Cytochrome c oxidase deficiency is a genetic condition that can affect several parts of the body, including the muscles used for movement (skeletal muscles), the heart, the brain, or the liver. Signs and symptoms of cytochrome c oxidase deficiency usually begin before age 2 but can appear later in mildly affected individuals.

The severity of cytochrome c oxidase deficiency varies widely among affected individuals, even among those in the same family. People who are mildly affected tend to have muscle weakness (myopathy) and poor muscle tone (hypotonia) with no other related health problems. More severely affected people have problems in multiple body systems, often including severe brain dysfunction (encephalomyopathy). Approximately one-quarter of individuals with cytochrome c oxidase deficiency have a type of heart disease that enlarges and weakens the heart muscle (hypertrophic cardiomyopathy). Another possible feature of this condition is an enlarged liver (hepatomegaly), which may lead to liver failure. Most individuals with cytochrome c oxidase deficiency have a buildup of a chemical called lactic acid in the body (lactic acidosis), which can cause nausea and an irregular heart rate, and can be life-threatening.

Many people with cytochrome c oxidase deficiency have a specific group of features known as Leigh syndrome. The signs and symptoms of Leigh syndrome include loss of mental function, movement problems, hypertrophic cardiomyopathy, eating difficulties, and brain abnormalities. Cytochrome c oxidase deficiency is one of the many causes of Leigh syndrome.

Many individuals with cytochrome c oxidase deficiency do not survive past childhood, although some individuals with mild signs and symptoms live into adolescence or adulthood.

Mutations in more than 20 genes have been found to cause cytochrome c oxidase deficiency. Most genes are found in DNA in the cell's nucleus (nuclear DNA). However, some genes are found in DNA in specialized cell structures called mitochondria. This type of DNA is known as mitochondrial DNA (mtDNA). Most cases of cytochrome c oxidase deficiency are caused by mutations in genes found within nuclear DNA; however, in some rare instances, mutations in genes located within mtDNA cause this condition.

The genes associated with cytochrome c oxidase deficiency are involved in energy production in mitochondria through a process called oxidative phosphorylation. Mutations in these genes affect an enzyme complex called cytochrome c oxidase, which is responsible for one of the last steps in oxidative phosphorylation. Cytochrome c oxidase is made up of two large groups of enzymes (complexes) called holoenzymes, which are each composed of multiple protein parts (subunits). Many other proteins are involved in assembling these subunits into holoenzymes.

In most cases, cytochrome c oxidase deficiency is caused by mutations that alter the proteins that assemble the holoenzymes. As a result, the holoenzymes are either partially assembled or not assembled at all. Without complete holoenzymes, cytochrome c oxidase cannot form. Less frequently, mutations alter the holoenzyme subunits, leading to a nonfunctional version of cytochrome c oxidase. Whether cytochrome c oxidase is not formed or not functional, this missing enzyme complex disrupts the last step of oxidative phosphorylation, causing a decrease in energy production.

Researchers believe that impaired oxidative phosphorylation can lead to cell death by reducing the amount of energy available in the cell. Certain tissues that require large amounts of energy, such as the brain, muscles, and heart, seem especially sensitive to decreases in energy. Cell death in these and other sensitive tissues likely contribute to the features of cytochrome c oxidase deficiency.

Cytochrome c oxidase deficiency can have different inheritance patterns depending on the gene involved.

When this condition is caused by mutations in genes within nuclear DNA, it is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

When this condition is caused by mutations in genes within mtDNA, it is inherited in a mitochondrial pattern, which is also known as maternal inheritance. This pattern of inheritance applies to genes contained in mtDNA. Because egg cells, but not sperm cells, contribute mitochondria to the developing embryo, children can only inherit disorders resulting from mtDNA mutations from their mother. These disorders can appear in every generation of a family and can affect both males and females, but fathers do not pass traits associated with changes in mtDNA to their children.

There is currently no cure for cytochrome C oxidase (COX) deficiency. Management of all forms of COX deficiency generally focuses on the specific symptoms present in the affected individual and is largely supportive. The goals of treatment are to improve symptoms and slow progression of the disease; the effectiveness of treatment varies with each individual. Treatment generally does not reverse any damage that has already occurred. Prognosis varies depending on the form of COX deficiency present. Individuals with benign infantile mitochondrial myopathy may experience spontaneous recovery (although early diagnosis and intensive treatment is still needed until this point), while there may be rapid demise in individuals with Leigh syndrome.

It is often recommended that individuals with mitochondrial disorders such as COX deficiency avoid fasting. Dehydration due to vomiting or illness may be treated with intravenous fluid if the individual is not able to take fluids orally. Seizures are typically controlled with anticonvulsants. Some affected individuals may benefit from physical, occupational, and speech therapies that are specifically tailored to their needs. Dietary supplements including certain vitamins and cofactors have shown varying degrees of benefit in individual cases.

Individuals interested in specific management recommendations for themselves or relatives should speak with their healthcare providers.

In Eastern Europe, cytochrome c oxidase deficiency is estimated to occur in 1 in 35,000 individuals. The prevalence of this condition outside this region is unknown.