Miyoshi myopathy

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Miyoshi myopathy is a muscle disorder that begins with weakness in the muscles that are located away from the center of the body (distal muscles), such as those in the legs. During early to mid-adulthood, affected individuals typically begin to experience muscle weakness and wasting (atrophy) in one or both calves. If only one leg is affected, the calves appear different in size (asymmetrical). Calf weakness can make it difficult to stand on tiptoe.

As Miyoshi myopathy slowly worsens, the muscle weakness and atrophy spread up the leg to the muscles in the thigh and buttock and can also involve the upper arm and shoulder muscles. Eventually, affected individuals may have difficulty climbing stairs or walking for an extended period of time. Some people with Miyoshi myopathy may eventually need wheelchair assistance.

Rarely, abnormal heart rhythms (arrhythmias) have developed in people with Miyoshi myopathy. Individuals with Miyoshi myopathy have highly elevated levels of an enzyme called creatine kinase (CK) in their blood, which often indicates muscle disease.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Miyoshi myopathy is caused by mutations in the DYSF or ANO5 gene. When Miyoshi myopathy is caused by ANO5 gene mutations it is sometimes referred to as distal anoctaminopathy; when this condition is caused by DYSF gene mutations it is known as a dysferlinopathy. The DYSF and ANO5 genes provide instructions for making proteins primarily found in muscles that are used for movement (skeletal muscles). The protein produced from the DYSF gene, called dysferlin, is found in the thin membrane called the sarcolemma that surrounds muscle fibers. Dysferlin is thought to aid in repairing the sarcolemma when it becomes damaged or torn due to muscle strain.

The ANO5 gene provides instructions for making a protein called anoctamin-5. This protein is located within the membrane of a cell structure called the endoplasmic reticulum, which is involved in protein production, processing, and transport. Anoctamin-5 is thought to act as a channel, allowing charged chlorine atoms (chloride ions) to flow in and out of the endoplasmic reticulum. The regulation of chloride flow within muscle cells plays a role in controlling muscle tensing (contraction) and relaxation.

DYSF and ANO5 gene mutations often result in a decrease or elimination of the corresponding protein. A lack of dysferlin leads to a reduced ability to repair damage done to the sarcolemma of muscle fibers. As a result, damage accumulates and leads to atrophy of the muscle fiber. It is unclear why this damage leads to the specific pattern of weakness and atrophy that is characteristic of Miyoshi myopathy. The effects of the loss of anoctamin-5 are also unclear. While chloride is necessary for normal muscle function, it is unknown how a lack of this chloride channel causes the signs and symptoms of Miyoshi myopathy.

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 exact prevalence of Miyoshi myopathy is unknown. In Japan, where the condition was first described, it is estimated to affect 1 in 440,000 individuals.

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.

Difficulty climbing stairs · Frequent (79-30%)

Reduced ability to climb stairs.

Distal lower limb amyotrophy · Frequent (79-30%)

Muscular atrophy of distal leg muscles.

Distal lower limb muscle weakness · Frequent (79-30%)

Reduced strength of the distal musculature of the legs.

Distal upper limb amyotrophy · Frequent (79-30%)

Muscular atrophy of distal arm muscles.

Exercise-induced myalgia · Frequent (79-30%)

The occurrence of an unusually high amount of muscle pain following exercise.

Gait disturbance · Frequent (79-30%)

The term gait disturbance can refer to any disruption of the ability to walk.

Pelvic girdle muscle weakness · Frequent (79-30%)

Weakness of the muscles of the pelvic girdle (also known as the hip girdle), that is, lack of strength of the muscles around the pelvis.

Proximal amyotrophy · Frequent (79-30%)

Amyotrophy (muscular atrophy) affecting the proximal musculature.

Other findings in the same source

From: Orphanet

Additional reported features include Proximal muscle weakness in lower limbs (Frequent (79-30%)); Quadriceps muscle weakness (Frequent (79-30%)); Shoulder girdle muscle weakness (Frequent (79-30%)); Tibialis atrophy (Frequent (79-30%)); Tibialis muscle weakness (Frequent (79-30%)); Difficulty standing (Frequent (79-30%)); Calf muscle hypertrophy (Occasional (29-5%)); Foot dorsiflexor weakness (Occasional (29-5%)); Loss of ambulation (Occasional (29-5%)); Myalgia (Occasional (29-5%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing Miyoshi myopathy is Neurology, with a neurologist as the relevant type of clinician. General physician / Family Medicine; paediatrician for children. Referral depends on symptoms.

Additional services that may be relevant, depending on the findings, include: Clinical Genetics.

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.

• Which nervous-system findings help explain the symptoms?
• Would an assessment of walking, communication or daily function be helpful?
• Are rehabilitation or other specialist services relevant?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Miyoshi myopathy. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

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.