Urticaria and Angioedema — Classification, Mechanisms and Hereditary Angioedema

Urticaria, or hives, is a pattern of itchy, raised wheals caused by oedema in the superficial dermis. The lesions may be pale or skin-coloured centrally, with surrounding erythema. They change shape, migrate and disappear. Angioedema is deeper swelling involving the lower dermis, subcutaneous tissues or mucosa; the lips, eyelids and extremities are common sites.

These findings can occur together, but the presence of angioedema does not automatically mean an allergic mechanism. Histamine-mediated angioedema commonly accompanies urticaria and itch. Bradykinin-mediated swelling usually lacks both. That distinction is central to examination stems because the investigations and response to common allergy medicines differ.

Use two clocks. The first is the lifetime of one wheal: an ordinary urticarial lesion usually resolves within 24 hours without persistent pigmentation. The second is the overall duration of recurrent episodes: acute urticaria lasts fewer than 6 weeks; chronic urticaria lasts 6 weeks or longer. A chronic disease does not mean that each individual lesion remains fixed.

Chronic spontaneous and inducible urticaria can coexist. The spontaneous label does not mean that the symptoms are imaginary, and it does not prove that the condition is caused by a hidden food allergy. Chronic urticaria is a mast-cell-mediated disorder, often with autoimmune mechanisms, and broad searches for external allergens may not identify a cause.

Inducible urticaria is classified by the stimulus that reliably produces symptoms. A useful history asks what happened immediately before the lesions and whether the same exposure reproduces them. This is different from attributing every flare to an incidental meal or medication without a consistent relationship.

Provocation testing may help confirm a suspected inducible subtype, but the test should be selected from the history and performed in an appropriate setting. In an image question, a linear wheal is a clue to dermographism; in a vignette, exercise-associated sweating and small wheals suggest cholinergic urticaria.

Mast-cell activation releases histamine and other mediators that increase vascular permeability, producing wheals and deeper swelling. Some acute episodes follow IgE-mediated allergy, but mast cells can also be activated through other pathways. The morphology of a wheal therefore does not identify the trigger by itself.

In chronic spontaneous urticaria, autoimmune processes can activate mast cells. Antibodies against IgE or its high-affinity receptor are relevant exam associations. This explains why recurrent spontaneous hives may continue without a reproducible exposure to food, pollen or another external allergen. It also explains the role of anti-IgE treatment in persistent disease.

Take a focused history of medicines, infections, potential triggers and systemic symptoms. NSAIDs may aggravate symptoms in susceptible patients. Avoidance should target a plausible, demonstrated relationship rather than an expanding list of foods imposed solely because hives are present. The aim is to find clinically meaningful triggers while avoiding unnecessary investigations.

For revision, connect the mechanism with the main treatment family: H1 antihistamines oppose histamine-mediated symptoms. Their benefit in ordinary urticaria does not mean that they will reliably treat hereditary angioedema, which uses the bradykinin pathway. The same visible symptom of swelling can therefore lead to very different drug choices.

ACE inhibitors can cause bradykinin-mediated angioedema by reducing bradykinin degradation. This is a medication-associated mechanism, not proof of an inherited C1-inhibitor disorder. Ask about the drug history before interpreting isolated lip or tongue swelling as a new food allergy.

Acquired C1-inhibitor deficiency can resemble hereditary disease and may be associated with lymphoproliferative or autoimmune conditions. Later onset and an absent family history are clues, but they are not diagnostic alone. The laboratory pattern and clinical context must be interpreted together.

There is no single bedside feature that safely replaces a complete assessment. A patient with uncertain acute swelling may still need emergency anaphylaxis treatment if that diagnosis is plausible. The key exam distinction is that antihistamines, corticosteroids and epinephrine have little effect on confirmed bradykinin-mediated swelling; airway protection must not wait for those drugs to work.

Hereditary angioedema associated with deficient or dysfunctional C1 inhibitor is usually an autosomal dominant disorder related to SERPING1. C1 inhibitor restrains complement and contact-system proteases. When the contact pathway is insufficiently controlled, excess bradykinin increases vascular permeability and produces angioedema.

