Myoclonic Epilepsy and Ragged-Red Fibres (MERRF)

MERRF is very rare. Experts believe it affects fewer than 1 in 100,000 people. Even when someone has the MERRF gene change, they might never develop symptoms. That means the actual number of people with classic MERRF may be even smaller.⁶

MERRF affects many parts of the body. Not everyone will have the same symptoms.

The main signs and symptoms include:

Myoclonus

Myoclonus is sudden, uncontrollable muscle twitches or jerks and is often the first sign of MERRF. It’s often triggered by bright lights and can get worse with movement or other triggers. Myoclonus can also come and go or be constant.

Epilepsy (seizures)

Seizures happen when the brain has sudden bursts of electrical activity. This can cause the body to jerk, go stiff, or stop moving for a short time. Some people lose awareness during a seizure.

Ataxia

Ataxia is a movement-coordination disorder. It is a neurological condition caused by a problem in the part of the brain that controls movement (cerebellum dysfunction). Ataxia may cause poor coordination and bad distance judgment. It can also make it hard to do quick, alternating movements.

Muscle weakness

Many people with MERRF have muscle weakness and can feel very tired even after light activity.

This can lead to:

• Trouble walking or lifting objects
• Shortness of breath during activity
• Needing more rest than usual

This happens because mitochondria can’t produce enough energy. Pacing, rest, and gentle activity can help manage these symptoms.

Learn more about exercise and mito

Peripheral neuropathy

Peripheral neuropathy means the nerves outside the brain and spinal cord aren’t working properly. These nerves help you feel things and move your muscles.

When these nerves are damaged, you might feel:

• Tingling or “pins and needles”
• Numbness
• Burning or sharp pain
• Weakness, especially in your hands or feet

It can make everyday things like walking, picking things up, or feeling heat or cold more difficult.

Other signs and symptoms of MERRF are:

• Vision loss (optic atrophy)
• Muscle changes seen under a microscope (ragged-red fibres)
• Hearing loss (sensorineural)
• Heart problems (cardiomyopathy)
• Thinking and memory problems (cognitive changes or dementia)
• Shorter than average height (short stature)

Some people with MERRF may also have stroke-like episodes, problems with eye movement, or changes in vision. This is because MERRF can share symptoms with other types of mito, like MELAS and KSS. These overlaps can make it harder to diagnose MERRF.

This section combines information from trusted sources.^{1,3,6,10–13} The explanations have been adapted into plain language by Mito Foundation to support understanding within the mito community.

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Understand the signs and symptoms of mito

MERRF is a genetic condition caused by genetic changes that affect the mitochondria. These are tiny parts inside our cells that turn food into energy. When mitochondria don’t work properly, cells can’t make enough energy. This can cause damage to different parts of the body.

Why does this happen?

MERRF is usually caused by a genetic change. This means a small change in the DNA that helps mitochondria work. This change is usually found in mitochondrial DNA (mtDNA),⁶ which is passed down from your mother.

Even if a mother has this change, she might not have any symptoms. But she can still pass it on to her children. If a father has the change, he won’t pass it on.

The most common cause

Most people with MERRF have a change in a gene called MT-TK.^14,15 The MT-TK gene contains instructions to make a molecule called transfer RNA (tRNA). You can think of tRNA as a tiny courier — it collects protein building blocks (amino acids) and delivers them to where proteins are made in the cell. MT-TK makes a specific type of tRNA called tRNALys, which delivers the amino acid lysine to help mitochondria build the proteins they need for energy.¹⁶

The most common genetic change in the gene is called m.8344A>G. This means one tiny letter in the mtDNA is swapped. About 80% of people with MERRF have this change.^6,14,15

Figure 1: Explanation of the common genetic change for MERRF. The most common genetic change that causes MERRF is found in the mtDNA genome (m), affecting a specific point in the MT-TK gene (.8344), with the sequence of letters changed from A to G (A>G).

When this gene doesn’t work properly, the mitochondria can’t make energy well. This can lead to symptoms like muscle jerks, seizures, and balance problems.

Can this happen by chance?

A genetic change can occur randomly during conception. This means it is seen for the first time in a person and has not appeared in other family generations. This kind of change is called de novo, which means new.

