Pyruvate dehydrogenase (PDH) deficiency is one of the most common mitochondrial disorders. Patients with this genetic condition have difficulty utilising carbohydrates to produce energy and develop a combination of problems including seizures, poor balance, developmental delay, disability and have a reduced life expectancy. As for most mitochondrial disorders there is a lack of effective treatments. It is essential to understand the mechanisms underlying the disease in order to identify new treatments, and to understand the natural history of disease in order to prepare for clinical trials. To date, a natural history study of PDH deficiency has not been undertaken in the UK. The researchers aim to undertake the first natural history study of PDH deficiency in the UK, to describe the spectrum of symptoms, genetics, management and outcomes in both children and adult patients.