Metabolic Myopathies

PURPOSE OF REVIEW

Metabolic myopathies are genetic disorders that impair intermediary metabolism in skeletal muscle. Impairments in glycolysis/glycogenolysis (glycogen-storage disease), fatty acid transport and oxidation (fatty acid oxidation defects), and the mitochondrial respiratory chain (mitochondrial myopathies) represent the majority of known defects. The purpose of this review is to develop a diagnostic and treatment algorithm for the metabolic myopathies.

RECENT FINDINGS

The metabolic myopathies can present in the neonatal and infant period as part of more systemic involvement with hypotonia, hypoglycemia, and encephalopathy; however, most cases present in childhood or in adulthood with exercise intolerance (often with rhabdomyolysis) and weakness. The glycogen-storage diseases present during brief bouts of high-intensity exercise, whereas fatty acid oxidation defects and mitochondrial myopathies present during a long-duration/low-intensity endurance-type activity or during fasting or another metabolically stressful event (eg, surgery, fever). The clinical examination is often normal between acute events, and evaluation involves exercise testing, blood testing (creatine kinase, acylcarnitine profile, lactate, amino acids), urine organic acids (ketones, dicarboxylic acids, 3-methylglutaconic acid), muscle biopsy (histology, ultrastructure, enzyme testing), MRI/spectroscopy, and targeted or untargeted genetic testing.

SUMMARY

Accurate and early identification of metabolic myopathies can lead to therapeutic interventions with lifestyle and nutritional modification, cofactor treatment, and rapid treatment of rhabdomyolysis.

The case of stainless statins

Two cases of hypercholesterolaemic patients are presented in whom raised plasma creatine kinase was noted during treatment with statins. The plasma creatine kinase failed to fall following cessation of therapy. Further investigation revealed the aetiology of the raised plasma creatine kinase to be due to previously undiagnosed glycogen storage diseases (McArdle's and Pompe's diseases).

[A case of acute renal failure secondary to late-onset McArdle's disease]

INTRODUCTION

Mc Ardles disease, also known as Type V glycogen storage disease, is a rare deficiency of the enzyme glycogen phosphorylase in muscle cells, inherited as an autosomal recessive trait. In the absence of this enzyme, muscles cannot break down glycogen during exercise, so in patients affected by McArdles disease even moderate physical activity produces cramps, pain and fatigue. Anaerobic activity leads to severe fixed contractures and rhabdomyolisis with myoglobinuria and raised serum creatine-kinase, which, in turn, can lead to acute renal failure. Disease onset is usually in early childhood, although diagnosis is often not made until the second or third decade.

CASE REPORT

We present the case of a 68-year-old man who presented to the Emergency Room with fatigue, vertigo, diarrhea and oliguria. The patient underwent five daily hemodialysis sessions, diuresis reappeared and there was progressive recovery of renal function. The patient described episodes of fatigue and muscular pain occurring since childhood: the positive personal history, together with persistently raised CPK levels in the absence of any infective or toxic cause of myositis, led us to suspect the presence of this rare metabolic disease, which was subsequently confirmed by muscle biopsy.

CONCLUSION

To date, there is no specific treatment for type V glycogenosis, although a diet rich in protein and saccarose, vitamin B6 supplementation and creatine administration are generally recommended. Moderate physical activity can help manage symptoms by improving exercise tolerance and blood supply to the muscles, ensuring provision of glucose and free fatty acids for the muscle fibers.

[Myalgia during warming-up in a 12-year-old boy]

INTRODUCTION

McArdle disease is a hereditary myopathy caused by muscle phosphorylase deficiency. Patients experience painful muscle cramps after strenuous exercise; the condition is sometimes associated with rhabdomyolysis, myoglobinuria and resulting acute renal failure.

CASE DESCRIPTION

A 12-year-old boy visited the Paediatric and Neurology outpatients' departments with symptoms of leg myalgia, occurring during the warming-up phase of soccer practice, which disappeared after a short rest. Detailed history-taking revealed that he already experienced pain while walking during early childhood. An elevated serum creatine kinase concentration in combination with the typical presentation led to the presumptive diagnosis 'McArdle disease', which was confirmed by molecular genetic analysis.

