Medicine – Myotonic Dystrophy

Myotonic dystrophy, also called dystrophia myotonica, is an inherited multisystem disorder characterised by myotonia, progressive muscle weakness and wasting, cataracts, endocrine abnormalities, cardiac conduction disease, and characteristic facial features.

It is usually inherited in an autosomal dominant pattern.

1. Inheritance

Myotonic dystrophy is classically autosomal dominant.

The commonest form, myotonic dystrophy type 1 (DM1), is caused by expansion of a CTG trinucleotide repeat in the DMPK gene.

The number of repeats can increase when passed from one generation to the next, producing anticipation.

Anticipation means that the disease may appear earlier and more severely in successive generations.

2. Myotonia

Myotonia means delayed relaxation of skeletal muscle after voluntary contraction.

For example, a patient may tightly grip an examiner’s hand but then have difficulty releasing it promptly.

Therefore:

Muscle contracts normally → relaxation is delayed.

This is one of the defining features of myotonic dystrophy.

3. Percussion Myotonia

Myotonia can also be demonstrated by percussion.

Tapping certain muscles, such as the thenar muscles, may produce a prolonged contraction before relaxation.

This is called percussion myotonia.

4. Myotonic Facies

Patients may develop a characteristic appearance known as myotonic facies.

This results from weakness and wasting of the facial and temporal muscles.

Typical features include:

Long, thin face.

Temporal wasting.

Hollowed cheeks.

Weak facial expression.

Ptosis.

The overall appearance may become quite distinctive in established disease.

5. Bilateral Ptosis

Bilateral ptosis is common because of weakness of the muscles that elevate the upper eyelids.

Unlike myasthenia gravis, the ptosis is generally more persistent and occurs alongside other characteristic muscular and systemic features.

6. Facial Muscle Weakness

Weakness of facial muscles may cause:

Reduced facial expression.

Difficulty closing the eyes tightly.

Weakness of smiling or whistling.

Difficulty with speech or swallowing in advanced disease.

Muscle wasting around the temples and jaw contributes to the characteristic facial appearance.

7. Frontal Balding

Premature frontal balding, especially in men, is a classic associated feature.

It is not caused simply by muscle weakness but forms part of the multisystem phenotype of the disorder.

Thus, an examination combination such as:

Myotonia + frontal balding + cataracts + distal weakness

should strongly suggest myotonic dystrophy.

8. Pattern of Muscle Weakness

Muscle weakness is usually progressive.

In classic DM1, weakness often begins predominantly in the distal muscles, particularly:

Hand muscles.

Forearm muscles.

Ankle dorsiflexors.

Over time, more proximal muscle groups may become affected.

9. Muscle Wasting

Progressive weakness is accompanied by muscle wasting.

Commonly affected regions include:

Distal limbs.

Facial muscles.

Temporal muscles.

Sternocleidomastoid muscles.

Advanced disease can therefore produce substantial functional impairment.

10. Cataracts

Cataracts are a classic extramuscular manifestation.

A characteristic appearance is the so-called Christmas-tree cataract, which contains brightly coloured, iridescent lens opacities.

Not every patient has this exact appearance, but early cataract formation is strongly associated with myotonic dystrophy.

11. Insulin Resistance

Patients may develop insulin resistance and other metabolic abnormalities.

This may lead to impaired glucose tolerance or diabetes mellitus.

Insulin resistance is part of the broader endocrine involvement seen in myotonic dystrophy.

12. Other Endocrine Features

Other endocrine and reproductive abnormalities may occur.

These can include:

Testicular atrophy.

Reduced fertility.

Hypogonadism.

Thyroid dysfunction in some patients.

The disorder therefore affects much more than skeletal muscle.

13. Cardiac Involvement

An important feature not included in the original note is cardiac conduction disease.

Patients may develop:

PR prolongation.

Bundle branch block.

Atrioventricular block.

Atrial or ventricular arrhythmias.

Because conduction abnormalities can cause syncope or sudden cardiac death, cardiac surveillance is an important part of management.

