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What are the types and causes of muscle cramps?
Skeletal muscle cramps can be categorized into four major types. These include "true" cramps, tetany, contractures, and dystonic cramps. Cramps are categorized according to their different causes and the muscle groups they affect.
True cramps involve part or all of a single muscle or a group of muscles that
generally act together, such as the muscles that flex several adjacent fingers.
Most authorities agree that true cramps are caused by hyperexcitability of the
nerves that stimulate the muscles. They are overwhelmingly the most common type
of skeletal muscle cramps. True cramps can occur in a variety of circumstances
Injury: Persistent muscle spasm may occur as a protective
mechanism following an injury, such as a broken bone. In this
instance, the spasm tends to minimize movement and stabilize the area
of injury. Injury of the muscle alone may cause the muscle to spasm.
Vigorous activity: True cramps are commonly associated
with the vigorous use of muscles and muscle fatigue (in sports or
with unaccustomed activities). Such cramps may come during the
activity or later, sometimes many hours later. Likewise, muscle
fatigue from sitting or lying for an extended period in an awkward
position or any repetitive use can cause cramps. Older adults are
at risk for cramps when performing vigorous or strenuous physical activities.
Rest cramps: Cramps at rest are very common, especially in older adults, but may be experienced at any age, including childhood. Rest cramps often occur during the night. While not life threatening, night cramps (commonly known as nocturnal cramps) can be painful, disruptive of sleep, and they can recur frequently (that is, many times a night, and/or many nights each week). The actual cause of night cramps is unknown. Sometimes, such cramps are initiated by making a movement that shortens the muscle. An example is pointing the toe down while lying in bed, which shortens the calf muscle, a common site of
Dehydration: Sports and other vigorous activities can cause excessive fluid loss from
perspiration. This kind of dehydration increases the likelihood of true
cramps. These cramps are more likely to occur in warm weather and can be an early sign of
heat stroke. Chronic volume depletion of body fluids from diuretics (medicine that
promote urination) and poor fluid intake may act
similarly to predispose to cramps, especially in older people. Sodium depletion has
also been associated with cramps. Loss of sodium, the most abundant
chemical constituent of body fluids outside the cell, is usually a
function of dehydration.
Body fluid shifts: True cramps also may be experienced in
other conditions that feature an unusual distribution of body
fluids. An example is cirrhosis of the liver, which leads to the accumulation of fluid in the
abdominal cavity (ascites). Similarly, cramps are a relatively
frequent complication of the rapid body fluid changes that occur
during dialysis for kidney failure.
Low blood calcium, magnesium: Low blood levels of either
calcium or magnesium directly increase the excitability of both the
nerve endings and the muscles they stimulate. This may be a
predisposing factor for the spontaneous true cramps experienced by
many older adults, as well as for those that are commonly noted
during pregnancy. Low levels of calcium and magnesium are common in
pregnant women unless these minerals are supplemented in the
diet. Cramps are seen in any circumstance that decreases the
availability of calcium or magnesium in body fluids, such as taking
diuretics, hyperventilation (overbreathing), excessive vomiting,
and/or magnesium in the diet, inadequate calcium absorption due to
vitamin D deficiency, poor function of the parathyroid glands (tiny
glands in the neck that regulate calcium balance), and
Low potassium: Low potassium blood levels occasionally cause muscle cramps, although it is more common for low potassium to be associated with muscle weakness.
In tetany, all of the nerve cells in the body are activated, which
then stimulate the muscles. This reaction causes spasms or cramps
throughout the body. The name tetany is derived from the effect of
the tetanus toxin on the nerves. However, the name is now commonly
applied to muscle cramping from other conditions, such as low blood
levels of calcium and magnesium. Low calcium and low magnesium,
which increase the activity of nerve tissue nonspecifically, also
can produce tetanic cramps. Often, such cramps are accompanied by
evidence of hyperactivity of other nerve functions in addition to
muscle stimulation. For instance, low blood calcium not only causes spasm of the muscles of the hands and wrists, but it can also cause a sensation of numbness and tingling around the mouth and other areas.
Sometimes, tetanic cramps are indistinguishable from true cramps. The accompanying changes of sensation or other nerve functions that occurs with tetany may not be apparent because the cramp pain is masking or distracting from it.
Contractures result when the muscles are unable to relax for an even more extended period than a common muscle cramp. The
constant spasms are caused by a depletion of adenosine triphosphate
(ATP), an energy chemical within the cell. This prevents muscle
fiber relaxation. The nerves are inactive in this form of muscle
Contractures can result from inherited (for example, McArdle's disease, which is a defect of
the breakdown of glycogen to sugar within the muscle cell) or
from acquired conditions (for example, hyperthyroid myopathy, which is a muscle disease that is
associated with an overactive thyroid). Cramps of this category are
The final category is dystonic cramps, in which muscles that are not
needed for the intended movement are stimulated to contract. Muscles
that are affected by this type of cramping include those that
ordinarily work in the opposite direction of the intended movement,
and/or others that exaggerate the movement. Some dystonic cramps
usually affect small groups of muscles (eyelids, jaws, neck, larynx,
etc.). The hands and arms may be affected during the performance of
repetitive activities such as those associated with handwriting
(writer's cramp), typing, playing certain musical instruments, and
many others. Each of these repetitive activities may also
produce true cramps from muscle fatigue. Dystonic cramps are not
as common as true cramps.