Doctors use blood tests to measure enzymes released from muscles that are injured or inflamed by disease processes. Creatnine kinase (CK) is an enzyme essential to chemical reactions that allow cells in muscle tissues to function properly in response to nerve impulses. Normal CK levels are between 60 and 400 U/ml. According to Dr. Marc Miller of UpToDate.com, three types of CK are specific to skeletal muscles and the heart, brain and liver, giving doctors clues about the source of elevated enzymes. Aminotransferase (AST), alanine aminotransferase (ALT) and aspartate lactate dehydrogenase (LD) are other muscle enzymes that may be elevated in certain neuromuscular diseases. Neuromuscular refers to conditions affecting nerves and muscles, such as muscular dystrophies, motor-neuron diseases and myopathies.
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The most severe form of muscular dystrophy (MD) strikes mostly male children under the age of 6. Characterised by initial weakness of voluntary muscles, this progressive disease can affect the heart and respiratory muscles, making survival beyond age 30 unlikely. The disease is genetic, carried by asymptomatic women and passed on to sons. There are at least half a dozen less severe forms of MD, some of which have onsets in adolescence or adulthood and are passed down through families with slightly different mechanisms. According to Dr. Miller, enzymes expected to be elevated in MD are CK of the myofibrillar mitochondrial (MM) type, aldolase and lactate dehydrogenase (LDH). Further testing could include electromyography (emg), which studies nerve and muscle function, and muscle biopsy.
Motor-neuron diseases (MND) are a group of disorders that affect nerve cells associated with muscles used during speaking, swallowing, breathing, walking and other movements. Lou Gehrig's disease, or amyotrophic lateral sclerosis (ALS), is the most referred to motor-neuron disease. MND also includes primary lateral sclerosis, progressive muscular atrophy and two types of bulbar palsey. Age of onset is typically in adults between 50 and 70 years old. Although muscle enzymes may be elevated, a neurologist must rule out other neuromuscular conditions and base a diagnosis on history, examination and symptoms. There is no single test for MND.
Polymyositis and Dermatomyositis
Inflammatory myopathies, such as polymyositis and dermatomyositis, are rare. According to Medline Plus, they occur in approximately one in 100,000 people. According to Dr. Sushma Podila, a neurologist at Columbia Presbyterian Medical Center, polymyositis might present with CK as much as 50 times the normal reference. Levels of aldolase, AST, ALT and LDH may also be elevated. According to Medline Plus, polymyositis is more common in females and blacks between the ages of 30 to 50. This neuromuscular disease destroys connective tissue. It is characterised by difficulty swallowing and weakness in voluntary muscles. Dr. Podila found that in dermatomyositis, CK levels can be as high as several 100s to 1,000 U/ml. AST, ALT, LDH and aldolase levels may also be elevated. In addition to symptoms in common with polymyositis, dermatomyositis is characterised by a distinctive rash. It most often occurs in children between the ages of 5 and 15, or in adults age 40 to 60.
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