTS AND METHODS: We assessed corticomotoneuronal function by analyzing the primary peak in the peristimulus time histograms (PSTHs) in 12 PLS and 12 ALS patients. Surface recorded motor evoked potentials (MEPs) and central motor conduction time (CMCT) were determined. PSTHs were constructed from 4-5 different, voluntarily recruited motor units in each patient and the onset latency, number of excess bins, duration and synchrony of the primary peak were measured.
RESULTS: The mean cortical threshold of single motor units in PLS was 73.6%, significantly higher than in ALS (60.3%; p < 2.2 x 10(-5)). Profoundly delayed primary peaks occurred in both PLS and ALS. Onset latency and desynchronization of the primary peak were similar in PLS and ALS, but the duration of the primary peak was significantly longer in PLS (p < 0.04).
CONCLUSIONS: Desynchronized primary peaks indicate dysfunction or demise of corticomotoneurones. Higher threshold and longer duration of the primary peak in PLS probably reflect different excitability and greater loss of corticomotoneuronal connections than in ALS.