CONCLUSION

There is no effective gene therapy; a multidisciplinary approach by a neurologist, paediatrician, physiotherapist and nutritionist is advised.

Diagnostic evaluation of clinically normal subjects with chronic hyperCKemia

The authors analyzed muscle biopsy specimens of 104 patients with creatine kinase activity greater than 500 UI/L (normal 10 to 170 UI/L) without signs of muscle weakness. They achieved a definite or probable diagnosis in 55% of cases. The most frequently identified diseases were glycogen storage diseases, muscular dystrophies, and inflammatory myopathies. The probability of making a diagnosis was higher in children and when creatine kinase level was greater than 2,000 UI/L.

Abnormal blood lactate accumulation after exercise in patients with multiple mitochondrial DNA deletions and minor muscular symptoms

STUDY OBJECTIVES

Muscle is one of the most commonly affected organs in mitochondrial disorders, and the symptoms are often exercise related. The cardiopulmonary exercise test with the determination of lactic acid formation could give supplementary information about the exercise-induced metabolic stress and compensatory mechanisms used in these disorders. The aim of this study was to evaluate the exercise capacity and lactate kinetics related to exercise in subjects with two genetically characterized mitochondrial disorders (multiple mitochondrial DNA deletions with PEO, MELAS) compared with lactate kinetics in subjects with metabolic myopathy (McArdle's disease) and in the healthy controls.

DESIGN

The subjects were consecutive, co-operative patients of Department of Neurology of Helsinki University Hospital. Molecular genetic analyses were used for group classification of the mitochondrial myopathy.

STUDY SUBJECTS

The study groups consisted of 11 patients with multiple deletions (PEO) and five patients with a point mutation in the mitochondrial DNA (MELAS), four patients with a muscle phosphorylase enzyme deficiency (McArdle's disease) and 13 healthy controls. The clinical disease of the patients was relatively mild.

MEASUREMENTS AND RESULTS

A graded exercise test with ventilatory gas analyses and venous blood lactic acid analyses was performed. The main finding was the prolonged accumulation of blood lactate after the exercise in the PEO and MELAS groups compared with the controls. An overcompensation in ventilation was found in the MELAS and PEO group.

CONCLUSIONS

The blood lactate accumulation after exercise occurs in patients with multiple mitochondrial DNA deletions or MELAS even in patients with only mild exercise intolerance. Cardiopulmonary exercise can be used in the diagnostic process of patients with mitochondrial myopathies.

No spontaneous second wind in muscle phosphofructokinase deficiency

OBJECTIVE

The spontaneous second wind in myophosphorylase deficiency (MD, McArdle's disease) represents a transition from low to a higher exercise capacity attributable to increased oxidation of blood-borne fuels, principally glucose and free fatty acids. Muscle phosphofructokinase deficiency (PFKD) blocks the metabolism of muscle glycogen and blood glucose. The authors inquired whether the additional restriction in glucose metabolism in PFKD prevents a spontaneous second wind.

METHODS

The authors compared the ability of 29 patients with MD and 5 patients with muscle PFKD to achieve a spontaneous second wind during continuous cycle exercise after an overnight fast. Patients cycled at a constant workload for 15 to 20 minutes (3 MD patients, 3 PFKD patients) and at variable workloads in which peak exercise capacity was determined at 6 to 8 minutes of exercise and again at 25 to 30 minutes of exercise (29 MD patients, 4 PFKD patients). Heart rate was monitored continuously, and perceived exertion (Borg scale) was recorded during each minute of exercise. Oxygen utilization and blood levels of lactate and ammonia were determined at rest and during peak workloads.

RESULTS

All variables in both patient groups were similar at 6 to 8 minutes of exercise. Thereafter exercise responses diverged. Each MD patient developed a second wind with a decrease in heart rate and perceived exertion and an increase in work and oxidative capacity. In contrast, no PFKD patient developed a spontaneous second wind.

CONCLUSIONS

Patients with muscle phosphofructokinase deficiency are unable to achieve a spontaneous second wind under conditions that consistently produce one in patients with McArdle's disease. The authors conclude that the ability to metabolize blood glucose is critical to the development of a typical spontaneous second wind.

