F response in vascular and degenerative upper motor neuron lesions

The F response represents the recurrent discharge of a small percentage of the motoneuron pool activated antidromically by any single impulse delivered to the nerve. We studied F waves from median nerve stimulation in 22 controls, in 30 patients with spasticity after cerebrovascular accident (CVA), and in 4 patients with familial spastic paraplegia (FSP). The following parameters were analyzed: median nerve conduction velocity, median nerve M response amplitude (M), average amplitude of 16 F responses (F16); persistence of F response (Fp) defined as the percentage of measurable responses to 16 stimuli, average amplitude of F response expressed as percentage of maximal M amplitude: (F16/M%). Results showed that Fp, F16 and F16/M% values were significantly increased on the spastic side of CVA patients and in FSP. Our findings indicate that after upper motor neuron lesions (whatever etiology and course) a change in the frequency of recurrent discharge and/or in the total number of lower motor neurons capable of backfiring occurs. Therefore, F response reflects the hyperexcitability state of the lower motor neurons in spasticity.