Historical note and nomenclature

In 1951, Brian McArdle gave a remarkably precise description of the metabolic problem of muscle phosphorylase deficiency (GSD V) on the basis of clinical observations and a few critical laboratory tests in one patient (86). He noted that ischemic exercise resulted in painful cramps of forearm muscles and that no electrical activity was recorded from the shortened muscles, indicating that they were in a state of contracture. He also noted that oxygen consumption and ventilation were normal at rest but increased more than normal with exercise. The astute observation that venous lactate and pyruvate did not increase after exercise led McArdle to conclude that his patient's disorder was "characterized by a gross failure of the breakdown of glycogen to lactic acid." Nor was the specific involvement of muscle lost to McArdle, who noted that epinephrine elicited a normal rise of blood glucose and "shed blood" in vitro accumulated lactate normally, leading him to conclude that "the disorder of carbohydrate metabolism affected chiefly, if not entirely, the skeletal muscles."

In its typical presentation, muscle phosphofructokinase deficiency (GSD VII), first described by Tarui and colleagues in a Japanese family (137) and soon thereafter by Layzer and colleagues in an Ashkenazi Jewish-American patient (71), can closely resemble McArdle disease. Certain clinical and laboratory findings can help separate these conditions.

Several less common glycolytic defects similarly involve symptoms similar to those of McArdle disease and Tarui disease.

Muscle nonlysosomal glycogenoses with fixed weakness have been associated early with debrancher deficiency (GSD III) and brancher deficiency (GSD IV), but new interest has been generated by polyglucosan myopathy in juvenile patients with cardiomyopathy and in late-onset patients without cardiac involvement.

Glycogenoses causing exercise intolerance and myoglobinuria

Phosphorylase kinase (PhK) deficiency (GSD IX). PhK is a key regulatory enzyme in glycogen metabolism because it activates glycogen phosphorylase in response to neuronal or hormonal stimuli. PhK deficiency can cause four distinct clinical presentations, which are distinguished by tissue involvement (liver, muscle, heart, or liver and muscle) and mode of inheritance (autosomal or X-linked). This clinical and genetic heterogeneity is explained by the complexity of the enzyme, a protein composed of four sets of subunits (αβγδ)4. In addition, there are two isoforms for the α subunit (muscle and liver, A1 and A2), both encoded by genes on the X-chromosome, and two isoforms for the γ subunit (muscle and testis, G1; liver, G2). Both G isozymes and the B subunit are encoded by autosomal genes (54).

The cardiac variant demonstrates frequent thickening of myocardium by echocardiography (124). The purely myopathic variant of PhK deficiency (GSDIXd) has an estimated prevalence of 1 in 100,000 (114). It has a variable age of onset and usually manifests as a milder form of myophosphorylase deficiency (GSDV, McArdle disease). Predominant symptoms include exercise intolerance with stiffness, cramps, and occasional myoglobinuria. Cognitive impairment accompanying myopathy and exercise intolerance has also been reported (18). One distinguishing laboratory feature of this condition is the lactate response to the forearm exercise test, which is usually flat in patients with McArdle disease, whereas it is normal or blunted in patients with PhK deficiency. Most cases reported of myopathic PhK deficiency have been men consistent with X-linked inheritance (Xq13.1). In agreement with this concept, all molecular defects identified so far are in PHKA1 (107).

The liver and muscle variant of PhK (GSD IXb) is an autosomal recessive disorder dominated by short stature, hepatomegaly, and fasting hypoglycemia with minimal muscle involvement (weakness and hypotonia): in one female and four male patients, five distinct nonsense mutations were identified in the PHKB gene on chromosome 16q (23). No specific treatment is available, but prevention of hypoglycemia with uncooked starch, high protein, and complex carbohydrate diet has been suggested (114).

Myophosphorylase deficiency (glycogenosis type V, McArdle disease)

Clinical presentation. The clinical picture of McArdle disease is dominated by exercise intolerance manifested by myalgia, premature fatigue, and stiffness or weakness of exercising muscles, which is relieved by rest (77). The type and amount of exercise needed to precipitate these symptoms varies considerably from patient to patient, possibly in relation to training and diet, but two types of exertion are likely to cause problems: brief intense isometric exercise, such as lifting heavy weights, or less intense but sustained dynamic exercise, such as walking uphill. Moderate exercise, such as walking on level ground, is usually well tolerated. On the other hand, strenuous exercise often results in painful cramps and muscle swelling, which can last for hours. Myoglobinuria is seen in about half of the patients. In fact, McArdle disease is the second most common metabolic cause of recurrent myoglobinuria after CPT II deficiency (141). An interesting clinical clue, described by patients with McArdle disease and useful for diagnosis (147), is the "second wind" phenomenon: if at the first appearance of exercise-induced myalgia they slow down or rest briefly (5 to 15 minutes), tolerance is improved when resuming activity. This is due to increased blood flow to muscles and metabolic switch to free fatty acids as source of energy.

The severity of symptoms varies markedly in different patients: some have neither cramps nor myoglobinuria, but some degree of exercise intolerance is almost universal. Symptoms start before age 15 years in most cases. About 20% to 47% of patients have progressive weakness, usually starting after age 40 years (109; 61). Severe axial myopathy and prominent proximal and paraspinal muscle involvement have been reported (159; 07; 75). Masticatory weakness is present in 5% of cases (61).

A distinct clinical variant reported in four children is characterized by severe generalized weakness at or soon after birth, respiratory insufficiency, and death in infancy (33; 87).

