Unlike nuclear genetic defects, which can often be diagnosed with a simple blood test, the investigation of mitochondrial disease is far more complex, and molecular genetic studies play only one part in the multidisciplinary approach that should be used for these patients.

Most of the mitochondrial disorders present with neurological symptoms and signs, but the clinical features are often complex and heterogenous. This clinical heterogeneity creates difficulties for the clinician in diagnosing mitochondrial diseases. The clinical features of the mitochondrial disorders are endless, however, some of the well-known clinical presentations include: