Fever

From Ganfyd

Synonyms: febrile pyrexia hyperthermia.

Fever is one of the commonest reasons for patients to consult health services.

Normal body temperature is less precise and fixed than is commonly averred.

Elevation of the body temperature is seen in the response to infection, but can also be a manifestation of a non-infective inflammatory process. The mechanism is complex, in that there are exogenous pyrogens eg bacterial lipopolysaccharide, but it is also driven by cytokines, especially Tumour necrosis factor alpha, IL-1 and IL-6. There is evidence that viral replication may be suppressed by a rise in body temperature but it remains unclear whether this is its function as such, or whether it is simply a by-product of inflammatory processes.

The autonomic regulation of body temperature is controlled from the hypothalamus but in mammals behaviour, which in humans includes choosing clothing and adjusting heating and air-conditioning, is a major element of the control of body temperature. Illness or anaesthesia hamper behaviour, and may also affect autonomic homeothermy, and therefore medical and nursing atendants must take over control in these situations.

Definitions of pyrexia vary, eg any elevation above the normal body temperature, else above a specific point, such as 37.5 or 38 degrees Celsius.

This is more complicated than generally assumed, due to variation in body temperature between individuals, within individuals (diurnally temperature rises in the evening, and it also varies according to menstrual cycle). It also depends on how and where you measure it.

Measurement of temperature

Mercury thermometers are now to be avoided. Their replacements provide less thinking time and do not silence the patient so effectively. In truth, an educated hand is very good at excluding a significant fever, and touching the patient has as much magic as using the bolometer.

Core temperatures

• Rectal - historically the core temperature relied upon. Can rarely be justified clinically. Vaginal temperature may be more aesthetically acceptable in the attempt to predict ovulation.
• Mouth - but must be held for 5 seconds so difficult for young children, who may also try to bite it. Not recommended for children.
• Tympanic membrane - can reasonably be assumed to be at brain temperature, but must be measured by bolometry - measuring the infra-red radiation coming off it. Incorrect angulation will give misreadings; and cerumen plugs are a less good indicator of brain temperature. Probably miss about 1 in 8 fevers. ^[1]
• Temporal artery - electronic devices are available.

Peripheral

• Axilla - generally about 0.5 degrees lower than core, but varies. Pretty close to core in neonates. Electronic or chemical dot methods are recommended. ^[2]
• Forehead temperature - not very reliable unless fever present but exceedingly convenient! Although parents using touch alone tend to overestimate the presence of fever, they are about 90% correct when it is present.^[3]

Clinical

Fever is often accompanied by flushing, sweating, tachypnoea and tachycardia. The peripheries may be cool, even cyanosed, and in young fair children the lips too can look cyanosed (acrocyanosis). At higher fevers, there may be delirium and rigors. Fever lowers seizure threshold and may precipitate seizures in those at risk eg febrile convulsion in young children, epilepsy.

Note that cold extremities may also reflect circulatory failure, and are a poor prognostic sign in meningococcal septicaemia. However, differentiating between that and normal cardiovascular function in the presence of fever can be difficult, especially since the septicaemic patient may not look particularly unwell prior to rapid decompensation. One strategy in the absence of other markers of serious infection would be to re-assess after administration of an anti-pyretic.

Pattern of fever

• High spiking - sometimes called hectic, typical of bacterial infection esp abscesses, cholangitis, UTI
• Nocturnal - classically tuberculosis (see also night sweats)
• Daily spiking - also called hectic! If temperature drops below baseline before the spike, this is the typical pattern of adult Stills disease or systemic onset juvenile idiopathic arthritis (SOJIA)
• Tertian - every 3 days. Typical of Pl. vivax malaria
• Quartian - every 4 days. Typical of Pl. malariae malaria
• Stepwise incremental - over a period of a week, classically typhoid
• Relapsing - several days or even weeks between episodes of fever, seen with borreliosis and the Periodic fevers, an increasingly well understood group of inflammatory conditions including TRAPS, Familial Mediterranean Fever.

These patterns generally have very low sensitivity and specificity, however.

The duration of fever has some significance too.

• If the history is very brief eg less than 24 hours and the patient looks unwell, then meningococcal septicaemia must be considered.
• In an unwell young child, a fever of more than 5 days duration must prompt consideration of Kawasaki disease.
• A fever of 38.3 plus of unknown cause for more than 3 weeks, despite hospital admission for at least a week, is a conventional definition of fever of unknown origin (FUO or PUO). This definition is only really useful for research.

Causes

However defined, the presence of fever generally suggests infection. The height of the fever is vaguely related to the type of infection, in that among child with hyperpyrexia (> 41.1.degC) 20% will have serious bacterial infection. At more commonly encountered temperatures however, the predictive value for bacterial infection is poor; furthermore, in the current era of universal conjugate pneumococcal vaccination, the fear (and risk) of occult pneumococcal sepsis is now much reduced.

Absence of fever does not assure us of the absence of infection in the very young and the old. Neonates may respond to severe infection with instability or reduction in body temperature.

Non-infectious fever

• Exertion - the rise in temperature with heavy exercise can be considerable.
• Environmental stress (too hot) - especially non-mobile children, leading to heat stroke
• Drug fever - especially penicillin
• Drug toxicity - especially MDMA, but also malignant hyperpyrexia related to anaesthetic agents and neuroleptic drugs (although see below)
• Ricin poisoning (The Bulgarian umbrella killing of Gregor Markov)
• Malignancy esp lymphoma
• Autoimmune disease
• Periodic fever

Preventing, treating or modifying fever

There are physical methods of cooling as well as antipyretic agents. Whether it is necessary to treat mild fever in an otherwise well patient is debatable.

Anti-pyretic drugs are sometimes given prophylactically, to reduce the risk of febrile convulsion either following child immunisation, or else during a febrile illness. There does not appear to be any good evidence that they work in these roles and are not recommended for these indications alone. ^[4][5].^[6]

References

UMLS definitions