Q & A: Pseudomyotonia in Dogs with Cushing's Syndrome?

One of my canine patients is a much-loved Collie-cross dog that has Cushing’s syndrome and pseudomyotonia. I really would like to learn more about the connection between these diseases but have not had much luck!

Could you update me on the latest on pseudomyotonia in dogs with Cushing’s disease?

My Response:

Pseudomyotonia is a very rare disorder affecting less than 1% of all dogs with Cushing’s syndrome. This disorder is characterized by continued muscle contraction after cessation of a stimulus or voluntary effort. The relaxation of the involved muscle(s) are delayed and associated with persistent, repetitive electrical activity.

Muscle changes associated with pseudomyotonia include weakness, stiff and stilted gait, distal muscle wasting, and sometimes muscle enlargement. Muscle spasms may be elicited when a muscle belly is tapped gently with a percussion hammer.


There are two forms of myotonia: acquired and congenital subtypes. Congenital myotonia is likely inherited in most breeds, and clinical signs appear at a young age. I personally have not seen such a case, since these dogs do not have any underlying endocrine disease.

Acquired myotonia is secondary to some underlying disease, usually hyperadrenocorticism (Cushing’s syndrome), and that’s the form of the disease that I’ve seen. That said, it is an extremely rare clinical feature of Cushing’s syndrome, and the cause is not at all clear.

Diagnosis of pseudomyotonia is based on characteristic clinical signs, characteristic electromyographic (EMC) findings, and muscle biopsy. Myotonic, bizarre, high-frequency discharges are noted on the EMG, Histologic findings in the musculature are characterized by a noninflammatory degenerative myopathy.

Most of the Cushing’s dogs that develop pseudomyotonia also have the more common clinical features seen in dogs with hyperadrenocorticism including polyuria, polydipsia, pot belly, hepatomegaly, and hair loss. In general, these dogs with pseudomyotonia have severe and advanced Cushing's syndrome.

With spontaneous (endogenous) hyperadrenocorticism, myotonic signs generally partially improve with effective mitotane or trilostane therapy. However, in my experience, the signs of myotonia never completely resolve. However, others have suggested that this disease is not a simple consequence of Cushing’s syndrome but is a full-fledged disease in itself (see paper by Siliart et al in reference list).

I’ve listed some references that I could find below. When available, I’ve put a link to the abstract of the article so that you can get additional information.

References:

1. Vite CH: Myotonia and disorders of altered muscle cell membrane excitability. Vet Clin North Am Small Anim Pract 2002 Vol 32 (1) pp. 169-187.
   
2. Blot S: Disorders of the skeletal muscles. Textbook of Veterinary Internal Medicine, Diseases of the Dog and Cat, 5th ed. Philadelphia, WB Saunders 2000 Vol 1 pp. 684-690.
   
3. Braund KG, Dillon AR, Mikeal RL: Subclinical myopathy associated with hyperadrenocorticism in the dog. Vet Pathol 1980 Vol 17 (2) pp. 134-148.
   
4. Duncan ID, Griffiths IR, Nash AS: Myotonia in canine Cushing's disease. Vet Rec 1977 Vol 100 pp. 30-31.
   
5. Kortz G: Canine myotonia. Semin Vet Med Surg (Small Anim) 1989 Vol 4 (2) pp. 141-145.
   
6. Swinney GR, Foster SF, Church DB, Malik R: Myotonia associated with hyperadrenocorticism in two dogs. Aust Vet J 1998 Vol 76 (11) pp. 722-724.
   
7. Siliart B, Marouze C, Martin L, Gayet C: Pseudomyotonia Associated with Hyperadrenocorticism in the French Poodle: 151 clinical cases (1993-2000). 12th EVCIM-CA/ESVIM Congress 2002. (see abstract below)

Pseudomyotonia Associated With Hyperadrenocorticism in the French Poodle: 151 Clinical Cases (1993-2000)
Objectives: To understand why pseudomyotonia affects only 1% of dogs with hyperadrenocorticism (HAC); why the breed affected is usually the French poodle; and why damage is generally limited to hind leg extensors.
Materials: 151 French poodles with pseudomyotonia referred on suspicion of Cushing's syndrome and 498 dogs of varied breeds affected by typical Cushing's syndrome. (HAC was confirmed based on stimulation by ACTH, when post-ACTH cortisol value was higher than 450 nmol/l after 90 minutes).
Results: Most myotonic French poodles were toy (75%), apricot coat (69%), female (66.2%), and old (11 ± 5 years) poodles. A genealogical study of the pedigrees suggested recessive autosomal transmission. Myotonia developed before typical signs of HAC appeared (6 months to 4 years before) and it developed first on the hind legs (hyperextension). The typical signs (cutaneous signs, polyuria-polydipsia, abdominal distension, and obesity) were less intense than in dogs with simple HAC, but cardiac disorders were more frequent. Biological signs: 13% of the dogs did not exhibit HAC and mean AKLP and cortisol values were lower than in simple HAC. Treatment of HAC had no effect on pseudomyotonia, even though cortisolemia decreased.
Conclusion: This disease is not a simple consequence of HAC, but a full-fledged disease and it is strongly suspected to be of genetic origin.