There have been a few rare de novo cases of MERRF.¹⁷ These changes are not maternally inherited but may be passed down to future generations if the affected individual is female.

Are there other gene changes?

Yes. While most people with MERRF have a change in MT-TK, other genes can also cause MERRF. These genes help make different building blocks (called amino acids) or help the cell produce energy. They include:¹

• MT-TF, MT-TH, MT-TI, MT-TL1, MT-TP, MT-TS1, MT-TS2 – These help make amino acids
• MT-ND5 – This helps make energy in the mitochondria.¹

What is a genetic carrier?

A genetic carrier has the gene change that causes MERRF but does not have any symptoms.

Each cell in the body has many mitochondria. Some of these mitochondria may carry the gene change, while others are healthy. This is called heteroplasmy.

You can think of it as having a mix of working and broken machines. If most are working, the person may not have any symptoms. But if more machines are broken, the body may not get enough energy, and symptoms can appear.

Because of this:

• A person can carry the gene change and not have symptoms
• One family member may have symptoms, while another doesn’t
• It’s hard to predict how severe the condition will be or who will be affected

A genetic test is needed to determine someone’s level of heteroplasmy. This test can show how many mitochondria have the gene change. Even then, predicting how the condition will affect someone is not always possible.

Even if a mother has no symptoms, she can still pass the gene change to her children. Each child may be affected differently.

For more information

Learn more about genetics and mito

Information to understand your diagnosis

Information to understand your child’s diagnosis

Diagnosing MERRF is based on your medical history, signs and symptoms, a physical examination, and other specialist tests.

Physical exam

A health professional will check for signs that may suggest MELAS. This can include:

• Watching for muscle jerks (quick, sudden twitches)
• Asking about seizures
• Checking your balance and coordination
• Seeing if you have trouble with tasks like walking or picking things up

Other tests may include:¹

• Blood or spinal fluid tests

These check for high levels of lactate and pyruvate, which can be signs of MERRF.

• Brain scans

Magnetic Resonance Imaging (MRI) and/or Computed Tomography (CT) can show changes in the brain linked to MERRF. Magnetic Resonance Spectroscopy (MRS) is another type of brain scan that looks specifically for lactate.

• Brain tests

An electroencephalogram (EEG) records brain activity to check for seizures.

• Heart tests

An electrocardiogram (ECG) checks your heart rhythm, or an echocardiogram (echo) looks at the heart muscle.

• Muscle biopsy

A small sample of muscle is looked at under a microscope. A finding called “ragged red fibres” may be seen in some people with MERRF.

Genetic testing

If your doctor suspects MERRF, they will arrange genetic testing to look for any changes associated with the condition. The genetic change for MERRF is found in DNA. This is usually in the form of a blood or saliva sample. But sometimes, other samples like skin, hair, urine, or muscle are needed due to heteroplasmy (see causes of MERRF).

Genetic testing is highly specialised, and it may take up to three to four months to receive the results. Genetic counselling is important to discuss what these results mean for you and your family.

For more information

Learn more about genetics and mito

If you have MERRF, you’ll likely need a team of health professionals to manage your care. Here are some health professionals you might work with:

• General Practitioner (GP): your main point of contact. They can help with referrals, tests, and coordinate your care
• Neurologist: specialises in nervous system disorders. Manages seizures, myoclonus, and other neurological symptoms
• Clinical Geneticist: to provide genetic counselling and manage the mitochondrial aspects of the disorder
• Cardiologist: for monitoring and treating any heart-related issues, such as cardiomyopathy
• Endocrinologist: to address any metabolic or endocrine abnormalities
• Ophthalmologist: for monitoring and treating vision problems
• Orthoptist: works with an ophthalmologist to test vision and use specialised diagnostic tests to detect and examine the visual system's function.
• Audiologist: to manage hearing loss
• Physiotherapist: to help maintain muscle strength and mobility
• Occupational Therapist: to assist with daily living activities and improve quality of life
• Speech Therapist: If there are issues with speech or swallowing
• Genetic Counsellor: provides education and support for people with genetic health conditions
• Mental health professional: offers mental health and wellbeing support

For more information

Information to build your care team

Information to build your child’s care team

For most people with mito, regular tests are important. These tests help monitor how mito is affecting you. Learn more about monitoring mito for adults, or children.

