Creatine kinase (CK) is one of the key enzymes of the energy cell system. CK catalyses the conversion of creatine and
consumes adenosine triphosphate (ATP) to create phosphocreatine and adenosine diphosphate (ADP) and vice versa.
Phosphocreatine serves as an energy reservoir, especially in active tissues, CK serum level is considered as the most sensitive
laboratory parameter for muscular diseases. The highest CK elevations are seen in rhabdomyolysis from any cause.
In Duchenne and Becker muscular dystrophies, limb girdle muscular dystrophies and Miyoshi myopathy, the serum
CK is markedly elevated in early stages but declines later on as the muscle mass is reduced. In polymyositis and dermatomyositis
the serum CK typically falls within weeks of starting steroids but weakness may take much longer to improve.
It is important to remember that CK elevation occurs in anterior horn cell disorders such as amyotrophic lateral sclerosis,
spinal muscular atrophy III and IV, bulbospinal muscular atrophy, and also in some neuropathies. Controversely,
some muscle diseases do not cause an increase in CK, e.g. mitochondrial myopathies and some congenital myopathies.
CK may be normal in two adult muscular dystrophies, namely facioscapulohumoral muscular dystrophy and myotonic
dystrophy. There are some causes of hyperCKemia unrelated to a recognized neuromuscular disease – strenuous
muscle exercise, epileptic seizures, muscle injury, repeated intramuscular injection, surgery, medications (e.g. statins),
toxins, viral infections, hypothyroidism, hypokalemia, malignant hyperthermia. Idiopathic asymptomatic hyperCKemia
is a term used to describe the finding of an elevated serum CK concentration in the absence of symptoms that can be
attributed to an underlying neuromuscular disease. All cases of asymptomatic hyperCKemia with no apparent medical
explanation should undergo the assessment of acid α-glucosidase activity on dried blood spot. According to the results of
some studies the selective screening in asymptomatic hyperCKemia allowed the identification of several Pompe patients
in a pre-clinical stage. Muscle biopsy may be indicated in asymptomatic hyperCKemia if one or more of the following are
present: the CK is ≥ 3x normal, electromyogram is myopathic or patients are < 25 years of age. In addition, women
with CK 3 times over the normal may be offered DNA testing because of the possibility of carrying a dystrophin mutation.