Clinical Diagnostic Criteria, Intramuscular Adrenaline Dosing, and Refractory Anaphylaxis

Paediatric Anaphylaxis: Clinical Recognition, Pharmacotherapy, and Emergency Resuscitation

Anaphylaxis is an acute, potentially fatal systemic hypersensitivity reaction characterized by rapid onset of life-threatening airway, breathing, or circulatory problems. In children, food allergens—predominantly peanut, tree nuts, cow's milk, hen's egg, fish, and shellfish—represent the most frequent triggers, followed by venom stings (Hymenoptera) and medications (e.g., beta-lactam antibiotics and neuromuscular blocking agents). Prompt recognition and immediate administration of intramuscular adrenaline form the foundation of successful emergency management.

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Diagnostic Criteria: WAO / EAACI Consensus Guidelines

EAACI 2021 retains the following three NIAID/FAAN clinical patterns. WAO 2020 uses two patterns: acute skin/mucosal involvement plus respiratory compromise, reduced BP or severe gastrointestinal symptoms (especially after non-food triggers); or acute hypotension, bronchospasm or laryngeal involvement after a known/highly probable allergen, even without skin signs. This latter bronchospasm criterion excludes isolated lower-airway symptoms from common inhalant allergens or perceived food-inhalation reactions without ingestion. Do not wait for a rash before treating life-threatening anaphylaxis.

The three EAACI patterns are:

Presentation 1: Acute Multiorgan Presentation with Skin/Mucosal Involvement

Acute onset (within minutes to several hours) of symptoms involving the skin, mucosal tissue, or both (e.g., generalized urticaria, intense pruritus, flushing, swollen lips, tongue, or uvula),

AND AT LEAST ONE OF THE FOLLOWING:

1. Respiratory compromise: Dyspnoea, tachypnoea, wheezing, bronchospasm, stridor, reduced peak expiratory flow, or hypoxaemia.
2. Reduced blood pressure or end-organ dysfunction: Hypotension (age-specific criteria), hypotonia, syncope, incontinence, or marked diaphoresis.

Presentation 2: Rapid Multiorgan Reaction to a Likely Allergen

Two or more of the following occurring rapidly (within minutes to several hours) after exposure to a likely allergen for that specific patient:

1. Sudden involvement of the skin or mucosal tissues (urticaria, itch, flushing, angioedema).
2. Respiratory compromise (dyspnoea, wheeze, stridor, hypoxaemia).
3. Reduced blood pressure or associated symptoms of end-organ hypoperfusion (collapse, syncope).
4. Persistent gastrointestinal symptoms: Severe crampy abdominal pain, repetitive projectile vomiting (particularly common in food-triggered paediatric anaphylaxis).

Presentation 3: Hypotension Following Known Allergen Exposure

Reduced blood pressure occurring rapidly after exposure to a known allergen for that specific patient:

• Infants and children: Low systolic blood pressure (age-specific 5th percentile) or a $>30\%$ decrease from baseline systolic blood pressure.
• Adolescents and adults: Systolic blood pressure $<90\text{ mmHg}$ or a $>30\%$ decrease from baseline.

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Cellular Pathophysiology and Vascular Collapse

Classic anaphylaxis is a Type I IgE-mediated hypersensitivity reaction:

[Allergen Re-exposure]
           |
           v
[Cross-linking of Specific IgE on FcεRI Receptors]
           |
           v
[Explosive Degranulation of Mast Cells & Basophils]
  /                                             \
[Preformed Granule Mediators]       [Newly Synthesized Lipid Mediators]
- Histamine (H1, H2)                - Leukotrienes (LTC4, LTD4, LTE4)
- Neutral Proteases (Tryptase)      - Prostaglandin D2 (PGD2)
- Heparin, Proteoglycans            - Platelet-Activating Factor (PAF)
  \                                             /
   +--------------------+----------------------+
                        |
                        v
          [Systemic End-Organ Manifestations]
  1. Massive Microvascular Endothelial Permeability
     -> Up to 35% intravascular volume lost in 10 minutes
  2. Systemic Arteriolar Vasodilation
     -> Profound distributive & hypovolaemic shock
  3. Bronchial Smooth Muscle Spasm
     -> Severe lower airway obstruction & wheeze
  4. Pharyngeal / Laryngeal Mucosal Edema
     -> Upper airway compromise & fatal asphyxiation

Mediators and Microvascular Leak

• Histamine: Acts via $H_1$ receptors to stimulate bronchial smooth muscle contraction, increase vascular permeability, and provoke sensory nerve activation (pruritus). Acts via $H_2$ receptors to promote gastric acid secretion, enhance vascular permeability, and augment cardiac chronotropy and coronary vasodilation.
• Cysteinyl Leukotrienes ($LTC_4, LTD_4, LTE_4$): Previously known as "slow-reacting substance of anaphylaxis," leukotrienes are 1,000 times more potent than histamine in triggering sustained bronchoconstriction, airway mucous secretion, and vascular leakage.
• Extravasation Dynamics: Widespread endothelial separation allows the rapid shift of up to $35\%$ of effective circulating intravascular volume into the interstitial third space within the first 10 minutes of symptom onset. This produces a combined distributive and profound hypovolaemic shock state.

