Calcium is one of the most important minerals in your body. As a major constituent of your bones and teeth, calcium lends strength to these structures. According to the Linus Pauling Institute at Oregon State University, nearly 99 percent of your total calcium stores are located in your skeleton. The remaining 1 percent, which is distributed throughout your soft tissues and body fluids, is tightly regulated to permit normal function of your heart, nerves and muscles. When calcium concentrations in your tissues fall, your muscles may contract uncontrollably, a condition known as tetany.
Muscle Contraction
Normal muscle, nerve and heart function are dependent on proper concentrations of electrically charged ions, or electrolytes, on both sides of your cellular membranes. Muscle cells possess pores in their membranes that permit the rapid inward movement of calcium ions whenever the cells are stimulated by an electrical impulse. As calcium moves into your cells, it binds to specialized proteins that stimulate the release of even more calcium from storage compartments inside the cells. The sudden increase in intracellular calcium triggers a cascade of events that leads to muscular contraction. As your muscle relaxes, calcium is pumped back into storage or out of the cell until the next impulse arrives.
Tetany
The back-and-forth movement of calcium ions that generates muscular contraction is a coordinated, near-instantaneous event that can happen many times within 1 second. However, when calcium concentrations in your bodily fluids change, membrane potentials are disrupted and your muscles begin to function abnormally. Tetany is a condition characterized by uncontrollable, prolonged and sometimes painful muscular contractions. It can stem from different causes – infection with Clostridium tetani is a classic cause of tetany – but a drop in your blood calcium level can lead to tetanic muscle contractions.
Hypocalcemia
Hypocalcemia, or low serum calcium levels, can occur with vitamin D deficiency, magnesium deficiency, loss of parathyroid function, kidney disease, severe infections or acute pancreatitis. When it is mild, hypocalcemia is usually asymptomatic. As your calcium level falls to 7 to 8 mg/dL – 8.8 to 10.4 mg/dL is normal – you may develop tetanic contractions when your muscles are lightly tapped or otherwise stimulated. Serum calcium levels below 7 mg/dL can cause spontaneous, unremitting tetanic spasms.
Considerations
Hypocalcemia is only one potential cause of tetanic muscle contractions. Tetanus and severe magnesium deficiency are other well-defined conditions that cause tetany, but all of these are unusual in countries where nutrition is adequate and most people are immunized against C. tetani. Persistent muscle twitches, which are often due to muscular irritability rather than hypocalcemia, can be mistaken for tetanic muscle spasms. If you experience persistent, painful muscle contractions, see your doctor for an evaluation.
References
- Linus Pauling Institute at Oregon State University: Calcium
- “The Merck Manual of Diagnosis and Therapy, 18th Edition: Hypocalcemia”; Mark H. Beers, M.D., Editor-in-Chief; 2006
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