Etiology and pathogenesis. Myophosphorylase deficiency is an autosomal recessive disease due to mutations of the muscle isoform of glycogen phosphorylase (PYGM) on chromosome 11q (77; 100). Prevalence has been estimated to be 1 in 100,000 to 140,000 patients (119). Family history is positive in approximately a quarter of cases (61). The apparently autosomal dominant transmission in a few families may be explained by at least two mechanisms: (1) the presence in subsequent generations of homozygous and manifesting heterozygous individuals (122; 103; 85); or (2) the presence of one homozygous and one heterozygous parent (145).

The molecular heterogeneity of myophosphorylase deficiency is striking: from the description of the first molecular defects (16; 145), more than 150 different mutations have been reported (77; 146; 131). The most common mutation in North America and Northern Europe (approximately 50% of cases in Caucasian population) appears to be a cytosine-to-thymine substitution in codon 50 of exon 1, converting an arginine to a stop codon (R50X). This has allowed the use of molecular genetic analysis in leukocytes for diagnostic purposes, thus, avoiding the need of a muscle biopsy (40). However, as more and more mutations in the PYGM gene are described and blood DNA is increasingly used for diagnosis (44), it becomes important to establish the relative frequency of the different mutations in ethnic groups (146). For example, the R50X mutation has never been observed in Japan, where the most common mutation appears to be a 3-bp deletion, TTC at codon 708/709 (133).

The genotype/phenotype relationship in McArdle disease remains unclear: the most common genetic defect in typical McArdle patients, R50X, was also found in an infant who had the fatal myopathic variant (145) and in a child who died of sudden infant death syndrome (SIDS) (39). Of course, one cannot exclude that these unusual presentations may be due to additional gene defects.

Phosphorylase activity is virtually absent in muscle biopsies when determined either histochemically or biochemically. Biochemical studies show that the enzyme is lacking in most patients, which is consistent with the most common genetic error, a nonsense mutation (R50X). As a consequence, glycogen of normal structure accumulates in muscle, reaching concentrations two or three times higher than normal. Morphologically, the stored glycogen collects mostly at the periphery of muscle fibers, where it forms subsarcolemmal "blebs." Vacuolar myopathy and increase glycogen by periodic acid Schiff staining are the classic pathological findings.

Phosphorylase catalyzes the first step in glycogenolysis by removing 1,4-glucosyl residues phosphorolytically with liberation of glucose-1-phosphate. Hence, lack of phosphorylase impedes glycolysis, as shown by the flat venous lactate response to the ischemic forearm exercise test. Similarly, 31P-nuclear magnetic resonance spectroscopy shows lack of acidification during aerobic or ischemic exercise and a greater than normal drop of the phosphocreatine/inorganic phosphate ratio (48). In addition to blunting of lactate, there is typically an abnormal increase in ammonia following exercise. This is due to an increased reliance on adenosine monophosphate (AMP)-deaminase pathway as an alternative way to provide the necessary ATP.

The nonischemic forearm exercise test (grip test) has a sensitivity of 100%: a normal lactate/ammonia response after proper exercise eliminates the possibility of McArdle disease (57). A provocative test (long exercise test) can be used as a complement in the neurophysiological evaluation. A postexercise decrease in compound muscle action potential was present in 23 of 25 patients with McArdle disease (123).

Three main pathophysiological mechanisms underlie the exercise intolerance of myophosphorylase deficiency and, in fact, of all glycogenoses with exertional premature fatigue (148): (1) decreased or absent muscle acidosis, which, by decreasing chloride permeability, maintains membrane excitability; (2) loss of the osmotic effect of lactate accumulation, leading to increased extracellular potassium concentration; and (3) excessive accumulation of ADP, likely leading to inhibition of Na+/K+ and Ca++ ATPases. In agreement with the concept that oxidative phosphorylation is curtailed by decreased substrate availability, oxygen extraction and maximal oxygen uptake are decreased in myophosphorylase deficiency but may be at least partially restored by intravenous glucose infusion (51). The pathogenesis of contracture and myoglobinuria remains unknown: depletion of high-energy phosphate compounds, especially ATP, has long been postulated to occur, but experimental evidence is lacking.

Resting serum creatine kinase (CK) is consistently elevated (5- to 13-fold) in patients with McArdle disease (61), indicating that individual fiber necrosis probably occurs even with daily activities, a concept supported by morphological observations. The cumulative effect of this focal muscle damage along the years may explain the appearance of fixed weakness in older individuals. Muscle imaging with whole body MRI shows fatty replacement of paravertebral and proximal muscles in middle-aged patients (07; 75).

Treatment and management. Many patients adapt their lifestyles to their limited exercise tolerance and, within these limitations, can lead nearly normal lives. Late in the course of the disease, fixed weakness, although usually moderate, may become a problem.

Acute renal insufficiency occurs in about 50% of patients with myoglobinuria. In these cases, abundant fluid intake to induce diuresis may suffice, but renal dialysis is often necessary. Uncomplicated episodes of myoglobinuria are followed by complete functional recovery.

There is no specific therapy for McArdle disease. Naturally, patients should be warned about the risks of strenuous exercise and advised to seek medical attention at the first appearance of pigmenturia, especially if accompanied by oliguria.

Oral sucrose loading 30 to 40 minutes prior to exercise helps with initial lack of glucose release from deficient glycogenolysis (149) but must be used sparingly to avoid weight gain (08). Regular moderate aerobic training (resulting in a heart rate of no more than 60% to 70% of maximal) to develop mitochondrial oxidation is recommended (47). Vitamin B6 bounds to phosphorylase enzyme and, therefore, was a promising agent in cases with residual phosphorylase activity (105; 59; 120). A metanalysis found no difference of B6 compared to placebo (110). Low-dose creatine has been shown to enhance power output (155).

