Glycogen storage disease type V: delayed diagnosis of a cause of exercise intolerance in a patient with hereditary haemorrhagic telangiectasia

Hereditary haemorrhagic telangiectasia (HHT) is a genetic disorder characterised by epistaxis, mucocutaneous telangiectasias and arteriovenous malformations. Iron deficiency due to chronic bleeding events is a common manifestation that produces a range of nonspecific symptoms. We report on a patient with HHT with longstanding fatigue and exercise intolerance, which was persistently attributed to iron deficiency, who was revealed to have glycogen storage disease type V, an autosomal recessive metabolic myopathy caused by deficiency of myophosphorylase due to <i>PYGM</i> variants. Genetic testing revealed a pathogenic common exon mutation of one allele and a pathogenic intronic mutation of the other, possibly suggestive of a milder phenotype. We not only detail the first case of concurrent HHT and glycogen storage disease in the literature but more importantly emphasise the need for clinician awareness of the disorders to avoid perpetuating a biased clinical impression and delay in diagnosis as well as prevent potentially harmful interventions.