The combined multipoint incremental method involves the use of a single recording site for a nerve/muscle pair, and a defined set of stimulation points, typically three [13]. At each stimulation site, the nerve is stimulated at low currents that are increased slowly, as in the multipoint approach. However, once the first motor unit action potential is obtained and recorded, the current should continue to be increased slowly until a second incremental (“all or none”) jump in the action potential amplitude occurs. Once the second is recorded, then the stimulation is again increased slowly until a third incremental jump occurs. This protocol is repeated at each of the three stimulation sites, yielding a total of 9 measurements. At each site, the first motor unit amplitude is equivalent to the first action potential obtained, while the second is calculated by subtracting the first from the second, while the third is calculated by subtracting the second from the third. These nine motor unit amplitudes are then averaged and used for the standard MUNE calculation (Fig. 12.2). Redundant motor unit action potentials are tolerated in this protocol, as opposed to the multipoint stimulation technique.

MUNE is increasingly used as an outcome measure for clinical trials in spinal muscular atrophy [7, 14, 15], and has yielded interesting information about the disease, including natural history data [16, 17] and evidence suggesting that new motor development may be occurring as a compensatory mechanism in spinal muscular atrophy [8]. Thus far, it does not appear to have obvious diagnostic applications in routine clinical settings, as it is not clear that it would detect signs of motor unit dysfunction that would not be seen by other means, such as CMAP amplitudes and signs of denervation/reinnervation on needle examination. However, as more data accumulate regarding the correlation of MUNE values with disease progression, such applications may be proposed in the future.