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Mitochondrial complex III deficiency
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

Mitochondrial complex III deficiency

Mitochondrial complex III deficiency is a genetic condition that affects energy production in various body systems, including the brain, liver, kidneys, and muscles.

Overview

Mitochondrial complex III deficiency is a genetic condition that can affect several parts of the body. Symptoms usually begin in infancy but can appear later, and the severity varies widely among individuals.

Symptoms

  • Muscle and movement: Muscle weakness (myopathy) and extreme tiredness (fatigue), particularly during exercise.
  • Systemic issues: Liver disease (potentially leading to failure), kidney abnormalities (tubulopathy), and heart disease (cardiomyopathy).
  • Neurological: Brain dysfunction (encephalopathy) causing delayed development of mental and motor skills, movement problems, weak muscle tone (hypotonia), and communication difficulties.
  • Chemical imbalances: Buildup of lactic acid (lactic acidosis), ketones (ketoacidosis), or high blood glucose levels (hyperglycemia).

Causes & Risk Factors

The condition is caused by mutations in genes such as MT-CYB and BCS1L, which help assemble or function within complex III. Complex III is located in mitochondria and is essential for oxidative phosphorylation, the process by which cells derive energy. Impaired complex III activity reduces energy availability and increases harmful reactive oxygen species, leading to tissue damage.

Inheritance

Mitochondrial complex III deficiency is usually inherited in an autosomal recessive pattern, requiring mutations in both copies of the gene. Some cases caused by MT-CYB mutations are not inherited (new mutations), while others follow a maternal inheritance pattern, as the gene is located in mitochondrial DNA (mtDNA) passed only from mothers to their children.

Frequently Asked Questions

Diagnosis usually involves a combination of biochemical assays (measuring complex III activity in muscle or skin fibroblasts), genetic testing for pathogenic variants in MT‑CYB, BCS1L or related nuclear genes, and sometimes a muscle biopsy to assess mitochondrial morphology.
Management is largely supportive: optimizing nutrition, avoiding fasting, using antioxidants such as coenzyme Q10 or riboflavin when recommended, treating organ‑specific complications (e.g., diuretics for cardiomyopathy, liver‑protective measures), and providing physical therapy for muscle weakness.
Seek immediate care if the child develops rapid breathing, severe vomiting, lethargy, unexplained low blood sugar, sudden worsening of liver function tests, or signs of metabolic acidosis (e.g., fruity breath, confusion), as these may indicate life‑threatening lactic or ketoacidosis.
When the condition follows an autosomal‑recessive pattern, each pregnancy has a 25 % chance of an affected child, a 50 % chance of a carrier, and a 25 % chance of an unaffected non‑carrier. If the mutation is in mitochondrial DNA, only mothers transmit it, and the risk depends on the proportion of mutated mtDNA in the egg.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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