Transient absence of F-waves in acute myelopathy: a potential source of diagnostic error

The frequent absence of F-waves in lesions of the nerve roots and proximal nerve is well known, with absence of F-waves occasionally the only electrophysiologic manifestation of early Guillain-Barré Syndrome. It is less well known that acute central nervous system lesions can cause disappearance of F-waves.

A 25 year old woman presented with quadriparesis and sensory loss progressive over several days. Hyporeflexia and hypotonia were present. Imaging studies were initially negative. Electrophysiologic testing was normal apart from the diffuse absence of F-waves. This led to strong consideration of the diagnosis of Guillain-Barré Syndrome, and treatment for this diagnosis. However, imaging studies ultimately revealed the diagnosis to be transverse myelitis. F responses normalized 6 weeks after the initial study.

F responses are significantly modulated by central nervous system factors. The relevant experimental and clinical literature is reviewed. The relevance of this to the diagnosis of Guillain-Barré Syndrome has not been previously emphasized, but our experience confirms that the absence of F-waves in a patient with acute weakness accompanied by hyporeflexia and hypotonia does not distinguish between peripheral nerve and central nervous system lesions.