F40. Myotonic discharges in a cohort of patients with centronuclear myopathies

Abstract

Electromyography (EMG) is a useful ancillary test in the diagnosis of neuromuscular disorders. EMG patterns may be helpful to identify specific subtypes of inherited muscle disease. There are few studies looking at the EMG profile in congenital myopathies, and centronuclear myopathy (CNM) in particular. The objective of this study is to describe clinical and EMG findings in patients with CNM from a tertiary Brazilian university hospital. We selected all patients with pathological confirmation (muscle biopsy) of CNM and regularly followed at UNICAMP (State University of Campinas), from 2016 to 2017. Individuals with X-linked CNM were not included. For each patient, we reviewed nerve conduction studies (NCS) and EMG findings at rest and during contraction of upper and lower limbs. We looked specifically at the presence of myotonic discharges. The total score of motor function measure scale (MFM32) was employed as a clinical examination marker of disease severity. Eleven patients were enrolled in this study. There were six children, seven females, mean age 18.1 years when first examined in our service; a boy with 4-years-old, was the youngest and a 54-year-old lady was the oldest. The mean/median of MFM32 total score was 65.0%.The motor and sensory NCS were normal for all nerves tested. Regarding EMG and the occurrence of abnormal spontaneous activity: the myotonic discharges, when present, predominated in distal muscles: tibialis anterior and abductor pollicis brevis, and all occurred at rest. During activation, all muscles presented short duration and polyphasic motor unit action potentials without proximal or distal preferential involvement. None of the patients had clinical myotonia (either percussion or action), but electrical myotonia was found in 6 out of the 11 patients (54%). Five women and one men had clinical phenotype similar to juvenile or classical Steinert disease (DM1). Two patients had negative genetic test for the disease, and in one of the was found a mutation in the DNM2 gene. We do highlight that two patients were followed up for decades as DM1 until properly diagnosed as CNM at our hospital. CNM must be included in the differential diagnosis of a patient with electrical, but not clinical myotonia. Clinical neurophysiologists must be aware of this profile to assist in the recognition of CNM. The frequency of abnormal spontaneous activity at EMG is similar to other studies.