Delayed Appearance of Conduction Block in Multifocal Motor Neuropathy—A Case Report

I ntroduction: Multifocal motor neuropathy (MMN) is a rare, treatable, immune-mediated neuropathy often associated with multifocal conduction block (CB). The hallmark electrodiagnostic feature is the presence of CB occurring at non-entrapment sites. However, MMN without CB has also been described and can be diagnosed, even in the absence of CB. Therefore, it is crucial to diagnose and identify MMN cases without CB, as it is a treatable disorder. Case presentation: We present a case with progressive symptoms of asymmetric distal upper and lower extremity weakness with no sensory deficits. Intravenous immunoglobulin (IVIG) therapy was initiated, as the patient fulfilled the criteria for probable MMN, despite the absence of CB. The patient’s symptoms demonstrated a relative plateau phase in response to IVIG. Although the patient lost follow-up visits, repeated electrodiagnostic study, conducted 11 years after initial presentation, revealed new CB in nerve segments that previously did not show any evidence of CB. Conclusion: This case emphasizes the importance of early diagnosis and respectively initiating early IVIG treatment in MMN, in order to maintain the clinical function. Underdiagnosis of clinically suspected MMN, based on absence of CB, will result in denial of treatment to potential IVIG responders.