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==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 [[Oral Cavity - Tongue - Anatomy & Physiology|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).