Mitochondrial Myopathy

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton  

Introduction

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Mitochondrial myopathies are any of a group of myopathies associated with an increased number of enlarged, often abnormal, mitochondria in muscle fibres. Mitochondrial diseases are caused by defects in mitochondria, which are energy factories found inside almost all the cells in the body. Mitochondrial myopathies affect 1 in 4300 people.[1]

  • Dysfunctions of the respiratory chain (responsible for oxidative phosphorylation and ATP energy production in the inner mitochondrial membrane) cause mitochondrial diseases.
  • Manifest as multisystem disorders with predominant involvement of muscles and nerves.
  • In isolated mitochondrial myopathy without involvement of other tissues, patients can exhibit myalgia, fatigue, exercise intolerance, proximal and distal muscle weakness, and elevated serum CK
  • Other clinical manifestations include the chronic progressive external ophthalmoplegia, in which a slowly progressive paresis of the extra ocular muscles is the most important phenotype and severe encephalomyopathy of infancy or childhood, in which brain and skeletal muscle tissue are involved, producing marked hypotonia, respiratory muscle weakness, and feeding difficulty[2]

Mitochondria

Mitochondria are membrane-bound cell organelles (mitochondrion, singular) that generate most of the chemical energy needed to power the cell's biochemical reactions. Chemical energy produced by the mitochondria is stored in a small molecule called adenosine triphosphate (ATP). Mitochondria contain their own small chromosomes. Generally, mitochondria, and therefore mitochondrial DNA, are inherited only from the mother.[3]
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A typical human cell relies on hundreds of mitochondria to meet its energy needs.  The symptoms of mitochondrial disease vary, because a person can have a unique mixture of healthy and defective mitochondria, with a unique distribution in the body.  In most cases, mitochondrial disease is a multisystem disorder affecting more than one type of cell, tissue, or organ.

Because muscle and nerve cells have especially high energy needs, muscular and neurological problems are common features of mitochondrial disease.  Other frequent complications include impaired vision, cardiac arrhythmia (abnormal heartbeat), diabetes, and stunted growth.  Usually, a person with a mitochondrial disease has two or more of these conditions, some of which occur together so regularly that they are grouped into syndromes[4].

Mitochondrial Myopathies - Types

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Examples include

  • Mitochondrial encephalopathy, lactic acidosis, and strokelike syndrome (MELAS). This syndrome has a childhood to early adulthood onset. Features:  The hallmarks of MELAS are encephalomyopathy with seizures and/or dementia, lactic acidosis, and recurrent stroke-like episodes.  These episodes are not typical strokes, which are interruptions in the brain’s blood supply that cause sudden neurological symptoms.  However, the episodes can produce stroke-like symptoms in the short term (such as temporary vision loss, difficulty speaking, or difficulty understanding speech) and lead to progressive brain injury.  The cause of the stroke-like episodes is unclear. Inheritance pattern: maternal
  • Mitochondrial DNA depletion syndromes (MDDS). Onset: infancy Features: A myopathic form of MDDS is characterized by weakness that eventually affects the respiratory muscles.  Some forms of MDDS, such as Alpers syndrome, are marked by brain abnormalities and progressive liver disease.  The anticonvulsant sodium valproate should be used with caution in children with Alpers syndrome because it can increase the risk of liver failure. Inheritance pattern: autosomal[4]
  • Others (Kearns-Sayre syndrome; Leber hereditary optic neuropathy; Myoclonic epilepsy with ragged red fibers; Leigh syndrome and neuropathy, ataxia, and retinitis pigmentosa; mtDNA deletion and depletion syndromes; Chronic progressive external ophthalmoplegia; etc.)[1]

Treatment

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Instead of focusing on specific complications of mitochondrial disease, some treatments under investigation aim at fixing or bypassing the defective mitochondria.  These treatments are nutritional supplements based on three natural substances involved in ATP production in our cells.

  1. Creatine, normally acts as a reserve for ATP by forming a compound called creatine phosphate. When a cell’s demand for ATP exceeds the amount its mitochondria can produce, creatine can release phosphate (the “P” in ATP) to rapidly enhance the ATP supply. In mitochondrial myopathies creatine monohydrate 5-10 g/day may benefit improving the symptoms[1]
  2. Carnitine, generally improves the efficiency of ATP production by helping import certain fuel molecules into mitochondria and cleaning up some of the toxic byproducts of ATP production.  Carnitine is available as an over-the-counter supplement called L-carnitine.
  3. Coenzyme Q10, also called CoQ10, is a component of the mitochondrial respiratory chain (which uses oxygen to manufacture ATP).  CoQ10 is also an antioxidant.  Some mitochondrial diseases are caused by CoQ10 deficiency, and CoQ10 supplementation is clearly beneficial in these cases[4].Coenzyme Q10 replacement still needs more consistent research results to prove significance[1].

Prognosis

  • The treatment goals for most myopathies are to slow or stop the progression of the disease concerning these congenital myopathies. eg Physiotherapy, see Myopathies.
  • Some of the complications could be fatal eg cardiomyopathies, recurrent infections, and sepsis, neuropathies, respiratory failure, or renal failures.
  • The health-related quality of life with congenital myopathies is impaired[1].

References

  1. 1.0 1.1 1.2 1.3 1.4 Nagy H, Veerapaneni KD. Myopathy. StatPearls [Internet]. 2020 Sep 1.Available from:https://www.ncbi.nlm.nih.gov/books/NBK562290/ (last accessed 22.11.2020)
  2. González-Jamett AM, Bevilacqua JA, Díaz AM. Hereditary Myopathies. Muscle Cell and Tissue-Current Status of Research Field. 2018 Apr 12.Available from:https://www.intechopen.com/books/muscle-cell-and-tissue-current-status-of-research-field/hereditary-myopathies (last accessed 22.11.2020)
  3. NIH Mitochondria Available from:https://www.genome.gov/genetics-glossary/Mitochondria (last accessed 22.11.2020)
  4. 4.0 4.1 4.2 NINDS Mitochondrial myopathies Available from:https://www.ninds.nih.gov/disorders/patient-caregiver-education/fact-sheets/mitochondrial-myopathy-fact-sheet (last accessed 22.11.2020)