ACAD9 deficiency

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

ACAD9 deficiency is a condition that varies in severity and can cause muscle weakness (myopathy), heart problems, and intellectual disability. Nearly all affected individuals have a buildup of a chemical called lactic acid in the body (lactic acidosis). Additional signs and symptoms that affect other body systems occur in rare cases.

Mildly affected individuals with ACAD9 deficiency usually experience nausea and extreme fatigue in response to physical activity (exercise intolerance). People with ACAD9 deficiency who are moderately affected have low muscle tone (hypotonia) and weakness in the muscles used for movement (skeletal muscles). Severely affected individuals have brain dysfunction combined with myopathy (encephalomyopathy); these individuals usually also have an enlarged and weakened heart muscle (hypertrophic cardiomyopathy), which is typically fatal in infancy or childhood.

Individuals with ACAD9 deficiency who survive past early childhood often have intellectual disability and may develop seizures. Rare signs and symptoms of ACAD9 deficiency include movement disorders and problems with liver and kidney function.

Some individuals with ACAD9 deficiency have had improvement in muscle strength and a reduction in lactic acid levels with treatment.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

ACAD9 deficiency is caused by mutations in the ACAD9 gene. This gene provides instructions for making an enzyme that is critical in helping assemble a group of proteins known as complex I. Complex I is found in mitochondria, which are the energy-producing structures inside cells. Complex I is one of several complexes that carry out a multistep process called oxidative phosphorylation, through which cells derive much of their energy.

The ACAD9 enzyme also plays a role in fatty acid oxidation, a multistep process that occurs within mitochondria to break down (metabolize) fats and convert them into energy. The ACAD9 enzyme helps metabolize a certain group of fats called long-chain fatty acids. Fatty acids are a major source of energy for the heart and muscles. During periods without food (fasting), fatty acids are also an important energy source for the liver and other tissues.

Some ACAD9 gene mutations disrupt complex I assembly as well as long-chain fatty acid oxidation, while others affect only complex I assembly. The mutations that affect both of the enzyme's functions tend to be associated with the most severe signs and symptoms of ACAD9 deficiency, such as encephalomyopathy and hypertrophic cardiomyopathy. Although the exact mechanism is unclear, it is likely that cells that are less able to produce energy die off, particularly cells in the brain, skeletal muscle, and other tissues and organs that require a lot of energy. The loss of cells in these tissues is thought to lead to the signs and symptoms of ACAD9 deficiency.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

This condition 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.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

The prevalence of ACAD9 deficiency is unknown. At least 25 people with this condition have been described in the scientific literature.

Reported clinical features and what the terms mean

The following findings are associated with this condition in Orphanet. They are not a checklist for diagnosing yourself, and they do not all occur in every affected person. Some are examination, imaging or laboratory findings that cannot be recognised at home.

The frequency labels describe how often a finding was reported among people with the condition in the source. They do not give the chance that a person with that symptom has the condition. Definitions below reproduce HPO terminology; they explain the term, not the likely severity in an individual.

Decreased activity of mitochondrial complex I · Obligate (100%)

A reduction in the activity of the mitochondrial respiratory chain complex I, which is part of the electron transport chain in mitochondria.

Congestive heart failure · Frequent (79-30%)

The presence of an abnormality of cardiac function that is responsible for the failure of the heart to pump blood at a rate that is commensurate with the needs of the tissues or a state in which abnormally elevated filling pressures are required for the heart to do so. Heart failure is frequently related to a defect in myocardial contraction.

Decreased circulating carnitine concentration · Frequent (79-30%)

The concentration of carnitine in the blood circulation is below the lower limit of normal.

Dilated cardiomyopathy · Frequent (79-30%)

Dilated cardiomyopathy (DCM) is defined by the presence of left ventricular dilatation and left ventricular systolic dysfunction in the absence of abnormal loading conditions (hypertension, valve disease) or coronary artery disease sufficient to cause global systolic impairment. Right ventricular dilation and dysfunction may be present but are not necessary for the diagnosis.

EMG: myopathic abnormalities · Frequent (79-30%)

The presence of abnormal electromyographic patterns indicative of myopathy, such as small-short polyphasic motor unit potentials.

Elevated circulating hepatic transaminase concentration · Frequent (79-30%)

Elevations of the levels of SGOT and SGPT in the serum. SGOT (serum glutamic oxaloacetic transaminase) and SGPT (serum glutamic pyruvic transaminase) are transaminases primarily found in the liver and heart and are released into the bloodstream as the result of liver or heart damage. SGOT and SGPT are used clinically mainly as markers of liver damage.

Elevated plasma acylcarnitine levels · Frequent (79-30%)

The concentration of fatty acylcarnitine in the blood circulation is above the upper limit of normal.

Encephalopathy · Frequent (79-30%)

Encephalopathy is a term that means brain disease, damage, or malfunction. In general, encephalopathy is manifested by an altered mental state.

Other findings in the same source

From: Orphanet

Additional reported features include Failure to thrive (Frequent (79-30%)); Fatigable weakness (Frequent (79-30%)); Generalized hypotonia (Frequent (79-30%)); Generalized muscle weakness (Frequent (79-30%)); Hepatic steatosis (Frequent (79-30%)); Hyperammonemia (Frequent (79-30%)); Hypertrophic cardiomyopathy (Frequent (79-30%)); Increased circulating lactate concentration (Frequent (79-30%)); Increased circulating lactate dehydrogenase concentration (Frequent (79-30%)); Lactic acidosis (Frequent (79-30%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing ACAD9 deficiency is Metabolic Medicine, with a metabolic specialist / clinical geneticist as the relevant type of clinician. Paediatrician (children) or physician (adults), with metabolic specialist / clinical geneticist referral.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• Is a biochemical or molecular result needed to clarify the diagnosis?
• Does this condition require an individual plan for illness or reduced food intake?
• Should nutrition advice come from a specialist metabolic dietitian?

Treatment discussions and follow-up

Where the source describes treatments, these are an overview of possible care, not a prescription for an individual. Ask which option applies to the confirmed diagnosis, what benefit is expected, what adverse effects to watch for and how progress will be assessed. Availability, approvals and local practice can differ from the country described in the source.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.