The F wave disappears due to impaired excitability of motor neurons or proximal axons in inflammatory demyelinating neuropathies.
OBJECTIVE
Investigation of pathophysiology of F wave disappearance in demyelinating neuropathies.


METHODS
The peripheral motor nerve conduction was studied by motor evoked potential (MEP) on transcranial magnetic stimulation as well as conventional nerve conduction studies before and after the treatment in 26 patients with inflammatory demyelinating neuropathies. In addition, serum antiganglioside antibodies in the acute or active stage were examined.


RESULTS
The F wave was abolished in 10 patients. Seven of the 10 patients showed motor evoked potentials (MEPs) on transcranial magnetic stimulation that ranged from 1-4 mV. In six of them the F wave reappeared in the recovery stage, but the MEP size did not change. This may be caused by humoral factors, because the F wave reappeared immediately after plasma exchange or intravenous immunoglobulin treatment. A correlation of F wave disappearance with the presence of serum antiganglioside antibodies was found.


CONCLUSIONS
The major pathophysiology of F wave disappearance in demyelinating neuropathies is impairment of motor neuron excitability or prolonged refractoriness of the most proximal axon for backfiring. The conventional interpretation that absent F waves suggest a conduction block at the proximal site is often inadequate.
Yokota
T
T
Department of Neurology, Tokyo Medical and Dental University, Japan.
Inaba
A
A
Yuki
N
N
Ichikawa
T
T
Tanaka
H
H
Saito
Y
Y
Kanouchi
T
T
D000208:Acute Disease; D000293:Adolescent; D000328:Adult; D000368:Aged; D000906:Antibodies / Q000276:immunology; D001369:Axons; D001921:Brain / Q000503:physiopathology*; D002648:Child; D002675:Child, Preschool; D003711:Demyelinating Diseases / Q000175:diagnosis* / Q000188:drug therapy / Q000503:physiopathology*; D004797:Enzyme-Linked Immunosorbent Assay; D019054:Evoked Potentials, Motor; D005260:Female; D005677:G(M1) Ganglioside / Q000097:blood / Q000276:immunology; D006801:Humans; D007136:Immunoglobulins / Q000008:administration & dosage / Q000627:therapeutic use; D007275:Injections, Intravenous; D008297:Male; D008875:Middle Aged; D009046:Motor Neurons; D009431:Neural Conduction; D010951:Plasma Exchange
D000906:Antibodies; D007136:Immunoglobulins; D005677:G(M1) Ganglioside