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Genetic neuromuscular condition

Myotonic dystrophy type 1 (Steinert disease): a multisystem guide

Myotonic dystrophy type 1, also called Steinert disease, is the most common adult-onset muscular dystrophy. It affects far more than muscle: the heart, eyes, swallowing, sleep, thinking and the gut can all be involved. This guide describes the genetics, how it varies between family members and how care is organised.

Last checked: 2026-10-03

Key facts

At a glance.

  • Gene

    An expanded CTG repeat in the DMPK gene. The Myotonic Dystrophy Support Group gives 5 to 37 repeats as normal and 50 or more as typically symptomatic.

  • Inheritance

    Autosomal dominant: a parent with DM1 has a 50 percent chance, in each pregnancy, of passing the changed gene on.

  • Anticipation

    The repeat can lengthen between generations, so symptoms may start earlier and be more severe in children than in the parent.

  • Frequency

    The UK Myotonic Dystrophy Support Group describes myotonic dystrophy as affecting around 1 in 8,000 adults, with DM1 generally more common than DM2.

  • Multisystem

    Cataracts, heart conduction problems, diabetes, sleepiness and swallowing difficulty are recognised features alongside muscle symptoms.

Myotonia, weakness and why this condition feels different

Myotonia is a delay in muscle relaxation after a contraction. A person may find it hard to release a handshake or to let go of a door handle. Alongside it, DM1 produces progressive wasting and weakness, particularly in the face, neck, forearms, hands and lower legs. That distal pattern differs from most other dystrophies, in which the shoulder and hip girdles are affected first.

MedlinePlus explains that the repeat expansion produces unusually long messenger RNA that clumps inside cells and interferes with the production of many other proteins. That is the reason a single gene change leads to effects in so many organs.

Effects outside the muscles

DM1 is best managed as a whole-body condition. The features that most often alter care are listed below. Not everyone has all of them, and severity varies widely even within a single family.

  • Heart: conduction defects, where electrical signals travel slowly through the heart, can cause fainting and sometimes need a pacemaker or similar device. Routine ECG follow-up is standard.

  • Eyes: early cataracts are characteristic, and the MDSG lists the eyes among organs commonly affected.

  • Sleep and energy: marked daytime sleepiness and fatigue are common and may reflect both sleep-disordered breathing and a central effect of the condition.

  • Swallowing: weak throat muscles can cause choking or chest infections.

  • Endocrine: diabetes and other hormone changes can occur, and MedlinePlus lists diabetes mellitus among the features.

  • Thinking and behaviour: reduced initiative, difficulty planning and low motivation are frequent and often misread as laziness.

Forms of DM1 and anticipation

The condition ranges from a mild adult-onset form with cataracts and little else, through classic adult-onset disease in the twenties to thirties, to a severe congenital form seen in newborns. The Myotonic Dystrophy Support Group notes that paternal transmission of the severe form is very uncommon, whereas when the change is passed on through the mother the condition often becomes more severe in later generations.

This phenomenon, anticipation, means a grandparent with cataracts alone can have a grandchild with serious muscle weakness at birth. It is the reason genetic counselling is recommended before pregnancy and why a diagnosis in one person needs to be carefully explained to relatives.

How DM1 is diagnosed

Clinicians suspect DM1 from the combination of myotonia, a typical facial appearance, weak hands and a family history that may include cataracts or heart rhythm problems. A genetic blood test measuring the DMPK repeat size confirms it. NHS genetic testing is free when arranged by a hospital specialist, and the NHS offers counselling to help people understand what a result means for relatives.

Electromyography can show typical myotonic discharges but is no longer essential for diagnosis. Because the repeat is the thing being measured, testing at-risk relatives before symptoms appear (predictive testing) is possible, and in the UK this is done through clinical genetics with counselling.

Care and monitoring in the UK

There is no cure and no treatment that removes the repeat. Care is directed at monitoring and treating complications: periodic ECGs and, when indicated, ambulatory rhythm monitoring or a pacemaker; eye review for cataract; sleep assessment and treatment for sleep apnoea; swallowing assessment; diabetes screening; and physiotherapy and occupational therapy to preserve function.

Medicines such as mexiletine are sometimes prescribed for troublesome myotonia, but they are prescription-only and only suitable after heart assessment. Anaesthesia is a particular risk area. Muscle weakness and sensitivity to sedatives and some anaesthetic agents mean that hospital teams should be told about the diagnosis in advance. The Myotonic Dystrophy Support Group offers an alert card for exactly this purpose and runs a UK patient registry for research.

Pregnancy and family planning

Women with DM1 may have a higher-risk pregnancy and should be looked after by obstetric and neuromuscular teams who know the condition. The risk of a child with congenital DM1 is mainly relevant when the mother carries the expansion. Reproductive options, including prenatal diagnosis and pre-implantation genetic testing, are discussed in clinical genetics.

Frequently asked

Questions people ask.

Quick, plain-English answers to the questions we hear most.

  • What is the difference between DM1 and DM2?

    DM1 is caused by a CTG repeat in DMPK and usually affects hands, face and lower legs first, with more brain involvement. DM2 is caused by a CCTG repeat in CNBP, is usually milder and affects the shoulders and hips.

  • Can DM1 skip a generation?

    The gene itself does not skip, but a parent with very mild features may not know they are affected. A child can then present with a more severe form because the repeat has lengthened.

  • Do I need a heart test if I feel fine?

    Yes. Conduction problems can be silent, and routine ECG surveillance is part of standard follow-up for DM1.

  • Why am I so sleepy?

    Daytime sleepiness in DM1 can come from sleep apnoea, from a central effect of the condition, or both. A sleep assessment helps separate the causes and decide on treatment.

  • Is anaesthesia dangerous?

    It needs planning. People with DM1 can be unusually sensitive to sedatives and may have breathing and swallowing risks afterwards, so surgical and anaesthetic teams must know the diagnosis beforehand.

  • Can DM1 be passed from fathers?

    Yes, as it is autosomal dominant, but the Myotonic Dystrophy Support Group notes that paternal transmission of the severe congenital form is very uncommon.

  • Does DM1 affect the brain?

    It can. Reduced motivation, attention problems and difficulty planning are recognised, and they may affect work and relationships as much as the muscle weakness does.

  • What does the repeat size tell me?

    A longer repeat generally goes with earlier onset and more severe disease, but it cannot predict an individual's course precisely.

  • Is there a UK registry?

    The Myotonic Dystrophy Support Group describes the UK DM Patient Registry, which links people with the condition to research opportunities.

Where this comes from

Sources and checking.

This guide is for information, not medical advice.

Your GP or consultant knows your history and can tell you which parts apply to you. If your symptoms are severe, rapidly worsening, or come with red-flag features, seek urgent medical care. Figures and availability change, so treat them as a guide and confirm with the provider.

Published 2026-10-03, next review 2027-10-03.

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