Movement disorders are common among patients with mitochondrial diseases (MIDs). From mild tremors to severe involuntary movements, a broad spectrum of symptoms can be indicative of mitochondrial disease and should prompt an investigation as a possible diagnosis.
Many people have the genetic mutations associated with MID development. They can occur in mitochondrial genes inherited from the mother or in nuclear genes associated with the maintenance or expression of mitochondrial DNA (mtDNA). But it is not clear why some tissues are more affected than others, and more specifically involved in MIDs.
In a study titled “Movement disorders in mitochondrial diseases,” published in the journal Revue Neurologique, the authors reviewed some movement disorders that can be found in people with mitochondrial diseases and should be taken into account for a diagnosis.
The nervous system and muscles are tissues that require high levels of energy to function normally. Since mitochondria are the cellular organelles responsible for the production of energy, this can in part explain why mitochondrial diseases are often associated with movement disorders. Epilepsy, muscular contraction, involuntary jerking movements, double vision, and muscle dysfunction, or myopathy, are among the most common neurological signs in MID patients.
The authors focused on four specific movement disorders:
- Cerebellar ataxia: The inability to coordinate balance, walk, and eye movements. It is one of the most prevalent neurological mitochondrial signs and, as such, MIDs should be investigated if the patient presents these symptoms.
- Myoclonus: Also a classical sign of MID, it is characterized by the sudden and involuntary jerking movement of muscles.
- Dystonia: Uncontrollable muscle contraction.
- Parkinsonism: Similar to symptoms seen in Parkinson’s disease patients, such as tremor, slow movement, impaired speech or muscle stiffness.
Though parkinsonism is seen more commonly in patients with POLG1 mutations and myoclonus in patients with myoclonic epilepsy with ragged red fibers (MERFF), it is not possible to directly link a neurological sign or movement disorder with a specific mutation. Indeed, a single MID-associated mutation can be related to a wide variety of movement symptoms, either isolated or in combination, and with different levels of manifestation.
Movement disorders are highly associated with mitochondrial diseases, and so mild movement alterations or other neurological signs, such as migraines, cognitive disorders, or brain lesions should be considered as possible symptoms of MID.
“In cases of a particular clinical [neurological] spectrum (LHON, MERFF, Kearns-Sayre, SANDO, SPG7, ARCA2, ARSACS), a search for the most frequently implicated mutation(s) is recommended. In other cases, muscle biopsies followed by metabolic and genetic studies may be useful for arriving at a [MID] diagnosis,” the authors wrote.