The effect of oral sucrose on exercise tolerance in patients with McArdle's disease

BACKGROUND

Energy metabolism in muscles relies predominantly on the breakdown of glycogen early in exercise. In patients with McArdle's disease, blocked glycogenolysis in muscles results in low exercise tolerance and can lead to muscle injury, particularly in the first minutes of exercise. We hypothesized that ingesting sucrose before exercise would increase the availability of glucose and would therefore improve exercise tolerance in patients with McArdle's disease.

METHODS

In a single-blind, randomized, placebo-controlled crossover study, 12 patients with McArdle's disease drank 660 ml of a beverage that had been sweetened with artificial sweeteners (placebo) or with 75 g of sucrose after an overnight fast. Thirty to 40 minutes later, the patients rode a stationary bicycle at a constant workload for 15 minutes while the heart rate, level of perceived exertion, and venous blood glucose levels were monitored.

RESULTS

Supplemental sucrose increased the mean plasma glucose level by more than 36 mg per deciliter (2.0 mmol per liter) and resulted in a marked improvement in exercise tolerance in all patients. The mean (+/-SE) heart rate dropped by a maximum of 34+/-3 beats per minute (P<0.001), and the level of perceived exertion fell dramatically when the patients ingested glucose as compared with when they received the placebo.

CONCLUSIONS

This study suggests that the ingestion of sucrose before exercise can markedly improve exercise tolerance in patients with McArdle's disease. The treatment takes effect during the time when muscle injury commonly develops in these patients. In addition to increasing the patients' exercise capacity and sense of well-being, the treatment may protect against exercise-induced rhabdomyolysis.

A 1-year-old infant with McArdle disease associated with hyper-creatine kinase-emia during febrile episodes

A 14-month-old girl was hospitalized due to repeated hyper-creatine kinase (CK)-emia during pyrexia. Mild hypotonia was observed, but other physical and neurological findings were unremarkable. The serum CK level was normal at rest or normothermia. Open muscle biopsy was performed on the rectus femoris, and showed glycogen storage and complete lack of phosphorylase activity histochemically and biochemically, establishing the diagnosis of McArdle disease. The diagnosis of McArdle disease in early infancy is uncommon. Until this study there have been no reports of clinical symptoms or muscle biopsy findings for McArdle disease in early childhood. This disease must be considered when transient hyper-CKemia is observed in infants, even if glycogen storage is unremarkable as compared with adult cases.

A nonischemic forearm exercise test for McArdle disease

Ischemic forearm exercise invariably causes muscle cramps and pain in patients with glycolytic defects. We investigated an alternative diagnostic exercise test that may be better tolerated. Nine patients with McArdle disease, one with the partial glycolytic defect phosphoglycerate mutase deficiency, and nine matched, healthy subjects performed the classic ischemic forearm protocol and an identical protocol without ischemia. Blood was sampled in the median cubital vein of the exercised arm. Plasma lactate level increased similarly in healthy subjects during ischemic (Delta5.1 +/- 0.7mmol L(-1)) and non-ischemic (Delta4.4 +/- 0.3) tests and decreased similarly in McArdle patients (Delta-0.10 +/- 0.02 vs Delta-0.40 +/- 0.10mmol L(-1)). Postexercise peak lactate to ammonia ratios clearly separated patients and healthy controls in ischemic (McArdle, 4 +/- 2 [range, 1-12]; partial glycolytic defect phosphoglycerate mutase deficiency, 6; healthy, 33 +/- 4 [range, 17-56]) and non-ischemic (McArdle, 5 +/- 1 [range, 1-10]; partial glycolytic defect phosphoglycerate mutase deficiency, 5; healthy, 42 +/- 3 [range, 35-56]) protocols. Similar differences in lactate to ammonia ratio between patients and healthy subjects were observed in two other work protocols using intermittent handgrip contraction at 50% and static handgrip exercise at 30% of maximal voluntary contraction force. All patients developed pain and cramps during the ischemic test, and four had to abort the test prematurely. No patient experienced cramps in the non-ischemic test, and all completed the test. The findings indicate that the diagnostic ischemic forearm test for glycolytic disorders should be replaced by an aerobic forearm test.

