Workshop 4: Advanced Neurophysiology Techniques WS4.1. My Approach to Motor Unit Number Estimation (with Demonstration)

Abstract

In motor neuron disease such as amyotrophic lateral sclerosis, the motor neurons die from the disease process. They cannot be replaced. The loss of motor units (MUs) is partially compensated by reinnervation. As a result, the number of MUs is reduced and their size is increased. Reinnervation cannot sustain and may not be sufficient as the MU loss continues. This leads to progressive weakness. As we look for treatments, we need a biomarker to track the disease progression. This had led to the methods of “motor unit number estimation (MUNE)” over the last 5 decades. If a treatment is effective, the rate of MU loss should decrease. In this workshop I will review MUNE methods that were based on recording and measuring individual surface recorded motor unit potentials (SMUPs). They are called “incremental stimulation”, “multiple point stimulation”, etc. The methods are time consuming due to low amplitude signals and have been used to study small distal muscles. Furthermore, they are not automated. In the last 15 years we have seen new methods based on mathematical models. Hence the calculation is described as “motor unit number index (MUNIX)”. In MUNIX the compound muscle action potential (CMAP) and surface EMG interference pattern are analyzed. The procedure is automated, requires less than 5 minutes per muscle, and can be used for large muscles such as tibialis anterior. By adding results from several muscles, one can better track the disease progression. The methods also provide a measure for the amplitude of SMUP. The motor unit size index (MUSIX) increases with reinnervation. Another set of methods is based on ‘CMAP scan’. The stimulus intensity is reduced in fine steps from supramaximal level to low intensity where no response is recorded. The amplitude of 500 responses is analyzed using different algorithms. This method is also automated for recording the data. All methods have their advantages and disadvantages. They provide different numerical results. However, each method shows disease progression when serial investigations are performed. Such assessment requires careful measurements of the CMAP regardless of the method.