Rigors are episodes in which your temperature rises - often quite quickly - whilst you have severe shivering accompanied by a feeling of coldness ('the chills'). The fever may be quite high and the shivering may be quite dramatic.
When the shivery part of the rigor ends, things change quite quickly. The affected person (who now has a fever) will suddenly feel too hot. They will want to cool down, will look flushed and may sweat profusely. Gradually their body temperature will fall again. Rigors may occur only once in an illness, or can occur several times in a row.
Are rigors serious?
A rigor is not serious in itself, although it feels unpleasant. However, some of the causes of rigors are serious and some need immediate treatment. For this reason you should always seek medical advice if you or your child experience a rigor.
Is there anything else rigors could be?
The shivering, pale face and sensation of cold in the shivery phase of rigors are similar to the shivering, pale face and sense of cold patients experience when their body temperature is actually too low (hypothermia). You are unlikely to confuse the two, as a person with hypothermia will be cold to the touch, and their recent exposure to a cold environment is likely to be obvious. However the possibility of hypothermia should not be forgotten in someone who is shivering. Hypothermia can come on gradually in elderly people in their own homes, particularly during spells of cold weather.
What causes a rigor?
Our body temperature is controlled by a part of the brain called the hypothalamus. This acts as a thermostat and 'sets' the normal body temperature to around 37°C (98.6°F). The body then does all it can to obey the hypothalamus and hold its temperature at this setting. It generates heat through the processes of its metabolism. If it starts to get cold it takes steps to warm up; if it starts to get too hot it takes steps to cool down.
Rigors are triggered by the presence of chemicals called pyrogens in the blood which 'turn up' the body's thermostat setting, telling the body to aim for a higher target temperature. Pyrogens are produced by our own immune systems in response to certain triggers, of which infection is the most common.
The symptoms of a rigor are caused by the body responding to its internal thermostat having been turned up.
Why do we shiver with a rigor?
When the body's thermostat has been set higher than the body's actual temperature, the mechanisms that kick in are those that the body uses to warm itself up. These include:
- Feeling a sensation of cold and so behaving in a way that increases temperature (wrapping up, curling up, seeking warmth).
- Hairs standing on end to hold in heat, causing 'goosebumps'.
- Reduced sweating.
- Reducing heat loss from the skin by shrinking down (constricting) the tiny blood vessels in the skin, leading to a pale face and fingers that feel cold to the touch.
All of these tend to raise body temperature to match the new thermostat setting. Shivering raises our temperature because it is a muscular activity - running around would have a similar effect. Once the body reaches that new set temperature, shivering stops and you stop feeling cold.
As your body fights the infection, the immune system stops releasing pyrogens, and the hypothalamus drops the thermostat setting back towards normal. This tends to happen when the numbers of bacteria or viruses circulating in the blood fall. Your body then uses various mechanisms to lower your temperature again. These include:
- Feeling hot and behaving in a way to lower temperature (fanning, taking off clothes).
- Expanding or opening up (dilation) of the blood vessels in the skin to increase heat loss, making you look flushed and feel warm and clammy to the touch.
If the process is repeated because a new rush of pyrogens is released into the blood, there will be another rigor. Some infections tend to cause repeated rigors, generally because there are repeated releases of infective agents. Malaria typically causes repeated rigors with intervals of several days between them, as the malaria parasite is released into the blood in sudden rushes every few days. Parasites are living things (organisms) that live within, or on, another organism.
What type of infection can cause a rigor?
Any infection that pushes the temperature up quickly can cause a rigor. Some people are more likely than others to have rigors, and the same infection will not cause a rigor in everyone. Children with a temperature and rigors are slightly more likely to have a bacterial infection than children with a temperature without rigors, but the difference is small.
What are some possible causes of rigors?
A germ (bacterium) called Streptococcus pneumoniae, which typically causes respiratory tract and ear infections, is the most common infectious bacterium to cause rigors in children. There are many other possible causes, although some are unusual (or not seen in the UK) and some occur only in adults.
Common infectious causes of rigors
This is not a complete list but it includes the most likely causes of rigors in children and adults, together with some of the more uncommon ones:
- Bacterial infection including ear infection, chest infection, kidney and bladder (urinary tract) infection, gastroenteritis.
- Many common viral infections, including winter flu (influenza).
- Childhood viruses - for example, measles, mumps, chickenpox, rotavirus, roseola.
- Widespread skin infection - for example, infected eczema, cellulitis, impetigo.
