Symmetry of phrenic nerve motor response in amyotrophic lateral sclerosis

To test for interside differences and ipsilateral correlation of compound muscle action potentials (CMAP) from muscles of the cervical region according to El Escorial criteria, we stimulated the phrenic, axillary, and ulnar nerves bilaterally in 67 patients with amyotrophic lateral sclerosis. The diaphragm CMAP was symmetric, but it did not correlate with deltoid or abductor digiti minimi (ADM) CMAPs. The deltoid CMAP in all groups and ADM CMAP in bulbar- and lower-limb-onset patients showed significant interside correlation. The ADM CMAP is asymmetric in upper-limb-onset patients. Unilateral stimulation is sufficient to monitor the phrenic nerve response; its degree of CMAP decrease does not correlate with other cervical muscle involvement.