ANAPHYLAXIS
DEFINITION
Anaphylaxis is an immunological description of a type I
hypersensitivity reaction mediated by IgE or IgG.
Clinically, the term is used to describe a group of symptoms (see
"clinical diagnosis" for complete description) irrespective of the
mechanism. Where an immunological basis for the syndrome is unproven,
the term "anaphylactoid" is used.
Clinical expression of anaphylaxis is variable in severity but
cardiovascular collapse is the most common life-threatening feature
and bronchospasm occurs frequently.
TOXIC CAUSES
Innumerable substances may cause anaphylaxis. Common causative agents
include:
Antivenoms
Blood products
Chemicals
Drugs: Antibiotics
Antiinflammatory agents
Analgesics
Chemotherapeutic agents
Local anaesthetic agents
Neuromuscular blocking drugs
Radiocontrast agents
Foods: Nuts
Shellfish
Insect stings
Latex
Plants
CLINICAL FEATURES
History of exposure to a substance capable of producing anaphylaxis
and appropriate clinical manifestations. These may include:
hypotension, bronchospasm, upper airways obstruction, pulmonary
oedema, angioedema, generalized oedema, pruritus, rash, vomiting,
diarrhoea and abdominal pain. In some cases, there may not be a clear
history of exposure.
DIFFERENTIAL DIAGNOSIS
Anxiety
Asthma
Bronchospasm or laryngeal oedema due to inhalation of irritant gases.
Cardiogenic shock
Foreign body in upper airway
Hypovolaemia
Pulmonary embolism
Vasovagal episode
RELEVANT INVESTIGATIONS
In such an emergency situation, no biomedical investigations are
required to commence treatment.
TREATMENT
Epinephrine (Adrenaline)
Epinephrine is the preferred treatment for anaphylaxis and should be
administered as soon as practicable while assessing and supporting
vital functions. It is usually given intramuscularly but may be given
subcutaneously in mild cases. Intravenous administration is only
indicated in severe cases because of the risk of ventricular
dysrhythmias. In the intubated patient, endotracheal installation is
possible if intravenous access is unavailable.
Doses of epinephrine:
1) Intramuscular/subcutaneous: Adult: 0.5 to 1.0 mg
Paediatric: 0.01 mg/kg
or
Age Epinephrine 1:1000 solution
< 1 year 0.05 mL
1 year 0.1 mL
2 years 0.2 mL
3-4 years 0.3 mL
5 years 0.4 mL
6-12 years 0.5 mL
> 12 years 0.5 to 1 mL
The appropriate dose should be repeated every 3 to 10 minutes until an
adequate response in pulse and blood pressure is observed.
2) Intravenous: Adult: 0.1 mg (1 mL of 1:10000 solution made
by diluting 1 mg of epinephrine in 10 mL of
normal saline) over 2 to 3 minutes.
Paediatric: 0.01 mg/kg over 2 to 3 minutes.
The appropriate dose should be repeated until an adequate response in
pulse and blood pressure is observed.
Notes on epinephrine therapy:
The dosage of epinephrine is NOT one ampoule.
For the intubated patient, if intravenous injection is not
possible, intratracheal instillation (1 to 3 mg) is an
alternative.
Patients on beta-blockers may require larger doses of
epinephrine.
Oxygen
Supplemental oxygen should be administered to all patients. In
severe cases, especially those with airways obstruction, establishment
of an adequate airway (endotracheal intubation or tracheotomy) and/or
assisted ventilation may be necessary.
Fluids
In patients presenting with hypotension, one to two litres of
intravenous fluids should be given as soon as intravenous access is
established. Colloid is preferable to crystalloid but either is
acceptable. Persistent hypotension should be treated with further
doses of epinephrine.
Further administration of intravenous fluids should be cautious and
ideally titrated against central venous pressure.
Corticosteroids are not life-saving and are never the primary
therapy of acute anaphylaxis. They may be useful in the treatment of
bronchospasm and in the prevention of relapses. An intravenous dose
of 200 to 300 mg of hydrocortisone (or equivalent dose of another
corticosteroid) may be given.
Nebulized salbutamol (albuterol) may be useful for refractory
bronchospasm, particularly in children.
CLINICAL COURSE AND MONITORING
There is usually a rapid response to therapy and recovery is complete.
Pulse, blood pressure, respiration and oxygen saturation must be
monitored until full recovery.
LONG TERM COMPLICATIONS
Recurrent episodes of anaphylaxis.
The agent that caused the anaphylaxis should be identified where
possible and the patient adequately advised regarding the avoidance of
further reactions. A warning device or letter should be issued.
Patients who suffered life-threatening anaphylaxis should be
instructed in the self-use of epinephrine. The need for
desensitization to the allergen should be considered.
AUTHOR(S)/REVIEWERS
Author: Dr R. Fernando, National Poison Information Centre,
Colombo, Sri Lanka.
Reviewers: Cardiff 3/95, Berlin 10/95: A. Jaeger, R. Dowsett, J.
Szajewski, V. Danel, A. Wong.
Cardiff 9/96: V Afanasiev, T Della Puppa, J Huang, G
Muller, L Murray, J Szajewski, C Warden.