Hyperadrenocorticism (HAC) or Cushing’s disease is a common cause of muscle weakness. Biopsies have shown that type II muscle fiber atrophy is common and clinical signs have included weakness in all four limbs, along with muscle atrophy. A less common form of HAC myopathy is a severe muscle stiffness (SMS) that results in a stiff or stilted gait. A recent study evaluated the signalment, presentation and treatment options along with long-term outcome in a collection of 37 dogs diagnosed with HAC.
What is HAC induced Severe Muscle Stiffness?
As the name suggests, the clinical manifestation of the unusual form of HAC myopathy includes stiffness of pelvic or all four limbs. The etiology is unknown but electrodiagnostic evaluation suggests a myotonia. Myotonia is defined as a sustained contraction after percussion (hammer reflexes) or voluntary contraction. The muscle will occasionally ripple visibly, but usually a prolonged limb stiffness can be seen. Dogs with HAC induced SMS have a ridged, stiff-legged gait when moving and don’t demonstrate appropriate joint flexion. According to Golinelli et al (DOI: 10.11.11/jvim.16620) over 60% of dogs had only the pelvic limbs affected at the time of diagnosis, the remainder had all four limbs affected. An elevated CK was noted in 75% of the dogs, suggesting muscle membrane breakdown or injury.
It was interesting to note that the diagnosis of SMS was more common in small breed dogs, weighing less than 15 kg. The time of SMS diagnosis varied in reference to the HAC diagnosis. Almost 62% were diagnosed 1 month to 3 years after being diagnosed with HAC, 8% were diagnosed with HAC and SMS at around the same time and 30% were diagnosed with SMS 1 month to 1 year BEFORE being diagnosed with HAC. There are many reasons for these findings including owner or veterinarian awareness of SMS, lack of ability to test for or diagnose HAC, or perhaps a pathologic change/trigger required to convert from HAC with weakness to SMS.
How is it treated?
In short, poorly. In the report cited above, no single treatment resulted in effective resolution of the clinical SMS. Therapeutic intervention included benzodiazepine drugs (19% of cases), physiotherapy (16% of cases), acupuncture (5% of cases), and one or two dogs received mexiletine, NSAIDs, dantrolene, botulin toxin, L-carnitine, methocarbamol, gabapentin, and cannabinoids. Mild improvement was reported with some treatments. Survival of dogs with HAC induced SMS was unchanged compared to survival of dogs with HAC only.
Key points:
Be aware that SMS can occur with HAC
The reason a dog will have SMS instead of muscle weakness is unknown
No specific treatment resolved signs in all dogs, however there are several reasonable treatment choices listed above to try.
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