Glycogen-storage and carbohydrate metabolism disorders — branching viva

Opening framework

My framework has three layers. First, the recognition — a child with hepatomegaly and fasting hypoglycaemia with lactic acidosis is a hepatic glycogen storage disease until proven otherwise, and the metabolic fingerprint decides which one. Second, the mechanism — the block's position along glycogenolysis determines whether the child starves the blood of glucose (the hepatic GSDs) or swells a lysosome (Pompe) or starves exercising muscle (McArdle). Third, the management — the unifying move is to prevent fasting and catabolism, with Pompe as the enzyme-replacement exception. [1]

The GSD I tetrad and the enzyme block

The diagnosis here is glycogen storage disease type Ia (von Gierke disease, glucose-6-phosphatase deficiency). Glucose-6-phosphatase performs the final step of glycogenolysis and gluconeogenesis — dephosphorylating glucose-6-phosphate to free glucose for hepatic release. When it is deficient the tetrad arises inevitably: free glucose cannot leave the liver so the blood glucose falls (hypoglycaemia); the trapped glucose-6-phosphate is shunted to glycolysis and lactate (lactic acidosis); the back-up feeds glycerol and triglyceride synthesis (hypertriglyceridaemia) and purine turnover (hyperuricaemia); and because glycolysis is fluxing, ketogenesis is suppressed, producing the non-ketotic picture that distinguishes GSD I from the ketotic hypoglycaemia of GSD III. The low ketones are the discriminating bedside clue. [1]

Confirmation and the GSD Ib pitfall

I confirm with molecular genetic testing of G6PC, which I favour over a liver biopsy because it is non-invasive, defines the variant for cascade family testing and prenatal or preimplantation diagnosis, and avoids the morbidity of a biopsy in a child with a bleeding tendency from liver dysfunction. The critical pitfall is to forget GSD Ib (SLC37A4, glucose-6-phosphate translocase), which shares the metabolic tetrad but adds neutropenia with recurrent infections and a Crohn-like inflammatory bowel disease that itself requires granulocyte colony-stimulating factor — so I check the neutrophil count and consider SLC37A4 sequencing in every suspected GSD I. [1]

The 'prevent fasting' management and surveillance

The cornerstone is to prevent fasting and prevent catabolism: frequent daytime feeds supplemented with uncooked cornstarch to extend the fasting interval, continuous overnight glucose via nasogastric or gastrostomy feed to prevent early-morning hypoglycaemia, and restriction of fructose and galactose because they feed the trapped pool. Every family holds a written sick-day plan: at the first sign of illness, stop fasting, give glucose or cornstarch, present early — because catabolism is the enemy and a minor illness can become a metabolic emergency within hours. The long-term surveillance targets the complications that drive adult morbidity: hepatic adenoma with hepatocellular carcinoma risk (annual or biennial liver imaging), renal disease (glomerular hyperfiltration to proteinuria to renal failure), osteoporosis, anaemia, growth failure and hyperuricaemic gout. For the child with refractory disease or adenoma with malignant risk, liver transplantation corrects the metabolic defect. [1] [15]

Branch: the floppy infant with a big heart (Pompe)

For the second child — hypotonia, macroglossia, a creatine kinase of 4200 and concentric left ventricular hypertrophy — the diagnosis is infantile-onset Pompe disease (acid α-glucosidase deficiency). Blood glucose is normal because acid α-glucosidase is a lysosomal hydrolase: it degrades glycogen inside the lysosome, not in the cytosol, so it does not participate in hepatic glucose output. Instead the lysosome accumulates glycogen, swells, ruptures, and damages cardiac, skeletal and respiratory muscle — which is exactly why Pompe causes cardiomyopathy and hypotonia rather than hypoglycaemia, and why it responds to enzyme replacement, which delivers functional enzyme to the lysosome via mannose-6-phosphate receptors. [5]

The treatment is enzyme replacement therapy with alglucosidase alfa, started early before irreversible muscle damage; untreated infantile Pompe is fatal in infancy, but early treatment achieves long-term survival with motor and cardiac function. The cipaglucosidase alfa plus miglustat regimen from the PROPEL trial offers improved efficacy, and newborn screening for Pompe (GAA on the bloodspot) is the key to detecting presymptomatic infants before the damage is done. [5] [6]

Closing: the toxic-sugar disorders and the trap to avoid

The closing comparison is with classic galactosaemia, a toxic-metabolite disorder in which galactose-1-phosphate accumulates and poisons the liver, kidney, lens and ovary, presenting in the neonate on milk feeds with jaundice, liver dysfunction, cataracts, failure to thrive and E. coli sepsis — managed by removing galactose. Hereditary fructose intolerance behaves analogously when fructose, sucrose or sorbitol is introduced at weaning. The trap to avoid is reassuring a galactosaemia family that the disease is "cured" by diet, when the long-term cognitive, speech, ovarian and bone outcomes remain imperfect despite strict adherence — a humbling reminder that metabolic control modifies but does not always normalise outcome. [12]