The following recommendations have been adapted from the Australian Patient Care Standards for mito:¹⁸

Watching for serious illness (Critical care)

During a sudden illness, you must be monitored closely and regularly, and your lactate levels should be closely monitored. Lactate levels refer to the amount of lactic acid in your blood. Elevated levels of lactic acid can be harmful. But, monitoring the lactic acid in your blood (serum lactate) can help your health care team intervene early to reduce the risk of serious illness.

Brain and nerve care (Neurology)

Seizures (Epilepsy)

If you notice changes in seizure frequency, speak to your health care team. They may request an Electroencephalogram (EEG).

For treating seizures, it is important to know that treatments such as valproate may be harmful (contraindicated). Medications like levetiracetam and benzodiazepines are preferred. If you experience severe continuous seizures (status epilepticus), your health professional may check for metabolic issues.

Additionally, if you have optic atrophy, a seizure medication called vigabatrin should be used with caution. If you are unsure, always speak to your health professional.

Movement disorder

Regular neurological assessments are important to manage movement disorders and muscle stiffness. If new or sudden movement issues arise, it’s important to check for progression or other problems. Your health professional may request an MRI, electrophysiological tests, and lab investigations.

Physiotherapy is recommended to maintain mobility, prevent joint issues, and reduce pain. In some cases, deep brain stimulation might be considered, depending on your overall health.

Heart care (Cardiology)

You may need an initial heart check-up, including an ECG, 24-hour ECG monitoring, and/or an echocardiogram. Annual ECGs are important to monitor your heart health. The frequency of other tests will be personalised based on your specific needs and family history.

For irregular heartbeats, various treatments and procedures are available. ACE inhibitors, which are commonly prescribed for high blood pressure, can help manage heart failure. But it’s important to have your kidney function monitored regularly.

Exercise testing or stress echocardiography can evaluate how your heart responds to physical activity.

Hearing care (Audiology)

Regular hearing checks are important. These assessments should be done initially and then every one to two years. Avoid loud noises to prevent worsening hearing loss, which might not be reversible. Some antibiotics, like aminoglycosides, can worsen hearing loss.

Monitor your hearing closely during serious illnesses or acute episodes. Hearing aids can help with moderate to severe hearing loss, while cochlear implants might be an option for very severe cases. Talk to your health professional about these options. Hearing loss can affect speech and learning, especially in children.

Looking after your mental health (Psychology)

Taking care of your mental health is as important as managing physical symptoms when dealing with MERRF. If you have any psychiatric symptoms, such as anxiety, depression, or memory loss, speak to your health professional.

For more information

Access the Patient Care Standards for Mito

Mito Foundation provides information to help people affected by mito understand clinical trials and how to find opportunities that may be relevant to them.

Clinical trials help researchers test new ways to prevent, diagnose, treat and manage mito. Some trials test whether a treatment is safe and whether it may help people.
Other clinical studies may look at symptoms, diagnosis, genetics, quality of life, or how mito affects people over time.
Clinical trials are carefully planned. The therapies being studied may be at different stages. Trials may not be available for every type of mito, and not every trial will be right for you. Some trials are only open to people with a confirmed genetic diagnosis, specific symptoms, a certain age range, or the ability to attend study appointments.

Mito Foundation’s emerging therapies information explains therapies in development, how clinical trials work, and what you may wish to consider before taking part. You can also use the Therapy Finder to explore therapies in development and see what may be relevant to you.

Learn more about taking part in clinical trials
Use the Therapy Finder

Staying informed

One way to hear about opportunities to take part in clinical trials is to join the Mito Registry.

The Mito Registry helps connect people with research opportunities that may be relevant to them. It also helps Mito Foundation understand how many people in Australia are affected by different types of mito.

This information can help support future research, services and advocacy.

Join the Mito Registry

National Organisation for Rare Disorders: MERRF

Genetic and Rare Diseases Information Centre: MERRF

Mito Support Community – Facebook group

Mito Support Community for Mito Families – Facebook group

As of July 2025, no active support groups dedicated to MERRF were found in Australia.

MERRF – Facebook group