Comparison of ultrasonography and pharmacokinetic analysis of creatine kinase release for quantitative assessment of postinjection muscle damage in sheep

Objectives-To investigate and validate noninvasive methods for the quantitative evaluation of postinjection muscle damage. Animals-5 adult sheep. Procedures-Muscle lesions were induced twice in the lumbar region of the longissimus dorsi muscles (2 sides) by IM administration of a 20% formulation of long-acting oxytetracycline (20 mg/kg of body weight). Clinical signs and local cutaneous temperature above the injection site were recorded. Muscle lesions were quantitatively evaluated by ultrasonography and by use of pharmacokinetic analysis of plasma creatine kinase activity, and both were compared with a comprehensive planimetric computer-assisted analysis of the injection sites after euthanasia. Results-Transient cutaneous hypothermia (temperature change, -3.9 +/- 0.62 C) and subsequent persistent hyperthermia (3.1 +/- 1.35 C) were observed after the administrations. Despite coefficient of variation < 10% for precision of ultrasonographic measurement of normal muscle, measurements of the lesions, with coefficient of variation > 60% for precision, were systematically underestimated. Quantitative evaluation of muscle damage by use of pharmacokinetic analysis of creatine kinase (12.1 +/- 4.96 g) was in agreement with results of macroscopic planimetric evaluation (10.8 +/- 3.64 g). Conclusions and Clinical Relevance-Ultrasonography cannot be used for quantitative assessment of postinjection muscle damage. Pharmacokinetic analysis of creatine kinase provides an accurate quantitative evaluation of macroscopic muscle damage after IM administration of drugs.