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First-Line Pharmacotherapy: Intramuscular Adrenaline

Adrenaline (epinephrine) is the only evidence-based life-saving medication in anaphylaxis. It must be administered immediately upon clinical suspicion.

Pharmacodynamic Mechanisms

• $\alpha_1$-adrenergic receptor stimulation: Induces potent peripheral vasoconstriction, reversing systemic vasodilation, increasing diastolic blood pressure, improving coronary perfusion, and dramatically reducing mucosal oedema in the larynx and upper airway.
• $\beta_1$-adrenergic receptor stimulation: Increases myocardial contractility (positive inotropy) and heart rate (positive chronotropy), restoring stroke volume and cardiac output.
• $\beta_2$-adrenergic receptor stimulation: Induces rapid bronchial smooth muscle relaxation (bronchodilation) and, critically, inhibits further mast cell and basophil degranulation by upregulating intracellular cyclic adenosine monophosphate (cAMP).

Exact Dosing and Administration Protocol

Auto-Injector Weight Brackets

When using fixed-dose pre-filled adrenaline auto-injectors (AAIs):

• Infants $<7.5\text{ kg}$: Exact manual syringe draw ($0.01\text{ mL/kg}$) is ideal; or $0.1\text{ mg}$ device where approved.
• Children $7.5\text{ kg to }25-30\text{ kg}$: $0.15\text{ mg}$ auto-injector (e.g., EpiPen Jr, Jext 150).
• Children $25-30\text{ kg to }50\text{ kg}$: $0.3\text{ mg}$ auto-injector (e.g., EpiPen, Jext 300).
• Adolescents $>50\text{ kg}$ and Adults: $0.3\text{ to }0.5\text{ mg}$ auto-injector (e.g., Emerade 500, EpiPen 300).

Vastus Lateralis versus Deltoid and Subcutaneous Routes

Intramuscular mid-thigh adrenaline is preferred. In the Simons 1998 paediatric pharmacokinetic study, mean time to peak plasma concentration was 8 ± 2 minutes with IM injection and 34 ± 14 minutes with subcutaneous injection. Separately, comparative studies support higher concentrations after thigh than deltoid injection. These findings support prompt IM delivery into the anterolateral mid-thigh.

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Patient Posture and the "Empty Ventricle Syndrome"

Patient positioning is a critical, frequently overlooked determinant of survival in anaphylaxis:

      [Patient with Severe Anaphylaxis Placed Abruptly Upright]
                                  |
                                  v
         Massive Capillary Leakage & Arteriolar Vasodilation
                                  |
                                  v
           Venous Blood Pools Heavily in Lower Extremities
                                  |
                                  v
          Critical Reduction in Venous Return and Ventricular Filling
                                  |
                                  v
           [The "Empty Ventricle Syndrome" / Cardiac Arrest]
      - Severely reduced preload and cardiac output
      - Ischaemic myocardial collapse
      - Immediate Pulseless Electrical Activity (PEA) Arrest

The Empty Ventricle Mechanism

Systemic vasodilation and rapid fluid extravasation reduce central venous filling. Sudden sitting, standing or walking can pool blood in the splanchnic and leg veins, sharply reducing venous return, cardiac output and myocardial perfusion. This severe preload loss—the “empty ventricle” mechanism—can precipitate collapse and PEA arrest.

Clinical Management Rules for Posture

1. Circulatory instability: Keep the patient flat, with legs raised if tolerated, to support venous return.
2. Unconscious but breathing normally, or vomiting: Use a lateral recovery position and protect the airway; monitor breathing continuously.
3. Predominant breathing difficulty: A supported semi-recumbent position may help breathing, with legs supported where possible. Avoid sudden postural change. Do not allow standing or walking during the acute reaction, including transfers.

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Second-Line Adjunctive Therapies: What They Do and Do Not Do

Adjunctive medications play a secondary, supportive role and must never be administered prior to or in place of adrenaline:

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Management of Refractory Anaphylaxis

Refractory anaphylaxis is defined as persistent respiratory compromise or circulatory shock despite two or more properly administered doses of intramuscular adrenaline accompanied by aggressive fluid resuscitation.

