To the Editor:
We read with interest the article by Rajabally et al.1 describing a patient with an alleged acute motor conduction block neuropathy (AMCBN). The authors claim that conduction blocks (CBs) were not reversible in this patient, as we and others previously described,2 but were followed by axonal degeneration and poor recovery. Interestingly, they argue that this case strengthens the idea that acute motor axonal neuropathy (AMAN) and AMCBN, both associated with Campylobacter jejuni infection and anti-GM1 antibodies, represent “electrophysiologic forms of the same disorder”.1 We have some methodologic and conceptual remarks.
The authors found motor CBs 6 days after onset, but 8 weeks later CBs disappeared whereas amplitudes of distal and proximal compound motor action potentials (CMAPs) equalized.1 These observations do not necessarily imply that CBs were followed by axonal degeneration but may merely reflect a length-dependent reduction of CMAP amplitude due to ongoing Wallerian degeneration.3,4 At the first electrophysiologic examination, axonal degeneration and nerve inexcitability could not have yet reached distal stimulation sites producing pseudo-CBs which disappeared as degeneration proceeded.3,4
Rajabally et al. did not perform serial conduction studies to provide convincing evidence that abnormal amplitude reduction of proximal CMAPs persisted beyond the time required to complete Wallerian degeneration. Therefore axonal degeneration could have developed from the beginning and the patient could simply have a typical AMAN.
We have suggested that AMCBN may be an “arrested” AMAN and that anti-GM1 antibodies may induce reversible conduction failure or axonal degeneration, possibly depending on the intensity of local immune reaction or complement deposit.2 Reversible CBs have been reported in some nerves of patients with AMAN and anti-GM1-antibodies.5 Thus, AMCBN with reversible CBs and prompt recovery and typical AMAN may represent the ends of the same immunopathologic process. However, we believe they should be distinguished because of prognosis and pathophysiologic mechanisms.
We would like to underline that serial electrophysiologic studies are mandatory for careful identification of Guillain-Barré syndrome (GBS) variants and to elucidate the pathophysiologic mechanism of weakness among demyelination, physiologic conduction failure, and Wallerian degeneration. For diagnosis of AMCBN and, in general, for distinction between true and pseudo-CBs conduction studies should be repeated within a few days. Moreover, to determine if physiologic conduction failure or demyelination underlie CBs, recordings should be repeated through some weeks in order to document increased CMAP duration due to excessive temporal dispersion, which is the electrophysiologic correlate of remyelination.2
Reply from the Authors:
We thank Drs. Uncini and Capasso for their interest in our article. We stated in the case report that the progression in our patient was similar to that observed in AMAN and accepted the possibility of the mechanism being one of acute primary axonal degeneration.1 In such cases, CB can be present in the initial stages, followed by low distal CMAP.6
The issues raised by Drs. Uncini and Capasso are conceptual and relate to different definitions for CB, AMCBN, and AMAN.
CB has been defined as the inability of an action potential to propagate beyond a specific region of a nerve.7 This can occur for a number of reasons including Wallerian degeneration.7 The term pseudoCB as used previously in vasculitic neuropathy relates to focal axonal infarctive injury causing localized conduction failure.3 Regardless of pathophysiology (physiologic conduction failure or axonal degeneration), blocks of conduction occurred in our patient as in theirs,2 electrophysiologically, in the initial stages.
Drs. Uncini and Capasso appear to define AMCBN as a disorder with obligatory transient, rapidly resolving CB and favorable outcome. They otherwise state that AMAN conveys the idea of axonal degeneration,2 which is probably why they believe our patient could simply have what they call typical AMAN. However, others have used the term AMAN to classify patients electrophysiologically, regardless of outcome.5,8 We have also considered these terms in their strict electrophysiologic sense, at presentation.
CB, as defined above, is generally rare in the first days of the disease in GBS.9 CB appears infrequently in anti- GM1 positive axonal GBS, and was absent between wrist and elbow for median and ulnar nerves, in a series of 25 recently published cases.8 We reported this case on these electrophysiologic grounds.
In our patient, axonal degeneration was later found, indicating that an electrodiagnosis of AMCBN at presentation cannot be predictive of good prognosis. Similarly, early detection of low CMAPs indicating an initial electrophysiologic diagnosis of AMAN is not suggestive of poor outcome as rapidly resolving distal CBs could be present. We agree with Drs. Uncini and Capasso that serial studies might be helpful in establishing prognosis. However, we believe that AMCBN and AMAN are electrophysiologic variants at presentation of the motor neuropathy resulting from Campylobacter jejuni infection with anti-GM1 positivity, the prognosis of which is initially unknown in either case.
Footnotes
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Disclosure: The authors report no conflicts of interest.
Disclosure: The authors report no conflicts of interest.
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