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Muscle fibrillations are the rapid, irregular, and unsynchronized contractions of muscle fibres. They occur when a muscle fibre loses contact with its innervating axon, resulting in a spontaneous action potential that causes the muscle fibre to contract. Fibrillations can occur in healthy individuals and are not visible under the skin, only detectable through needle electromyography (EMG) and ultrasound. They are, however, a major symptom in acute and severe peripheral nerve disorders and can indicate the severity and magnitude of neural injury.
| Characteristics | Values |
|---|---|
| Definition | Small, local muscular contractions that occur in response to the spontaneous activation of single muscle fibres. |
| Appearance | Small, irregularly oscillating movements of muscle tissue in all directions, while the overall shape of the muscle is preserved. |
| Cause | Muscle fibres lose contact with their innervating axon, producing a spontaneous action potential. |
| Detection | Needle electromyography (EMG) and ultrasound. |
| Types | Rhythmic and irregular fibrillation. |
| Occurrence | Skeletal muscle fibrillations, atrial fibrillation, ventricular fibrillation. |
| Symptoms | Muscle twitching, rapid and irregular contractions of muscle fibres, lack of synchronism between heartbeat and pulse. |
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What You'll Learn
Fibrillation and peripheral nerve disorders
Fibrillations are spontaneous depolarizations of the membrane of individual muscle fibres that result in their contraction. They occur when a muscle fibre loses contact with its innervating axon. Fibrillations are invisible from the outside and can only be detected by electrodiagnostic testing, such as needle electromyography (EMG). EMG is considered the gold standard for fibrillation detection, but it can miss focal areas with fibrillations within a muscle due to sampling error.
Fibrillations are considered a defining characteristic of motor nerve disruption to the skeletal muscle fibre. They are often used clinically to determine the severity and magnitude of neural injury and nerve regeneration status. Fibrillations are also an important sign of axonal loss in peripheral nerve disorders.
In the 1930s, more reliable means of recording fibrillation potentials became available, enabling more intense investigation of this phenomenon. In 1937, Brown unilaterally sectioned an unspecified number of cat sciatic nerves. After 4 to 27 days of denervation, he examined the gastrocnemius muscles both visually and electrically for fibrillations and fibrillation potentials. From this work, Brown concluded that fibrillation potentials are "to be attributed to the activity of single muscle fibres".
In humans, Feinstein reported a large number of nerve injury cases related to the Second World War in 1944. Subsequently, in 1945, Feinstein et al. and Denny-Brown and Pennybacker in 1938 published work that formed the basis for the accepted wisdom regarding the timing of fibrillation potential onset. They determined that fibrillation was "found most active two to four weeks following nerve lesion".
Despite the frequent use of this 1- to 4-week guideline in human clinical care, it is important to note that the evidence for this timing primarily comes from animal studies. As a result, an understanding of these limitations aids in the clinical application of this guideline.
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Fibrillation detection methods
Muscle fibrillations are small, local muscular contractions that occur in response to the spontaneous activation of single muscle fibres, which then contract independently of surrounding fibres. They are not visible through the skin surface and can only be detected by electrodiagnostic testing.
The detection of muscle fibrillations has been an area of interest for researchers since the 1930s, when more reliable means of recording fibrillation potentials became available.
Electromyography (EMG)
Electromyography (EMG) is a technique that involves inserting needle electrodes into the muscle to detect fibrillation potentials. EMG is considered the gold standard for fibrillation detection, but it can miss focal areas with fibrillations within a muscle due to sampling error.
Dynamic Echomyography (DyEM)
Dynamic Echomyography (DyEM) is an ultrasound muscle approach that has been recently designed and developed to explore denervated and reinnervating muscles. Ultrasound visualises fibrillations as small, irregularly oscillating movements of muscle tissue in all directions, while the overall shape of the muscle is preserved. The sensitivity of ultrasound for detecting fibrillations depends on the amount of fibrillations present and the limb temperature.
Atrial Fibrillation Detection
Atrial Fibrillation (AF) is a common heart arrhythmia that can be detected using Electrocardiogram (ECG) signals and RR intervals. Recent advances in computational power and deep learning algorithms have led to new AF detection methods with improved accuracy, robustness, and reliability. These algorithms can be incorporated into wearable devices such as smartwatches and smartphones for AF screening outside of clinical settings.
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Fibrillation in cardiac muscle
Fibrillation is the rapid, irregular, and unsynchronized contraction of muscle fibres. Fibrillations can occur in skeletal muscle, but fibrillation of the heart muscle, or cardiac fibrillation, is particularly significant. There are two major types of cardiac fibrillation: atrial fibrillation (AFib) and ventricular fibrillation (VFib).