Ketone bodies constitute an alternative energy source that is independent of glycogenolysis. However, a placebo-controlled study failed to demonstrate improved exercise capacity when patients with McArdle disease were administered exogenous ketone bodies (76). The ketogenic diet improved exercise tolerance in an anecdotal report and in one open-label study (24; 74). A complex carbohydrate-rich diet resulted in better exercise performance compared to a protein-rich diet (110). Animal models have demonstrated that sodium valproate can increase muscle expression of brain glycogen phosphorylase isoenzyme. However, an open-label study of 16 patients failed to show improvement in any of the biochemical or functional outcomes at 6 months (121).

Phosphofructokinase (PFK) deficiency (glycogenosis type VII, Tarui disease)

Clinical presentation. Typically, there is intolerance to intense exercise, often accompanied by cramps of exercising muscles, which is relieved by rest (35). The onset of disease is typically between 3 and 25 years of age (95). Symptoms are more likely to occur with isometric exercise (such as pushing a stalled car) or intense dynamic exercise (such as walking uphill). The exercise intolerance appears to worsen with high carbohydrate intake (50). In contrast to patients with McArdle disease, patients with PFK deficiency do not experience a "second wind" (52).

A few patients may be jaundiced as a consequence of the hemolytic trait that accompanies PFK-M deficiency, and a few may suffer from gouty arthritis due to hyperuricemia. Increased bilirubin without anemia is reflective of compensated hemolytic anemia.

Fixed weakness of late onset has been described in a few patients who suffered from exercise intolerance earlier in life (125; 53; 29; 14; 82; 153).

A strikingly different clinical presentation consists of severe myopathy in infancy or early childhood, with respiratory failure and death before 2 years of age. This variant has been reported in several children (35).

Studies of 31P-nuclear MRS show the accumulation, even with mild exercise, of glycolytic intermediates in the form of phosphorylated monoesters, which occurs also in other defects of glycolysis, but not in myophosphorylase deficiency (13).

Etiology and pathogenesis. PFK is a tetrameric enzyme under the control of three autosomal loci: a locus on chromosome 1 encodes the muscle (M) subunit; a locus on chromosome 21 encodes the liver (L) subunit, and a locus on chromosome 10 encodes the platelet (P) subunit (35). The three subunits are variably expressed in different tissues. Mature human muscle expresses only the M subunit and contains only the homotetramer M4, whereas erythrocytes express both the M and L subunits and contain five isozymes: the two homotetramers, M4 and L4, and three hybrid isoforms. In patients with typical PFK deficiency, genetic defects of the M subunit cause total lack of activity in muscle, but only partial PFK deficiency in red blood cells, where the residual activity (approximately 50% of normal) is accounted for by the L4 isozyme.

This disease is inherited as an autosomal recessive trait. The first molecular defect in PFK deficiency (a splice junction mutation resulting in a large deletion) was identified in the Japanese family originally described by Tarui and coworkers (97). Soon thereafter, Raben and coworkers described two mutations, a splicing defect and a nucleotide deletion, which are common among Ashkenazi Jewish patients (113; 126). About 20 distinct mutations have been identified in patients of different ethnic origins (95), although in the United States, the disease predominantly affects individuals of Ashkenazi Jewish origin.

Genetic defects of PFK-M cause virtual absence of PFK in muscle biopsies when the activity is determined either histochemically (20) or biochemically. Tissues such as liver and platelets express predominantly or exclusively in the non-M PFK subunits and are not affected. The lack of frequent involvement of the heart is puzzling because the PFK-M is abundantly represented in cardiac tissue (38).

As a consequence of PFK deficiency, glycogen accumulates in muscle, reaching concentrations two to three times greater than normal. Morphologically, the stored glycogen is mostly seen at the periphery of the fibers, where it is revealed by the periodic acid-Schiff (PAS) histochemical reaction and is normally digested by preincubation with diastase. However, a peculiarity of PFK deficiency is the additional finding of an abnormal polysaccharide, which stains intensely with PAS, but is not digested by diastase (polyglucosan). Ultrastructurally, polyglucosan affects about 10% of type 1 fibers (53) and is composed of finely granular and filamentous material, similar to the amylopectin-like polysaccharide that accumulates in branching enzyme deficiency. The presence of this material is likely due to the accumulation in muscle of glucose-6-phosphate, an activator of the enzyme glycogen synthetase, which alters the delicate ratio between the two main glycogenosynthetic enzymes, synthetase and branching enzyme (01; 112).

PFK is the rate-limiting enzyme of glycolysis, and PFK deficiency blocks glycolysis, thus, explaining the flat venous lactate response to forearm ischemic exercise. The accumulation in muscle of phosphorylated monoesters, observed by 31P-nuclear MRS in PFK but not in myophosphorylase deficiency, is explained by the sites of the two metabolic blocks: midway in the glycolytic pathway for PFK deficiency and upstream of glycolysis for myophosphorylase deficiency.

As in myophosphorylase deficiency, PFK deficiency also impairs both anaerobic and aerobic glycogen metabolism and blocks the fall in muscle pH that normally accompanies heavy exercise. It also results in high levels of adenosine diphosphate (ADP) and increased adenine nucleotide degradation with exaggerated production of ammonia and myogenic hyperuricemia during exercise (89). It causes substrate-limited oxidative metabolism with fluctuations in exercise and oxidative capacity related to the availability of blood-borne fuels (50). Finally, PFK deficiency causes exaggerated sympathetic neural responses to exercise associated with enhanced mobilization of extramuscular fuels (152) and exaggerated heart rate, cardiac output, and blood flow relative to the muscle capacity to use oxygen (73).