Continuous Intravenous Adrenaline Infusion

• After two appropriate IM doses with persistent respiratory/circulatory compromise, seek urgent expert help and start a titrated IV adrenaline infusion in a suitably monitored setting. Continue IM adrenaline every 5 minutes while the infusion is organised or if it cannot safely be given.
• Infusion Dosing: Start at $0.05\text{ to }0.1\text{ micrograms/kg/min}$, titrated every 2 to 3 minutes by increments of $0.02\text{ to }0.05\text{ mcg/kg/min}$ based on continuous arterial blood pressure, heart rate, and tissue perfusion (titrated up to $1.0\text{ mcg/kg/min}$ in severe collapse).
• Monitoring: A trained clinician must supervise continuous ECG/SpO2, frequent BP measurement (non-invasive at least every 5 minutes until an arterial line is established) and a dedicated infusion-pump line. Check the infusion site and titrate to response. Obtain critical-care support and invasive monitoring when practicable; do not delay treatment for PICU transfer or arterial access.

The Patient on Beta-Blocker Therapy: Glucagon

Patients receiving chronic beta-adrenergic antagonist therapy (e.g., propranolol for migraines, infantile haemangiomas, or arrhythmias) exhibit marked resistance to adrenaline because adrenergic receptors are competitive blocked. These patients develop profound, intractable bradycardia and hypotension.

• Management: Administer Intravenous Glucagon:
  • Bolus dose: $20\text{ to }30\text{ micrograms/kg}$ (maximum single dose $1\text{ mg}$) infused over 5 minutes.
  • Continuous infusion: Follow with $5\text{ to }15\text{ micrograms/min}$ titrated to response.
• Mechanism: Glucagon bypasses the blocked beta-adrenergic receptor by directly activating adenylate cyclase via specific non-adrenergic G-protein-coupled receptors, driving intracellular cyclic AMP production and restoring myocardial inotropy and chronotropy.

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Biphasic Reactions and Inpatient Observation Protocols

A biphasic anaphylactic reaction is defined as the recurrence of clinical anaphylaxis symptoms within 1 to 72 hours (most commonly within 4 to 12 hours) after the complete clinical resolution of the initial event, occurring without any re-exposure to the trigger. Biphasic reactions occur in approximately $5\%\text{ to }20\%$ of paediatric presentations.

Risk Factors for Biphasic Anaphylaxis

• Severe initial presentation (profound hypotension or respiratory arrest).
• Requirement for more than one dose of adrenaline to control initial symptoms.
• Delay in receiving the first dose of adrenaline ($>30\text{ to }60\text{ minutes}$ after symptom onset).
• Wide pulse pressure at initial presentation.
• Co-existing diagnosis of poorly controlled asthma.

Observation According to Risk and Local Protocol

• EAACI 2021: Observe patients with respiratory compromise for at least 6–8 hours; after hypotension, observe for at least 12–24 hours.
• RCUK 2021: Use risk-stratified observation from symptom resolution. Selected patients with a rapid, complete response to one early IM dose, two auto-injectors with training, and adequate supervision may leave after 2 hours. Observe at least 6 hours after two IM doses or a previous biphasic reaction, and at least 12 hours after a severe reaction requiring more than two doses, severe asthma/respiratory compromise, or other ongoing risk. Persistent instability requiring an infusion needs critical care.

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Comprehensive Discharge Protocol

Prior to discharging a child who has stabilized following an anaphylactic event, the multidisciplinary clinical team must verify the five pillars of paediatric allergy discharge:

1. Two Adrenaline Auto-Injectors (The "Rule of Two"): Prescribe and dispense two pre-filled adrenaline auto-injectors calibrated to the child's exact weight. (Two devices are mandatory because up to 20% of patients require a second dose before emergency medical services arrive).
2. Practical Training: Provide direct hands-on training to both the parents/caregivers and the child (if developmentally capable) using an unmedicated trainer device, ensuring proper technique and needle disposal.
3. Written Anaphylaxis Action Plan: Supply a personalized, written Paediatric Anaphylaxis Emergency Action Plan detailing symptoms, trigger avoidance, and step-by-step instructions for auto-injector delivery for home, school, and childcare environments.
4. Trigger Avoidance Education: Offer clear dietary or environmental elimination guidance.
5. Subspecialty Allergy Referral: Arrange a formal, urgent outpatient appointment with a paediatric allergy specialist for diagnostic skin prick testing, serum-specific IgE quantification, component-resolved diagnostics, and consideration of allergen-specific immunotherapy (e.g., venom subcutaneous immunotherapy or peanut oral immunotherapy).