Respiratory chain enzyme analysis in muscle and liver

Respiratory Chain (RC) enzyme analysis remains the mainstay for diagnosis of children suspected of having a RC disorder. A previous international workshop suggested a set of criteria for the ideal approach to diagnosis but concluded that probably no single centre fulfilled all these criteria. Major practical issues relate to the range of tissues tested, whether frozen tissue biopsies can be used reliably, assay methods, difficulty in defining realistic reference ranges, and the lack of an external quality assurance scheme. We discuss these issues and describe our experience over the last decade with assaying RC enzymes in over 600 skeletal muscle and 300 liver biopsies from patients, a range of different controls (other known inborn errors, end-stage liver disease, post-mortem samples) and single donated normal muscle and liver samples assayed on more than 100 occasions over 5- to 10-year periods. Our experience is that 'sick' tissues have wider 'normal' ranges than 'healthy' tissues. Caution is therefore needed to ensure that secondary RC defects are not misdiagnosed as primary RC defects. We describe diagnostic criteria that integrate the results of RC enzyme assays with clinical, histological, metabolic and molecular investigations to determine whether the overall diagnostic certainty is possible, probable or definite.