Differentials
Septic shock
SIGNS / SYMPTOMS
Absence of previous allergic reactions, lack of allergen exposure, slower onset, fever, and other signs of localised infection often differentiate septic shock from anaphylaxis.
INVESTIGATIONS
Increased WBC count and increased temperature.
Chest x-ray with a pulmonary infiltrate is suggestive of underlying pneumonia as source of infection.
Cardiogenic shock
SIGNS / SYMPTOMS
Age, risk factors for coronary artery disease, previous angina episodes, absence of an allergic history, no indication of allergen exposure, typical cardiac signs and symptoms (such as chest pain on exercise) help to distinguish. Severe anaphylactic reactions can trigger cardiac events.
INVESTIGATIONS
Elevated cardiac enzymes (CK and troponin). ECG may show signs of myocardial ischaemia (elevated ST segments or flipped T waves). Chest x-ray may show signs of congestive heart failure (e.g., pulmonary oedema, changes in the cardiac silhouette).
Hypovolaemic shock
SIGNS / SYMPTOMS
Water or fluid loss occurs via heat exposure, profuse sweating, significant blood loss, vomiting, and diarrhoea. Patients report thirst and a drop in urinary output. Anaphylaxis is a form of hypovolaemic shock secondary to intravascular fluid shifts. Establishing an alternative cause of hypovolaemia will differentiate from anaphylaxis.
INVESTIGATIONS
A drop in haematocrit suggests blood loss.
A serum urea/creatinine ratio >20 suggests volume depletion.
Vasovagal reaction
SIGNS / SYMPTOMS
The characteristic feature is hypotension, with pallor, weakness, nausea, vomiting, and diaphoresis. The sudden onset, the cardiovascular collapse, and unconsciousness are all also typical of anaphylaxis.
May be differentiated by the lack of cutaneous manifestations, the absent allergic history, and the presence of bradycardia instead of the tachycardia, even though either can be absent or misleading.
INVESTIGATIONS
There is no differentiating test.
Asthma
SIGNS / SYMPTOMS
Absence of allergen exposure, or of cutaneous/digestive findings or hypotension, and a history of previous asthma episodes help to differentiate. A rapid onset is suggestive of anaphylaxis and should at least trigger referral to higher-level medical care.
INVESTIGATIONS
There is no differentiating test.
Mast cell activation syndrome (MCAS)
SIGNS / SYMPTOMS
Characterised by recurrent episodes of anaphylaxis. Consider the possibility of MCAS whenever a patient presents with a history of recurrent anaphylaxis, or with recurrent episodes of other acute, severe symptoms of mast cell activation affecting at least two organ systems.
INVESTIGATIONS
There is no differentiating test.
Acute COPD exacerbation
SIGNS / SYMPTOMS
Absence of allergen exposure, or of cutaneous or digestive findings, and a history of established chronic lung disease help to differentiate.
INVESTIGATIONS
There is no differentiating test.
Hereditary angio-oedema/bradykinin-mediated angio-oedema
SIGNS / SYMPTOMS
Characterised by recurrent episodes of slowly progressing angio-oedema of the skin, mucosa, and submucosal tissue that may be accompanied by abdominal pain. There is no urticaria, hypotension, or history of allergen exposure as in anaphylaxis. Family history is positive. The episode may be precipitated by ingestion of an ACE inhibitor.[95]
Patients do not respond to adrenaline (epinephrine).
INVESTIGATIONS
Deficiency or underactivity of the C1 esterase inhibitor enzyme.
Serum complement C4 and CH50 are low.
Vocal cord dysfunction syndrome
SIGNS / SYMPTOMS
Cutaneous signs, digestive findings, hypotension, and allergen exposure are absent.
INVESTIGATIONS
Pharyngeal endoscopy by an ear, nose, and throat consultant will show vocal cord adduction instead of laryngeal oedema.
Foreign body aspiration
SIGNS / SYMPTOMS
No allergen exposure, no cutaneous or digestive findings, but a history of foreign body aspiration differentiate this diagnosis.
INVESTIGATIONS
Imaging and bronchoscopy are used to locate and document the foreign body. If the object inhaled is radio-opaque (e.g., a small coin), a chest x-ray may show its location.
Monosodium glutamate (MSG) ingestion
SIGNS / SYMPTOMS
MSG symptom complex may occur following MSG exposure. One or more of the following is present: burning sensation in the back of the neck, forearms, and chest; numbness in the back of the neck, radiating to the arms and back; tingling, warmth, and weakness in the face, temples, upper back, neck, and arms; facial pressure or tightness; chest pain; headache; nausea; rapid heartbeat; bronchospasm; drowsiness; and weakness. No urticaria, angio-oedema, or hypotension occurs.
INVESTIGATIONS
Serum histamine levels and mast cell tryptase are not elevated.
Carcinoid syndrome
SIGNS / SYMPTOMS
On examination, there is an associated right heart murmur.
INVESTIGATIONS
Urinalysis will show high levels of hydroxy indole acetic acid.
Postmenopausal hot flushes
SIGNS / SYMPTOMS
Flush appears several times a day and lasts for 4 to 5 minutes. There are no respiratory tract symptoms and no hypotension.
INVESTIGATIONS
There is no differentiating test.
Red man syndrome
SIGNS / SYMPTOMS
Usually appears within 4 to 10 minutes after start, or soon after the completion, of a vancomycin infusion. It is characterised by flushing and/or an erythematous rash that affects the face, neck, and upper torso. Hypotension and angio-oedema may also occur, but less frequently.
INVESTIGATIONS
There is no differentiating test.
Panic disorder
SIGNS / SYMPTOMS
In some, functional stridor develops as a result of forced adduction of the vocal cords; however, there is no urticaria, angio-oedema, or hypotension.
INVESTIGATIONS
There is no differentiating test.
Scombrotoxic food poisoning
SIGNS / SYMPTOMS
Symptoms resemble IgE-mediated food allergy, but are most prominent on upper torso and face. Erythematous rash, but no urticaria noted. Resolves untreated within 12 hours.
INVESTIGATIONS
Skin test to suspected seafood allergy is negative. Histamine level in consumed fish is elevated.
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