HyperCKemia as the only sign of McArdle's disease in a child

An asymptomatic 13-year-old boy, who never complained of exercise intolerance or myalgia, was found to have markedly elevated serum creatine kinase (CK) levels during a routine check-up. General physical and neurologic examinations were normal. Surprisingly, histochemical and biochemical analysis of muscle showed myophosphorylase deficiency and genetic analysis showed that the patient was homozygous for the most common mutation encountered in McArdle's disease (R49X). This case illustrates the fuzzy correlation between molecular defect and clinical phenotype in patients with McArdle's disease, and suggests that a thorough study of the muscle biopsy is important in patients with idiopathic hyperCKemia for correct diagnosis and careful follow-up.

Blood ammonia and ventilation at maximal exercise

This study, intended to evaluate the role of ammonia (NH3) as a ventilatory stimulus, was conducted in three groups of subjects: 14 sedentary individuals, 12 triathletes, 5 patients with a glycolytic deficiency (Mc Ardle disease). All subjects performed maximal exercise tests on a cycle ergometer. Ventilation measured at maximal oxygen consumption (VE 100%) was correlated with lactatemia (lactate 100%) and ammonemia (NH3 100%) in the sedentary group, but only with ammonemia in triathletes, although NH3 100% and lactate 100% were correlated in both groups, which suggests that correlation between VE 100% and NH3 100% is not a false correlation. In patients with Mc Ardle disease, unable to produce lactate during exercise, VE 100% was correlated with NH3 100%. NH3 may act indirectly by increasing the production of lactate in cereberal tissue. Another hypothesis rests on the fact that the catabolism of ammonia leads to an increase in intracerebral glutamate which may act as a ventilatory stimulus.

Angina in McArdle's disease

McArdle's disease (myophosphorylase deficiency) results in the inability to metabolise skeletal muscle glycogen to lactate. A patient with this condition developed angina and therefore offered a unique opportunity to explore the differential expression of the defective myophosphorylase gene in skeletal and cardiac muscle.

Diagnosis of McArdle's disease by molecular genetic analysis of blood

We analyzed leukocyte DNA from 32 patients with suspected McArdle's disease, 24 of whom had biochemically or histochemically proven myophosphorylase deficiency. We found that 19 were homozygous for the most common mutation at codon 49, 2 were compound heterozygotes, and 1 was a manifesting heterozygote. In six patients, we could find only one mutant allele, suggesting a still unidentified mutation on the second allele. We were unable to identify any of the known mutations in four patients. Our findings indicate that the diagnosis of McArdle's disease can be established in approximately 90% of patients using DNA isolated from leukocytes, thereby avoiding muscle biopsy.

Mcardle's disease. A case report

McArdle's disease is a hereditary, metabolic myopathy characterized by weakness and muscle cramps after exercise, appearing mostly in the second or third decade of life. Due to myophosphorylase deficiency in skeletal muscle, glycogen cannot be used and deposited in the sarcolemmal spaces, leading to lack of endurance to sustained work. The ischemic exercise test is a screening procedure for muscle energy disorders, and the diagnosis is confirmed by reduced enzyme activity in muscle biopsy. In this report, a family with one child having enzyme assay-proven McArdle's disease and two other children demonstrating a positive ischemic exercise test is presented.

A double blind, placebo controlled, crossover trial of D-ribose in McArdle's disease

To determine whether seven days oral D-ribose would improve exercise tolerance in a group of 5 patients with McArdle's disease, we performed a double blind placebo controlled crossover trial. Subjects performed weekly treadmill exercise tests with expired gas analysis until their times were reproducible. They then received 60 g D-ribose daily or placebo for seven days. Exercise testing was repeated on completion of this period. A seven day washout period then followed. Subjects then performed a new baseline exercise test prior to starting the other solution. Again after seven days the exercise test was repeated. There was no significant difference between pre-treatment exercise tests for peak oxygen consumption or level of leg fatigue. Patients did not like taking the ribose and D-Ribose does not appear to be of benefit to patients with McArdle's disease.

