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    55 Creatine kinase MM (CK-MM)
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    55 Creatine kinase MM (CK-MM)

    Creatine kinase MM: skeletal muscle isoenzyme, causes of raised CK, rhabdomyolysis and statins.

    Creatine kinase (CK) is an enzyme that transfers a phosphate group between ATP and creatine. The resulting phosphocreatine is a rapid energy reserve for muscle contraction. CK consists of two subunits, M (muscle) and B (brain), which form three isoenzymes: CK-MM (skeletal muscle, also most of the CK in heart muscle), CK-MB (heart muscle) and CK-BB (brain, smooth muscle). Almost all CK normally found in blood is CK-MM from skeletal muscle. Its level rises whenever muscle cells are damaged.

    MuscleEnzymesRhabdomyolysis
    Share of total CK≈ 95% or more
    Reference rangedepends on sex, ethnicity, muscle mass
    Peak after injury≈ 1–3 days
    Main risk of very high CKacute kidney injury
    Creatine kinase isoenzymes: CK-MM from skeletal muscle forms almost all CK in blood, CK-MB comes mainly from heart muscle, CK-BB from brain and smooth muscle Creatine kinase isoenzymes CK-MM skeletal muscle (also most of heart CK) ≈ 95% or more of blood CK CK-MB heart muscle (small amount in skeletal muscle) CK-BB brain, smooth muscle rarely found in blood Each isoenzyme is a pair of M and B subunits.
    Figure 1. The three isoenzymes of creatine kinase and their main sources.

    Causes of raised CK-MM

    • Exercise and muscle strain, especially unaccustomed or eccentric exercise; intramuscular injections, falls, seizures
    • Rhabdomyolysis: crush injury, prolonged immobilisation, extreme exertion, heat stroke, alcohol and drugs, some infections; CK can rise to many thousands of U/L
    • Muscle diseases: muscular dystrophies (Duchenne, Becker), inflammatory myopathies (polymyositis, dermatomyositis)
    • Medicines: statins (especially with interacting drugs), fibrates, antipsychotics (neuroleptic malignant syndrome)
    • Hypothyroidism, low potassium or phosphate, malignant hyperthermia

    Values are naturally higher in men, people of African ancestry and muscular people, and lower in older and inactive people.

    Seek emergency care

    for severe muscle pain and weakness with dark (cola-coloured) urine, reduced urine output or swelling, especially after injury, heavy exertion or heat exposure. Myoglobin released from damaged muscle can cause acute kidney injury.

    Taking statins?

    Muscle pain or weakness on statins should be discussed with a doctor, who may check CK and review the dose or the medicine. Do not stop the medicine without advice.

    References

    1. Bosch X, Poch E, Grau JM. Rhabdomyolysis and acute kidney injury. N Engl J Med. 2009;361(1):62–72. doi:10.1056/NEJMra0801327
    2. Stroes ES, Thompson PD, Corsini A, et al. Statin-associated muscle symptoms: impact on statin therapy. European Atherosclerosis Society Consensus Panel Statement on Assessment, Aetiology and Management. Eur Heart J. 2015;36(17):1012–1022. doi:10.1093/eurheartj/ehv043

    This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

    Published on 29 April 2024