Overview of Clonus
Clonus is a medical term referring to a series of rhythmic, shaking movements that often occur in response to sudden muscle stretching. While we are not completely sure why it occurs, it has been linked to issues with the upper motor neurons, the nerve cells responsible for carrying signals from the brain to the muscles in your body. Clonus often goes hand-in-hand with hyperreflexia, an exaggerated reflex response.
Doctors often use a clonus test as part of the neurological physical exam, to assess how well a patient’s nervous system is working. This test can be performed on many different joints, but the ankle joint is the most common one tested. For example, a doctor might observe clonus by quickly bending the patient’s foot upwards (a movement known as dorsiflexion). If clonus is present, they would see a repeated series of obvious extensions and relaxations of the foot. This can look similar to tapping the knee and observing the knee jerk reflex, but the movements continue for multiple rounds.
Performing a clonus test doesn’t require any special equipment, can be done very quickly, and doesn’t cost anything. It is particularly useful because clonus is indicative of issues with the central nervous system (comprised of the brain and spinal cord), rather than the peripheral nervous system (nerves outside the brain and spinal cord). This can help doctors diagnose what’s going wrong in the body. Additionally, checking for the presence of clonus can also help predict seizures in patients who have overdosed on certain drugs, and can inform decisions on when to start seizure prevention treatment.
Anatomy and Physiology of Clonus
Clonus is a condition that isn’t fully understood, but there are two primary ideas about what’s likely causing it. One suggests that clonus results from the constant reactivation of certain muscle stretch reactions in the body, with each “beat” of clonus inducing another. The other theory believes there’s some outside stimulus that triggers a reflex in the body to stretch, and then the central nervous system tells the muscle to keep making that motion even when it’s unnecessary. Scientists believe the real explanation might be a mixture of these two theories.
The onset of clonus usually involves the lessening of control over certain cells, called motor neurons, that are involved in a specific reflex in our bodies related to muscle stretching. This usually happens when there’s damage to certain pathways of the nervous system that go from the brain to the spinal cord. These nerves are supposed to manage the motor neurons but if they’re damaged, they can’t do their job. This leads to what we call a “hyper excitatory state” where the muscle stretching reflex is overactive. Clonus is often seen as evidence of some damage to these upper nerves in the body, which is also why it’s usually accompanied by other signs of overactive reflexes.
The “beat” of clonus in a certain joint depends on the anatomy of the nerves involved in that particular joint’s stretching reflex. Longer nerves mean signals take more time to travel to and from the spine, so the clonus “beats” take longer too. For instance, the wrist or fingers may show a faster frequency, or shorter time between beats, than the ankle, which is further away from the spine.
Medical professionals often check for clonus in particular areas because of the specific nerves involved. The most common site is the ankle using the Achilles tendon reflex (connected to the S1/S2 nerve routes). But there are other common testing areas, like the jaw using the Trigeminal nerve at the chin, the knee using the L2 to L4 nerves (mostly L4) just below the kneecap, the biceps using the C5 to C6 nerves near the elbow’s front, and the triceps using the C7 to C8 nerves (mostly C7) near the elbow’s back.
Equipment used for Clonus
Clonus, a series of involuntary muscle contractions, is a helpful tool for physical examinations because it’s easy to test without needing any special equipment. The test for clonus is mostly done at the ankle. The doctor simply rapidly bends the foot upwards (a process called dorsiflexion). No tools are needed for this.
For other joints like the knee, your doctor might gently tap the knee cap tendon with their fingers or with a handy object like a stethoscope or phone. Clonus indicates overactive reactions to stimuli (hyperreflexia), meaning it can generally be spotted with ease in a patient and doesn’t need a lot of force so the doctor’s fingers might be the best tool to use.
If it’s necessary, any type of reflex hammer can be used to test this reflex. It just goes to show, fancy instruments aren’t always necessary to test your health!
How is Clonus performed
Clonus is a type of muscle response that doctors test for to understand the health of your nervous system. This response is similar to the reaction that occurs when a doctor taps your knee with a reflex hammer. The doctor can make your muscles respond in a similar way, but instead of tapping, they will apply and hold pressure at certain joints, such as the knee and ankle.
To check for clonus at your ankle, the doctor will have you relax while they hold your leg. They’ll then move your foot upwards in a quick motion and continue to apply pressure. Against this pressure, they may feel your foot respond with several quick “beats” as it moves downwards and then relaxes. Your doctor will note the rhythm and number of these beats, which usually start out longer and then become similar in length after the fourth beat.
Your doctor would check for clonus at the knee in a similar way. They’ll have you straighten and relax your leg. Then they’ll quickly move your kneecap downwards and apply light pressure in that direction. With each “beat” of clonus, they’ll feel the kneecap move upwards and then relax. These checks help the doctor understand the health of the muscles and nerves in your leg.
What Else Should I Know About Clonus?
Clonus is a condition that causes involuntary and rhythmic muscle contractions. This is sometimes normal in newborn babies, where occasional clonus can be expected. However, it’s not commonly seen in most infants, and even infants who later develop a condition called cerebral palsy, a disorder that affects movement and muscle tone, don’t often show signs of clonus. In adults, this condition is typically abnormal and could be a sign of underlying health issues.
If the muscle contractions last longer than 10 beats, it is called “sustained clonus”. This condition can be a minor inconvenience or a severe hindrance affecting daily life.
Clonus often indicates hyperreflexia, a condition of overactive reflexes, often associated with an upper motor neuron syndrome, a type of nerve damage. Alongside clonus, people might also experience spasticity, a condition where certain muscles are continuously contracted; or weakness. These signs may indicate damage to the central nervous system from a variety of causes like stroke, infection, congenital conditions like cerebral palsy, autoimmune diseases like multiple sclerosis, or trauma.
Clonus can also occur as a side effect of certain drugs or chemical imbalances in the body. It is often evaluated in suspected serotonin syndrome, which could be caused by certain medications or recreational drugs. Serotonin syndrome is a group of symptoms that can occur when there is too much serotonin, a chemical your neurons produce, in your brain. This condition can be caused by a variety of drugs, including certain types of antidepressants and recreational drugs like cocaine, ecstasy, and amphetamines. Clonus has also been used to predict seizures in patients who have taken too much of a certain kind of medication (SNRI) called tramadol.