McArdle's disease and gout

We report the first case of McArdle's disease (muscle phosphorylase deficiency) and tophaceous gout. To examine the contribution of adenine nucleotide degradation to the disturbance of uric acid metabolism, we labeled the adenine nucleotide pool with [8-14C]adenine, and measured plasma and urine purines following vigorous exercise tests. Plasma and urinary hypoxanthine and xanthine concentrations and the specific radioactivity of urinary purines increased markedly, but plasma urate levels and uric acid excretion were not substantially modified. We suggest that, in this patient, the association of McArcle's disease with gout is coincidental.

Effect of deficient muscular glycogenolysis on extramuscular fuel production in exercise

Hormonal, metabolic, and cardiovascular responses to 21 min of cycling in three saline- or glucose-infused men with McArdle's disease were compared with those of matched controls to elucidate whether mobilization of extramuscular fuel is enhanced to compensate for the lack of intramuscular glycogenolysis in patients with McArdle's disease. During exercise, all saline-infused patients compared with controls working at both the same absolute and at similar relative work rates had higher glucose production (31 +/- 7 vs. 19 +/- 5 and 26 +/- 4 mumol.min-1.kg-1) and utilization (34 +/- 8 vs. 22 +/- 2 and 28 +/- 4 mumol.min-1.kg-1); higher plasma glycerol (155 +/- 19 vs. 75 +/- 20 and 90 +/- 22 mumol/l), free fatty acids (487 +/- 175 vs. 295 +/- 47 and 202 +/- 52 mumol/l), growth hormone (7.7 +/- 2.8 vs. 2.6 +/- 1.1 and 3.6 +/- 3.4 mU/l), and cortisol (530 +/- 168 vs. 268 +/- 8 and 367 +/- 80 nmol/l), greater decrease in insulin (delta 57 +/- 4 vs. delta 11 +/- 8 and delta 11 +/- 23 pmol/l), and similar glucose concentrations. Furthermore, norepinephrine, epinephrine, and adrenocorticotropic hormone levels were higher and heart rate and cardiac output were higher during exercise in all patients than in controls at the same absolute work rate. Glucose infusion induced hyperglycemia and hyperinsulinemia in patients and inhibited the exercise-induced increases in glucose production, glycerol, free fatty acids, catecholamines, growth hormone, cortisol, and heart rate. In conclusion, feedback from metabolism in contracting muscle enhances hormonal responses and extramuscular substrate mobilization during exercise in McArdle's disease.

Relationship between ammonia, heart rate, and exertion in McArdle's disease

We studied plasma ammonia and exercise tolerance in six patients with McArdle's disease (myophosphorylase deficiency, type V glycogenosis) during incremental cycle ergometry. Tests were performed either in the postabsorptive state or after supplementation with branched-chain amino and 2-oxoacids and glucose. Glucose and branched-chain 2-oxoacid combined increased total work performed from control 49 +/- 22 to 80 +/- 36 kJ (P less than 0.05). Glucose alone also improved total work performed from 49 +/- 22 to 64 +/- 33 kJ (P less than 0.05). Branched-chain 2-oxoacids alone had a variable effect, and branched-chain amino acids were of no benefit. Correlations between plasma ammonia and heart rate for individual patients were r = 0.99, P less than 0.01; r = 0.95, P less than 0.01; r = 0.84, P less than 0.01; r = 0.76, P less than 0.01; r = 0.73, P less than 0.01; and r = 0.63, P less than 0.05 and between ammonia and perceived exertion for all patients combined was r = 0.70, P less than 0.0001. In two patients, correlation of ammonia with heart rate at a power output of 60 W was r = 0.91, P less than 0.001 and at 40 W was r = 0.77, P less than 0.001. We conclude that ammonia is either a mediator or a marker of the metabolic events leading to fatigue.

