Neurology · MU Sofia

78. Myotonic dystrophy, myotonias

Explains myotonia as delayed muscle relaxation, then moves to myotonic dystrophy: its type 1 and type 2 genetics, the pathogenesis through abnormal RNA, and the clinical features including the hatchet face, systemic cardiac, ocular, endocrine and CNS involvement. Also covers EMG, genetic testing and ECG diagnosis, treatment with mexiletine and cardiac monitoring, and the non-dystrophic myotonias myotonia congenita and paramyotonia congenita.

Inside: 2 sections · 1 labelled picture · 15 flashcards · 2 min

Topic 78 of 88 in the MU Sofia syllabus for Neurology (Medicine, Year 4). The free AceMed note covers it in 2 sections, about 252 words (2 min of reading), with 15 flashcards.

In this note

  1. Myotonic dystrophy. Myotonias
  2. Myotonic Dystrophy (DM)

Myotonic dystrophy. Myotonias

Definition

Myotonia: Myotonia is delayed relaxation of a muscle after voluntary contraction or percussion.

Example:

  • Difficulty releasing a hand grip after shaking hands.
  • Delayed opening of eyes after forceful closure.

Myotonic Dystrophy (DM)

Definition: Myotonic Dystrophy is the most common adult muscular dystrophy, characterized by:

  • Myotonia. Progressive muscle weakness. Multisystem involvement

Genetics

Type 1 (Steinert Disease) – Most Important: Autosomal dominant. CTG trinucleotide repeat expansion (DMPK gene). Anticipation present

Type 2: Milder form. Proximal muscle involvement

Pathogenesis

Genetic mutation → abnormal RNA accumulation → dysfunction of multiple tissues:

  • Skeletal muscle. Heart. Endocrine organs. Eye. CNS

Clinical Features

A. Myotonia (Hallmark): Delayed muscle relaxation. Hand-grip myotonia. Percussion myotonia

B. Muscle Weakness: Slowly progressive. Distal muscles affected first (DM1). Facial and neck weakness

C. Characteristic Facial Appearance: Long thin face. Ptosis. Temporal muscle wasting. “Hatchet face”

D. Systemic Manifestations

  • Cardiac: Conduction defects. Arrhythmias
  • Ocular: Early cataracts
  • Endocrine: Diabetes mellitus. Testicular atrophy. Infertility
  • CNS: Cognitive impairment. Excessive daytime sleepiness

Examination

Muscle Findings: Grip myotonia. Percussion myotonia. Distal weakness. Muscle wasting

Diagnosis

  • EMG (Most Characteristic): Myotonic discharges. “Dive-bomber” sound
  • Genetic Testing: Confirms diagnosis
  • ECG: Detects cardiac conduction abnormalities

Treatment

  • Symptomatic: Mexiletine (for myotonia). Physiotherapy
  • Cardiac Monitoring: Essential because arrhythmias are a major cause of death

2.Non-Dystrophic Myotonias: These disorders have myotonia without muscular dystrophy.

A. Myotonia Congenita

Cause: Chloride channel (CLCN1) mutation

Clinical Features: Muscle stiffness improves with repeated movement.

Warm-Up Phenomenon: Repeated contractions improve stiffness.

Muscle Appearance: Well-developed, athletic musculature

B. Paramyotonia Congenita

Cause: Sodium channel mutation

Clinical Features: Myotonia worsens with repeated activity. Triggered by cold exposure

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