Myotonic Muscular Dystrophy (MMD)

What is Myotonic Muscular Dystrophy?

Myotonic Muscular Dystrophy is a genetic, progressive neuromuscular disorder characterized by muscle weakness, myotonia (delayed relaxation of muscles after contraction), and multi-system involvement affecting muscles and other organs.

It is the most common form of adult-onset muscular dystrophy.

Types of Myotonic Muscular Dystrophy

1. Type 1 (DM1) - Steinert's Disease

   • Caused by a CTG trinucleotide repeat expansion in the DMPK gene on chromosome 19.

   • Symptoms typically start in adulthood but can be congenital.

   • More severe and common than DM2.

2. Type 2 (DM2) - Proximal Myotonic Myopathy (PROMM)

   • Caused by a CCTG repeat expansion in the CNBP gene on chromosome 3.

   • Usually milder than DM1, with adult onset.

   • Proximal muscles are more affected.

Symptoms

• Muscle symptoms:

  • Myotonia (difficulty relaxing muscles after use)

  • Progressive muscle weakness and wasting, especially in the face, neck, and distal limbs (DM1) or proximal limbs (DM2)

  • Difficulty swallowing and speaking

  • Drooping eyelids (ptosis)

  • Muscle stiffness and cramps

• Non-muscle symptoms:

  • Cataracts

  • Cardiac conduction abnormalities (arrhythmias)

  • Endocrine disturbances (diabetes, thyroid problems)

  • Cognitive impairments or learning difficulties (especially in congenital DM1)

  • Gastrointestinal problems

  • Respiratory difficulties due to muscle weakness

Causes and Genetics

• Autosomal dominant inheritance.

• Expansion of unstable nucleotide repeats (CTG for DM1, CCTG for DM2).

• The number of repeats correlates with severity and age of onset (anticipation phenomenon, especially in DM1).

Diagnosis

• Clinical evaluation (history, physical exam showing myotonia and muscle weakness).

• Electromyography (EMG) shows myotonic discharges.

• Genetic testing confirms repeat expansions in DMPK or CNBP genes.

• Cardiac evaluation (ECG, echocardiogram).

• Muscle biopsy (rarely needed).

Treatment

There is no cure, but management focuses on symptom relief and complications:

• Symptomatic treatments:

  • Medications for myotonia (e.g., mexiletine).

  • Physical therapy to maintain muscle function.

  • Cardiac monitoring and pacemaker implantation if needed.

  • Cataract surgery.

  • Respiratory support if required.

• Supportive care:

  • Occupational therapy.

  • Speech therapy.

  • Endocrinological management of diabetes or thyroid issues.

Prognosis

• Progressive disease with variable severity.

• Life expectancy can be normal or reduced depending on cardiac and respiratory complications.

• Regular multidisciplinary follow-up is important.