Anaphylactic shock

Anaphylactic shock is a medical emergency; it is a life- threatening type I hypersensitivity reaction.

• Anaphylaxis is characterised by:
  • Sudden onset and rapid progression of symptoms.
  • Airway and/or Breathing and/or Circulation problems.
  • Usually, skin and/or mucosal changes (flushing, urticaria, angioedema)
  • diagnosis is supported if a patient has been exposed to an allergen known to affect them. However, in up to 30% of cases there may be no obvious trigger
  • note that (1)
    • Skin or mucosal changes alone are not a sign of anaphylaxis.
    • Skin and mucosal changes can be subtle or absent in 10–20% of reactions
      (e.g. some patients present initially with only bronchospasm or hypotension).
  • Gastrointestinal symptoms (e.g. nausea, abdominal pain, vomiting) in the absence of Airway and/or Breathing and/or Circulation problems do not usually indicate anaphylaxis. Abdominal pain and vomiting can be symptoms of anaphylaxis due to an insect sting or bite.
    
• the main characteristics are rapidly developing life-threatening circulatory collapse and airway impairment in response to an allergen to which the patient has previously been sensitized.Usually skin and mucosal changes are associated.

In the UK it is estimated that 500,000 people have had a venom-induced anaphylactic reaction and 220,000 people up to the age of 44 have had a nut-induced anaphylactic reaction:

• available UK estimates suggest that approximately 1 in 1333 of the population of England has experienced anaphylaxis at some point in their lives
• there are approximately 20 deaths from anaphylaxis reported each year in the UK, with around half the deaths being iatrogenic, although this may be an underestimate

Key recommendations for clinical practice (1):

• Anaphylaxis is a potentially life-threatening allergic reaction.
• Recognise anaphylaxis based on:
  • sudden onset and rapid progression of symptoms
  • Airway and/or Breathing and/or Circulation problems
  • skin and/or mucosal changes (flushing, urticaria, angioedema) – but these may be absent in up to 20% of cases

The diagnosis is supported if a patient has been exposed to an allergen known to affect them

Treat life-threatening features, using the Airway, Breathing, Circulation, Disability, Exposure (ABCDE) approach:

• Adrenaline is the first-line treatment for anaphylaxis. Give intramuscular (IM) adrenaline early (in the anterolateral thigh) for Airway/Breathing/Circulation problems.
• A single dose of IM adrenaline is well-tolerated and poses minimal risk to an individual having an allergic reaction. If in doubt, give IM adrenaline.
• Repeat IM adrenaline after 5 minutes if Airway/Breathing/Circulation problems persist.
  
  • Intravenous (IV) adrenaline must be used only in certain specialist settings, and only by those skilled and experienced in its use.
  • IV adrenaline infusions form the basis of treatment for refractory anaphylaxis: seek expert help early in patients whose respiratory and/or cardiovascular problems persist despite 2 doses of IM adrenaline
• All patients should be referred to a specialist clinic for allergy assessment.
• Offer patients (or, if appropriate, their parent and/or carer) an appropriate adrenaline injector as an interim measure before the specialist allergy assessment (unless the reaction was drug-induced).
• Patients prescribed adrenaline auto-injectors (and/or their parents/carers) must receive training in their use, and have an emergency management or action plan
  • at discharge following emergency treatment for suspected or known anaphylaxis, ensure the adult, young person or child has 2 in-date adrenaline auto-injectors (via a prescription, if needed) and knows when and how to use them, unless the anaphylaxis was due to a drug allergy and the drug can be easily avoided (2)

Documenting suspected anaphylaxis (2)

• the acute clinical features of suspected anaphylaxis should be documented, that is, rapidly developing, life-threatening problems involving:
  • one of more of the following:
    • the airway (pharyngeal or laryngeal oedema)
    • breathing (bronchospasm with tachypnoea)
    • the circulation (hypotension or tachycardia or both)
  • in most cases, associated skin and mucosal changes
• record the time of onset of suspected anaphylaxis
• record the circumstances immediately before the onset of symptoms to help to identify the possible trigger

Timing of blood samples post anaphylaxis (2)

• after suspected anaphylaxis in an adult or young person aged 16 years or older, take timed blood samples for mast cell tryptase testing as follows:
  • a sample as soon as possible after emergency treatment has started
  • a second sample ideally within 1 to 2 hours (but no later than 4 hours) from the onset of symptoms
• after suspected anaphylaxis in a child younger than 16 years, consider taking blood samples for mast cell tryptase testing as follows if the cause is thought to be venom-related, drug-related or idiopathic:
  • a sample as soon as possible after emergency treatment has started
  • a second sample ideally within 1 to 2 hours (but no later than 4 hours) from the onset of symptoms
• the adult, young person or child (or their parent or carer, as appropriate) should be informed that a blood sample may be required at follow-up with the specialist allergy service to measure baseline mast cell tryptase

Period of observation (2)

• a suitably qualified and experienced healthcare professional should consider discharging the adult, young person or child after 2 hours of observation, starting from resolution of airway swelling and resumption of normal breathing and stable blood pressure and heart rate if:
  • there was a good response (within 5 to 10 minutes) to a single dose of intramuscular (IM) adrenaline given within 30 minutes of the onset of suspected anaphylaxis and
  • symptoms have completely resolved, and
  • the person already has 2 in-date adrenaline auto-injectors and knows how and when to use them, and
  • there is adequate supervision from an appropriate adult, if needed, following discharge
• observe the adult, young person or child for a minimum of 6 hours after resolution of all symptoms if:
  • 2 doses of intramuscular (IM) adrenaline were needed to treat the anaphylaxis or
  • there is a history of biphasic reaction
• observe the adult, young person or child for a minimum of 12 hours after resolution of all symptoms if:
  • the person had severe anaphylaxis requiring more than 2 doses of adrenaline, or
  • the person has severe asthma or had anaphylaxis that involved severe respiratory compromise, or
  • there is a possibility of continuing absorption of allergen, for example, slow-release medicines, or
  • the person presents out-of-hours, or
  • the person may not be able to respond in the event of a deterioration in their condition, or the person would be discharged to a geographical area where access to
    emergency care is difficult
• a suitably qualified and experienced healthcare professional should consider discharging the adult, young person or child after 2 hours of observation from resolution of anaphylaxis following a supervised allergy challenge even if 2 doses of IM adrenaline were needed

Reference:

• (1) Emergency treatment of anaphylactic reactions.Guidelines for healthcare providers.Working Group of the Resuscitation Council (UK) 2021
• (2) NICE (May 2026). Anaphylaxis: assessment to confirm an anaphylactic episode and the decision to refer after emergency treatment for a suspected anaphylactic episode.