Does a markedly elevated serum creatine kinase activity in a cat always mean muscle disease?

Contributed by Dr. Diane Shelton
University of California San Diego
La Jolla, CA

Clinical History

A 6-year old FS domestic short-haired cat presented with a 1 week history of poor appetite and vomiting. No other past medical problems were reported.

Physical and Neurological Examination

No abnormalities were found on physical or neurological examination.

Diagnostic Tests

The serum creatine kinase (CK) activity was markedly elevated at 30,000 IU/L (reference <895). Serological testing for Toxoplasma gondii infection was negative. A muscle biopsy was collected and was normal.

Clinical Course and Outcome

With only supportive care the cat improved over the following following week and was clinically normal. The serum CK activity returned to the reference range.

What is the significance of the markedly elevated CK activity?

In veterinary medicine, CK activities are most often used in the diagnosis of muscle diseases such as inflammatory myopathy (myositis), muscular dystrophy, necrotizing myopathies and weakness associated with electrolyte abnormalities such as in hypokalemic myopathy. Creatine kinase catalyzes the reaction that makes adenosine triphosphate (ATP) available for muscle contraction through the phosphorylation of adenosine diphosphate (ADP) from creatine phosphate (see figure below). CK also catalyzes the transfer of a high-energy phosphate bond from ATP to creatine in resting muscle.

However, during starvation, the body breaks down skeletal muscle to provide amino acids for protein synthesis and to supply glucose for tissues that have an obligatory glucose requirement. This process is accelerated by the "physiologic stress of illness". A paper was published in 1997 by Fascetti et al (J Vet Intern Med 11;9-13) describing a study to identify serum CK activities in anorectic cats and to evaluate the usefulness of CK activities in the nutritional assessment and monitoring of these patients. The CK activities in anorectic cats ranged from mildly elevated to >150,000 IU/L (reference 10-100 IU/L). Conclusion from this study were that 1) serum CK activities may serve as a marker in the assessment and monitoring of nutritional status in cats and, 2) that serum CK activity may be an unreliable indicator of myositis or other degenerative myopathies in anorectic cats. Based on this study and the cat of this report, a diagnosis of muscle disease should not be considered based on serum CK alone until nutritional status has been normalized in cats.