There are numerous forms of glycogen storage diseases, but the common end-result is inability to store glycogen in either the liver and/or muscles due to enzyme deficiencies that are transmitted by an autosomal recessive pattern of inheritance. Symptoms are diverse, but hepatosplenomegaly, failure to thrive and hypoglycemia are the most common. The diagnosis is confirmed by genetic testing, while treatment depends on the subtype.
Clinical presentation somewhat depends on the type of GSD and a broad classification into liver and muscle glycogenoses aids the physician in differentiating between various forms . Symptoms may appear at any age, but peak in childhood, adulthood and mid-adolescence. Liver disease is present in patients with type I, III, IV, VI, IX, XI, and 0, hepatomegaly being the most prominent symptom . Hypoglycemia, hyperlipidemia and growth retardation are commonly observed, but some symptoms are specific for certain subtypes, such as elevated blood lactate (type I), profound neutropenia (type Ib), ketosis (types VI and 0) and cardiomyopathy (type II). On the other hand, skeletal glycogenoses (types V, VII) are characterized by muscle cramping, fatigue, myoglobinuria during strenuous exercise and anemia. Pompe disease (type II) has a distinct clinical presentation, encompassing both muscular and hepatic manifestations, together with rapidly progressive cardiac and respiratory failure .
Making the diagnosis of GSD may be difficult, but patients with progressive liver disease and/or muscle cramping, fatigue and poor general condition without an identifiable cause, a high suspicion to one of the GSDs should be present. Initial laboratory findings may reveal elevated liver transaminases and impaired synthetic function, anemia, as well as hypoglycemia, elevated triglycerides, cholesterol, and lactate. The gold standard, however, is either biopsy or detection of reduced enzymatic activity in the target tissue, while magnetic resonance imaging (MRI) can provide important clues as well .
Treatment principles almost strictly depend on the type of GSD:
Management of hypoglycemia through dietary changes and additional symptoms is imperative for other GSDs, but in general, this approach is favored across all subtypes so that the metabolic needs for glycogen and energy are fulfilled.
The prognosis of patients with GSDs significantly depend on the subtype. Type Ib patients may develop recurrent infections that can be fatal due to persistent neutropenia, while Pompe disease is often fatal during childhood due to respiratory and cardiac failure  . Rapid liver failure that necessitates transplantation is seen in type IV patients, whereas a mild and relatively benign clinical course may be observed in type III and type VI . Some types (I and VI) have been associated with hepatocellular carcinoma . In all other forms, disease manifestations may range from benign and mild to severe and severely debilitating. For these reasons, it is important to identify the exact subtype in order to instate appropriate therapy and prevent further complications.
Enzyme deficiency that impairs normal glycogen degradation is the principal cause of all GSDs (except in type 0, where glycogen synthase deficiency results in impaired glycogen storage in the liver) . Enzyme deficiencies are acquired through autosomal recessive pattern of inheritance in virtually all types, but rare cases (type IX) have shown to occur as a result of X-linked transmission . For all diseases, the exact enzyme deficiencies have been identified. Glucose-6-phosphatase (type I), acid alpha-glucosidase (type II), glycogen debranching enzyme (type III), glycogen branching enzyme (type IV), glycogen phosphorylase (type V), liver phosphorylase (type IV), phosphofructokinase (type VII), liver phosphorylase kinase (type IX) GLUT2 (type XI), glycogen synthase (type 0) and several other enzyme deficiencies have been established.
Incidence and prevalence rates significantly depend on the subtype, but overall estimations suggest that 1 per 25,000 individuals develop some form of GSD . Type II (Pompe disease) is estimated to develop in 1 per 40,000 births, whereas type III occurs in approximately 1 per 5,400 births, with a significant predilection toward Sephadric Jews of North Africa  . On the other hand, some subtypes have shown to be extremely rare, like type XI and 0, as only a handful of cases described in literature  . Gender distribution is usually diverse, but in type V, a male predominance is observed .
