McArdle's Disease Presenting as Unexplained Dyspnea in a Young Woman

Differential control of respiratory frequency and tidal volume during exercise

The lack of a testable model explaining how ventilation is regulated in different exercise conditions has been repeatedly acknowledged in the field of exercise physiology. Yet, this issue contrasts with the abundance of insightful findings produced over the last century and calls for the adoption of new integrative perspectives. In this review, we provide a methodological approach supporting the importance of producing a set of evidence by evaluating different studies together-especially those conducted in 'real' exercise conditions-instead of single studies separately. We show how the collective assessment of findings from three domains and three levels of observation support the development of a simple model of ventilatory control which proves to be effective in different exercise protocols, populations and experimental interventions. The main feature of the model is the differential control of respiratory frequency (f_R) and tidal volume (V_T); f_R is primarily modulated by central command (especially during high-intensity exercise) and muscle afferent feedback (especially during moderate exercise) whereas V_T by metabolic inputs. Furthermore, V_T appears to be fine-tuned based on f_R levels to match alveolar ventilation with metabolic requirements in different intensity domains, and even at a breath-by-breath level. This model reconciles the classical neuro-humoral theory with apparently contrasting findings by leveraging on the emerging control properties of the behavioural (i.e. f_R) and metabolic (i.e. V_T) components of minute ventilation. The integrative approach presented is expected to help in the design and interpretation of future studies on the control of f_R and V_T during exercise.

Respiratory Frequency during Exercise: The Neglected Physiological Measure

The use of wearable sensor technology for athlete training monitoring is growing exponentially, but some important measures and related wearable devices have received little attention so far. Respiratory frequency (f_R), for example, is emerging as a valuable measurement for training monitoring. Despite the availability of unobtrusive wearable devices measuring f_R with relatively good accuracy, f_R is not commonly monitored during training. Yet f_R is currently measured as a vital sign by multiparameter wearable devices in the military field, clinical settings, and occupational activities. When these devices have been used during exercise, f_R was used for limited applications like the estimation of the ventilatory threshold. However, more information can be gained from f_R. Unlike heart rate, V˙O₂, and blood lactate, f_R is strongly associated with perceived exertion during a variety of exercise paradigms, and under several experimental interventions affecting performance like muscle fatigue, glycogen depletion, heat exposure and hypoxia. This suggests that f_R is a strong marker of physical effort. Furthermore, unlike other physiological variables, f_R responds rapidly to variations in workload during high-intensity interval training (HIIT), with potential important implications for many sporting activities. This Perspective article aims to (i) present scientific evidence supporting the relevance of f_R for training monitoring; (ii) critically revise possible methodologies to measure f_R and the accuracy of currently available respiratory wearables; (iii) provide preliminary indication on how to analyze f_R data. This viewpoint is expected to advance the field of training monitoring and stimulate directions for future development of sports wearables.

McArdle disease: what do neurologists need to know?

McArdle disease (also known as glycogen storage disease type V) is a pure myopathy caused by an inherited deficit of myophosphorylase, the skeletal muscle isoform of the enzyme glycogen phosphorylase. The disease exhibits clinical heterogeneity, but patients typically experience exercise intolerance, that is, reversible, acute crises (early fatigue and contractures, sometimes with rhabdomyolysis and myoglobinuria) triggered by static muscle contractions (e.g. lifting weights) or dynamic exercise (e.g. climbing stairs or running). In this Review, we discuss the main features of McArdle disease, with the aim of providing neurologists with up-to-date, useful information to assist their patients. The topics covered include diagnostic tools-for example, molecular genetic diagnosis, the classic ischemic forearm test and the so-called 'second wind' phenomenon-and current therapeutic options-for example, a carbohydrate-rich diet and carbohydrate ingestion shortly before strenuous exercise, in combination with medically supervised aerobic training of low to moderate intensity.

McArdle disease presenting as acute renal failure

In 1951, McArdle described a glycogen storage disorder which presents primarily as a myopathy. It is characterized by muscle pain, weakness and exercise intolerance with elevated creatine kinase from rhabdomyolysis. The pathophysiology involves a deficiency of myophosphorylase enzyme resulting in an inability to degrade glycogen stores. We present a novel case of McArdle disease (glycogen storage disease V [GSDV]) in a patient who had sickle cell trait and bulimia. The disease went unrecognized despite several admissions to the hospital with increased creatine kinase and muscle myopathy until the patient's initial presentation with acute renal failure.

Molecular genetics of McArdle's disease

This review highlights recent advances in our understanding of McArdle's disease, including the mechanisms involved in the regulation of the clinical phenotype. The latest molecular genetic studies have demonstrated the genetic heterogeneity of the disorder, with more than 65 mutations identified to date. There is not a specific treatment for McArdle's disease, but some nutritional treatments in combination with aerobic conditioning could improve the quality of life in most patients.

Variable presentation of the clinical phenotype of McArdle's disease in a kindred harbouring a novel compound genotype in the muscle glycogen phosphorylase gene

We report a Spanish family with muscle glycogen phosphorylase (PYGM) deficiency (McArdle's disease) harbouring a novel compound genotype (A659D/L586P). Four individuals who had the same genotype for PYGM, showed a wide variability in the presentation of the clinical phenotype, including one patient with a restrictive respiratory pattern, which is unusual in McArdle's disease. Moreover, these patients were studied for the insertion/deletion (I/D) trait in the angiotensin converting enzyme (ACE) which has been suggested to be a strong modulator of severity in McArdle's disease. Our results indicate no association of the I/D ACE trait in this family, suggesting that other factors would be more relevant in determining the severity of the clinical presentation.