Mitochondrial disorders can present in many different ways—and not only to the neurologist. The general rule of thumb “multisystem? ... think mitochondrial” is a useful prompt, but it comes to mind all too often in routine neurological practice, particularly as the population continues to age and multiple pathologies become the norm in the neurology outpatient clinic. How can we sort out the “wheat from the chaff” in a simple and efficient way? Recent advances in molecular genetic testing have made it much easier to diagnose some mitochondrial disorders, but the broadening clinical phenotype presents new challenges. However, it is still important to rely on clinical judgement and not exclusively on the results of complicated and sometimes unreliable tests. If you really think it may be a mitochondrial disorder, do not give up at first base.

I will present a contemporary view on this difficult problem, a no nonsense guide for the general neurologist who asks the question “Could this patient have a primary mitochondrial disorder?” Unfortunately, “no nonsense” does not necessarily mean simple, and quick screening tests are rarely comprehensive. It is also important to be aware of the many secondary defects of mitochondrial biochemistry and mitochondrial DNA (mtDNA), which can present clinically in a similar way, but these are not the topic of this review and nor is the rapidly evolving clinical management of mitochondrial disease.¹