Acute motor conduction block neuropathy followed by axonal degeneration and poor recovery

Acute motor neuropathies with conduction block are rare and often fully recover within weeks without residual axonal involvement.^1,2 We report a case occurring after Campylobacter jejuni infection, with anti-GM1 antibody positivity, which presented axonal degeneration. Clinical improvement remained minimal 6 months after onset.

This 50-year-old woman presented, 4 weeks after a gastrointestinal illness, with weakness of the lower limbs, which progressed to the arms over 3 days. Her past personal and family medical histories were unremarkable. On examination, she had weakness proximally and distally in all limbs (grade 2 on the Medical Research Council [MRC] scale). Sensory examination showed mild distal hypoesthesia to pinprick and vibration sense up to the ankles. She was areflexic, and plantar reflexes were flexor. Cranial nerve functions were unimpaired. All routine blood investigations were normal. C. jejuni serology for IgM (1:640) and IgG anti-GM1 antibodies were positive. CSF examination done 6 days after onset of the weakness showed no cells and protein level of 0.36 g/L (normal 0.20 to 0.45). Nerve conduction studies also performed on day 6 revealed a pure motor neuropathy with multiple definite or probable partial conduction blocks (table). Maximal temporal dispersion including at Erb point did not exceed 20%. Distal sensory studies were normal. Needle electromyography (EMG) showed no spontaneous activity.

A diagnosis of acute motor conduction block neuropathy was made. Minimal sensory signs were present, but electrophysiologically the involvement was purely motor. The patient did not undergo a search for sensory conduction blocks. The criteria for definite or probable partial motor conduction block according to the American Academy of Electrodiagnostic Medicine³ were fulfilled by five of eight motor nerves. F waves were all absent.

The patient was treated with IV immunoglobulins (400 mg/kg/day for 5 days). No substantial improvement occurred. Nine weeks into the illness, electrophysiology was repeated (see table). Compound muscle action potentials (CMAPs) were all reduced. F waves remained absent. Sensory potentials remained within normal limits. Needle EMG showed active denervation in all tested muscles. Only minimal functional progress was achieved after a further 14 weeks, with persisting, diffuse MRC grade 3 weakness.

Few cases of acute conduction block neuropathy have been reported,^1,2 most of which had a rapidly favorable outcome. Two patients described recently each recovered completely in 6 weeks.¹ Although conduction blocks were found in some nerves for each patient, both also had nerves with initially reduced CMAPs. The conduction blocks resolved, and in the case of the second patient, the CMAPs improved to at least twice the baseline value for median and ulnar nerves without temporal dispersion. No major indication of demyelination was observed in either patient, and the authors concluded that the conduction blocks observed could have resulted from immune-mediated conduction failure at the nodes of Ranvier, without underlying demyelination.

The initial electrophysiologic picture in our patient resembled that of the two described above. In our case, however, instead of reversible block of conduction, axonal degeneration appeared to have occurred. Such progression can be observed in typical acute motor axonal neuropathy (AMAN) where hypothesized distal conduction failure, either resolving rapidly or, alternatively, leading to axonal degeneration, could explain prognostic variability.⁴ Alternatively, the mechanism in AMAN of poor prognosis, as in our patient, could be one of acute primary axonal degeneration, which can also show, electrophysiologically, conduction blocks in the initial stages, followed by low distal CMAPs.⁵ Other causes of conduction block followed by axonal degeneration such as vasculitic neuropathy⁶ were otherwise most unlikely in our patient, given the history, pure motor involvement, positive C. jejuni serology and anti-GM1 antibodies, as well as negative vasculitic markers.

Preceding C. jejuni infection and anti-GM1 positivity have been associated with typical AMAN, acute motor neuropathy with reversible conduction blocks¹ and in a syndrome described as acute multifocal motor neuropathy with conduction blocks,² where conduction blocks persist irrespective of clinical outcome. The common etiologic and prognostic factors for these three entities, as illustrated by our case, make it possible that they, in fact, represent electrophysiologic forms of the same disorder. Conduction blocks, possibly distal and undetectable in AMAN or more proximal as in our patient, may disappear with normalization of conduction and complete clinical improvement or lead to axonal degeneration, with poor recovery; or persist with or without recovery, or with the disorder becoming chronic and relapsing. The precise pathophysiology underlying this prognostic variability are unknown. It is possible that there are two distinct mechanisms, with either reversible conduction failure or primary axonal degeneration. Interestingly, our patient was areflexic and had an unfavorable course, whereas normal reflexes or hyperreflexia are associated with better outcome.⁷

Acknowledgment

The authors thank Mr. G. Swana (Department of Immunology, St. Thomas’ Hospital, London, UK), where the immunologic studies were performed.