The negative effect of high-carbohydrate meals on exercise tolerance is attributed to the fact that glucose lowers the blood concentration of free fatty acids and ketones, which are alternative fuels in PFK-deficient patients. Thus, not only is glucose ineffective in alleviating exercise intolerance (in contrast to myophosphorylase deficiency), but it is actually harmful, a situation dubbed "out-of-wind" phenomenon (50).

Evaluation and diagnosis. This entity should be differentiated from myophosphorylase deficiency. The latter is the prototype of glycogenolytic disorder and Tarui disease, a typical glycolytic disorder.

Table 1. Clinical Differences Between McArdle Disease (GSD V) and Tarui Disease (GSD VII)

Clinical differences include lack of a typical "second wind" phenomenon, more common nausea and vomiting accompanying exercise-induced myalgia, detrimental effect of glucose administration, and lower frequency of myoglobinuria attacks in Tarui disease. More useful in distinguishing PFK deficiency from McArdle disease are a few simple laboratory tests, such as increased uric acid, bilirubin concentration, and reticulocyte count in the former.

Needle electromyography may be normal or show "myopathic" abnormalities (small and short-duration motor unit action potentials with early recruitment). Furthermore, no electrical activity is recorded from maximally shortened muscles during contractures induced by ischemic exercise.

The forearm exercise test is a valuable, but not specific, diagnostic test. In patients with an abnormal forearm exercise test, laboratory evidence of hemolytic anemia suggests PFK deficiency. Of note, an increase in lactate to two to three times the basal level but still lower than control can be seen in Tarui disease (106). CK is typically elevated greater than 1000 U/L (95). Until recently, definitive diagnosis required biochemical documentation of the enzyme defect in muscle (keeping in mind that PFK is notoriously labile and PFK deficiency is often a spurious finding if muscle is not flash-frozen at the time of biopsy). Present knowledge of multiple molecular defects in the PFK-M gene allows identification of many suspected patients using genomic DNA isolated from white blood cells.

Treatment and management. Most patients adapt their lifestyles to their limited exercise tolerance and may lead nearly normal lives. In the fifth or sixth decade, some patients develop proximal limb weakness, which can limit their functional independence, but is rarely disabling.

Although myoglobinuria is rare in this condition, acute renal insufficiency may occur in patients with myoglobinuria, requiring forced diuresis or renal dialysis. Uncomplicated episodes of myoglobinuria are followed by complete functional recovery.

There is no specific therapy. Glucose administration prior to exercise is detrimental (50). Supplementation with aneplerotic substrate (triheptanoin) did not improve exercise performance in a placebo-controlled study (111).

A 2-year-old boy with the infantile form of PFK deficiency benefited remarkably – as had a patient with McArdle disease (24) – from a ketogenic diet, which was instituted to provide muscle and brain with ketone bodies as alternative fuels (135). Improvement of exercise performance and ammonia peak reduction was seen in an adult treated long-term with a ketogenic diet (127). Overall, ketogenic diet should be tried in infants or children with severe deficiency and could be considered in the milder form.

Aldolase (ALD) deficiency (glycogenosis type XII)

Two children with muscle and erythrocyte aldolase deficiency have been reported (69; 160). Both had transfusion-requiring nonspherocytic hemolytic anemia, muscle weakness, exercise-induced myalgia, and increased serum CK, especially during febrile illnesses (2620 U/L in one patient and 13,800 U/L in the other). One patient was alive at 4.5 years (69); the other died at 4 years during an episode of myoglobinuria and hyperkalemia (160). Both children were compound heterozygous for mutations in ALDOA, the gene that encodes aldolase A, the only aldolase isozyme present in muscle and erythrocytes. The increased thermolability of the mutant enzyme probably explains the vulnerability of patients to febrile illnesses (84).

Phosphoglycerate kinase (PGK) deficiency

Primary myopathy is not a common presentation of PGK deficiency, an X-linked recessive disorder most commonly presenting as nonspherocytic hemolytic anemia and central nervous system (CNS) dysfunction. However, purely myopathic presentation is the second most frequent presentation, whereas isolated hemolytic anemia was reported in six patients and the association of myopathy and CNS dysfunction in four patients (129). All myopathic patients complained of exercise intolerance, with cramps and myoglobinuria. Molecular defects have been documented in several patients and one, T378P (PGK Afula), caused the peculiar association of myopathy and severe juvenile parkinsonism (128; 37).

The wide spectrum of clinical phenotypes in PGK deficiency is difficult to explain because PGK is a monomeric enzyme encoded by a single gene on Xq13 and expressed in all tissues except the testis (a testicular isozyme, PGK2, is encoded by a gene on chromosome 19). Different amounts of residual activities in different tissues do not explain the clinical heterogeneity (129). Although lack of myoglobinuria in patients with severe hemolytic anemia and brain dysfunction may be attributed to their inability to exercise, it is more difficult to explain the converse situation, lack of blood or brain involvement, in patients with myopathy. The highly perturbed catalytic properties of a mutation (I371K) may account for the PGK deficiency found in an Italian patient with hemolytic anemia, intellectual disability, and severe myopathy (42). A further unusual clinical manifestation of PGK deficiency is retinal dystrophy in a 6-year-old boy with deteriorated vision, but no hemolytic anemia associated with a known mutation (D164V) (92).

Phosphoglycerate mutase (PGAM) deficiency (glycogenosis type X)

In contrast to PGK deficiency, PGAM deficiency affects only muscle, causing exercise intolerance, cramps, and recurrent myoglobinuria with onset on childhood or adolescence (36). This is because PGAM is a dimeric enzyme composed of a muscle-specific (M) and a brain-specific (B) subunit, and normal muscle contains predominantly the MM homodimer, which accounts for 95% of the total activity. Most cases have been reported in the United States (96). All patients in the U.S. have been black, and they harbor one common mutation (W78X) in the PGAM-M gene (encoded by a gene on chromosome 7), suggesting a founder effect (96). Different mutations were found in two Italian families (142; 140), in a Japanese family (45), and in a Pakistani patient (151; 96).

