[A motor unit number estimate (MUNE)--a quantitative and pathophysiologic parameter for amyotrophic lateral sclerosis (ALS)].

We performed rate studies to develop a new method to obtain a MUNE reflecting the number of spinal alpha-motor neurons (MN). In the physiological part, six unitary muscle action potential (uMAPs) of the medial gastrocnemius muscle (MG) were averaged to obtain the mean uMAP, and a MUNE was calculated by dividing the area of its compound muscle action potential (CMAP) by the area of the mean uMAP. In the anatomical part, cholera toxin was injected into the MG muscle, and we counted the number of MG MNs identified by immunohistochemical staining of the toxin in their somata. We found that a MUNE of the MG muscle was consistent with the number of MG MNs. In the clinical application, we obtained MUNEs in 10 patients with ALS and 20 control subjects. In each patient we recorded 10 uMAPs and CMAP at each of the hypothenar and extensor digitorum brevis muscles using fine needle electrodes placed in these nerves. MUNEs for these muscles were calculated in the same way as described above. In the ALS patient group at one year post-onset of symptoms, the MUNEs for these two muscles were decreased to about 30% of the norm. We concluded that progression of ALS could be quantified by MUNEs.