Myotonic dystrophy type 2 (DM2): proximal weakness, pain and stiffness

Both conditions are caused by an unstable repeat of DNA bases, and in both the abnormal RNA that results interferes with the cell's handling of many other genes. The clinical picture is different, though. DM2 usually causes less facial weakness, less intellectual and behavioural involvement and a milder overall course. Pain is often a leading symptom, which is unusual among muscular dystrophies.

Because it is milder and the symptoms are non-specific, DM2 is frequently under-diagnosed. It may be labelled as fibromyalgia, arthritis, or simply ageing, until a clinician notices cataracts, myotonia and a raised creatine kinase together.

The characteristic features are thigh and hip girdle weakness, which makes stairs and rising from a chair difficult; muscle stiffness that eases with repeated movement; deep muscular pain, often in the thighs; and tiredness. Calf enlargement is common in DM2 and may be noticeable on examination. Tremor, muscle cramps and fluctuating weakness are also described.

• Proximal weakness in the thighs, hips, neck and shoulders

• Myotonia, which can be subtle and is often described as stiffness

• Aching muscle pain, sometimes severe

• Early cataracts, particularly the posterior sub-capsular type

• Heart conduction changes and, in some people, rhythm problems

• Raised blood sugar or diabetes

Diagnosis depends on a blood test for the CCTG repeat expansion, which the Myotonic Dystrophy Support Group describes as the way DM2 is confirmed. The CCTG repeat can be extremely long and difficult to measure, so specialist laboratories are used and results may take time. Electromyography can show myotonic discharges, and a muscle biopsy is rarely needed now.

Before the genetic test became available, doctors relied on clinical suspicion, EMG and biopsy, so some older diagnoses of unclassified myotonic myopathy have since been re-labelled DM2. NHS genetic testing is free when arranged by a hospital specialist, and counselling is provided alongside it.

The Myotonic Dystrophy Support Group says management involves cardiorespiratory, ocular and endocrine screening and genetic counselling, while noting that formal evidence for specific protocols remains limited. In practice, many neuromuscular teams offer periodic ECGs, an eye examination for cataracts, glucose or HbA1c checks and review of any breathing or sleep symptoms.

Cardiac conduction defects are the most important safety issue. A history of fainting, palpitations or dizziness needs prompt assessment and a collapse needs an emergency response. Anaesthetic planning is also advised because muscle conditions can affect sedation, breathing and swallowing after surgery.

There is no disease-modifying treatment. Physiotherapy helps preserve strength and flexibility. Pain is treated with the usual stepwise approach, adjusted for medicines that are suitable alongside heart conduction problems. Mexiletine or other membrane-stabilising drugs are sometimes considered for myotonia, although stiffness in DM2 often fluctuates and some people find warming up and pacing activity more helpful than medicines.

Occupational therapists can advise on work adjustments, energy conservation and equipment. Many people with DM2 work for most of their lives, and the course is often slowly progressive rather than rapidly disabling.

DM2 is autosomal dominant, so each child of an affected parent has a 50 percent chance of inheriting the expansion. Because the repeat size does not predict severity, and no congenital form has been documented, the pattern across generations is different from DM1, but relatives can still be affected to varying degrees. Clinical genetics can offer predictive testing and discuss reproductive options.