Myotonic Dystrophy: An Australian Guide to DM1 and DM2

Myotonic dystrophy: the most common adult muscular dystrophy, and the least understood

If you take one thing from this guide, take this: the most important appointment for somebody with myotonic dystrophy is an annual ECG, and it is the one most often missed.

Conduction defects in the heart’s electrical system are found in around 90 per cent of people with myotonic dystrophy type 1 in some series, and they are a major cause of early death — including sudden death, in people who were walking, working and had been told their muscle condition was mild. A pacemaker fitted on the basis of a routine ECG prevents that. An ECG not done does not.

This matters because myotonic dystrophy is systematically underestimated, by patients and by clinicians. It is the most common adult-onset muscular dystrophy. It is often mild in its muscle symptoms. And it is not really a muscle disease at all — it is a multisystem condition that affects the heart’s conduction system, the eyes, the endocrine system, the gut, the brain, sleep regulation and respiration, with muscle weakness as one feature among many.

Somebody told they have “a mild form of muscular dystrophy” and sent away has been given dangerously incomplete information.

This guide covers the two types, the genetics and why it gets worse through the generations, the forms it takes, the organ-by-organ picture, the surveillance schedule, the anaesthetic risk that every person with this condition should carry a card about, and the practical business of living with it.

What it is, and the two types

Myotonic dystrophy type 1 (DM1), also called Steinert disease, is caused by an expanded CTG repeat in the DMPK gene on chromosome 19. Type 2 (DM2) is caused by a repeat expansion in a different gene, CNBP, and is generally milder with less severe weakness, no congenital form, and more prominent muscle pain. Most of what follows concerns DM1, which is the more common and more serious.

The mechanism is unusual and explains the multisystem picture. The expanded repeat produces a toxic RNA that sequesters proteins needed to process dozens of other genes correctly. So the consequences are not confined to muscle — they turn up wherever those misprocessed genes matter, which is almost everywhere.

Myotonia is the feature that gives it its name: muscle that contracts and will not relax promptly. People describe a hand that locks onto a steering wheel or a jar lid, a grip that will not let go, difficulty releasing a handshake, or stiffness that eases after a few repetitions — the “warm-up” phenomenon. It is often more of a nuisance than a disability, and it is treatable.

Prevalence. Worldwide estimates for DM1 are around 1 in 20,000, ranging from about 1 in 100,000 in parts of Japan to about 1 in 10,000 in Iceland, with founder effects raising rates in some regions. But a 2021 New York State newborn screening study found CTG expansions of 50 or more repeats in about 4.76 per 10,000 — considerably more than the clinical prevalence estimates — with over half of that group having 50 to 150 repeats, which means many of them may have few or no symptoms and will never be diagnosed. Premutations of 35 to 49 repeats were found in about 19.1 per 10,000.

The implication is that there are more Australians with myotonic dystrophy than anybody has counted, many of them undiagnosed, many of them not having the annual ECG that might save their life.

The genetics, and anticipation

Inheritance is autosomal dominant. Each child of an affected parent has a 50 per cent chance of inheriting the expanded allele.

The repeat ranges:

Penetrance approaches 100 per cent by age 50 when subtle signs are looked for, though mild cases are frequently missed.

Anticipation. The repeat is unstable when passed on and tends to expand, so children can inherit a longer repeat than their parent had — producing earlier onset and more severe disease in each successive generation. This is the single most important thing for families to understand, because it means a grandparent with cataracts and a bit of stiffness, a parent with weakness and diabetes, and a grandchild with congenital myotonic dystrophy can all be the same condition in one family line.

Congenital myotonic dystrophy is usually inherited from the mother, though paternal transmission can also produce large expansions — in one study of parents with 50 to 100 repeats, paternal transmissions showed a larger median increase (425 repeats) than maternal ones (200).

And a point that catches families out: virtually everybody with DM1 inherited it from a parent who also had an abnormal repeat — even where that parent appears unaffected. A mild or premutation parent can be entirely unaware. Which means a new diagnosis in a family should prompt assessment of the parents and siblings, not just the children.

Repeat length does not reliably predict severity. Ranges overlap between the clinical forms, and somatic mosaicism means the repeat size measured in blood may not reflect what is happening in muscle or heart. So a number is informative and not predictive.

The forms

Classic DM1 is what most adults have. Weakness is distal first — hands, forearms, feet and ankles — which is the opposite of the limb-girdle pattern and is a useful diagnostic clue. Facial weakness produces a characteristic appearance: a long thin face, drooping eyelids, weak eyelid closure and a weak smile. Early frontal balding in men. Neck flexor weakness. Fatigue, which is pervasive and frequently the most disabling symptom.

