Mitochondrial Disorders


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Neurological Institute MDA/Neuromuscular Clinic

Mitochondrial diseases are chronic, usually genetic disorders that occur when the mitochondria of the cell fails to produce enough energy for cell or organ function. The course of disease for patients with mitochondrial myopathies (muscle diseases) varies greatly, depending on the type of disease and how it impacts various organs. 

Symptoms of mitochondrial disease are often apparent at birth, but can occur at any age.  This group of diseases is often diagnosed incorrectly; however, with advances over the last 10 years in genetics and molecular biology we have a better appreciation of the complexity in mitochondrial disorders.  

Causes of Mitochondrial Disease
The underlying cause of a specific mitochondrial disorder may be complex, and symptoms might be either more or less severe in people with a family member who shares the same defect. Most of the genetic information in our cells is contained in the DNA we inherit from both of our parents, and that is true for the 3000+ genes that are needed to make a mitochondrion . This DNA is referred to as nuclear DNA (nDNA). However, mitochondria also carry their own DNA that is inherited only from the mother.  This DNA is referred to as mitochondrial DNA (mtDNA) and it accounts for an additional 37 genes.  With so many genes potentially involved, and the facts that mitochondria are present in almost every cell in your body (red blood cells are the exception) yet function differently in different tissues, it is not surprising that mitochondrial diseases are very diverse. This is not a single disease, but a constellation of diseases that may affect each person differently. Genetic changes (mutations) that cause disease can occur in several different ways.
  • A gene may be inherited. 
    • If the defect is in the nuclear DNA (nDNA), inheritance is similar to other genetic diseases: 
      • A recessive trait inherited from one parent may result in no other family members appearing to be affected
      • A dominant trait often results in disease that occurs in other family members.
    • If the defect is in the mother’s mitochrondrial DNA (mtDNA), every child has the mutation, because all mitochondria are inherited from the mother (not the father). 
    • A combination of mtDNA and nDNA defects may also occur.   
  • A random mutation (also called sporadic or de novo event) may cause disease in an individual without affecting other family members.   
In addition to genetic changes, medicines or other toxic substances can trigger mitochondrial disease.

 Symptoms of Mitochondrial Disease

Diseases of the mitochondria appear to cause the most damage to cells in the brain, heart, liver, skeletal muscles, kidney and the endocrine and respiratory systems, according to the United Mitochondrial Disease Foundation

While some mitochondrial disorders only affect a single organ, many involve multiple organ systems and often present with prominent involvement of nerves (neurologic or neuropathic features) and muscle (myopathic features). Many individuals display a cluster of clinical features that fall into a discrete clinical syndrome that affects several organs.

Some clinical features of mitochondrial disease – whether involving a mitochondrial or nuclear gene – are common:
  • Ptosis (drooping of the upper eyelid)
  • Paralysis of the muscles that move the eyeball
  • Muscle weakness 
  • Exercise intolerance
  • Cardiomyopathy (weakness of the heart’s pump)
  • Deafness
  • Vision problems
  • Diabetes mellitus 
Certain central nervous system findings are common in mitochondrial diseases:
  • Fluctuating encephalopathy (degeneration of the brain that seems to improve, as well as worsen)
  • Seizures
  • Dementia
  • Migraine
  • Stroke-like episodes
  • Ataxia  (uncoordinated muscle movements/unsteady gait)
  • Spasticity   (movement disorder ranging from muscle stiffness to painful muscle spasms and uncontrolled jerking or scissoring movement ) 
Children with mitochondrial disease may begin an activity with plenty of energy, but “run out of steam” quickly, becoming mentally and/or physically exhausted. 

Diagnosis of Mitochondrial Disease
No single hallmark symptom— muscle weakness, exercise intolerance, hearing impairment, ataxia, seizures, learning disabilities, cataracts, heart defects, diabetes and stunted growth — is unique to mitochondrial disease. A combination of three or more of these symptoms, however, strongly points to mitochondrial disease, especially when the symptoms involve more than one organ system.

Mitochondrial diseases can resemble other neuromuscular disorders, and, as with symptoms, there is no single test that provides a definitive diagnosis. Metabolic testing, muscle testing, genetic testing, and brain imaging may all provide information to be interpreted by the experienced clinician.  Genetic testing and interpretation of diagnostic tests by doctors with extensive experience in managing mitochondrial diseases is available at Houston Methodist. 

The most important of test for diagnosing mitochondrial disorders involves a muscle biopsy, removing a small sample of muscle tissue to examine. When treated with a dye that stains mitochondria red, muscles affected by mitochondrial disease often show ragged red fibers — muscle cells (fibers) that have excessive numbers of mitochondria. Other stains can detect the absence of essential mitochondrial enzymes in the muscle. 

Treatment Options for Mitochondrial Disease

Although there is no specific treatment for any of the mitochondrial myopathies, many of the varied complications that may arise in patients with different mitochondrial disorders can be managed even if progression cannot be prevented. Physical therapy may extend the range of movement of muscles and improve dexterity. Related symptoms, such as seizures or attention problems, can be managed with various medications, although the mitochondrial disease itself is unchanged. Vitamin therapies such as riboflavin, coenzyme Q, and carnitine (a specialized amino acid) have little to no effect on most patients, according to the National Institute of Neurological Disorders and Stroke . Many of these supplements are available without a prescription, but you should always consult your doctors rather than trying to treat your disease yourself, even with these non-prescription medications.

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