Estudo da condução nervosa motora na doença de McArdle: relato de caso

Abstract

RESUMO -A doença de McArdle (glicogenose tipo V) é miopatia metabólica com sintomas de intolerância ao exercício, causados pela deficiência da enzima miofosforilase. Nesses pacientes, o estudo da condução n e rvosa motora após período de esforço muscular máximo ou ao estímulo repetitivo pode revelar achados característicos da doença. Descrevemos o caso de um homem de 37 anos com sintomas de intolerância aos e x e rcícios, fadiga muscular e cãibras no início da atividade física com a presença do fenômeno de "second wind". O estudo da condução nervosa motora apresentava redução na amplitude do potencial de ação muscular composto após esforço de 30 e 90 segundos em nervos mediano, ulnar e fibular profundo e d e c remento após 200 estímulos a 40 Hz em nervo fibular profundo. A eletromiografia de agulha apre s e n t a v a padrão miopático e durante o exercício isquêmico não se evidenciou silêncio elétrico. Discutimos as características eletrofisiológicas enfatizando a importância do estudo da condução nervosa motora e teste de estimulação repetitiva nos pacientes com suspeita de miopatia metabólica.PA L AV R A S -C H AVE: doença de McArdle, glicogenose tipo V, potencial de ação muscular composto, condução nervosa motora, teste de estimulação repetitiva. Motor nerve conduction study in McArdle disease: case reportABSTRACT -McArdle disease (glycogenosis type V) is a metabolic myopathy with symptoms of exercise intolerance caused by deficiency of the enzyme myophosphorylase. In these patients, the motor nerve conduction studies after a short period of maximal voluntary muscle contraction or repetitive stimulation reveals characteristic findings of the disease. A 37-years-old man presented symptoms of exercise intolerance, muscular fatigue and cramps in the beginning of the physical activity with "second wind" phenomenon. The motor nerve conduction studies after a voluntary contraction of 30 and 90 seconds presented decrease in the amplitude of the compound muscle action potential in median, ulnar and deep peroneal nerves; and d e c rement after 200 stimulation at the 40 Hz in deep peroneal nerve. The electromyography pre s e n t e d myopathic pattern and during the ischemic exercise electric silence was not proven. The characteristic of electrophysiological studies are discussed with emphasis at the importance of the motor nerve conduction studies in the patients with suspicion of metabolic myopathy.KEY WORDS: McArdle disease, glycogenosis type V, compound muscle action potential, motor nerve conduction, repetitive nerve stimulation test.A doença de McArdle (DM) ou glicogenose do tipo V é miopatia metabólica autossômica re c e s s i v a com sintomas de intolerância ao exercício causados pela deficiência da enzima miofosforilase, que metaboliza o glicogênio para contração muscular durante o exerc í c i o 1 , 2 . O diagnóstico é confirm a d o pela biópsia muscular e estudo genético, porém o q u a d ro clínico associado à dosagem seriada do áci-do lático e as alterações do estudo eletro f i s i o l ó g ico, durante o exerc...

Neuromuscular, cognitive and metabolic implications of McArdle Syndrome: a global overview

“…The disease manifestations vary from muscle fatigue, cramps, contracture and intolerance to exercise [24], [20] to more severe conditions, such as myoglobinuria (the presence of myoglobin in the urine, making it darker) and rhabdomyolysis (the breakdown of muscle tissue that leads to the release of muscle fiber contents into blood [31]. The action potential is also affected in patients with MS. Lorenzoni et al [32] reported one case of MS in which needle electromyography showed a reduction of the compound muscle action potential amplitude following a short period of voluntary muscle activity or high-frequency repetitive stimulation. Those findings may have been observed due to absence of glycogen, which normally serves as an energy substrate for the active transport of ions through the neuronal membrane.…”

“…When clinical complaints are indicative of disease presence, diagnostic testing must be performed. One of the methods used to investigate the presence of MS is histochemical analysis of the muscle tissue, which may detect the subsarcolemmal accumulation of glycogen [20], [32], as was found in several cases. Such an accumulation may be due to excess glycogen that cannot be broken down.…”

“…For muscle contraction to occur normally, an action potential must travel along the membranes of the nerve fibers. According to Lorenzoni [32], electromyography has been used for the clinical assessment of individuals with suspected metabolic myopathy ever since the first description of MS in 1951. Indeed, this method is practical, and simple, and plays a relevant role in the diagnosis and follow-up of conditions that lead to fatigue of the muscle tissue [61].…”

“…The mechanism of impaired Na + /K + ATPase functioning in MS is not fully elucidated, but it may be related to the inadequate supply of ATP produced by glycolysis to the muscle fiber Na + /K + ATPase pump [63]. In a study with MS patients, Lorenzoni et al [32] found changes in the motor nerve conduction following short-term voluntary muscle exertion or high-frequency repetitive stimulation. In most patients, the amplitude of the muscle action potential exhibited a significant reduction after a 30-or 90-second effort, returning to its normal level after a period of rest.…”