Myotonia

Myotonia

• Myotonia is the term given to sustained muscle contraction.
• Myotonia may be congenital or acquired.

Congenital

• Myotonia congenita is caused by an ion channel defect.
• Certain breeds of dogs are predisposed to myotonia congenita.
  • Chow chow
    • The condition is caused by a defect in the sodium channel.
  • Schnauzer
    • The condition is caused by a defect in the chloride channel.

Clinical Signs

• Stiffness is present from birth.
  • Signs improve with exercise.
• Hypertrophy of the tongue and appendicular muscles.
• Percussion dimple formation
• Biopsy results may:
  • Be normal, or
  • Show type 1 fibre predominance or hypertrophy.

Treatment

• Exercise avoidance.
• Drugs:
  • Metilxantine
  • Procainamide

Acquired

• Acquired myotonia may be caused by:
  • Herbicides
    • For example, (2,4-D) and dicamba.
  • Hyperadrenocorticoidism
    • This may be natural (i.e. increased endogenous steroid production), or iatrogenic (i.e. prolonged corticosteroid administration).