Congenital DM1 is a neonatal emergency in its severe form — hypotonia, feeding difficulty and respiratory failure at birth. Survivors usually do learn to walk, and strength often improves through early childhood before declining in the second decade. Intellectual disability is common. These children need paediatric neuromuscular, respiratory, developmental and educational support from the start.

And classic DM1 can show signs in childhood — myotonia, drooping lids, weak eyelid closure, a weak smile, a thin face — which are easy to attribute to nothing in particular.

Organ by organ, which is how this condition has to be managed

The heart — the most important. Conduction defects are common (90 per cent in one series) and are a major cause of early death including sudden death. First-degree heart block, bundle branch block, atrial fibrillation and flutter, and ventricular arrhythmias. Pacemakers are sometimes needed, and in some people an implantable defibrillator. Dilated cardiomyopathy occurs but is less common than conduction disease. Atrial fibrillation raises stroke risk and needs managing accordingly.

Crucially, cardiac risk does not track muscle severity. Somebody with mild weakness can have dangerous conduction disease.

The respiratory system. Lung function can decline progressively, sometimes to the point of needing mechanical ventilation. Diaphragm weakness and aspiration risk matter in advanced disease. Central as well as peripheral mechanisms are involved, which is part of why sleepiness is so prominent.

Sleep and daytime sleepiness. Hypersomnia and sleep apnoea are well recognised, often appearing later, and frequently mistaken for depression, laziness or poor motivation. In one study, 20 of 40 people assessed had obstructive sleep apnoea. This is one of the most quality-of-life-destroying features of the condition and one of the most treatable — a sleep study and, where indicated, ventilation or CPAP can transform somebody’s function. No routine psychostimulant has shown proven benefit, so chasing the sleep disorder itself is the better route.

The eyes. Nearly everybody with DM1 develops posterior subcapsular cataracts, with a characteristic multicoloured “Christmas tree” appearance on slit lamp examination. Visual symptoms usually begin in the thirties or forties. They are surgically treatable, and cataracts in a young adult should prompt the question of whether myotonic dystrophy has been considered.

The endocrine system. Insulin resistance and diabetes, thyroid dysfunction, calcium dysregulation, testicular atrophy and infertility, and possible growth hormone abnormalities. Secondary hyperparathyroidism has been reported in up to 18 per cent. Most are rarely clinically dominant, and diabetes is common enough to warrant annual screening.

The gut. Dysphagia, constipation, pseudo-obstruction and diarrhoea. Gallstones are common and liver enzymes are frequently elevated. The gastrointestinal symptoms are among the most under-discussed and most troublesome in daily life.

Cognition and mental health. Minor deficits are common, with reduced executive function, working memory and processing speed. Full-scale IQ tends to be lower in mild and classic DM1. Anxiety, depression and specific personality features are frequent, and quality of life can be seriously impaired. There is also a well-described tendency to apathy and avoidance that gets read as lack of motivation — which, combined with the sleepiness, means people with DM1 are routinely misjudged as not trying. That misjudgement reaches employers, clinicians and sometimes families.

What should be checked, and how often

This is the practical core of myotonic dystrophy management. From the GeneReviews recommendations:

ECG annually, to detect asymptomatic conduction defects. A number of centres add 24-hour Holter monitoring each year even where there are no cardiac complaints. Where there is fainting, palpitation or chest pain, an ECG and Holter are both indicated, with echocardiography and a cardiology opinion to be weighed.

Annual fasting glucose and HbA1c, with diabetes treated if present.

Ophthalmology review every two years.

Nutrition and swallowing monitored, with speech pathology and dietetic input where there is difficulty.

Polysomnography (a sleep study) for sleep complaints — and given how common and how treatable sleep-disordered breathing is here, a low threshold for requesting one is appropriate.

Genetic testing offered to at-risk relatives, which allows early detection and treatment of cardiac disease, diabetes and cataracts. This is a condition where identifying a relative early has concrete medical benefit, which distinguishes it from predictive testing in conditions with no treatment.

The practical instruction: this schedule will not be delivered to you automatically. Keep a copy, know what is due, and ask. A GP with a recall system set up for these items is worth more than almost any specialist appointment.

The anaesthetic warning

Every person with myotonic dystrophy should know this and should carry something in writing.

People with myotonic dystrophy are at increased risk during anaesthesia, and the risk is not obvious to an anaesthetist who does not know the diagnosis. The main points from the published anaesthetic recommendations:

Avoid succinylcholine (suxamethonium).

