Myotonic dystrophy type 1 (DM1)

Overview

Myotonic dystrophy type 1 (DM1) causes both progressive muscle weakness and myotonia — a stiffness where muscles are slow to relax after contracting. It's genuinely a whole-body condition, affecting the heart, lungs, digestive system and brain as well as skeletal muscle, and usually starts in adulthood. It's both more common and typically more severe than the related condition DM2.

Symptoms

Weakness often starts in the face, eyelids, jaw and neck, affecting facial expression, speech and chewing; weak forearms affect grip, and weak ankles cause foot drop. Myotonia mainly affects the hands and jaw and eases with repeated use — a recognisable 'warm-up' effect, for example when letting go of something gripped tightly. Heart involvement can cause various arrhythmias (a slow heart rate, heart block, atrial fibrillation, or a fast rhythm from the lower chambers). Breathing can be affected by muscle weakness, and swallowing difficulties raise the risk of aspiration pneumonia; speech can become slurred or slow. Gut symptoms — constipation, diarrhoea, bloating — are common, as are early cataracts, squints, drooping eyelids and dry eyes. Cognitive effects can include difficulty concentrating, memory problems, and trouble with decision-making or planning, alongside a higher chance of depression, and excessive daytime sleepiness is common too.

Inheritance

DM1 is autosomal dominant — a single changed copy from one parent is enough — caused by an abnormally long repeated DNA sequence (a CTG repeat) in the DMPK gene on chromosome 19. Generally, more repeats mean more severe symptoms and an earlier start. DM1 also shows 'anticipation': the repeat tends to lengthen further when passed from parent to child, so the condition often becomes more severe and starts earlier in each successive generation — worth discussing with a specialist team if family planning is on the horizon.

Getting a diagnosis

A GP refers to a neurologist, who bases the initial diagnosis on symptoms, examination and family history, then confirms it with a genetic blood test measuring the CTG repeat length in the DMPK gene (typically over 50 repeats in DM1).

Management and outlook

Because DM1 affects so many systems, care is genuinely multidisciplinary. Annual ECGs help catch heart problems early, with further tests if needed; rhythm problems may be treated with ablation, a pacemaker or an implantable defibrillator. Breathing is monitored with lung function tests and sleep studies, with non-invasive ventilation and cough-assist devices used as needed; annual pneumococcal, flu and COVID-19 vaccines are recommended. Speech and language therapy helps with speech and swallowing, with dietitian input for nutrition, and gut symptoms are managed with a high-fibre diet, more fluids, medication or further investigation if they're severe.

Physiotherapy — stretching, strengthening, low-impact aerobic exercise, and orthotic support as the condition progresses — helps maintain function. Pregnancy needs careful planning with a specialist team, and preimplantation genetic diagnosis is worth discussing where relevant. Before any surgery, the anaesthetic team needs to know about the diagnosis well in advance: suxamethonium should be avoided, and sedatives and opioids used with particular caution.