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The Problem with a Name

The classification of mitochondrial diseases is lacking and remains under development.  Available classifications (e.g., based on molecular etiology or to functional deficit) do not necessarily have a meaningful clinical application where the patient is concerned.  Disorders are called by their enzymatic defect, or by the DNA lesion, or by an acronym (usually describing certain significant symptoms), or by eponyms based on the name of a clinician or scientist who played a major role in identifying one or more patients with a particular phenotype.  However, as a rule, there is broad genetic and phenotypic heterogeneity within this nomenclature system so that the name is not only of limited meaning to the clinician and patient but it can actually be confusing and even misleading.  For example a sizable proportion of patients with MELAS (Mitochondrial Encephalomyopathy, Lactic Acidemia, and Stroke-like episodes) syndrome assigned to mtDNA bp 3243 is maternally-inherited diabetes mellitus and deafness (Chae, 2004), symptoms not at all reflected in the name.

To complicate matters further, an abnormal biochemical or enzymatic finding does not necessarily imply a primary finding, i.e., a definite diagnosis.  For example, secondary defects in complex I are well described in patients with primary genetic diagnoses proven by molecular testing, including Prader-Willi syndrome, velocardiofacial syndrome, Rett syndrome, Alstrom syndrome, among others.  Current laboratory testing cannot necessarily differentiate whether or not the biochemical finding is a primary defect or secondary phenomenon.

Discussions with patients or families must include a comment about this lack of certainty, and support provided for the anxiety and confusion that arise as a result.  However, it should also be pointed out that even if the mitochondrial dysfunction is secondary, it does not mean it is insignificant.  Furthermore, the management approach developed for primary mitochondrial disease can also be of benefit for those with secondary dysfunction.

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