Muscle fasciculations are visible, fine and fast contractions of fine muscle fibers that occur spontaneously and intermittently. Injury to central or peripheral nerves can result in muscle fasciculations. The pathophysiology is different for different sites of the injury. It is thought that most fasciculations have a location away from (distal origin) the motor nerve in normal animals as well as patients with motor neuron diseases.
Fasciculations are known to be associated with hypersensitivity of denervated muscles and they are observed in some diseases such as amyotrophic lateral sclerosis in people. In horses the fine tremor of the face, muzzle, or lips is best associated with West Nile Virus infection.
Other triggers of fasciculations are progressive spinal muscular atrophy, neuromuscular junction disorders, electrolyte disorders, systemic diseases and some medications. Even healthy animals can have fasciculations-these are usually located in the forearm or the eye-lids. Fasciculations were thought to be a prelude to the onset of a progressive or lethal disease that involved the lower motor neuron. However, benign fascicular syndrome has been described in young healthcare professionals.
In normal individuals physical exercise, stress, fatigue, and caffeine abuse can cause or aggravate twitchy muscles. A diagnosis of benign fascicular syndrome can be diagnosed after five years. A benign diagnosis is made when there isn’t a progression to motor neuron disease and that takes 5 years.
Muscle tremors are abnormal and are motor disorders, although fasciculations are not classified as motor disorders. Some genetic diseases of the cerebellum associated with motor disorders are accompanied by fasciculations. A specific type of fasciculation with cramping occurs in peripheral axonal excitability. This occurs when adjacent neurons (to the damaged neuron) begin to re innervate partially denervated muscles. Sometimes in genetic disease, fasciculations affect the tongue as well as the trunk and limb muscles. The lower motor neurons are involved in these cases.
Rare cases of fasciculations occur when muscles fail to relax. Failure of muscle fiber relaxation can occur with neoplasia, immune-mediated disease, heredity, and degenerative disease. In these cases, the pathophysiology is hyperexcitability of the peripheral nerves and consequent continuous muscle fiber activity. Continuous activity occurs when potassium channels are dysfunctional. Potassium channels can be damaged when there is an antibody response against proteins in this structure. Cramps, stiffness, delayed muscle relaxation and excessive sweating can be seen clinically. Motor neuron disease is associated with fasciculations in people. These diseases are detected using EMG’s, electromyography, this machine measures the muscle response or electrical activity in response to nerve stimulation of a specific muscle. During the test, one or more small needles (also called electrodes) are inserted through the skin into the muscle.
Systemic disease, drugs and heavy metal toxicity (like lead) can also induce fasciculations. Low blood levels of phosphate (hypophosphatemia) and calcium disorders (secondary to hyperparathyroidism) sometimes result in fasciculations. Calcium disorders occur with some renal diseases. Neostigmine, a drug, can increase fasciculations in cats by increasing the concentration of acetylcholine (a direct effect) in the concentration of acetylcholine at the neuromuscular junction. Some anesthetic agents work using the same pathway at the neuromuscular junction and have the same results. Mercury toxicity should be considered in peripheral neuropathies of unknown origin that are also accompanied by tremor, ataxia, and depression.
There is no specific treatment for the muscle contractions because fasciculations are a symptom of an underlying condition. It is necessary to identify the origin of the underlying condition and that condition is the therapeutic target.