F-wave motor unit numbers do not change in diabetic polyneuropathy: does it indicate a pathophysiologic mechanism for generating F-waves?

Motor unit number estimation (MUNE) methods including the one using F-waves (F-MUNE) are used to detect axonal loss in polyneuropathies. The aim of this study is to evaluate the amount of axonal loss by repeater F-wave parameters and F-MUNE in patients with type 2 diabetes mellitus (DM). In 24 controls and 49 patients with diabetic polyneuropathy, 90 F-waves elicited with supramaximal and 300 F-waves with submaximal stimulation of the ulnar nerve were recorded from the abductor digiti minimi muscle. F-MUNE values were calculated using the automated software with the repeater F-waves elicited using submaximal stimulation. Ulnar repeater F-wave index and repeater neuron index were found to be significantly higher in patients with DM compared with the control group. The amplitudes of F-waves elicited using both sub- and supramaximal stimulus intensities were reduced in patients with polyneuropathy when compared with controls. The F-MUNE values were found to be similar between the two groups. Repeater F-wave parameters recorded from the upper extremity might indicate the presence of axon loss in DM-related polyneuropathy. However, F-MUNE failed to reveal axonal loss in diabetic polyneuropathy probably because of its length-dependent nature and the dominance of low-amplitude F-waves, which might be secondary to several yet unknown mechanisms, mainly related to sensory fiber dysfunction.