Despite the abundance of PGAM in muscle, exercise intolerance often occurs in heterozygous PGAM-deficient patients (45; 96; 118). A second unusual feature of PGAM deficiency is the frequent occurrence of tubular aggregates in muscle pathology (33% of patients), which has never been associated with other, more common glycogen storage diseases.

Cycle exercise responses in two patients were markedly different from those of patients with clinically similar McArdle disease: the PGAM-deficient patients had virtually normal cycle exercise and oxidative capacity, no second wind, and no improvement of their exercise capacity with lipid or lactate supplements (150).

Beta-enolase deficiency (glycogenosis type XIII)

Beta-enolase deficiency presents with adult-onset myalgias post-exertion, mildly elevated CK levels to recurrent rhabdomyolysis (94). The first case reported was a compound heterozygous with flat lactate response to forearm ischemic exercise (27). Muscle ultrastructure showed subsarcolemmal accumulations of glycogen, and muscle biochemistry revealed an isolated severe deficiency of enolase activity (5% of the normal mean). Over 90% of this muscle enzyme in the distal glycolytic pathway is accounted for by the beta-enolase form, which is encoded by the ENO3 gene on chromosome 17.

Phosphoglucomutase-1 deficiency (glycogenosis type XIV)

This enzyme is involved in glycogen metabolism and in protein glycosylation. There are two main phenotypes: one with predominant muscle involvement and one with multisystem disease. The first pathogenic mutation reported was present in a patient with episodes of exercise-induced rhabdomyolysis (132). Interestingly, this patient had a normal lactate response to forearm exercise but with increased ammonia levels. Muscle biopsy did not reveal vacuoles or increased glycogen content. Treatment with oral D-galactose should be considered in this disease (154).

Lactate dehydrogenase (LDH) deficiency (glycogenosis type XI)

The discovery of this glycogenosis was due to the astute observation that a patient with myoglobinuria had predictably sky-high values of serum CK, but extremely low values of LDH (63). This enzyme converts pyruvate to lactate in skeletal muscle and is composed of a muscle-specific subunit (LDH-A) and a cardiac subunit (LDH-B). LDH-A is encoded by a gene on chromosome 11, and three different mutations have been identified in Japanese patients (79; 81; 80), whereas the only two described white patients had two distinct mutations (143). In addition to muscle symptoms, three affected Japanese women suffered from dystocia necessitating cesarian sections, and a few patients had dermatologic problems (62). Generalized pustular psoriasis is attributed to abnormal activity of LDH (58).

Muscle glycogen synthetase (GS) deficiency (glycogenosis type 0)

Although glycogen synthetase (GS) deficiency of the liver was described several years ago and dubbed glycogenosis type 0 (102; 158), the first cases of muscle glycogenosis type 0 were reported in 2007 in a Swedish family (67). Of three siblings, a boy died of sudden cardiac arrest at 10 years of age, his brother suffered from exercise intolerance and hypertrophic cardiomyopathy, and his sister was asymptomatic. Muscle biopsies from the younger siblings showed conspicuous lack of glycogen by periodic acid Schiff (PAS) histochemistry and by electron microscopy, as well as increased numbers of mitochondria. All three children harbored a homozygous nonsense mutation in the gene (GYS1) encoding the muscle and heart isoform of the glycogen synthetase.

Two other reported of sudden cardiac death had preceding history of exertional syncope (25; 134). The diagnosis of glycogenosis type 0 is complicated by the mild muscle symptoms (exercise intolerance), the scarce attention paid by morphologists to lack of glycogen (as opposed to glycogen storage), and the misleading clue of the reactive mitochondrial proliferation.

Differential diagnosis

Exercise intolerance and recurrent rhabdomyolysis can be caused by other metabolic myopathies, especially disorders of lipid metabolism.

On purely clinical grounds, myophosphorylase deficiency is difficult to differentiate from other defects of glycolytic enzymes described above. Laboratory data may offer useful clues. Patients with PFK deficiency have a compensated hemolytic trait, with hyperuricemia, hyperbilirubinemia, and increased reticulocyte count, whereas patients with PGK deficiency often have overt nonspherocytic hemolytic anemia.

The forearm exercise test (grip test) is a common investigation performed in the workup of patients presenting with exercise intolerance and rhabdomyolysis. It consists of the measurement of lactate and ammonia from the antecubital vein before and after exercise. The exercise test with nonischemic conditions (grip with 70% of maximal contraction for 30 seconds) has been shown to be as useful as the ischemic one (with inflated blood pressure cuff) and avoids the risk of pain, compartment syndrome, or rhabdomyolysis (56). Normally, a nearly 4-fold increase in lactate and 2- to 3-fold increase in ammonia is present after exercise. Patients with McArdle disease typically demonstrate a null lactate response (less than 2-fold) with excessive Ammonia peak (more than 3-fold). In Tarui disease a low lactate response with increased ammonia is present (see Table 1). Patients with defects of terminal glycolysis (PGK, PGAM, and LDH deficiencies) have a blunted, but not absent, venous lactate response, and patients with LDH deficiency show low lactate, but excessive pyruvate, response.

