NSAID Hypersensitivity

Adverse reactions to nonsteroidal anti-inflammatory drugs mediated predominantly by pharmacological (cyclo-oxygenase inhibition-related) mechanisms rather than true IgE-mediated allergy in the majority of cases, though genuine selective IgE-mediated hypersensitivity to a single NSAID also occurs. Distinguishing the mechanism is essential since it determines whether cross-reactivity with other NSAIDs is expected.

Classification

• Cross-reactive reactions (COX-1 inhibition-mediated, the majority of reactions): occur with multiple structurally unrelated NSAIDs that share potent COX-1 inhibition, reflecting excess leukotriene production when the COX pathway is blocked and arachidonic acid is shunted toward the lipoxygenase pathway. Subtypes include:
• NSAID-exacerbated respiratory disease (NERD, formerly Samter triad/aspirin-exacerbated respiratory disease): asthma, chronic rhinosinusitis with nasal polyps, and NSAID sensitivity --- bronchospasm and upper airway symptoms on NSAID exposure in a patient with underlying chronic airway inflammatory disease.
• NSAID-exacerbated cutaneous disease: worsening of underlying chronic spontaneous urticaria with NSAID exposure.
• NSAID-induced urticaria/angioedema: occurs in patients without underlying chronic urticaria, cross-reactive across COX-1 inhibiting NSAIDs.
• Single-NSAID-induced (selective) reactions: true IgE-mediated (immediate urticaria/anaphylaxis) or T-cell-mediated (delayed reactions, including severe cutaneous adverse reactions) hypersensitivity to one specific NSAID or a closely related chemical class, with tolerance of other structurally distinct NSAIDs (including selective COX-2 inhibitors, which are generally well tolerated in this group given minimal COX-1 inhibition).
• Severe cutaneous adverse reactions (T-cell mediated, delayed): Stevens-Johnson syndrome/toxic epidermal necrolysis, drug reaction with eosinophilia and systemic symptoms (DRESS), fixed drug eruption --- mechanistically distinct, drug-specific, and not predictably cross-reactive across the NSAID class.

• Respiratory (NERD): rhinorrhoea, nasal congestion, and bronchospasm/wheeze developing within 30 minutes to a few hours of NSAID ingestion, in a patient typically with pre-existing asthma and nasal polyposis; reactions can be severe and life-threatening.
• Cutaneous: urticaria and/or angioedema, developing within minutes to hours, ranging from mild to severe with airway involvement; may be an isolated reaction or exacerbation of underlying chronic urticaria.
• Anaphylaxis: rapid-onset, potentially with hypotension, bronchospasm and angioedema, seen with true selective IgE-mediated single-NSAID allergy or occasionally as part of a severe cross-reactive reaction.
• Delayed severe cutaneous reactions: onset days to weeks after starting the drug, with widespread rash, mucosal involvement, skin detachment (Stevens-Johnson syndrome/toxic epidermal necrolysis), or fever with rash, eosinophilia and organ involvement (DRESS) --- medical emergencies requiring immediate drug cessation and specialist management.
• Gastrointestinal intolerance (dyspepsia, gastritis) is common with NSAID use but is not a hypersensitivity reaction and should be distinguished from true allergic/pharmacological hypersensitivity reactions.

• Primarily clinical, based on a careful history establishing the timing, nature of the reaction, and specifically whether reactions have occurred with more than one structurally distinct NSAID (supporting cross-reactive disease) or only ever with one specific agent (supporting selective hypersensitivity).
• Assess for underlying chronic respiratory disease (asthma, nasal polyposis) and chronic spontaneous urticaria, which point toward cross-reactive NERD/NSAID-exacerbated cutaneous disease.
• Skin prick and intradermal testing have limited reliable utility for cross-reactive reactions (since the mechanism is pharmacological, not IgE-mediated) but can be useful in evaluating selective IgE-mediated single-NSAID reactions.
• Oral drug provocation/challenge testing, performed in a specialist allergy setting with resuscitation facilities available, is the reference standard for confirming or excluding NSAID hypersensitivity and for establishing whether a specific alternative NSAID (or COX-2 selective inhibitor) is tolerated --- essential when NSAID or aspirin therapy is clinically important (e.g., cardiovascular disease requiring aspirin, or NERD patients being considered for aspirin desensitisation).
• Aspirin challenge is also diagnostic for NERD, and is the basis for subsequent desensitisation protocols.
• Severe cutaneous adverse reactions are diagnosed clinically (with skin biopsy in Stevens-Johnson syndrome/toxic epidermal necrolysis) and challenge testing is absolutely contraindicated given the risk of a severe, potentially fatal recurrence.

Acute reaction

• Standard anaphylaxis management (intramuscular adrenaline, airway support, fluids) for anaphylaxis/angioedema with airway compromise.
• Bronchodilators and standard asthma exacerbation management for acute bronchospasm.
• Antihistamines and corticosteroids for urticaria/angioedema.
• Immediate cessation of the causative drug in all cases, and avoidance of rechallenge, particularly critical and absolute for suspected severe cutaneous adverse reactions.

Long-term management

• Avoidance of the causative agent, and of the entire NSAID class where cross-reactive disease is diagnosed or suspected without formal testing.
• Paracetamol is generally safe as an alternative analgesic (weak COX inhibition), though high doses can occasionally provoke mild symptoms in some NERD patients and should be used cautiously with monitoring on first use.
• Selective COX-2 inhibitors (celecoxib, etoricoxib) are generally well tolerated in cross-reactive NSAID hypersensitivity given minimal COX-1 inhibition, but should ideally be introduced under medical supervision or after formal challenge testing given individual variability.
• Aspirin desensitisation is an effective and increasingly used treatment for NERD patients requiring aspirin for cardiovascular indications, or as therapy for the underlying nasal polyposis/asthma itself (aspirin desensitisation followed by maintenance high-dose aspirin significantly improves nasal and sinus disease control in NERD) --- performed as an inpatient/specialist procedure with graded dosing under close monitoring.
• For patients with selective single-NSAID hypersensitivity, other structurally unrelated NSAIDs can generally be used safely, ideally confirmed by supervised challenge before relying on this clinically.
• Referral to an allergy/immunology specialist for formal characterisation, challenge testing where clinically indicated, and desensitisation planning.
• Clear documentation and patient education (medical alert bracelet, written allergy status) distinguishing cross-reactive class hypersensitivity from selective single-drug reactions, given the very different practical implications for future drug choices.