The correlation between clinical manifestations and features of compound muscle action potentials in patients with amyotrophic lateral sclerosis

Objective To analyse the amplitude characteristics of the compound muscle action potentials (CMAP) in patients with amyotrophic lateral sclerosis (ALS), and explore the correlation between CMAP amplitude and muscle strength, disease duration and neurological functional score. Methods The 127 patients of EMG presenting generalized neurogenic amyotrophic lateral sclerosis during May 2001-Nov 2004 were collected. These patients were diagnosed as definite cases 43, assume cases 39, probable cases 13 and suspicious cases 32 according to El Escorial diagnosis criterion. Among them a total of 82 cases of definite and assume cases were enrolled for evaluation by the amyotrophic lateral sclerosis function rating scale and Appel's amyotrophic laterosclerosis scale. Then the association of compound muscle action potential (CMAP) with changes of muscls strength, disease duration and neurological functional score was comparatively analyzed. Results Decreasing of CMAP amplitude correlated strongly with weakening of muscle strength, the relation could be fitted by an exponential equation (r = 0.969, P = 0.001); CMAP amplitude declined sharply or slowly over time to the same patient, the slower decline in CMAP amplitude correlated with a longer disease duration. There was no significant difference of muscle action potential amplitude levels among the different patients (r =- 0.077, P = 0.502). There was significant positive correlation of decreasing CMAP amplitude level with neurological functional score (r = 0.412, P = 0.001) but significant negative correlation with Appel's scale (r = - 0.549, P = 0.001). There was no simple linear correlation of decreasing CMAP amplitude with motor nerver conduction velocity (CV) (r =- 0.087, P = 0.545). But it presented linear correlation between the square roots when converting the amplitude and conductive velocity to the percentage of the lower limit of their normal levels and calculated their square root (r = 0.382, P 0.001). Conclusion The changes of CMAP amplitude is resulted in both primary and secondary pathological changes of ALS. The CAMP significantly decreases when the exacerbation of ALS symptoms; but at that time, the conductive veloeity of motor nerve still keeps normal as the myelin sheath remains intact. Thus the mechanism of muscle strength decreasing in ALS is mainly due to the injury of motor nerve neurons. In the following up of patients, the CMAP amplitude is gradually decline as the disease course prolong in the same patient, but time-amplitude parallel relation curve could not be established yet. It shows the relation between CMAP amplitude change and disease course is rather complicate. The significant correlation of CMAP amplitude and the amyotrophic lateral sclerosis function rating scale of ALS patients or Apple's scale demonstrates that the slower of early decreasing amplitude, the milder the patient's condition.