Munro, A. Are rigors a sign of serious bacterial infection?, Don't Forget the Bubbles, 2019. Available at:
Noah is an 18m old boy who presents with fever since yesterday evening. He’s been eating and drinking a little less than usual but wetting nappies regularly. He’s been miserable when hot, but settles when his temperature comes down. His mum presented to A&E because whilst febrile this morning, he had an episode of shivering which lasted several minutes. He was conscious during the episode.
On examination he has an erythematous pharynx with bilateral tonsillar enlargement and a runny nose.
You think he has a viral illness, but you have heard that rigors may be associated with an increased risk of bacterial infection?
What is a rigor?
A rigor can be defined as involuntary shaking or shivering associated with, “chills”, which is the sensation of feeling cold when febrile (Tokuda, 2005).
It occurs as a heat gaining mechanism (muscle contraction), associated with the behaviours brought with “chills”, such as seeking extra clothing or blankets, and other heat retaining mechanisms such as pilo-erection (“goose pimples”) (Ogoina, 2011).
Are rigors clinically significant?
In adult medicine there seems to be an association with rigors and the risk of bacteraemia and invasive bacterial infections (Lee, 2012). This leads to the question about their significance in children, where fever is obviously a lot more common and the aetiology much more likely to be viral. After all, children are not just little adults.
Let’s take a look at the evidence…
There are two published studies looking as the risk of bacterial infection in children presenting with rigors.
One study was published from Israel in 1997 (Tal, 1997) – which automatically should be interpreted with extreme caution, as this was an era pre-HiB or Pneumococcal vaccines, which have transformed the landscape of invasive infections in children.
The study looked at children already admitted to hospital due to infective aetiology, then compared the prevalence of confirmed or presumed bacterial infections in those who had rigors (67%), to those who did not rigor (50%). They also compared the percentage of patients with positive microbiology in those who had rigors (15%) vs those who did not (6%). The results are statistically significant, however the clinical significance of a rigor among this cohort of patients already admitted is highly debatable.
A more recent case-control study also from Israel was published in 2017 (Erell, 2017) which more usefully looked to examine children presenting to an emergency department with fever, with and without rigors.
Children with rigors were more likely to be older and have a higher temperature.
There was no difference in rates of serious bacterial infections between the two groups.
This study also needs interpreting with caution, as there are only 84 patients in each group, there were no cases of bacteraemia in the study, and the rates of SBI were high (~20% in each group). What is worth noting however, is the majority of children presenting with rigors did not have a bacterial infection.
So what about the NICE guidelines?
The NICE guidelines of “Fever in under 5s: assessment and initial management” (NICE, 2013), notes rigors as an “amber” sign, meaning children should be considered at intermediate risk of serious infection based on the presence of rigors. However, the evidence summary starts by saying,
“The evidence suggested that children with rigors were not more likely to have a bacterial illness than children who did not have rigors”.
Confused? Me too.
In summary, the group decided there wasn’t enough evidence to put it as a “red” feature, but since rigors seem to be associated with higher temperature, and higher temperature is a risk factor for bacterial infection in itself…then rigors should be considered a risk factor. This is slightly confusing given the evidence summary for height of temperature states that, “on an individual basis, high temperature [is] not useful for detecting serious illness”.
So what does this all mean?
There is some, low quality and conflicting evidence suggesting a small increased risk of bacterial infection associated with the presence of rigors. However, this is perhaps because a rigor is a surrogate for a higher degree of fever. So really, however you feel about the height of a temperature signifying an increased risk of bacterial infection, a rigor is just a weaker reflection of this. It is worth remembering that SBIs are relatively rare, and rigors are relatively common; therefore most children who have an episode of rigor will still most likely have a viral illness.
Should the presence of a rigor be noted as part of your assessment? Yes.
Should it be a decisive factor? No.
Once age, the appearance of the child, and the presence or absence of a focus is taken into account, rigors add little to the assessment of the risk of bacterial infection in a febrile child.
Erell Y, Youngster I, Abu-Kishk I, et al. Shivering in Febrile Children: Frequency and Usefulness in Predicting Serious Bacterial Infections – A Prospective Case-Control Study. J Pediatr 2017;190:258–260.e1. doi:10.1016/j.jpeds.2017.06.075
Lee C-C, Wu C-J, Chi C-H, et al. Prediction of community-onset bacteremia among febrile adults visiting an emergency department: rigor matters. Diagn Microbiol Infect Dis 2012;73:168–73. doi:10.1016/j.diagmicrobio.2012.02.009