Fasciculations is contraction of one or more motor units (separate motoneuron and group of muscle fibers supplied by it) leading to rapid, visible to eye, contraction of muscle bundles (fascicular twitching or fasciculations). On EMG fasciculations are viewed as wide two-phase or multiphase action potentials. Simultaneous or sequential contraction of set of motor units causes wavelike contraction of muscles, known as myokymia.
Normally, relaxed muscle is not accompanied by bioelectric activity. If fasciculation is the only symptom, which is it is not accompanied by muscle atrophy and changes in reflexes, its clinical significance is small. With more severe motor neuron dysfunction all muscle fibers, which obtain innervation from it, are affected, that leads to atrophy of muscles (denervation atrophy), decrease in reflexes and is accompanied by fibrillation potentials, positive waves, fasciculations and amendments of motor unit potentials.
Main causes of fasciculations
- Motor neuron disease (ALS, progressive spinal amyotrophy, seldom - other diseases).
- Benign fasciculations.
- Syndrome of painful muscular fasciculations.
- Damage or compression of root and peripheral nerve.
- Face myokymia (multiple sclerosis, brain tumor, syringobulbia, seldom - other reasons).
- Neuromyotonia (Isaacs’s syndrome).
- Face hemispasm (some forms).
- Postparalytic contraction of facial muscles.
- Iatrogenic fasciculations.
Motor neuron diseases
Fasciculations are typical for diseases of motor neuron (ALS, progressive spinal amyotrophy). However, only presence of fasciculations without any signs of denervation is not sufficient for diagnosis of motor neuron disease. At amyotrophic lateral sclerosis on EMG widespread dysfunction of anterior horn cells is revealed, including clinically intact muscles, and clinically there are also symptoms of upper motor neuron (pyramidal signs) and progressive course of disease. Characteristic pattern of "asymmetric amyotrophies with hyperreflexia and progressive course" is revealed.
Progressive spinal amyotrophy are caused by degeneration of anterior horn cells, and is manifested only by symptoms of lower motor neuron (neuronopatia), there are no signs of upper motor neuron lesion. Amyotrophies are more symmetrical. Fasciculations are identified, but not always. Disease has more favourable course and prognosis. In diagnosis of motor neuron diseases EMG study is crucial.
Other disorders of motor neuron (tumors of brain stem and spinal cord, syringobulbia, OPTSA, Machado-Joseph disease, late manifestations of polio), sometimes fasciculations may be among other manifestations (usually more or less localized, for example, in trapezius and sternocleidomastoid muscles, perioral muscles, muscles in arms or legs).
Some isolated fasciculations occur in calf muscles or circular muscle of eye (sometimes they last up to several days) and occur in perfectly healthy people. Sometimes benign fasciculations acquire more generalized distribution and may occur within few months and even years. But at the same time reflexes are not changed, there is no violation of sensitivity, there is no decrease in rate of excitation of nerve, and on EMG there are no other abnormalities except fasciculations. Unlike ALS, benign fasciculations are more permanent in location; they are more rhythmic and possibly more frequent. Sometimes this syndrome is called "benign motor neuron disease."
Syndrome of painful muscular fasciculations
Syndrome of painful fasciculations is unsettled term which is intended to refer rare syndrome of fasciculation, cramps, myalgia, and poor exercise tolerance in case of degeneration of distal portions of axons of peripheral nerves (peripheral neuropathy). Sometimes this term is used to refer to previous syndrome, if accompanied by frequent painful cramps.
Damage or compression of root and peripheral nerve
Such damage can cause fasciculations, myokymia or cramps in muscles, which are innervated by root or nerve. These symptoms may persist after surgical treatment of compression radiculopathy.
Facial myokymia is a rare neurological symptom and often is the only finding in neurological status. Facial myokymia has great diagnostic value, as always refers to organic lesion of brain stem. Its onset is usually sudden, and duration varies - from few hours (for example, multiple scleroses) up to several months and even years. Arbitrary psychic activity, reflex automatism, sleep and other exogenous and endogenous factors have little or no influence on course of myokymia. It is manifested itself by small undulation (vermiform) contractions of muscles on one side of face and most often develops against background of multiple sclerosis or brain stem glioma. Less frequent face myokymia is observed in Guillain-Barre syndrome (can be double-sided), syringobulbia, facial nerve neuropathy, ALS and other diseases. EMG reveals spontaneous rhythmic activity in the form of single, double or group bits with relatively stable frequency.
Clinically face myokymia is usually easy to be distinguished from other facial hyperkinesis.
Differential diagnosis of face myokymia is carried out with facial hemispasm, mioritm, jacksonian seizures, benign fasciculations.
Neuromyotonia (Isaacs’s syndrome, syndrome of constant activity of muscle fibers) occurs in both children and adults and is characterized by gradually increasing stiffness, muscle tension and small muscle contractions (myokymia and fasciculations). These symptoms begin to appear in distal limbs, gradually extending proximally. They are preserved during sleep. Pains are rare, although discomfort in muscles is fairly typical. Hands and feet take position of permanent flexion or finger extensions. Body also loses plasticity and natural posture; gait becomes tense (stiff) and constrained.
Causes: syndrome is described as idiopathic (autoimmune) disease (inherited or sporadic) and as well in combination with peripheral neuropathy. In particular, Isaacs’s syndrome is sometimes observed in hereditary motor and sensory neuropathies, with CIDP, toxic neuropathies and neuropathies of unknown origin, in combination with malignancy formations without neuropatia combined with myasthenia.
Fasciculations and myokymia, along with myoclonia, make up core of main clinical manifestations in facial hemispasm. Clinically fasciculations are not always readily apparent, since they are overlapped by more massive muscle contractions.
Postparalytic contraction of facial muscles
The same can be said for postparalytic contractions of facial muscles ("syndrome of facial hemispasm after neuropatia of VII nerve"), which can manifest itself not only by persistant contraction of muscles of varying degrees of severity, but also by myoclonic local hyperkinesis and fasciculations in area of affected branches of facial nerve.
- Iatrogenic fasciculations
- Iatrogenic fasciculations are described in application of penicillin and overdose of anticholinergics.
Fasciculations can sometimes occur in hyperthyroidism, which, combined with muscle atrophy and weakness, may mimic amyotrophic lateral sclerosis.
Rattlesnake, scorpion, spider, "Black Widow" bites, and bites of some biting insects can cause cramps, myalgia, and fasciculations.
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