Atrial fibrillation is an irregular and uncoordinated contraction of the cardiac muscle of the atria, or upper chambers of the heart. In a typical heart rhythm, electrical signals travel from the atria to the ventricles, causing the ventricles to contract and pump blood. In atrial fibrillation, disorganized electrical impulses originating in the roots of the pulmonary veins overwhelm the normal electrical pulses from the sinus node, leading to irregular conduction of impulses to the ventricles. This results in an irregular heartbeat, or arrhythmia, which can cause heart palpitations, fatigue, trouble breathing, and dizziness. AFib is a progressive condition that can lead to stroke, and is often treated with anticoagulation or conversion to normal sinus rhythm. Risk factors for atrial fibrillation include high blood pressure, coronary artery disease, obesity, and increasing age.
Ventricular fibrillation is an irregular and uncoordinated contraction of the cardiac muscle of the ventricles, or lower chambers of the heart. During ventricular fibrillation, the ventricles contract in a very rapid and uncoordinated manner, causing the heart to abruptly stop pumping blood to the body. Ventricular fibrillation is a medical emergency that can lead to sudden cardiac death within minutes if left untreated. Emergency treatment for ventricular fibrillation includes cardiopulmonary resuscitation (CPR) and shocks to the heart with an automated external defibrillator (AED). Risk factors for ventricular fibrillation include cardiovascular disease, high blood pressure, obesity, and genetic factors.
In summary, fibrillation in cardiac muscle refers to the two main types of cardiac fibrillation, atrial fibrillation and ventricular fibrillation, which involve the rapid and irregular contraction of the upper and lower chambers of the heart, respectively. These conditions can have serious consequences, including arrhythmia, stroke, and sudden cardiac death, and require prompt diagnosis and treatment.
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Fibrillation in skeletal muscle
Skeletal muscle fibrillation refers to small, local muscular contractions that occur in response to the spontaneous activation of single muscle fibres, which then contract independently of surrounding fibres. This is in contrast to muscle fasciculations, which occur when muscle fibres of a motor unit are stimulated and contract synchronously.
Fibrillations are spontaneous depolarizations of the membrane of individual muscle fibres that result in their contraction. They occur when the fibre loses contact with its innervating axon, leading to a spontaneous action potential that causes the contraction. These contractions are not visible under the skin and can only be detected through needle electromyography (EMG) and ultrasound. EMG is considered the gold standard for fibrillation detection, but it can miss focal areas with fibrillations within a muscle, while ultrasound can visualise larger areas of the muscle to compensate for this.
Fibrillations typically repeat in a rhythmic pattern but can also occur irregularly. They are characterised by low-amplitude, short-duration potentials with an initial negative deflection. The exact mechanism producing fibrillations is not known, but they are believed to originate from local segments of abnormal muscle fibre membrane, which are usually near the neuromuscular junction but can occur anywhere along the length of the muscle fibre. These abnormal segments exhibit altered sodium conductance, which may cause the spontaneous depolarizations leading to fibrillation.
Fibrillations are an important sign of axonal loss in peripheral nerve disorders and can be used to determine the severity and magnitude of neural injury. They can also occur in inflammatory myopathies when muscle fibres are split or inflamed, separating part of the fibre from the endplate zone. Fibrillations can even occur in healthy individuals, but in these cases, they do not have pathological significance.
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Fibrillation in healthy individuals
Fibrillation refers to small, local muscular contractions that occur in response to the spontaneous activation of single muscle fibres, which then contract independently of surrounding fibres. This is distinct from muscle fasciculations, which occur when muscle fibres of a motor unit are stimulated and contract synchronously. Fibrillations are spontaneous depolarizations of the membrane of individual muscle fibres that result in their contraction.
Fibrillations are considered a defining characteristic of motor nerve disruption to the skeletal muscle fibre in humans and animal models of spinal cord injury and peripheral denervation. They are often used clinically to determine the severity and magnitude of neural injury and nerve regeneration status. Fibrillations can be detected by electrodiagnostic testing, such as electromyography (EMG), which is considered the gold standard for fibrillation detection. EMG can be combined with Dynamic Echomyography (DyEM), an ultrasound muscle approach, to explore denervated and reinnervating muscles.
While fibrillations are typically associated with muscle disorders and nerve injuries, it is important to note that they may also occur in healthy individuals. In healthy individuals, fibrillations can be observed as part of normal muscle function and are not indicative of any underlying pathology. These fibrillations are typically sporadic and may occur due to various factors, such as muscle fatigue, dehydration, or increased muscle tension.
For example, healthy individuals may experience muscle twitches or fasciculations, which are mild forms of fibrillations. These twitches are usually brief, involuntary contractions of a few muscle fibres and are generally not a cause for concern. They can be caused by stress, anxiety, excessive caffeine consumption, or intense exercise. Additionally, healthy individuals may also experience benign fasciculation syndrome (BFS), which is characterised by frequent muscle twitches or cramps that can occur in any part of the body. While BFS can be bothersome, it is typically harmless and does not indicate any serious underlying condition.
In summary, while fibrillations are typically associated with muscle disorders or nerve injuries, they can also occur in healthy individuals as part of normal muscle function or due to factors such as muscle fatigue, dehydration, or increased muscle tension. However, when fibrillations are persistent, frequent, or associated with other symptoms, it is essential to consult a healthcare professional to rule out any potential underlying causes or disorders.
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