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BACKGROUND:Amyotrophic lateral sclerosis (ALS) is the most common of all the motor neuron diseases and the absence of a biologic marker has made both diagnosis and tracking evolution of the disease difficult, Electrodiagnostic tests play a fundamental role in quantifying pathological changes in the motor unit pool.OBJECTIVE:We assessed distal-proximal Motor Unit (MU) loss and changes using the method of motor unit number estimation (MUNE).DESIGN, TIME AND SETTING:A case-control study was performed at the Department of Neuroscience, Pisa University Medical School, Italy from December 1999 to November 2009. PARTICIPANTS:A total of 50 ALS patients were recruited, 30 males:mean age (59.6 ± 13.3) years; 20 females:mean age (63.9 ± 11.7) years; range (30-82) years; all patients had probable or definite ALS. Thirty healthy volunteers were recruited from department staffs, including 20 males and 10 females; mean age (57.7 ± 13.8) years served as controls.METHODS:MUNE was performed for both the biceps brachii and abductor digiti minimi muscles of the same side. The technique used relayed substantially on manual incremental stimulation of the motor nerve, known as the McComas technique (50 ms sweep duration, a gain of 2 mV/Div for M wave, 0.5 mV/Div for each step; filters 10-20 kHz).MAIN OUTCOME MEASURES:MUNE results were measured.RESULTS:Functioning MU numbers, measured by MUNE, decreased in the biceps brachii and abductor digiti minimi muscles over the entire one-year follow-up period (one assessment every three months) compared with baseline determination, the rate of MU decrease was similar in both muscles, but steeper distally.CONCLUSION:MUNE is a feasible method for ALS patients both proximally and distally to track changes over time in muscle MUs during the disease's evolution.