- Eczema infected by the cold sore virus, which is called eczema herpeticum.
- Infection after surgery, particularly to the tummy (abdomen) or to a joint.
- Cat scratch disease.
- Joint infection (septic arthritis).
- Sinusitis (adults and older children only).
- Infection associated with kidney stones or gallstones (adults).
- Malaria - an important cause of rigors if you have visited a malarial zone.
- Mastitis and breast abscess when breastfeeding.
- Womb infection (endometritis) after childbirth or miscarriage.
- Infection of the prostate gland (prostatitis).
Uncommon infectious causes of rigors
- Meningococcal infection.
- Kawasaki disease.
- Lyme disease.
- Rheumatic fever.
- Infection of the heart valves (infective endocarditis).
- Some fungal infections (for example, histoplasmosis).
- Dental abscess.
- Other uncommon infections (in the UK), such as tuberculosis.
- Some 'tropical' diseases, such as dengue and typhoid.
Non-infectious causes of rigors
- Widespread skin inflammation (including sunburn, burns, psoriasis).
- Reactions to medicines (including some street drugs).
- Inflammatory conditions, such as Crohn's disease, ulcerative colitis.
- Some cancers.
- Blood transfusion (rarely).
- Rigors which can occur in pregnant women during labour.
What are the symptoms of a rigor?
A rigor is an episode of shivering and feeling very cold whilst your body temperature rises above normal, followed by an episode of feeling very warm whilst your body temperature falls again. During the 'feeling cold and shivery' phase, other people will feel that you are hot to the touch although you'll feel as if you are too cold. During the 'feeling hot and sweaty' phase, you will feel as if you are too hot, and other people will notice you are flushed and sweaty. Rigors last varying amounts of time, from minutes to hours. They may occur repeatedly.
Rigors in literature and film
If you read novels or watch TV dramas set in the pre-antibiotic era, you may be familiar with the idea that when people were sick with infectious diseases they would be put to bed, pale and unwell. Sometimes they were said to be 'delirious', as the temperature and illness made them confused. This was a frightening experience, because at this point they were becoming more unwell, as their temperature (fever) increased. In those days the most common causes were infections like pneumonia, which were often deadly.
Eventually the temperature would stop rising. Everyone would be relieved because 'the fever had broken'. 'Breaking' was the point at which the fever had reached its peak, the shivering stopped and the patient became flushed and started to sweat. This was greeted with relief, as it meant that the body temperature was starting to return towards normal. People believed that the battle against infection was won and the patient was recovering. Whilst this was not always the case, as sometimes further rigors occurred, it was often true that the worst was over.
What is being described is a rigor. In pre-antibiotic times the most common causes would have been bacterial pneumonia, and multiple different childhood infections like scarlet fever and measles.
How is a rigor different from a febrile convulsion or a seizure?
It is important to know the difference between a rigor, a febrile convulsion and a seizure (sometimes called a fit).
During a rigor the affected person will be shivery (and may be shaking violently). They may be confused (particularly when their temperature is high) but during a rigor they will be conscious, and will respond to you - for example, by obeying commands like: "Open your eyes."
Febrile convulsions are a type of seizure that can happen when a child, between the ages of 6 months and 6 years, has a high temperature (fever). During a febrile convulsion your child will not be conscious and will not respond to you. A febrile convulsion lasts anything from seconds to about five minutes. See the separate leaflet called Febrile Seizure (Febrile Convulsion) for more information.
A seizure is like a febrile convulsion but it may occur at any age. During a seizure the patient will not be conscious and will not respond to you. A seizure in a person who is unwell is suggestive of illness affecting the brain (such as stroke or meningitis). Seizures may also arise in people who have an increased tendency to seizures - a condition called epilepsy. See the separate leaflet called Epilepsy and Seizures for more information.
Who is most likely to experience rigors?
Rigors can happen in anyone who has an infection and experiences a sudden rise in temperature. Rigors are more common in children, who tend to develop higher temperatures than adults in response to infection. Older adults, particularly the very elderly, have less reactive immune systems, and are less likely to experience rigors.
What is a rigor seizure or rigor fit?
The term 'rigor seizure' or 'rigor fit' is widely discussed on the internet, with people asking what it is on several websites. Rigors and seizures are separate things. Some people refer to the violent shivering of a rigor as a 'rigor seizure' or 'rigor fit' - but these are confusing terms which are better avoided as they make it unclear what has happened (ie was it a rigor, or a seizure, or both?).
