Over the past decade we have seen a surge of interest in patients with mitochondrial disease.1 More than 100 pathological defects of mitochondrial DNA (mtDNA) have been characterised in patients with a wide range of different disorders,2-3 and we are only just beginning to recognise the impact of mitochondrial disease on both neurological and general medical practice.4 These disorders often present in early adult life, with a progressive disabling neurological syndrome which is responsible for considerable morbidity and premature death.5 Although the actual prevalence of mtDNA disorders is not known, over the past five years we have accumulated about 100 patients with probable mitochondrial disease in the Northern Region. We have identified a pathogenic mtDNA defect in over 50 of these, and on this basis we calculate the point prevalence of established mtDNA disease to be around 1 in 50 000. This figure is likely to be a highly conservative estimate as it is dependent on the recognition and referral of these patients to only one centre, but in practical terms, it means that each United Kingdom neurologist will have at least four patients with mtDNA defects within their own district.

Mitochondrial dysfunction and mtDNA mutations seem to have infiltrated every branch of medicine, and as a consequence, the field has become increasingly complex. In this editorial we discuss the relevance of these recent findings on adult neurological practice. We focus on three major clinical issues: which patients should we investigate for possible mitochondrial disease, how should we investigate them, and what are the management options?