The pathogenesis across all subtypes invariably includes inability to utilize glycogen as a source of energy due to deficiencies of enzymes that are either a part of its degradation or synthesis (type 0) . Under physiological circumstances, excess glucose ingested by food is up to a certain extent converted to glycogen by the action of glycogen synthase (enzyme deficient in type 0) and stored principally in the liver, while the skeletal muscles are also a site of its storage . Glycogen is further stored until the tissues in which it is stored reach maximal capacity (which is impaired in patients suffering from type IV GSD, as the enzyme responsible for its assembly, branching enzyme is deficient), but its conversion back to glucose in metabolic needs is an important source of fuel and provided rapid energy utilization. Various enzymes are involved in its breakdown and conversion to glucose, including debranching enzyme, liver and muscle phosphorylase kinases, acid maltase, phosphofructokinase, glucose-6-phosphatase and GLUT2 transporter . All of these enzymes are deficient in certain types of GSDs, with the common end-result being inability of the liver and muscles to degrade glycogen and provide the necessary energy for metabolic functions, which manifests in a variety of symptoms, depending on the subtype and the severity of enzyme deficiency.
Although exact enzyme deficiencies have been determined in virtually all subtypes, prevention of GSDs is currently not possible, as the triggers that are responsible for their development are unknown. Genetic counseling may be advisable for families with first-degree relatives that have GSDs, but prevention strategies should be focused on ensuring long-term management through adequate treatment.
Glycogen storage diseases (GSDs) result in impaired utilization of glycogen as a result of various enzyme deficiencies. Glycogen is converted from glucose in liver and skeletal muscles to some extent and these two organs are principally affected . Because of its role in energy production and utilization by many tissues, numerous symptoms may be encountered. Up to today, 23 GSDs have been established , and are classified into [1-15]:
Although each type is distinguished by deficiency of different enzymes, signs and symptoms that reflect hepatic and skeletal pathology without an evident cause can rise clinical suspicion. The initial diagnosis can be made by clinical criteria, whereas confirmation can be determined by genetic testing that may reveal mutated genes that led to enzyme deficiencies. Treatment depends on the subtype . For some GSDs, moderate exercise, vitamin supplementation and appropriate dietary changes are only options. Maintenance of blood glucose through introduction of corn starch is highly effective for type III and type I, although fructose and galactose intake should be limited for type I patients. On the other hand, enzyme supplementation has been introduced to patients suffering from type II GSD and has markedly improved patient outcomes . In general, the prognosis of GSDs range from mild to severe and rapidly fatal across different subtypes , but early recognition of the disease may prevent complications such as hepatic, respiratory and cardiac failure, which will invariably prolong the patient's life.
Today, more than 20 glycogen storage diseases (GSDs) are described in literature and they all cause the same metabolic disturbance - disruption of normal glycogen storage and inability of the body to utilize this source of energy for its needs. Glycogen is synthesized when excess concentrations of glucose are introduced through food, but the body can store limited amounts of glycogen. The liver and the skeletal muscles are sites where glycogen can be stored, but in the setting of various GSDs, enzymes that are involved in its creation from glucose are deficient. Consequently, impaired glycogen conversion to glucose leads to very low glucose levels (hypoglycemia), one of the most important manifestations of this group of diseases. Enzyme deficiencies occur as a result of genetic mutations that are transferred from parent to their child through an autosomal recessive pattern of inheritance. This means that the disease is present only if both parent transfer have a defective gene copy and transfer it to their child, whereas only one transferred copy implies that the child is a carrier but does not develop any symptoms. GSDs roughly develop in approximately 1 per 25,000 individuals and gender distribution is mostly equal. Based on clinical symptoms, GSDs are roughly divided into those that involve the liver and those in whom symptoms are mostly related to skeletal muscles, but both organs may be affected across various types. Liver enlargement, increased circulating values of lipids, and decreased blood sugar are the most common manifestations of GSDs, while muscle cramping, profound fatigue and weakness are also frequently encountered. Making the diagnosis may be quite difficult, but liver or skeletal muscle symptoms together with hypoglycemia that do not have an identifiable cause should rise suspicion toward GSDs. A definite diagnosis can be made by either biopsy or genetic testing for deficient enzymes. Treatment principles depend on the subtype. Changes in dietary habits through introduction of uncooked cornstarch is a very useful method to recover from persistent low sugar levels, whereas symptomatic therapy and even use of recombinant human enzymes has been accomplished in some subtypes. Many patients, however, suffer a poor prognosis, as several subtypes can be fatal within years due to heart or liver failure, which is why early recognition of this disease is imperative in prolonging survival rates.