F-response frequency in motor neuron disease and cervical spondylosis.

Abstract

SUMMARY During a train of 200 stimuli, F-response frequency, frequency of identical responses, and F-response shape were studied in the ulnar nerve of 17 patients with motor neuron disease (MND) and 16 patients with cervical spondylosis (CS). In MND patients, F-response frequency varied between 5% and 96% with a median of 39% which was significantly lower than controls (p < 0 001), and showed a significant rank correlation with the M-response amplitude (r = 0-62, p < 0.004). Identical responses occurred more frequently than in controls (median 308%, p < 0 001). F-response frequency was normal in CS patients (median 76-3%, range 35% to 97%), but the frequency of identical responses (median 6-3%) was higher than normal (p < 00 1). Reduced F-response frequency in MND was thought to reflect loss of lower motor neurons, while the presence of spacticity was probably the major factor underlying the increased frequency of identical responses in both disorders. F-response shape tended to be simpler in MND and rather more complex in CS patients than controls.Recent studies in normal human subjects have shown the production of F-responses in individual motor neurons to be an uncommon event,' 2 whereas in the motor neuron pool as a whole such recurrent responses occur relatively frequently.3 The production of these responses seems to depend mainly on the balance of excitatory and inhibitory activity at the appropriate spinal cord level.' In general, increased levels of excitability as seen in spasticity, lead to an increase in F-response production by responding motor neurons.' On the other hand frequency falls in the immediate postictal period following a cerebrosvascular accident during which muscle tone is decreased and the deep tendon reflexes are depressed.4It might also be expected that F-response frequency may be influenced by changes affecting the lower motor neuron, and for example, that a reduction in their number might lead to a decrease in F-response $Present address:

Electrophysiologic evaluation of upper motor neuron involvement in amyotrophic lateral sclerosis

“…[24][25][26][27][28][29][30] The main reason for this is probably that the association of upper and lower motor neuron signs in ALS makes the interpretation of the ndings markedly more dif cult than in 'pure' syndromes.…”

Nerve conduction studies in amyotrophic lateral sclerosis

“…A correlation between F-response frequency and M-response amplitude has been reported in ALS, with reduced Fresponse frequency in weak muscles. 45 We found that the F-wave frequency was progressively reduced in weaker muscles (Tables 1 and 2), being lowest in MRC 2-1 muscles (39% with no F waves). We found that F-wave dispersion was increased in weaker muscles.…”

“…These include increased F-wave latencies 3,12,36,41 with normal F-wave frequency 41 and increased F-wave amplitudes, 3 and slowing of F-wave velocities with decreased F-wave frequency. 13 Decreased F-wave frequency has been related to the degree of muscle atrophy, and an increased number of repeating responses 45 has been reported. Muscle strength in ALS is probably mainly related to lower motor neuron function, 7,25,35 with upper motor neuron dysfunction being less important.…”

Comparison of motor conduction abnormalities in lumbosacral radiculopathy and axonal polyneuropathy

“…Similar findings have been reported in patients with motor neuron disease, in whom F-response persistence correlated with CMAP amplitude. 11 It should be noted that if CMAP amplitudes were less than 500 mV, then F-response parameters were not included in the analyses. This was because we believe that the CMAP must be clearly present before commenting on the nature of the F-response abnormalities.…”