A rigor is a medical symptom defined as a sudden, severe episode of shivering accompanied by an intense sensation of coldness, typically occurring during the onset of a fever. This involuntary, whole-body shaking is the body’s method of rapidly increasing its core temperature, signaling a significant internal process, usually an infection. This symptom should not be confused with rigor mortis, the stiffness that occurs after death.
The Body’s Response: How Rigor Occurs
A rigor is a physiological event resulting from the body’s temperature regulation center being abruptly reset to a higher point. This process begins when fever-inducing substances called pyrogens enter the bloodstream. Pyrogens can originate externally from invading pathogens like bacteria, or internally from the body’s own immune cells responding to an infection.
These pyrogens act upon the hypothalamus, the small region in the brain that functions as the body’s thermostat. The pyrogens cause the hypothalamus to reset the normal core temperature set point, for example, from 98.6°F (37°C) to 103°F (39.4°C). Since the actual body temperature is now perceived as too low relative to this new setting, the body initiates immediate heat-generating mechanisms.
To bridge this temperature gap quickly, the body employs thermogenesis. The rigor is the most visible component of this heat production, characterized by intense, rapid, and involuntary muscle contractions. This muscle activity generates a large amount of heat energy, rapidly raising the core temperature until it matches the new, elevated hypothalamic set point.
The body also attempts to conserve heat through peripheral vasoconstriction, where blood vessels near the skin narrow. This shunts blood away from the extremities and toward the core, causing the patient to feel cold and look pale. Once the core temperature reaches the new set point, the rigor and shivering cease, signaling the maintenance phase of the fever.
Infections and Conditions That Trigger Rigor
The intense shaking of a rigor often points to a systemic process, such as bacteremia, which is the presence of bacteria in the blood. Rigors can be a predictor of a severe infection, especially when the bacterial load is high.
Severe bacterial infections in specific organs are frequent causes of rigor. Examples include pneumonia, particularly that caused by Streptococcus pneumoniae, or pyelonephritis, a severe infection of the kidney. Systemic infections like sepsis, the body’s overwhelming response to infection, are also commonly marked by rigors.
In tropical regions, intermittent rigors occurring in cycles are a classic presentation of malaria, a parasitic infection. Other infectious causes include viral illnesses like influenza, abscesses, or infections following surgical procedures. Non-infectious causes, such as reactions to certain medications, blood transfusions, or inflammatory conditions, can also provoke this intense shivering response.
Recognizing Severity and Seeking Help
Unlike a simple chill, a true rigor involves violent, uncontrollable shaking that can last for several minutes. Because rigor often manifests a serious underlying infection, recognizing signs of severity is important.
Immediate medical attention is warranted if the rigor is accompanied by signs of organ dysfunction or systemic compromise. These symptoms include difficulty breathing, severe confusion, a non-blanching rash, or a significant drop in blood pressure. Such signs suggest the infection is progressing toward septic shock, a medical emergency requiring urgent treatment.
Patients should monitor their temperature accurately, noting the exact timing and duration of the rigor, as this information is helpful for diagnosis. Communicating the severity of the shivering is also important when speaking with a healthcare professional. The underlying cause needs to be identified quickly to initiate appropriate treatment, which may involve antibiotics or other supportive care.