ISSN 2398-2969      

Myotonic myopathy (myotonia)

icanis

Introduction

  • Seen in Cavalier King Charles Spaniel Cavalier King Charles Spaniel , Chow Chow Chow Chow , Rhodesian Ridgeback Rhodesian Ridgeback , Australian Cattle dog Australian Cattle Dog , Jack Russell Terrier Jack Russell Terrier (American) , Miniature Schnauzer Schnauzer: miniature , Staffordshire Bull Terrier Staffordshire Bull Terrier , etc.
  • Cause: failure of normal myocyte chloride conductance.
  • Signs: presented as puppies with unusual gait (bunny-hop). Muscle stiffness without cramping and muscle dimpling after palpation/percussion.
  • Diagnosis: (myotonic dimple in muscle on percussion); electromyography shows characteristic myotonic discharges.
  • Treatment: procainamide, quinidine, phenytoin and mexillitine.
  • Prognosis: good with use of drugs although most likely to improve as opposed to normalize the condition.

Pathogenesis

Etiology

  • Diminished chloride conductance across the muscle membrane which compromises the resting membrane potential.
  • As the chloride channel contributes approximately two-thirds of the resting membrane conductance, a decrease in chloride channel conductance significantly compromises the resting membrane potential.
  • During normal muscle activation-depolarization, potassium ions accumulate in the sarcoplasmic reticulum and increase the probability of further depolarizations which is 'buffered' in the presence of normal high chloride conductance.
  • Loss of conductance tips the balance towards potassium-induced depolarization burst that manifest as myotonia.
  • Genetic defects in the skeletal muscle ion channels in some cases.
  • Hyperadrenocorticism Hyperadrenocorticism (Cushing's disease) may lead to acquired myotonic myopathy (pseudomyotonia).
  • Acquired myotonic myopathy has been reported secondary to exposure to herbicides containing 2,4-dichlorophenoxyacetic acid (2,4-D) Phenoxy herbicide poisoning or 2-methoxy-3,6-dichlorobenzoic acid (dicamba) and as an idiopathic condition associated with a myotonic dystrophy-like disorder in a Rhodesian Ridgeback dog.

Specific

  • Adrenal neoplasia.
  • Pituitary neoplasia Pituitary gland: neoplasia.
  • Exogenous administration of corticosteroids.
  • Exposure to herbicides containing 2,4-dichlorophenoxyacetic acid (2,4-D) or 2-methoxy-3,6-dichlorobenzoic acid (dicamba).

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Nagata N, Yuki M (2015) Long-term outcome of myotonia associated with hyperadrenocorticism in 2 dogs. Can Vet J 56 (9), 931-933 PubMed.
  • Lobetti R G (2009) Myotonia congenita in a Jack Russell terrier. J S Afr Vet Assoc 80 (2), 106-107 PubMed.
  • Bhalerao D P, Rajpurohit Y, Vite C H et al (2002) Detection of a genetic mutation for myotonia congenital among Miniature Schnauzers and identification of a common carrier ancestor. Am J Vet Res 63 (10), 1443-1447 PubMed.
  • Vite C H (2002) Myotonia and disorders of altered muscle cell membrane excitability. Vet Clin North Am Small Anim Pract 32 (1), 169-187 PubMed.
  • Rhodes T H, Vite C H, Giger U et al (1999) A missense mutation in canine C1C-1 causes recessive myotonia congenita in the dog. FEBS Lett. 456 (1), 54-58 PubMed.
  • Vite C H, Melniczek J, Patterson D, Giger U (1999) Congenital myotonic myopathy in the miniature schnauzer: an autosomal recessive trait. J Hered 90 (5), 578-580 PubMed.

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