Muscle Spasms and Cramps Health Article

Definition

Muscle spasms and cramps are spontaneous, often painful muscle contractions.

Description

The rapid, uncontrolled muscle contraction, or spasm, happens unexpectedly, with either no stimulation or some trivially small one. The muscle contraction and pain last for several minutes and then slowly ease. Cramps may affect any muscle but are most common in the calves, feet, and hands. While painful, they are harmless and, in most cases, not related to any underlying disorder. Nonetheless, cramps and spasms can be manifestations of many neurological or muscular diseases.

The terms cramp and spasm can be somewhat vague, and they are sometimes used to include types of abnormal muscle activity other than sudden painful contraction. These include stiffness at rest, slow muscle relaxation, and spontaneous contractions of a muscle at rest (fasciculation). Fasciculation is a type of painless muscle spasm, marked by rapid, uncoordinated contraction of many small muscle fibers. A critical part of diagnosis is distinguishing these different meanings and allowing the patient to describe the problem as precisely as possible.

Demographics

The exact incidence of muscle cramps and spasms is not known. They are more likely to occur in older children and teenagers who are participating in organized, competitive sports and strenuous aerobic activities.

Causes

Normal voluntary muscle contraction begins when electrical signals are sent from the brain through the spinal cord along nerve cells called motor neurons. These include both the upper motor neurons within the brain and the lower motor neurons within the spinal cord and leading out to the muscle. At the muscle, chemicals released by the motor neuron stimulate the internal release of calcium ions from stores within the muscle cell. These calcium ions then interact with muscle proteins within the cell, causing the proteins (actin and myosin) to slide past one another. This motion pulls their fixed ends closer, thereby shortening the cell and, ultimately, the muscle itself. Recapture of calcium and unlinking of actin and myosin allow the muscle fiber to relax.

Abnormal contraction may be caused by unusual activity at any stage in this process. Certain mechanisms within the brain and the rest of the central nervous system help regulate contraction. Interruption of these mechanisms can cause spasm. Motor neurons that are overly sensitive may fire below their normal thresholds. The muscle membrane itself may be overly sensitive, causing contraction without stimulation. Calcium ions may not be recaptured quickly enough, causing prolonged contraction.

Structural disorders such as flat feet, hyperextended knees (genu recurvatum), and hypermobility syndrome (joints that can move beyond the normal range of motion) may predispose a person to developing leg cramps. Prolonged sitting, inappropriate leg positioning during sedentary activity, or standing on concrete flooring for prolonged periods may be associated with an increased incidence of leg cramps.

Interruption of brain mechanisms and overly sensitive motor neurons may result from damage to the nerve pathways. Possible causes include stroke, multiple sclerosis, cerebral palsy, neurodegenerative diseases, trauma, spinal cord injury, and nervous system poisons such as strychnine, tetanus, and certain insecticides. Nerve damage may lead to a prolonged or permanent muscle shortening called contracture.

Changes in muscle responsiveness may be due to or associated with the following:

• Prolonged exercise: Curiously, relaxation of a muscle actually requires energy to be expended. The energy is used to recapture calcium and to unlink actin and myosin. Normally, sensations of pain and fatigue signal that it is time to rest. Ignoring or overriding those warning signals can lead to such severe energy depletion that the muscle cannot be relaxed, causing a cramp. The familiar advice about not swimming after a heavy meal, when blood flow is directed away from the muscles, is intended to avoid this type of cramp.
• Exercising or participating in activities in high or humid temperatures: Copious sweating during prolonged exercise can lead to heat cramps, a condition associated with brief, painful cramps, especially in the legs, sweating, and mild fever, usually less than 102°F. Heat cramps are more likely to occur when the child has not taken in enough fluids before, during, and after the activity. Exercising in high temperatures without adequate fluid intake may increase the risk of dehydration.
• Dehydration and salt depletion: This condition may be brought on by repeated bouts of vomiting or diarrhea or by copious sweating during prolonged exercise. Loss of fluids, salts, and minerals—especially sodium, potassium, magnesium, and calcium—can disrupt ion balances in both muscle and nerves. This imbalance can prevent the muscles and nerves from responding and recovering normally and can lead to cramping.
• Metabolic disorders that affect the energy supply in muscle: These are inherited diseases in which particular muscle enzymes are deficient. They include deficiencies of myophosphorylase (McArdle's disease), phosphorylase b kinase, phosphofructokinase, phosphoglycerate kinase, and lactate dehydrogenase.
• Myotonia: Myotonias include myotonic dystrophy, myotonia congenita, paramyotonia congenita, and neuromyotonia. These conditions cause stiffness due to delayed relaxation of the muscle but do not cause the spontaneous contraction usually associated with cramps. However, many patients with myotonia do experience cramping from exercise. Symptoms of myotonia are often worse in cold temperatures.

Fasciculation may be due to fatigue, cold, medications, metabolic disorders, nerve damage, or neurodegenerative disease, including amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's disease). Most people experience brief, mild fasciculation from time to time, usually in the calves.