Phenotypic expression of late-onset glycogen storage disease type II: identification of asymptomatic adults through family studies and review of reported families.

The intrafamilial variability of late-onset glycogen storage disease type II was studied in siblings of 18 patients and in reports in the literature. Siblings of seven of the 18 index cases opted for DNA testing or enzyme studies after being informed by the index case of the availability of testing, and after genetic counselling. Of the 12 siblings tested, five asymptomatic individuals were diagnosed (mean age, 32.8 years; range, 17-53). Intrafamilial variability in the age at onset (more than 10 years difference) or in the clinical symptoms was found in one of seven sibships tested in this study, and also in seven sibships reported in the literature. We advocate that testing should not be offered to healthy siblings of late-onset glycogen storage disease type II patients as a routine, because it is impossible to give a precise prognosis to an individual who is symptom-free, but has been identified with a glycogen storage disease type II genotype, nor is there any therapeutic intervention available.