In vivo 13C-NMR evaluation of glycogen content in a patient with glycogen storage disease

Glycogen storage disease was suspected in a 10-month-old boy. Initial technical problems did not permit the determination of the precise enzyme, deficiency, and type VI glycogen storage disease was only diagnosed at the age of 2 years. In the mean time, natural abundance 13C nuclear magnetic resonance evaluation of muscular and hepatic glycogen content indicated normal muscular glycogen and increased hepatic glycogen in our patient, a finding which strongly argued for the diagnosis of type VI glycogen storage disease. Even though the use of nuclear magnetic resonance might seem, in this situation, a somewhat circuitous means of reaching the diagnosis, it appears that nuclear magnetic resonance could provide a useful tool for a non-invasive diagnosis of glycogen storage diseases.

Changes in arterial K+ and ventilation during exercise in normal subjects and subjects with McArdle's syndrome

1. We have examined the relationship between ventilation (VE), lactate (La) and arterial plasma K+ concentrations [( K+]a) during incremental exercise in six normal subjects and in four subjects with McArdle's syndrome (myophosphorylase deficiency) who do not become acidotic during exercise. 2. In normal subjects, [K+]a rose to ca 7 mM at the point of exhaustion. The time courses of the increases in VE, La and [K+]a were all similar during the exercise period. La reached its peak concentration during the recovery from exercise when both VE and [K+]a were returning to resting levels. 3. McArdle's subjects, like normal subjects, had a non-linear ventilatory response during incremental exercise. Their [K+]a was closely related to VE throughout exercise and recovery. 4. The arterial pH of McArdle's subjects, rather than remaining constant, actually rose from the onset of exercise. 5. For a given level of exercise, the levels of VE and [K+]a were greater in the McArdle's subjects than in normal subjects. 6. These findings are consistent with the idea that hyperkalaemia may contribute significantly to the drive to breathe, especially during heavy exercise.

Metabolism of branched-chain amino acids and ammonia during exercise: clues from McArdle's disease

Patients with McArdle's disease (myophosphorylase deficiency) cannot use muscle glycogen as an energy source during exercise. They therefore are an ideal model to learn about the metabolic adaptations which develop during endurance exercise leading to glycogen depletion. This review summarizes the current knowledge of ammonia and amino acid metabolism in these patients and also adds several new data. During incremental exercise tests in patients with McArdle's disease, forearm venous plasma ammonia concentration rises to a value between 200 and 500 microM. Femoral arteriovenous difference studies show that muscle produces the ammonia. The leg release of both ammonia and glutamine (in mumol/min) has been estimated to be five- to tenfold larger in one of these patients than in healthy individuals exercising at comparable relative work load. Patients with McArdle's disease have a larger uptake of branched-chain amino acids (BCAA) by exercising leg muscles and show a more rapid activation of the muscle branched-chain 2-oxo acid dehydrogenase complex, a key enzyme in the degradation of the BCAA. In general, supplements of BCAA taken before the exercise test lead to a deterioration of exercise performance and a higher increase in heart rate and plasma ammonia during exercise, whereas supplements of branched-chain 2-oxo acids improve exercise performance and lead to a smaller increase in heart rate and plasma ammonia. At constant power output, patients with McArdle's disease show a rapid increase in heart rate and exertion perceived in the exercising muscles, which peak within 10 min after the start of exercise and then fall again ("second wind"). Peak heart rate and peak exertion coincide with a peak in plasma ammonia. Ammonia production during exercise in these patients is estimated to exceed the reported breakdown of ATP to IMP and therefore most likely originates from the metabolism of amino acids. Deamination of amino acids via the reactions of the purine nucleotide cycle and glutamate dehydrogenase are possible pathways. Deamination of glutamine, released by muscle, by glutaminase present in the endothelial cells of the vascular system may also contribute to the ammonia production. The observations made in these patients have led to the hypothesis that excessive acceleration of the metabolism of BCAA drains 2-oxoglutarate in the primary aminotransferase reaction and thus reduces flux in the citric acid cycle and impedes aerobic oxidation of glucose and fatty acids. This draining effect is normally counteracted by the anaplerotic conversion of muscle glycogen to citric acid cycle intermediates, a reaction which is severely hampered in these patients due to the glycogen breakdown defect.(ABSTRACT TRUNCATED AT 400 WORDS)

