asthma mimics

When “asthma” does not respond to appropriate therapy, reconsider the diagnosis. The most commonly missed mimics are vocal cord dysfunction, upper airway obstruction, and obesity-related dyspnoea. The flow-volume loop on pulmonary function tests is the single most useful screening tool.

when to suspect a mimic

Asthma diagnosed without objective airflow limitation
No response to adequate ICS-LABA trial (adherence and technique confirmed)
Fixed airflow obstruction (no BD reversibility, no variability)
Atypical features: stridor, inspiratory wheeze, acute onset without trigger
Onset in middle age with no atopic history

differential

vocal cord dysfunction (VCD) / inducible laryngeal obstruction (ILO)

Paradoxical adduction of vocal cords during inspiration (and sometimes expiration).

Symptoms: episodic dyspnoea, throat tightness, stridor (not wheeze), cough; often triggered by exercise, strong odours, stress
Key clues: symptoms localise to throat/neck; onset and offset are rapid; SpO2 preserved; SABA does not help
FVL: flattened or truncated inspiratory limb (variable extrathoracic obstruction pattern)
Diagnosis: direct visualisation via laryngoscopy during symptomatic episode (continuous laryngoscopy during exercise if exercise-triggered)
Management: speech language pathology (laryngeal control techniques), treat comorbid GERD/rhinosinusitis, psychology if stress-related
Exercise-induced laryngeal obstruction (EILO) is a specific subtype common in young athletes — diagnose with continuous laryngoscopy during exercise
Coexists with asthma in up to 40% — both diagnoses may be present

subglottic / tracheal stenosis

Fixed upper airway obstruction — flattening of both inspiratory and expiratory limbs on FVL
Idiopathic subglottic stenosis: typically middle-aged women (F:M ~10–20:1); often years of misdiagnosis as “asthma”; symptoms typically occur when airway narrows to < 5–6 mm (normal trachea 15–20 mm)
Other causes: post-intubation, granulomatosis with polyangiitis (GPA), relapsing polychondritis, goitre, tracheal tumour
Clue: “wheezing” is actually monophonic stridor localised to the neck; does not respond to bronchodilators
Diagnosis: CT neck/airway, bronchoscopy, ENT assessment
Management: balloon dilatation, surgical resection, stenting; depends on aetiology

BMI > 30 independently causes dyspnoea via reduced chest wall compliance, increased work of breathing, and reduced FRC
PFTs may show low FVC and FEV1 with preserved ratio (non-specific pattern) — often misinterpreted as restriction or asthma
DLCO: normal or supranormal (increased pulmonary blood volume)
Methacholine challenge is negative (unless true coexisting asthma)
May coexist with true asthma — but if obesity is the dominant contributor, bronchodilators will not help. Weight loss is the primary intervention

endobronchial lesions

Carcinoid tumour, endobronchial metastases, foreign body, mucous plugging
Clue: unilateral wheeze, haemoptysis, focal abnormality on CT
Diagnosis: CT chest, bronchoscopy

cardiac disease

Heart failure with pulmonary oedema (“cardiac asthma”) — orthopnoea, bilateral crackles, elevated BNP, cardiomegaly
Left atrial enlargement causing bronchial compression
Clue: responds to diuresis, not bronchodilators; wheeze is bilateral and associated with fluid overload

other mimics

Mimic	Key distinguishing feature
COPD	Fixed obstruction, smoking history, low DLCO, older age
Bronchiectasis	Chronic productive cough, CT shows airway dilation
Hyperventilation / anxiety	Normal PFTs, normal SpO2 during episodes, tingling, carpopedal spasm
Eosinophilic bronchitis	Cough (not wheeze), normal spirometry, elevated sputum eosinophils, responds to ICS
Tracheobronchomalacia	Expiratory central airway collapse; dynamic CT or bronchoscopy

diagnostic approach — “refractory asthma” that isn’t asthma

Review original diagnosis — was variable airflow limitation ever documented?
Full PFTs with FVL review — obstruction? restriction? FVL shape abnormal?
FeNO — low FeNO (< 25 ppb) off ICS makes eosinophilic asthma less likely
BD response and methacholine challenge — if both negative off ICS, asthma is unlikely (PC20 > 16 mg/mL essentially rules out asthma)
CT chest — bronchiectasis, endobronchial lesion, ILD, emphysema
Laryngoscopy — if VCD suspected (inspiratory symptoms, throat tightness, normal SpO2)
Echocardiography / BNP — if cardiac cause suspected
Consider: GERD investigation, sleep study (OSA), anxiety screening