Background and purpose: F-wave is a late response recorded from muscle elicited by electric impulse conveyed antidromically to alpha-motor neurons of the spinal cord. F-wave latency and frequency are assessed in routine electroneurography providing information of conduction in the proximal segment of the nerves. F-wave amplitude is rarely analyzed, while it could add valuable information on excitability of motor neurons in different disease states. This study was conducted to determine whether F-wave amplitude is indicative of the level of the peripheral nervous system lesion.
Material and methods: EMG recordings of 204 consecutive patients suspected of peripheral nerves system lesion were retrospectively analyzed. Based on the clinical diagnosis three groups were defined: neuropathy (N=100), myopathy (N=33), ALS (N=18), reference (musculoskeletal pain syndromes: N=53). F-wave amplitude and F/CMAP-ratio (CMAP-compound motor action potential) and their relation to parameters of impulse conduction in motor nerve fibers was analyzed.
Results: Mean F/CMAP ratio was 11.1% in ALS, 5% in myopathy (p=0.008), 7.1% in neuropathies and 5.6% in the reference group. Giant F-wave (more than 10% of CMAP) was observed in 30% of nerves in ALS, 15% in neuropathy and 10% in myopathy (p=0.036). F-wave amplitude correlated significantly with CMAP amplitude in all groups, while F/CMAP ratio was inversely related to CMAP amplitude in ALS (r=-0.43, p<0.01) and neuropathy (r=-0.37, p<0.0001). F-wave frequency was similar in all groups and correlated with CMAP amplitude.
Conclusions: F-wave amplitude is not indicative of the level of peripheral nervous system lesions. Giant F-wave is observed in neurogenic processes. It reflects an increase of motor unit size in the reinnervation process, but possibly also a change of excitability of motor neuron and its axon.