[McArdle disease: differential diagnosis of the increase in creatine kinase induced by the exercise test]

In a Turkish couple presenting atypical precordial pain, muscle pain and a massive increase of creatine kinase during and one day after bicycle ergometry, suspicion of McArdle's disease was confirmed by a pathologic ischemic forearm worktest, a pathologic serial stimulation test and by pathologic glycogen content with lack of myophosphorylase activity on histochemical examination of thigh muscle tissue. Characteristic signs of McArdle's disease such as muscle weakness, muscle pain and muscle swelling, especially after exertion, were detected only after specific questioning of the patients. McArdle's disease was also detected by phosphor nuclear resonance in the two male children. Frequent consanguinity in the small isolated mountain village where the family originated explains why all four members of two generations are affected by the autosomal recessive disease.

Myogenic hyperuricemia. A common pathophysiologic feature of glycogenosis types III, V, and VII

To identify the mechanism of hyperuricemia in glycogen storage diseases (glycogenoses) that affect muscle, we studied the effects of exercise and prolonged rest on purine metabolism in two patients with glycogenosis type III (debrancher deficiency), one patient with type V (muscle phosphorylase deficiency), and one patient with type VII (muscle phosphofructokinase deficiency). All had hyperuricemia except for one patient with glycogenosis type III. Plasma concentrations of ammonia, inosine, and hypoxanthine increased markedly in all the patients after mild leg exercise on a bicycle ergometer. The plasma urate concentrations also increased, but with a delayed response. Urinary excretion of inosine, hypoxanthine, and urate increased greatly after exercise, consistently with the increases in plasma levels. Hypoxanthine and urate concentrations were extremely high in the plasma and urine of the patient with glycogenosis type VII. With bed rest, the plasma hypoxanthine level returned to normal within a few hours, and the plasma urate concentration decreased from 18.6 to 10.6 mg per deciliter (1106 to 630 mumol per liter) within 48 hours. Similarly, the urinary excretion of these purine metabolites was reduced by bed rest. These findings indicate that muscular exertion in patients with glycogenosis types III, V, and VII causes excessive increases in blood ammonia, inosine, and hypoxanthine due to accelerated degradation of muscle purine nucleotides. These purine metabolites subsequently serve as substrates for the synthesis of uric acid, leading to hyperuricemia.

McArdle's disease with myoadenylate deaminase deficiency: observations in a combined enzyme deficiency

Exercise and work potential of a patient with coexistent myophosphorylase and myoadenylate deaminase (AMPDA) deficiency was compared with that of three patients with myophosphorylase deficiency alone. The patient with the combined defect failed to produce an abnormal rise in serum ammonia or hypoxanthine as seen in the other patients after forearm exercise. Maximum oxygen consumption and work rates during cycle ergometer testing were similar in all patients, but well below controls. The occurrence of two defects involving short-term energy metabolism in muscle presents an opportunity to define further the metabolic role of AMPDA.

Ischaemic exercise test in myoadenylate deaminase deficiency and McArdle's disease: measurement of plasma adenosine, inosine and hypoxanthine

Plasma adenosine, inosine and hypoxanthine concentrations were assayed in seven control subjects, five myoadenylate deaminase deficient (MADD) patients and six McArdle patients before and after ischaemic forearm exercise. The plasma adenosine increase was very low in all test groups and there were no significant differences. The MADD patients showed a significantly lower increase of plasma inosine and hypoxanthine after exercise as compared with the controls. In the McArdle patients the increase in plasma inosine and hypoxanthine after exercise did not differ significantly from the values measured in the controls. The ischaemic exercise test with measurement of plasma inosine and hypoxanthine might be of diagnostic value in MADD, but not in McArdle's disease.

Type V glycogen storage disease

We describe three children with type V glycogen storage disease, who were reluctant to climb hills. We suggest that this condition, usually described as being of adult onset, can often be diagnosed in childhood.