14. Respiratory Involvement

Respiratory muscles can also become weak.

In addition, patients may have impaired central respiratory drive or sleep-disordered breathing.

Consequences can include:

Nocturnal hypoventilation.

Sleep apnoea.

Daytime somnolence.

Respiratory failure in advanced disease.

15. Gastrointestinal Involvement

Smooth-muscle dysfunction may affect the gastrointestinal tract.

Patients may develop:

Dysphagia.

Gastro-oesophageal reflux.

Constipation.

Intestinal dysmotility.

These features reinforce the fact that myotonic dystrophy is a multisystem disorder, not simply a muscular disease.

16. Myotonic Dystrophy Type 1 and Type 2

There are two major forms.

Myotonic dystrophy type 1 (DM1) is the classic form and is associated with CTG repeat expansion in the DMPK gene.

DM1 more commonly produces distal weakness, facial weakness, myotonia, cataracts, and potentially congenital disease.

Myotonic dystrophy type 2 (DM2) is caused by a different repeat expansion and often produces more proximal muscle weakness and pain.

The phenotype can overlap considerably with DM1.

17. Congenital Myotonic Dystrophy

Severe DM1 can occasionally present at birth as congenital myotonic dystrophy.

Affected infants may have:

Severe hypotonia.

Respiratory weakness.

Feeding difficulties.

Delayed motor development.

Congenital disease is particularly associated with maternal transmission.

18. Diagnosis

Diagnosis is usually confirmed by genetic testing.

For DM1, testing identifies an expanded CTG repeat in the DMPK gene.

Electromyography may also show characteristic myotonic discharges.

These are often described as having a waxing-and-waning sound resembling a “dive bomber,” although this description is mainly an examination teaching aid.

19. Management

There is no universal curative treatment, so management is largely multidisciplinary and supportive.

Important areas include:

Physiotherapy and rehabilitation.

Cardiac monitoring.

Respiratory assessment.

Treatment of cataracts.

Management of endocrine abnormalities.

Genetic counselling.

20. Treatment of Myotonia

Myotonia does not always require drug treatment.

If it is functionally troublesome, medications such as mexiletine may be used under specialist supervision.

Treatment is aimed at improving muscle relaxation rather than reversing the underlying genetic disorder.

21. Cardiac Surveillance

Patients require regular assessment for conduction abnormalities.

This may include:

ECG.

Ambulatory rhythm monitoring when indicated.

Cardiology review.

Some patients eventually require a pacemaker or implantable cardiac device depending on the conduction disturbance and arrhythmia risk.

22. Cataract Management

Cataracts are treated in the usual way when they become visually significant.

This generally involves cataract extraction with intraocular lens implantation.

Because cataracts may occur relatively early, visual symptoms should be actively assessed.

23. Myotonic Dystrophy – Note Form

Inheritance: autosomal dominant.

Classic mutation: CTG repeat expansion in the DMPK gene in DM1.

Anticipation: disease may become earlier and more severe in successive generations.

Myotonia: delayed relaxation after muscle contraction.

Myotonic facies: long thin face, temporal wasting, facial weakness and ptosis.

Ptosis: usually bilateral.

Frontal balding: classic associated feature.

Muscle weakness: progressive and classically distal in DM1.

Muscle wasting: affects distal limbs, facial muscles, temporal muscles and sternocleidomastoids.

Cataracts: common; Christmas-tree cataracts are characteristic.

Metabolic feature: insulin resistance.

Cardiac complication: conduction defects and arrhythmias.

Respiratory complication: respiratory muscle weakness and sleep-related hypoventilation.

Diagnosis: genetic testing ± EMG showing myotonic discharges.

Key Clinical Pattern

Remember myotonic dystrophy as:

Autosomal dominant + myotonia + distal muscle weakness + frontal balding + bilateral ptosis + cataracts + insulin resistance.

The most characteristic examination clue is:

Patient grips strongly but cannot relax the hand promptly.

And the major systemic complication to remember is:

Myotonic dystrophy → cardiac conduction disease and arrhythmias.