The most common metabolic cause of recurrent myoglobinuria in adults is CPT II deficiency. In patients with myophosphorylase deficiency, myoglobinuria is invariably triggered by exercise, usually of high intensity. Patients with CPT II deficiency, however, have episodes of myoglobinuria not only after exercise (which can be of moderate intensity, but is usually prolonged), but also after prolonged fasting without exertion, or after the combination of exercise and fasting. The forearm exercise test is normal in patients with CPT II deficiency. CK level is typically normal between attacks in CPTII deficiency, unlike McArdle disease. The other two disorders of lipid metabolism that mimic CPT II deficiency and ought to be considered in the differential diagnosis of recurrent myoglobinuria are very long-chain acyl-CoA dehydrogenase (VLCAD) and trifunctional protein (TFP) deficiencies. Blood acylcarnitine profile can be helpful in distinguishing these disorders (low free carnitine with increased intermediates) specially if drawn at the time of rhabdomyolysis.

Exercise intolerance is one of the most common signs of mitochondrial encephalomyopathies and especially of mitochondrial myopathies (32). In these cases, there are no cramps, but rather shortness of breath and premature fatigue even with moderate exertion and, sometimes, recurrent myoglobinuria related to fasting or prolonged exercise. Recognition of the underlying mitochondrial etiology, especially in cases harboring mtDNA mutations, is obscured by the lack of the typical stigmata of mtDNA-related diseases: (i) most cases are sporadic without evidence of maternal inheritance; (ii) instead of being multisystemic disorders, these are isolated myopathies. Between episodes of myoglobinuria, serum CK is normal, but lactate is usually elevated (with high lactate/pyruvate ratio) and provides a good clue to the correct diagnosis. Plasma and urinary organic acids may also show abnormalities. Growth differentiating factor 15 (GDF-15) has been found to be elevated in mitochondrial disorders of respiratory chain (161), but several conditions that cause chronic ischemia (cancer, diabetes, cardiovascular disease) may give false positives.

Most cases of mitochondrial myopathies with recurrent myoglobinuria have been attributed to mutations in genes encoding subunits of complex I (12) and complex IV (64), but especially cytochrome b (MTCYB) of complex III (10; 09; 11).

Muscle biopsies show a mosaic pattern of succinate dehydrogenase (SDH)-hyperintense (“ragged blue”), Gomori trichrome (“ragged red”), and COX-positive fibers (except, of course, if the mutation affects a complex IV subunit, in which case the ragged-blue fibers are COX-negative). Electron-microscopic analysis often shows increased number of mitochondria that may contain paracrystalline inclusions. Genetic analysis for definitive diagnosis of mitochondrial myopathies is best done on muscle-derived material as mtDNA deletions are not commonly seen in white blood cell derived DNA.

The reason these disorders escape the rules of mitochondrial genetics is that they are due to de novo mutations that arise in myoblasts or myoblast precursors after germ-layer differentiation. One notable nucleus-related mitochondrial myopathy characterized by exercise-induced myoglobinuria was described in Swedish individuals and was associated with muscle deficiency of SDH and aconitase (49; 46). The peculiar pattern of biochemical abnormalities suggested a defect of non-heme iron-sulfur (FeS) clusters in the respiratory chain. This was confirmed by the documentation of homozygous mutations in the gene encoding the FeS scaffold protein, ISCU (90).

Exercise-related myoglobinuria may occur in some muscular dystrophies and in malignant hyperthermia. In patients with dystrophinopathies, serum CK levels between attacks are usually much higher than in patients with myophosphorylase deficiency, and transmission is X-linked recessive. Dysferlinopathy can also cause rhabdomyolysis; its baseline CK is high (more than 10-fold), and inheritance is autosomal recessive. In patients with malignant hyperthermia, there is usually a family history of characteristic attacks related to the administration of volatile anesthetics, and inheritance is autosomal dominant (RYR1 gene).

Intolerance to exercise without myoglobinuria rarely can be due to adenylate deaminase deficiency. In patients with adenylate deaminase deficiency, the ischemic forearm exercise test causes a normal rise of venous lactate, in contrast with a lack in the rise of ammonia. Because of the high frequency of the adenylate deaminase deficiency trait, an association with myophosphorylase deficiency (144; 115) or phosphofructokinase deficiency (21) may occur, and this "double trouble" may aggravate the clinical phenotype.

Exercise intolerance without myoglobinuria is a common complaint of malingering and functional disorders and is the cardinal symptom of "chronic fatigue syndrome."

Muscle phosphorylase and PFK deficiencies should be included in the differential diagnosis of late-onset proximal limb weakness. These patients usually have a history of lifelong exercise intolerance and cramps. Laboratory signs of a hemolytic trait, an abnormal forearm exercise test, and abundant polyglucosan in the muscle biopsy are clues to the diagnosis of PFK deficiency.

Glycogenoses causing fixed weakness

Debrancher deficiency (glycogenosis type III, Cori or Forbes disease). This is usually a benign disease of childhood characterized by hepatomegaly, growth retardation, and fasting hypoglycemia, which tend to resolve around puberty (66). Thickened myocardium, a risk factor for cardiomyopathy, has been observed (124). A few patients develop a myopathy later in life (third or fourth decade), which is often more distal, but a proximal predominant phenotype has also been reported (116). Wasting of leg muscles and intrinsic hand muscles often suggests the diagnosis of motor neuron disease or peripheral neuropathy. This clinical picture, the "mixed" myopathic and neurogenic EMG pattern, and the slowed nerve conduction velocity reinforce the impression that the weakness in these patients may have a neurogenic component (34; 157). A subacute presentation of respiratory weakness in a 47-year-old woman was reported (65). This patient had undergone strict fasting prior to the onset of weakness and weaning-off respirator was only possible after high-protein diet. The latter may be beneficial as gluconeogenesis is intact in this condition.