Metabolic control of cardiac output response to exercise in McArdle's disease

During dynamic exercise cardiac output (Q) normally increases approximately 5 liters per liter of increase in O2 uptake (Vo2) (i.e., delta Q/delta Vo2 approximately equal to 5), indicative of a tight coupling between systemic O2 transport and utilization. We studied four patients with muscle phosphorylase deficiency (McArdle's disease) in whom Q was normal at rest, but delta Q/delta Vo2 was 14.1 +/- 1.3 during bicycle exercise. Procedures designed to alter the availability of substrates were employed to test the hypothesis that the increased delta Q/delta Vo2 is linked to the abnormal metabolic state of skeletal muscle. Fasting plus prolonged moderate exercise was used to increase the availability of plasma free fatty acid (FFA) and resulted in a normalization of delta Q/delta Vo2 (5.3 +/- 0.4). Hyperglycemia (70% above control levels) partially normalized delta Q/delta Vo2. Nicotinic acid lowered plasma FFA concentration and dramatically increased delta Q/delta Vo2 (4.6 to 13.7) when administered after fasting plus prolonged exercise in one patient. Glucose infusion after nicotinic acid administration markedly lowered delta Q/delta Vo2. The results support the hypothesis and suggest that the metabolic state of skeletal muscle, possibly via activation of muscle afferents, participates in the regulation of systemic O2 transport.

Late-onset Mcardle's disease with unusual electromyographic findings

Symptoms of McArdle's disease (muscle phosphorylase deficiency) commonly begin in childhood or adolescence. Late onset of the disease is rare. We describe a 76-year-old man whose symptoms began at age 74 years with sudden onset of proximal muscle weakness and fatigability. Electromyography disclosed substantial spontaneous activity and myopathic features as seen in inflammatory muscle disease. The diagnosis of McArdle's disease was made by histochemical studies of muscle, an abnormal ischemic lactate test, and absence of myophosphorylase activity.

Exercise hyperventilation in patients with McArdle's disease

This study was undertaken to determine if patients who lack muscle phosphorylase (i.e., McArdle's disease), and therefore the ability to produce lactic acid during exercise, demonstrate a normal hyperventilatory response during progressive incremental exercise. As expected these patients did not increase their blood lactate above resting levels, whereas the blood lactate levels of normal subjects increased 8- to 10-fold during maximal exercise. The venous pH of the normal subjects decreased markedly during exercise that resulted in hyperventilation. The patients demonstrated a distinct increase in ventilation with respect to O2 consumption similar to that seen in normal individuals during submaximal exercise. However their hyperventilation resulted in an increase in pH because there was no underlying metabolic acidosis. End-tidal partial pressures of O2 and CO2 also reflected a distinct hyperventilation in both groups at approximately 70-85% maximal O2 consumption. These data show that hyperventilation occurs during intense exercise, even when there is no increase in plasma [H+]. Since arterial CO2 levels were decreasing and O2 levels were increasing during the hyperventilation, it is possible that nonhumoral stimuli originating in the active muscles or in the brain elicit the hyperventilation observed during intense exercise.

Lactate production in McArdle's disease

A case of McArdle's disease in a man is described in detail and a less complete study of his family is reported. This patient showed the classical features of McArdle's disease and the diagnosis was confirmed by muscle biopsy. Unlike other reported cases of this disorder, this case showed a normal rise in blood lactate levels on ischaemic exercise. This apparently paradoxical finding is discussed. It is suggested that a normal rise in the level of blood lactate on ischaemic exercise should not exclude myophosphorylase deficiency.

McArdle's syndrome: the reaction to a fat-rich diet

The effect of a diet enriched with fats on the muscle performance of a patient with McArdle's syndrome was studied. The tolerance to physical activity was studied during exercise (ergometric examination) and by sustained abduction to 90 degrees of the deltoid muscle, both after a three-day period on a normal diet and after a three-day period on a fat-rich diet. After the first period a woody spasm of the deltoid muscle was found which lasted several days. After a period on fat-rich diet the patient's physical fitness was increased and the recovery period after the acute load was shorter. Moreover, no induration of the deltoid muscle was observed after the sustained abduction to 90 degrees. It is argued that maximal strength is not improved by a fat-rich diet but tolerance to submaximal loads is increased by such treatment and recovery from non-lactate-produced muscle discomfort is hastened.