Whilst it is possible (although quite unusual) to have a rigor and a seizure at the same time (as the same underlying illness could cause both to occur simultaneously), they are two very different things.
How will the doctor work out what is causing the rigors?
Very often, listening to the history of exactly what happened, together with careful examination, will allow healthcare professionals to diagnose the location of infection which is the cause of rigors. If this is the case then treatment can begin without the need for further tests.
However, if the cause of the infection is not obvious - and particularly if your child seems unwell - then your child is likely to be sent to your local hospital for further investigation of the source of the infection. This is important, as any tests your GP surgery can do will not generate immediate results, and treatment may be needed straightaway.
What tests might they do in hospital?
If you or your child are admitted to hospital, they will assume there is probably an infectious cause for the rigors. You will therefore be kept separate from other patients on the ward, to prevent transmission of infection. The tests that will be done depend partly on what they think is likely after examining you or your child.
- All patients are likely to have blood tests. These look for signs of infection and help determine if it is a viral or a bacterial cause. Blood cultures look for bacteria circulating in the blood; these bacteria can be grown and tested to see which antibiotics will work against them. A blood sample may also be examined under a microscope to look for specific infections, such as malaria.
- Urine testing is usual - urine will be 'dipped' to see if there are proteins or blood cells present, both of which suggest the possibility of infection of the bladder or kidneys - and then further tested to see what that infection might be.
- Other tests which may be performed include:
- Lumbar puncture (a sample of fluid from close to the spine, to look for meningitis).
- Imaging using X-ray, computerised tomography (CT) scan or magnetic resonance imaging (MRI) scan to look for local signs of infection internally.
- Ultrasound to look for conditions such as appendicitis, gallstones or kidney stones, and abscess.
It is likely that you or your child will remain in hospital until diagnosis and treatment have been decided upon. Sometimes treatment will be started whilst you are awaiting firm diagnosis, on the basis that the doctors may have an idea of the most likely infection that you are experiencing.
How do I treat a rigor?
The most important thing is to work out what is causing the rigor and then to treat that cause. However, whilst the rigor is happening there are a few things you can do to make the person who is experiencing it more comfortable, calm and reassured.
- Give lots to drink. This helps to prevent a lack of fluid in the body (dehydration). You might find that a child is more willing to have a drink if they are not so irritable. It may help to give some paracetamol and then try again with drinks half an hour or so later.
- Tepid sponging is not recommended because the blood vessels under the skin become narrower if the water is too cold and this may further raise the temperature. People with rigors also find sponging uncomfortable in the shivery phase.
- Cold fans are not recommended for the same reasons, although cooling an over-warm room with adequate ventilation is sensible.
- Children with high temperature (fever) should not be underdressed or over-wrapped.
- Medicines like paracetamol and ibuprofen should not be used for fever unless your child appears distressed:
- Using paracetamol and ibuprofen does not prevent febrile convulsions.
- You can use either paracetamol or ibuprofen in children with fever who appear distressed.
- You should not use both at the same time. Paracetamol is usually recommended as the first option.
How do I treat the cause of the rigor?
This is likely to require advice from a healthcare professional.
If the cause of the rigor is a viral infection then once a healthcare professional has identified this, it's possible that there will be no specific treatment, but you will be given information about what to expect from the condition. Some viral infections, such as measles, can be serious, and your child may need to stay in hospital if they are unwell.
Sometimes 'empirical' treatments are used. This means that the doctors are not yet certain of the cause of the rigors but they treat the patient with a broad-spectrum antibiotic. This is often given intravenously, as this is fastest and most effective in serious illness. The antibiotic or antibiotics chosen will be those effective against the most likely cause of the rigors. In malarial zones (where malaria would be a highly likely cause of an unexplained temperature), malaria treatments may also be started before there is a firm diagnosis.
It is also likely that an intravenous drip will be set up. This allows you to be given extra fluid but also provides a useful route for giving intravenous antibiotics.
Will I experience rigors again?
You may experience a rigor again the next time you have an infection that causes a high temperature. The chance of reacting in this way to infection decreases slowly as you become older.
Further reading and references
Fever in under 5s: assessment and initial management; NICE Guidance (last updated November 2021)
Dall L, Stanford JF; Fever, Chills, and Night Sweats
Unger M, Karanikas G, Kerschbaumer A, et al; Fever of unknown origin (FUO) revised. Wien Klin Wochenschr. 2016 Sep 26.