The classic presentation is recurrent swelling of skin or mucosa, episodic abdominal pain and possible upper-airway involvement, usually without urticaria or itch. Trauma, procedures, infection, stress, estrogens and ACE inhibitors can be relevant triggers or aggravators. The absence of an obvious trigger does not remove the diagnosis from consideration.

The name HAE with normal C1 inhibitor is preferable to treating every such case as a single old “type III” disorder. Normal C1-inhibitor quantity and function do not establish one uniform genetic mechanism. This page focuses on the classic deficient and dysfunctional patterns most useful for comparison questions.

Ordinary urticaria is primarily a clinical diagnosis. For chronic disease, a focused evaluation may include a blood count and inflammatory markers, with additional investigations directed by the history. Extensive allergy panels, broad imaging or routine biopsy are not required in every uncomplicated case.

• Establish the duration of individual lesions and the overall recurrent illness.
• Look for reproducible physical triggers, new medicines and systemic symptoms.
• Consider skin biopsy when lesions are atypical, painful, persistent or suggest vasculitis.
• For recurrent angioedema without wheals, assess C4 and arrange C1-inhibitor antigen plus functional activity when C1-inhibitor disease is suspected.
• Use C1q in the appropriate context to help separate hereditary from acquired C1-inhibitor deficiency.

C4 is commonly used as an initial screen for HAE. The confirmatory assessment includes both protein quantity and function because a normal or increased antigen level can accompany dysfunctional protein in type II disease. One test therefore cannot answer both the quantity and quality questions.

C1q is generally normal in hereditary C1-inhibitor deficiency and is often low in acquired deficiency. Treat that distinction as a useful pattern rather than an absolute standalone diagnosis. A late-onset presentation with low C1q deserves consideration of acquired disease and its associated conditions.

The standard starting point is a second-generation H1 antihistamine, used regularly for persistent disease. Examples include cetirizine, loratadine and fexofenadine. Compared with older agents, they generally have fewer sedating and anticholinergic effects, making them preferable for routine symptom control.

When standard treatment is inadequate, guidelines support increasing a suitable second-generation antihistamine up to four times the standard dose under clinical supervision. Persistent disease may require specialist add-on treatment such as omalizumab. Cyclosporine is another specialist option, with nephrotoxicity and hypertension as important limitations. This is a core revision framework; specialist treatment options continue to evolve.

• Avoid a demonstrated trigger or aggravating exposure when practical.
• Start a second-generation H1 antihistamine and review response.
• Escalate antihistamine treatment when indicated rather than immediately committing to prolonged systemic steroids.
• Consider specialist therapy for symptoms that remain uncontrolled.
• Reserve systemic corticosteroids for short-term control of selected severe flares; they are not routine chronic maintenance.

The exam distinction is between suppressing histamine-mediated symptoms and treating a different mediator. Increasing antihistamines in established HAE is not equivalent to appropriate mechanism-specific care. Likewise, an ordinary wheal does not require a steroid prescription simply because it looks dramatic.

For an acute HAE attack, treatment targets the missing regulation or excess bradykinin. Options include C1-inhibitor concentrate, icatibant, a bradykinin receptor antagonist, and ecallantide, a kallikrein inhibitor. Choice depends on the clinical situation and availability. These medicines do not all serve the same role in long-term prevention.

Upper-airway swelling requires urgent airway assessment and protection alongside attack treatment. Do not wait for antihistamines or corticosteroids to reverse established bradykinin-mediated disease. For suspected anaphylaxis, intramuscular epinephrine remains appropriate; distinguishing mechanisms should support emergency care, not delay it.

Prevention is divided into short-term prophylaxis, such as planning before a triggering procedure, and long-term prophylaxis for suitable patients. C1-inhibitor replacement and kallikrein-directed agents such as lanadelumab or berotralstat are established examples. Patients also need a plan for breakthrough attacks, trigger review and access to on-demand treatment.

Attenuated androgens such as danazol are older prophylactic options with important adverse effects. They are not acute rescue medicines. This is a useful drug-role trap: a medicine that increases C1-inhibitor production for prevention does not provide the same immediate attack treatment as replacement or bradykinin-pathway blockade.