The Role of Cardiopulmonary Exercise Testing for Patients with Suspected Metabolic Myopathies and Other Neuromuscular Disorders

Abstract

Neuromuscular diseases (NMDs) can be associated with significant exercise limitation. As cardiopulmonary exercise testing (CPET) examines the integrated response to exercise, it has been investigated in multiple NMDs, proving itself particularly valuable in the evaluation of metabolic disorders, which exhibit typical patterns of exercise response. Disorders of carbohydrate metabolism usually demonstrate limitations of aerobic capacity and abnormalities of glycogen utilization. Lipid disorders seem to be associated with lesser aerobic limitation, while disorders of mitochondrial function seem to be associated with the greatest limitation in aerobic capacity. Furthermore, a hyperventilatory and hypercirculatory pattern seems to typify exercise response in these latter patients. As such, CPET can be used in the initial evaluation of patients with suspected carbohydrate or lipid metabolism as well as those with defects of mitochondrial function.Neuromuscular diseases can be associated with an abnormal muscle response to exercise. As such, these disorders may present with weakness, myalgia, abnormal fatigue, or exertional breathlessness [1, 2]. These disorders can be classified as those that are associated with impaired muscle energy metabolism, those associated with impaired muscle bulk, and those associated with impaired control of muscle contraction [2]. A general enumeration of these disorders is presented in table 1. As reviewed elsewhere in this book, exercise requires a close integration between skeletal muscle function, cardiovascular function with associated oxygen delivery, and ventilatory function with oxygenation of blood. As such, it is not surprising that myopathic disorders can significantly impact exercise capacity. This chapter reviews the pathophysiologic implications of myopathic disorders on exercise capacity and reviews clinical indications of exercise testing in the diagnosis and management of these disorders. Disorders of Muscle Energy MetabolismMetabolic myopathies are disorders of muscle energy metabolism. These disorders have been grouped into three broad categories: (1) defective carbohydrate utilization, (2) abnormal lipid utilization, and (3) mitochondrial

Effect of L-carnitine on exercise performance in patients with mitochondrial myopathy

“…Our patients also showed significantly lower maximum exercise capacity and peak compared to controls. Several studies evaluating exercise tolerance in MM have demonstrated markedly low peak ( 23 - 27 ), reflecting impaired muscle mitochondrial oxidative phosphorylation. The maximal exercise capacity of MM subjects has been shown to directly correlate with the degree of O 2 extraction in the impaired muscle, which enables the use of the systemic arterial to venous O 2 difference as a surrogate marker for mitochondrial oxidative capacity ( 3 , 4 ).…”