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Background Conduction block (CB) is described as a complete or partial failure in the propagation of the nervous impulse within the axon. It is diagnostic of focal demyelination, and suggestive of acute demyelinating neuropathies (typically inflammatory). Patients and methods We report three cases of demyelinating polyneuropathy with CBs in which their presence led to the erroneous diagnosis of inflammatory polyneuropathy. Results Case 1: 32-year-old woman with a history of slowly progressive gait disturbance and cavus foot, muscle weakness and musculoskeletal pain. In two repeated EMG studies presented a demyelinating polyneuropathy with CBs, and was diagnosed of a chronic inflammatory demyelinating polyneuropathy. Immunosuppressant treatment did not modify the symptoms neither did its withdrawal, finally being diagnosed with autosomal recessive form of Charcot-Marie-Tooth disease (CMT). Case 2: 60-year-old man with lower limb atrophy and absence of deep tendon reflexes. The first genetic test resulted negative for CMT. Later, genetic test presented a new variant of genes PMP22 and ARHGEF10. Case 3: 50-year-old woman with acute myeloid refractory leukemia, who presented a subacute generalized weakness. EMG demonstrated a demyelinating polyneuropathy, predominantly motor, involving proximal segments and with partial CBs. Cerebrospinal fluid studies were normal. Treatment with immunoglobulins was initiated without improvement. Finally, polyneuropathy was attributed to the chemotherapy treatment. Conclusions Although conduction blocks are typical of autoimmune polyneuropathies, their presence has been described in polyneuropathies of another causes. This must be taken into account when making the differential diagnosis.

Introduction Facioscapulohumeral (FSH) distrophy is an autosomal dominant inherited muscular disease, caused by the deletion of the chromosome 4 D4Z4. It is characterized by progressive muscular weakness and atrophy, affecting asymmetrically muscles of the face, shoulders and arms, and provokes deformities such as scapular winging. Methods We present the case of a 61-years-old male referred to our department with a history of muscular atrophy on the right arm, causing mild weakness, and in the last few months, a right drop foot. The referring physician suspected a motor neuron disease. Results In the lab results he presented a Creatine Kinase of 281 and an Aldolase of 7.60. EMG showed a myopathic pattern with spontaneous activity in the clinically affected muscles, suggestive of a muscular dystrophy. At the light of these results, a genetic study was performed, being positive for FSH dystrophy, with only 6 repetitions in D4Z4. Conclusion We present this case of FSH dystrophy with discordance between the mild symptoms and the marked genetic disturbance. Also we want to highlight the importance of have the disease in mind, due to the asymmetric presentation of the weakness and the atrophy, in the differential diagnosis of the motor neuron disease, in which the accurate diagnosis is essential due to its prognosis.