Clonus is a set of involuntary and
rhythm
Rhythm (from Greek , ''rhythmos'', "any regular recurring motion, symmetry") generally means a " movement marked by the regulated succession of strong and weak elements, or of opposite or different conditions". This general meaning of regular recu ...
ic muscular
contractions and relaxations. Clonus is a sign of certain
neurological
Neurology (from el, νεῦρον (neûron), "string, nerve" and the suffix -logia, "study of") is the branch of medicine dealing with the diagnosis and treatment of all categories of conditions and disease involving the brain, the spinal ...
conditions, particularly associated with
upper motor neuron lesion
A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals.
Types
There is no designated classifi ...
s involving descending motor pathways, and in many cases is accompanied by
spasticity (another form of hyperexcitability).
Unlike small spontaneous twitches known as
fasciculation
A fasciculation, or muscle twitch, is a spontaneous, involuntary muscle contraction and relaxation, involving fine muscle fibers. They are common, with as many as 70% of people experiencing them. They can be benign, or associated with more serio ...
s (usually caused by
lower motor neuron pathology), clonus causes large motions that are usually initiated by a
reflex
In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus.
Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...
. Studies have shown clonus beat
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
to range from three to eight
Hz on average, and may last a few seconds to several minutes depending on the patient’s condition.
Signs
Clonus is most commonly found at the ankle, specifically with a dorsiflexion/plantarflexion movement (up and down).
Some case studies have also reported clonus in the finger, toe, and
laterally in the ankle (as opposed to the typical up and down motion).
* Ankle (medial
gastrocnemius)
*
Patella
The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...
(
knee cap)
*
Triceps surae
*
Biceps brachii
The biceps or biceps brachii ( la, musculus biceps brachii, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join t ...
Cause
Clonus is typically seen in people with
cerebral palsy
Cerebral palsy (CP) is a group of movement disorders that appear in early childhood. Signs and symptoms vary among people and over time, but include poor coordination, stiff muscles, weak muscles, and tremors. There may be problems with sensa ...
,
stroke
A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...
,
multiple sclerosis
Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...
,
spinal cord damage and
hepatic encephalopathy
Hepatic encephalopathy (HE) is an altered level of consciousness as a result of liver failure. Its onset may be gradual or sudden. Other symptoms may include movement problems, changes in mood, or changes in personality. In the advanced stages ...
.
It can occur in
epilepsy
Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrical ...
as part of a
generalized tonic–clonic seizure, and in pregnant women presenting with severe
pre-eclampsia
Pre-eclampsia is a disorder of pregnancy characterized by the onset of high blood pressure and often a significant amount of protein in the urine. When it arises, the condition begins after 20 weeks of pregnancy. In severe cases of the disease ...
and
eclampsia.
It can also be an
adverse drug reaction, such as after ingestion of potent
serotonergic drugs, where clonus strongly predicts imminent serotonin toxicity (
serotonin syndrome).
Mechanism
Hyperactive stretch reflexes
The self re-excitation of hyperactive stretch reflexes theory involves a repetitive contract-relax cycle in the affected muscle, which creates oscillatory movements in the affected limb.
In order for self re-excitation to exist, both an increase in
motor neuron
A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...
excitability and
nerve
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.
A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
signal delay are required.
Increased
motor neuron
A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...
excitability is likely accomplished by alterations to the net inhibition of neurons occurring as a result of injury to the
central nervous system (CNS) (
stroke
A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...
/
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...
injury).
This lack of inhibition biases neurons to a net excitatory state, therefore increasing total signal conduction.
Signaling delay is present due to an increased nerve conduction time.
Long delays are primarily due to long reflex pathways, which are common in
distal
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
joints and
muscles.
This may therefore explain why clonus is typically found in
distal
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
structures like the ankle.
Frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
of clonus beats have been found to be directly proportional to the length of the reflex pathway it is found in.
Central oscillator
Clonus, with respect to the presence of a central oscillator, functions on the theory that when the central oscillator is turned on by a
peripheral
A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer. The term ''peripheral device'' refers to all hardware components that are attached to a computer and are controlled by the ...
event, it will continue to rhythmically excite
motor neurons, therefore creating clonus.
Although the two proposed mechanisms are very different in
heoryand are still debated, some studies now propose the potential of both mechanisms co-existing to create clonus.
It is thought that the
stretch reflex pathway may be stimulated first, and through its events, cause a decreased
synaptic current
threshold
Threshold may refer to:
Architecture
* Threshold (door), the sill of a door
Media
* ''Threshold'' (1981 film)
* ''Threshold'' (TV series), an American science fiction drama series produced during 2005-2006
* "Threshold" (''Stargate SG-1''), ...
.
This decreased synaptic current threshold would enhance
motor neuron
A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...
excitability as
nerve impulse
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, c ...
s would be more readily conducted, and thus turn on this central oscillator.
This theory is still being investigated.
Clonus and spasticity
Clonus tends to co-exist with
spasticity in many cases of
stroke
A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...
and
spinal cord injury
A spinal cord injury (SCI) is damage to the spinal cord that causes temporary or permanent changes in its function. Symptoms may include loss of muscle function, sensation, or autonomic function in the parts of the body served by the spinal cor ...
likely due to their common physiological origins.
Some consider clonus as simply an extended outcome of spasticity.
Although closely linked, clonus is not seen in all patients with spasticity.
Clonus tends to not be present with spasticity in patients with significantly increased
muscle tone
In physiology, medicine, and anatomy, muscle tone (residual muscle tension or tonus) is the continuous and passive partial muscle contraction, contraction of the muscles, or the muscle's resistance to passive stretch during resting state.O’Sull ...
, as the muscles are constantly active and therefore not engaging in the characteristic on/off cycle of clonus.
Clonus results due to an increased
motor neuron
A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...
excitation (decreased
action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
threshold
Threshold may refer to:
Architecture
* Threshold (door), the sill of a door
Media
* ''Threshold'' (1981 film)
* ''Threshold'' (TV series), an American science fiction drama series produced during 2005-2006
* "Threshold" (''Stargate SG-1''), ...
) and is common in muscles with long conduction delays, such as the long
reflex
In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus.
Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...
tracts found in distal muscle groups.
Clonus is commonly seen in the ankle but may exist in other distal structures as well.
Diagnosis
Clonus at the
ankle is tested by rapidly flexing the foot into dorsiflexion (upward), inducing a stretch to the
gastrocnemius muscle.
Subsequent beating of the foot will result, however only a sustained clonus (5 beats or more) is considered abnormal. Clonus can also be tested in the
knees
In humans and other primates, the knee joins the thigh with the leg and consists of two joints: one between the femur and tibia (tibiofemoral joint), and one between the femur and patella (patellofemoral joint). It is the largest joint in the hu ...
by rapidly pushing the
patella
The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...
(knee cap), towards the toes.
See also
*
Myoclonus
*
Generalized tonic–clonic seizure
*
Stretch reflex
*
Tetany
Tetany or tetanic seizure is a medical sign consisting of the involuntary contraction of muscles, which may be caused by disorders that increase the action potential frequency of muscle cells or the nerves that innervate them.
Muscle cramps cause ...
*
Tetanic contraction
A tetanic contraction (also called tetanized state, tetanus, or physiologic tetanus, the latter to differentiate from the disease called tetanus) is a sustained muscle contraction evoked when the motor nerve that innervates a skeletal muscle emits ...
s
References
External links
{{wiktionary
Dr. Gilman on Serotonin ToxicityVideo about Clonus at the ankleVideo about Clonus at the knee (patella)
Medical signs
Seizure types