It is surprising that an enzyme that acts "hand-in-hand" with muscle phosphorylase in the degradation of glycogen should not cause exercise intolerance and myoglobinuria, at least in those patients that are not severely weak. This conundrum has been partially resolved by the demonstration that patients without overt weakness gave results similar to those obtained in McArdle disease: decreased skeletal muscle carbohydrate oxidation, increased fatty acid oxidation, and exercise intolerance improved by fructose administration (108).

The debranching enzyme is a single protein that catalyzes two enzymatic reactions, an oligo-1,4-1,4-glucantransferase and an amylo-1,6-glucosidase, and is encoded by a gene on chromosome 1p21. There are three biochemical variants: a rare deficiency of the transferase activity alone (type IIId), a common deficiency of both enzyme activities in both muscle and liver (type IIIa), and a less frequent deficiency of both enzyme activities in liver but not in muscle (type IIIb) (35). In the myopathic form, pathology reveals extensive nonrimmed glycogen vacuoles of subsarcolemmal location (116).

Glycogen branching enzyme (GBE) deficiency (glycogenosis IV, Andersen disease). This glycogenosis has a wide spectrum of clinical phenotypes, considering that GBE is a single polypeptide (encoded by a gene on chromosome 3). The enzyme defect can be silent, or predominantly affect the liver, the heart, skeletal muscle, or the brain (35). GBE deficiency results in the deposition of an amylopectin-like polysaccharide that has fewer branching points and longer outer chains than normal glycogen and is known as polyglucosan. Polyglucosan is PAS-positive and only partially digested by diastase, which makes it easily recognizable in various tissues and offers an important clue to the correct diagnosis.

The presentation described in most textbooks as "typical" is in infancy with hepatosplenomegaly, progressive cirrhosis, and chronic hepatic failure.

As recognized in a seminal paper (22), there are two main infantile presentations. The first is a perinatal disorder called “fetal akinesia-deformation sequence” (FADS), characterized by multiple congenital contractures (arthrogryposis multiplex congenita), hydrops fetalis, pulmonary hypoplasia, craniofacial abnormalities, intrauterine growth retardation, abnormal amniotic fluid volume, and perinatal death. The second form, labeled “congenital” should probably be called “fatal infantile” because it presents at or soon after birth with hypotonia, muscle wasting, neuronal involvement, inconsistent cardiomyopathy, and early death.

It is becoming increasingly clear that patients with congenital GBE deficiency present a clinical continuum from FADS to a rapidly fatal congenital multisystem disorder dominated by profound hypotonia, respiratory failure, and inconsistent cardiomyopathy (06; 22; 60; 138; 04; 68; 15; 136). Detailed neuropathology in a few infants showed polyglucosan inclusions in neurons of the basal ganglia and thalamus, oculomotor and pontine nuclei, and periaqueductal neurons (136). In the medulla, polyglucosan deposits were found in the hypoglossal nucleus, the dorsal motor nucleus of the vagus, and the nucleus ambiguous (138; 68). The motor neurons of the spinal cord are also severely affected (55), thus, explaining how one of the patients we studied was initially diagnosed as spinal muscular atrophy type I until mutations in the SMN1 gene were ruled out (138).

Brain involvement also dominates the clinical picture in a characteristic late-onset (fifth or sixth decade) form of GBE deficiency known as "adult polyglucosan body disease" (APBD) manifesting as a leukoencephalopathy with a clinical triad of spastic paraparesis, neurogenic bladder, and peripheral neuropathy. There is also frequent cognitive involvement, which is characterized by frontal executive impairment and disinhibition (31). The polyglucosan bodies accumulate in the axons of brain neurons and of peripheral nerves, thus, impairing axonal flow. Presentation with asymmetric motor syndrome can mimic amyotrophic lateral sclerosis (156). Differential diagnosis also includes CMT neuropathy, but sural nerve or muscle biopsy can point toward the diagnosis (polyglucosan bodies). Although adult polyglucosan body disease is seen in various ethnic groups, it predominates among Ashkenazi Jews, in whom one common mutation (Y329S) probably reflects a founder effect (91). An interesting conundrum was that several patients with adult polyglucosan body disease carried a single Y329S mutation: these “manifesting heterozygotes” were found to harbor a second deep intronic GBE1 mutation (03).

An “intermediate” variant of adult polyglucosan body disease has been recognized in patients who have earlier adult onset and a subacute relapsing-remitting clinical course, often simulating multiple sclerosis. They are not Ashkenazi Jews, and harbor GBE1 mutations that are different from the Y329S that is the signature of typical adult polyglucosan body disease (17; 104; 117).

Measurement of leucocyte GBE activity can be helpful for diagnostic purposes, especially when a variant of unknown significance is found on genetic testing (31). There is no specific therapy for GBE deficiency, but the creation of transgenic mice models that faithfully recapitulate both the fatal infantile form (05) and the late-onset adult polyglucosan body disease (Orhan 03) will facilitate the development of therapeutic modalities.

Lafora disease. Lafora disease is an autosomal recessive progressive myoclonus epilepsy (PME) with onset in adolescence and severe course, characterized by myoclonic, absence, and generalized tonic-clonic seizures, followed by difficulties with speech and gait, and dementia. Death, often in status epilepticus, occurs about 10 years after the first appearance of symptoms (70; 88).

Lafora bodies, the hallmark of the disease, are polyglucosan bodies structurally identical to those found in the APBD variant of branching enzyme deficiency. They are seen in many tissues, including skeletal muscle, heart, liver, and brain. In brain neurons, Lafora bodies, at difference from APBDs (that are in axons), are localized in dendrites and in close proximity of synaptic vesicles. Skin biopsy can be used for diagnostic purposes by demonstration of Lafora bodies in apocrine sweat glands.

