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 re ...
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 c ...
conditions, particularly associated with
upper motor neuron
Upper motor neurons (UMNs) is a term introduced by William Gowers in 1886. They are found in the cerebral cortex and brainstem and carry information down to activate interneurons and lower motor neurons, which in turn directly signal muscles t ...
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 classif ...
s involving descending motor pathways, and in many cases is accompanied by
spasticity
Spasticity () is a feature of altered skeletal muscle performance with a combination of paralysis, increased tendon reflex activity, and hypertonia. It is also colloquially referred to as an unusual "tightness", stiffness, or "pull" of muscles ...
(another form of hyperexcitability).
Unlike small spontaneous twitches known as
fasciculations (usually caused by
lower motor neuron
Lower motor neurons (LMNs) are motor neurons located in either the anterior grey column, anterior nerve roots (spinal lower motor neurons) or the cranial nerve nuclei of the brainstem and cranial nerves with motor function (cranial nerve low ...
pathology), clonus causes large motions that are usually initiated by a
reflex. 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
The gastrocnemius muscle (plural ''gastrocnemii'') is a superficial two-headed muscle that is in the back part of the lower leg of humans. It runs from its two heads just above the knee to the heel, a three joint muscle (knee, ankle and subtalar ...
)
*
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 ...
(
knee cap
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 ...
)
*
Triceps surae
The triceps surae consists of two muscles located at the calf – the two-headed gastrocnemius and the soleus. These muscles both insert into the calcaneus, the bone of the heel of the human foot, and form the major part of the muscle of the p ...
*
Biceps brachii
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 sens ...
,
stroke,
multiple sclerosis,
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 stage ...
.
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 electrica ...
as part of a
generalized tonic–clonic seizure
A generalized tonic–clonic seizure, or GTCS, previously known as a Grand mal seizure, is a type of generalized seizure that produces bilateral, convulsive tonic and then clonic muscle contractions. Tonic-clonic seizures are the seizure type m ...
, and in pregnant women presenting with severe
pre-eclampsia and
eclampsia
Eclampsia is the onset of seizures (convulsions) in a woman with pre-eclampsia. Pre-eclampsia is one of the hypertensive disorders of pregnancy that presents with three main features: new onset of high blood pressure, large amounts of protein in ...
.
It can also be an
adverse drug reaction
An adverse drug reaction (ADR) is a harmful, unintended result caused by taking medication. ADRs may occur following a single dose or prolonged administration of a drug or result from the combination of two or more drugs. The meaning of this term ...
, such as after ingestion of potent
serotonergic
Serotonergic () or serotoninergic () means "pertaining to or affecting serotonin". Serotonin is a neurotransmitter. A synapse is serotonergic if it uses serotonin as its neurotransmitter. A serotonergic neuron ''produces'' serotonin. A substance is ...
drugs, where clonus strongly predicts imminent serotonin toxicity (
serotonin syndrome
Serotonin syndrome (SS) is a group of symptoms that may occur with the use of certain serotonergic medications or drugs. The degree of symptoms can range from mild to severe, including a potentiality of death. Symptoms in mild cases include high ...
).
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 excitability and
nerve signal delay are required.
Increased
motor neuron 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/
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 sp ...
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 joint
A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...
s and
muscles
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of musc ...
.
This may therefore explain why clonus is typically found in
distal 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 event, it will continue to rhythmically excite
motor neurons
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 indirectly ...
, 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
The stretch reflex (myotatic reflex), or more accurately "muscle stretch reflex", is a muscle contraction in response to stretching within the muscle. The reflex functions to maintain the muscle at a constant length. The term deep tendon reflex is ...
pathway may be stimulated first, and through its events, cause a decreased
synaptic current
threshold.
This decreased synaptic current threshold would enhance
motor neuron 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, ...
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
Spasticity () is a feature of altered skeletal muscle performance with a combination of paralysis, increased tendon reflex activity, and hypertonia. It is also colloquially referred to as an unusual "tightness", stiffness, or "pull" of muscles ...
in many cases of
stroke 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
Spasticity () is a feature of altered skeletal muscle performance with a combination of paralysis, increased tendon reflex activity, and hypertonia. It is also colloquially referred to as an unusual "tightness", stiffness, or "pull" of muscles ...
.
Although closely linked, clonus is not seen in all patients with
spasticity
Spasticity () is a feature of altered skeletal muscle performance with a combination of paralysis, increased tendon reflex activity, and hypertonia. It is also colloquially referred to as an unusual "tightness", stiffness, or "pull" of muscles ...
.
Clonus tends to not be present with
spasticity
Spasticity () is a feature of altered skeletal muscle performance with a combination of paralysis, increased tendon reflex activity, and hypertonia. It is also colloquially referred to as an unusual "tightness", stiffness, or "pull" of muscles ...
in patients with significantly increased
muscle tone, 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 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) and is common in
muscles with long conduction delays, such as the long
reflex 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
The ankle, or the talocrural region, or the jumping bone (informal) is the area where the foot and the leg meet. The ankle includes three joints: the ankle joint proper or talocrural joint, the subtalar joint, and the inferior tibiofibular joi ...
is tested by rapidly flexing the foot into dorsiflexion (upward), inducing a stretch to the
gastrocnemius
The gastrocnemius muscle (plural ''gastrocnemii'') is a superficial two-headed muscle that is in the back part of the lower leg of humans. It runs from its two heads just above the knee to the heel, a three joint muscle (knee, ankle and subtalar ...
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 ...
(knee cap), towards the toes.
See also
*
Myoclonus
Myoclonus is a brief, involuntary, irregular (lacking rhythm) twitching of a muscle or a group of muscles, different from clonus, which is rhythmic or regular. Myoclonus (myo "muscle", clonic "jerk") describes a medical sign and, generally, is ...
*
Generalized tonic–clonic seizure
A generalized tonic–clonic seizure, or GTCS, previously known as a Grand mal seizure, is a type of generalized seizure that produces bilateral, convulsive tonic and then clonic muscle contractions. Tonic-clonic seizures are the seizure type m ...
*
Stretch reflex
The stretch reflex (myotatic reflex), or more accurately "muscle stretch reflex", is a muscle contraction in response to stretching within the muscle. The reflex functions to maintain the muscle at a constant length. The term deep tendon reflex is ...
*
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 caus ...
*
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