A blood test, rather than an invasive muscle biopsy, may be sufficient to diagnose mitochondrial disease in some patients.
BRC researchers at the National Hospital for Neurology and Neurosurgery found that in patients with progressive external ophthalmoplegia (weakness of the eye muscles) due to mitochondrial disease, the presence of peripheral neuropathy (loss of sensation and muscle weakness in arms and legs) was correlated with the underlying genetic cause.
Mitochondria produce energy within the body required by cells to function correctly. If the cells are not fed energy, the tissues that the cells are made up of do not work properly. Each person who suffers from mitochondrial disease is affected differently but a common symptom is weakness of the muscles. Other organs or tissues typically affected include the nervous system (brain and nerves), the eyes, the heart, the kidney or the liver, all of which require high amounts of energy to function correctly.
The researchers reviewed the medical records of 116 mitochondrial disease patients with progressive external ophthalmoplegia.
The study, published in Brain, identified three main types of mutations among these patients: single deletion of mitochondrial DNA (67% of patients), point mutation in mitochondrial DNA (10% of patients) or a nuclear DNA defect (22% of patients).
The researchers analysed the clinical features of all patients and observed that the frequency of peripheral neuropathy was high in patients with a nuclear DNA defect (52% of patients) and slightly less common in those with a point mutation in mitochondrial DNA (44% of patients). In contrast, significantly fewer (2%) patients with a single deletion of mitochondrial DNA had peripheral neuropathy.
Lead author Dr Alex Horga said: “The probability of patients with progressive external ophthalmoplegia and neuropathy having a nuclear DNA defect was eight times higher than those with progressive external ophthalmoplegia but no neuropathy. We also confirmed that neuropathy is extremely rare in patients with progressive external ophthalmoplegia due to a common type of mitochondrial DNA defect called single deletion”.
Mathematical models and analyses showed that peripheral neuropathy, followed by family history and hearing loss, were predictors of an underlying nuclear defect. Age of onset, gender, progressive external ophthalmoplegia at clinical presentation and pigmentary retinopathy could not predict the mutations.
Dr Horga said: “In patients with progressive external ophthalmoplegia and peripheral neuropathy genetic tests in DNA obtained from blood should be the first approach. If this confirms the diagnosis, a muscle biopsy may not be necessary.”
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