Rigors are a sudden, severe episode of uncontrollable shivering and feeling intensely cold that occurs during a rapid rise in body temperature, typically accompanying a high fever. This physiological event is the body’s attempt to quickly generate heat to reach a new, elevated temperature set point. While the shaking can be violent, the rigor itself is a symptom, not a disease. A rigor signals that the body is actively fighting an underlying condition, usually a significant infection.
The Physiological Process Behind Rigors
The body’s temperature is tightly regulated by a control center in the brain called the hypothalamus, which acts like a thermostat. This hypothalamic thermostat normally maintains the core body temperature around 98.6°F (37°C). Rigors begin when fever-inducing substances, known as pyrogens, are released into the bloodstream, often by invading pathogens or the immune system itself.
These pyrogens act on the hypothalamus and effectively “reset” the thermostat to a higher temperature. Because the body’s actual temperature is now below this new, higher set point, the body perceives itself as being too cold. To close this temperature gap, the body initiates immediate, powerful heat-generating mechanisms.
One of the first compensatory actions is peripheral vasoconstriction, which narrows blood vessels near the skin to reduce heat loss to the environment. Simultaneously, the nervous system triggers involuntary, intense muscle contractions—the violent shaking known as a rigor—which rapidly generate heat through metabolic activity. This severe shivering continues until the body’s core temperature successfully reaches the new, elevated temperature set point established by the hypothalamus.
Underlying Medical Conditions That Cause Rigors
The sudden and intense nature of a rigor often indicates a significant, acute inflammatory response, most commonly due to an infection. The rapid release of pyrogens is a hallmark of certain bacterial infections, making rigors a predictive sign of a potential bloodstream infection, or bacteremia, in adults.
Severe bacterial infections like sepsis, pneumonia, or pyelonephritis (a kidney infection) are frequently associated with these shaking chills. Urinary tract infections (UTIs) can also progress to cause rigors as the infection spreads. Furthermore, infections following surgical procedures or the formation of an abscess often trigger this intense febrile response.
While less common, certain systemic infections like malaria are known for causing classic, cyclical rigors. Viral illnesses, including severe cases of influenza, can also lead to rigors. In addition to infections, non-infectious conditions such as some inflammatory disorders, certain cancers, or reactions to medications can also induce a rigor episode.
Managing Symptoms and Knowing When to Get Help
When a rigor begins, the immediate focus is on patient comfort and safety during the uncontrollable shaking episode. Although the person feels cold, excessive bundling or very warm blankets should be avoided, as this can interfere with the body’s temperature regulation once the rigor passes. After the shivering stops and the fever has spiked, the patient will often feel too hot, flushed, and begin to sweat profusely.
Once the rigor subsides, hydration is extremely important, as fever can lead to fluid loss and dehydration. Over-the-counter fever-reducing medications like acetaminophen or ibuprofen can be administered to help lower the elevated temperature and increase comfort, though they do not treat the underlying cause. Tepid sponging or cold water should be avoided, as the sudden cold can cause blood vessels to narrow and potentially make the person shiver again.
It is important to seek medical evaluation for any rigor episode, as they often signal a serious infection requiring specific treatment. Immediate medical attention is necessary if a rigor is accompanied by signs of severe illness, which serve as red flags for a life-threatening condition like sepsis. These urgent symptoms include:
- Confusion
- Difficulty breathing or shortness of breath
- A stiff neck
- Extreme lethargy
- Persistent, high-grade fever
- Chest pain
- Inability to keep down fluids