Mitochondrial diabetes is a group of rare forms of diabetes caused by mutations of the mitochondrial genome. The commonest form of this diagnosis is due to a mutation in the mitochondrial leucine-transfer RNA gene. They are clinically characterised by maternal transmission, and prevalence is between 0.5% and 2% of all diabetes.
Polyuria and polydipsia are the cardinal symptoms of diabetes and occur when the blood glucose level exceeds the renal threshold for glucose excretion - usually approximately 10 mmol/L.
Variable difficulty with focusing - particularly if present for only a short period and fluctuating - is due to osmotically driven swelling of the lens and varies as the degree of hyperglcaemia varies.
These are also classical symptoms of hyperglycaemia.
A thorough family history is essential in order to detect monogenic forms of diabetes such as MODY and mitochondrial diabetes.
The cardinal feature of the family history of diabetes due to mitochondrial mutations is that inheritance only occurs from the mother. However, a proportion of cases arise due to de novo mutations and thus a family history of the condition is not mandatory for diagnosis.
Sensorineural deafness is a common association of mitochondrial diabetes and generally precedes the onset of the diabetes.
Pigmentary retinopathy, myopathy and neuropathy can all also arise due to mutations in the mitochondrial genome.
Although common in diabetes in general, a history of cardiac or renal disease - especially if they ante-date the onset of diabetes - may be relevant as they can also arise due to mitochondrial genome mutations.
Peak age of onset of mitochondrial diabetes is before 40 years of age.
Other associations of mitochondrial diabetes include MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke), MERRF (myoclonic epilepsy and ragged red fibres) and Kearns-Sayre syndrome.
All of these conditions are also caused by mutations in the mitochondrial genome.
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