Clinical History
A 2-year-old spayed female Jack Russell terrier was presented
for evaluation of episodes of muscle twitching and rigidity
starting at one year of age. The dog had suffered two severe
episodes lasting about one hour and several milder episodes.
The attacks appeared to be triggered by excitement and were
characterized by visible rippling movements (myokymia)
Go To Video Clip in all skeletal muscle groups that progressed
to generalized muscle rigidity. She remained alert and did not
vocalize or seem painful during these attacks. Rectal temperature
was usually elevated (as high as 107° F) during the episodes. A detailed family history was
unavailable.
Physical and Neurological Examination
There was rippling in the gluteal muscles and mild
generalized tremor. The
dog was alert with appropriate mentation. With the exception
of the muscle contractions and tremor, her gait was normal
with no evidence of ataxia or weakness. Postural reactions,
spinal reflexes and cranial nerves were normal. There was
no pain, atrophy, or hypertrophy of the muscles. The clinical
signs resolved with diazepam (2.5 mg intravenously) and a
calm environment.
Diagnostic Testing
CBC, serum chemistry
profile, electrolytes, and serum creatine kinase: No significant abnormalities
Electromyography and motor nerve conduction velocity: Normal
with the exception of occasional positive sharp waves in the
biceps femoris muscles and occasional fibrillation potentials
in the middle gluteal muscles. The motor nerve conduction
velocity of the sciatic-tibial nerve was normal (83 m/s) with
a normal M-wave.
Muscle biopsy: No abnormalities identified within either
the biceps femoris or gastrocnemius muscles
Clinical Course
Diazepam (5 mg PO every 6-8 hours as needed) was dispensed
in an attempt to help control the episodes. Despite therapy,
the dog suffered several more attacks. Mexiletine (6 mg/kg
PO every 8 hours) was started. The severity of the episodes
decreased although the dog still had mild episodes with excitement.
Comments
Several Jack Russell terriers with similar clinical
signs have been evaluated at the University of Tennessee.
Dr. Luc Van Ham has reported 12 Jack Russell terriers with
episodes of "myokymia" progressing to muscle stiffness,
hyperthermia and collapse. In this case the term rippling
muscle is preferred over myokymia since the muscles were in
general electrically silent, unlike that of myokymia. Episodes
may be preceded by intense facial rubbing. Membrane stabilizing
agents (procainamide, mexiletine, phenytoin) may be helpful.
A similar syndrome has been reported by Reading et al in a
single Yorkshire terrier. The pathophysiology of this syndrome
is not fully understood.
References
Reading MJ, McKerrell RE. Suspected myokymia in a Yorkshire
Terrier. Vet Rec 1993;132:587-588
Van Ham L. Suspected neuromyotonia in 12 Jack Russell terriers.
Proceedings European Society of Veterinary Neurology 1998,
pgs 26-27.
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