Propofol injection pain can trigger myotonia.

Vecuronium has been linked to malignant hyperthermia in this population, though this is very uncommon.

Pulmonary complications are the principal perioperative concern, so careful postoperative monitoring is essential — people with DM1 can have delayed recovery of respiratory function and are at risk of aspiration and respiratory failure after procedures that would be routine in anybody else.

Sevoflurane has been used without reported problems.

And separately: statins should be avoided, because they can cause muscle pain and weakness in this population.

What to do about it. Carry a card or a letter stating the diagnosis, the anaesthetic risks, and the contact details of your neuromuscular clinic. Tell every surgeon, every dentist considering a general anaesthetic, and every anaesthetist. Make sure it is on your medical record. This includes day procedures and dental work — the risk is not confined to major surgery.

Treatment and what helps

There is no treatment that slows the condition. Everything available manages symptoms and protects organs.

Myotonia can be treated with mexiletine and some other agents where it is genuinely disabling, under specialist supervision and with cardiac status taken into account.

Cardiac management as above, including pacing where indicated.

Sleep-disordered breathing treated with CPAP or non-invasive ventilation, which is one of the highest-value interventions available.

Diabetes, thyroid disease and cataracts treated conventionally.

Ankle-foot orthoses for foot drop, which make a large practical difference to walking and falls.

Hand function support — built-up handles, adapted tools, occupational therapy. Distal weakness makes hands the limiting factor for many people.

Physiotherapy, with the caveat that excessive eccentric exercise is generally avoided; a physiotherapist with neuromuscular experience is worth seeking out.

Speech pathology and dietetics for swallowing.

Pain management, particularly in DM2 where muscle pain is prominent.

And treating the mood and the fatigue as real. Depression and anxiety are frequent, treatable, and under-addressed, and the apathy-plus-sleepiness picture is a clinical problem rather than a character trait.

The family conversation

Myotonic dystrophy is a family diagnosis more than most.

Autosomal dominant inheritance means 50 per cent risk to each child.

Anticipation means the next generation may be more severely affected, and it is why a woman with mild DM1 can have a baby with congenital myotonic dystrophy — an outcome that is entirely unexpected to a family who understood the condition as mild.

Parents and siblings should be assessed, because the transmitting parent is frequently unaware. When somebody is newly diagnosed, the generation above needs an ECG as much as the generation below does.

Pregnancy carries specific risks — polyhydramnios, preterm labour, and for the mother increased anaesthetic and cardiac risk. Pre-pregnancy counselling with an obstetric service that knows the condition is important, not optional.

Reproductive options include preimplantation genetic testing and prenatal diagnosis, and should be discussed before a pregnancy rather than during one.

Genetic counselling is free through state genetic services. See families, carriers and genetics.

Work, driving and the practical adjustments

Myotonic dystrophy mostly affects working-age adults, and the daily problems it causes are specific enough to have specific answers.

Hands first. Distal weakness and myotonia make grip the limiting factor long before walking is affected. What helps: built-up handles on tools and cutlery, lever taps rather than knobs, key turners, electric can and jar openers, a bidet seat, and voice-to-text on every device — which is free, built in, and preserves written work for years. An occupational therapy assessment focused specifically on hand function and task simplification is among the highest-value appointments available and is rarely offered.

Foot drop. Ankle-foot orthoses transform walking and reduce falls for a great many people with this condition, and they are frequently not prescribed until after a fall. Ask.

Fatigue and sleepiness at work. This is the issue that costs people jobs, and it is usually treated as a performance problem. Under the Disability Discrimination Act an employer must make reasonable adjustments, and the ones that work here are a later start, a predictable schedule, a quiet space, scheduled breaks, and reduced hours rather than reduced responsibility. JobAccess and the Employment Assistance Fund pay for workplace modifications and assistive technology, separately from any NDIS plan.

And get the sleep investigated before accepting that this is just how it is, because a sleep study and treatment can change daytime function substantially.

Driving. Myotonia in the hands, foot drop, reduced reaction time and daytime sleepiness all bear on fitness to drive, and you must notify the licensing authority of a condition likely to affect driving — non-disclosure can void insurance cover entirely. Notification does not mean automatically losing the licence: fitness is assessed against the Austroads standards as administered by your state, and an occupational therapy driving assessment, sometimes with vehicle modifications, is the usual route. The sleepiness is the part most likely to matter and the part people least want to raise.

Insurance and superannuation. Check every policy and every fund, including old ones, for total and permanent disablement and income protection cover, and read the definitions before lodging anything. Do not resign first — many income protection policies require the claim to arise while employed.