Two genes can be mutated in Lafora disease, EPM2A, encoding laforin, or NHLRC1, encoding malin. The precise pathogenic roles of mutated laforin and malin in causing polyglucosan accumulation are not known. In the absence of laforin, glycogen is hyperphosphorylated and becomes polyglucosan (28). The role of malin, a ubiquitin ligase, may be that of removing laforin: in its absence, laforin remains abnormally attached to glycogen and also leads to polyglucosan formation (139). Clearly, Lafora disease is both a neurodegenerative disease and a glycogen storage disease (30).

Differential diagnosis

As mentioned above, debrancher deficiency should be considered in the differential diagnosis of distal atrophies, including amyotrophic lateral sclerosis and Charcot-Marie-Tooth disease, which may be complicated by the mixed myopathic and neurogenic pattern of the EMG. Clues to the diagnosis of debrancher deficiency are a history of transient hepatomegaly in infancy or childhood, markedly increased serum CK levels, and abnormal electrocardiography even in the absence of overt cardiopathy. The muscle biopsy showing pools of extralysosomal glycogen bolsters the diagnosis, which has to be confirmed by biochemical and molecular analyses. However, debrancher deficiency should be considered more often as a benign form of myophosphorylase deficiency (ie, McArdle disease light), as described by Preisler and colleagues (108).

Branching enzyme deficiency should be considered in all infants with arthrogryposis multiplex congenita, congenital myopathy with or without cardiomyopathy, or spinal muscular atrophy without mutations in the SMN1 gene. Muscle biopsy showing storage of PAS-positive but amylase-resistant polysaccharide (polyglucosan) establishes the diagnosis, which will have to be confirmed by biochemical analysis (in white blood cells if muscle is not available) and by sequencing of the GBE1 gene. The APBD variant of branching enzyme deficiency should be part of the differential diagnosis in older patients with motor neuron disease, especially when they complain of urinary incontinence (it is often said that these patients see a urologist before seeing a neurologist), have cognitive problems, and are mostly (but not exclusively) of Ashkenazi Jewish origin. Lafora disease does not pose a differential diagnostic problem because of its characteristic clinical presentation, adolescent onset, and rapid downhill course with intractable epilepsy and dementia.

Polyglucosan myopathy and cardiomyopathy due to mutations in RBCK1 encoding a ubiquitin ligase. Ten patients from eight families had juvenile-onset myopathy, accompanied in eight patients by rapidly progressive cardiomyopathy, which required cardiac transplantation in four of them (98). There was extensive accumulation of polyglucosan in skeletal and cardiac muscle in patients with cardiomyopathy (now classified as Polyglucosan body myopathy type 1). All patients were homozygous or compound heterozygous for missense or truncating mutations in the gene RBCK1, which encodes a ubiquitin ligase (note that malin is also a ubiquitin ligase). Cognitive impairment with mild subcortical white matter disease accompanying hip girdle predominant muscle weakness has been reported in a teenager (26). Some patients with mutations in the same gene have a different phenotype characterized by failure to thrive, chronic autoinflammation, and recurrent episodes of sepsis. They also accumulate polyglucosan in muscle, heart, and liver (19). Careful genotype-phenotype correlation analysis will be needed to clarify the different tissue involvement in these allelic disorders. Prognosis of this relatively new disorder is poor.

Glycogenin deficiency (glycogenosis type XV). The primer of glycogen synthesis is a glycosyl-transferase that uses uridine diphosphate glucose (UDPG) as a substrate in an autoglycosylation reaction to generate a short (about 10 glucosyl units) glucose polymer, the kernel of the new glycogen molecule. There are two isoforms of glycogenin, the muscle isozyme glycogenin-1 and the liver isozyme glycogenin-2, which is also partially expressed in the heart.

The first pathogenic mutation in the gene encoding glycogenin-1 (GYG1) was described in 2010 in a young man who was slower than his peers as a child, suffered from exertional dyspnea, and, at 27 years of age, had a life-threatening episode of ventricular fibrillation and was equipped with a permanent defibrillator (93). Skeletal muscle was devoid of glycogen, whereas the heart showed large accumulation of a poorly structured PAS-positive material, presumably a novel form of abnormal polysaccharide due to the presence of some glycogenin in the heart.

More interestingly, seven patients with mutations in GYG1 (chromosome 3q) resulting in residual glycogenin-1, had no lack of glycogen in muscle but presence of abundant polyglucosan (30% to 40% of fibers) and no cardiomyopathy or respiratory involvement. Polyglucosan body myopathy type 2 has late-onset weakness affecting predominantly limb girdle muscles (41; 78; 02). The decreased amount of glycogenin-1 or its impaired interaction with glycogen synthase may explain the accumulation of polyglucosan (83). Proximal involvement can mimic limb-girdle muscular dystrophy (72), but muscle biopsy aids in the differential diagnosis. Polyglucosan bodies are round or oval subsarcolemmal or cytoplasmatic inclusions that appear pinkish matt.

There is no specific treatment for this disorder. Improved exercise tolerance with glucose infusion was seen in four patients (130), but the practical implications of this finding are not clear.

Conclusion

Although the glycogenoses were among the first metabolic myopathies to be identified and have been studied for almost a century, they are far from being a closed chapter, as shown by the identification of defects in glycogen synthesis (the so-called glycogenoses type 0) and by our relative ignorance about pathogenesis. Therapy is also inadequate, although enzyme replacement therapy (ERT) has been largely successful in the 1 lysosomal glycogenosis [a-glucosidase (AAG) deficiency, glycogenosis type II, Pompe disease]. Exercise and dietary regimens are of some help, and transgenic animal models for myophosphorylase deficiency (99) and for branching enzyme deficiency (05) and Lafora disease (43) offer excellent tools for therapeutic experimentation. Genetic therapy is still in experimental stages in muscle glycogenoses.