STEPTRACKERRESPIRATORY | PATHOLOGY + IMMUNOLOGY | HIGH-YIELD REVIEW

Asthma & Bronchiectasis

Reversible Th2-driven bronchospasm, and permanent bronchial dilation after destructive inflammation.

FOCUS ON
Asthma: IL-4 → IgE, IL-5 → eosinophils, IL-13 → mucus. Early-phase mast cells, late-phase eosinophils. Curschmann spirals and Charcot-Leyden crystals. A normal or rising PaCO₂ in an attack is ominous. Bronchiectasis: copious purulent sputum and tram-track airways — CF, Kartagener, ABPA, prior infection.
TOPIC
DETAIL
MECHANISM
CLUE
Asthma — definition
Episodic, reversible bronchoconstriction with airway hyperresponsiveness and inflammation.
Wheeze, dyspnea, chest tightness, cough (often at night)
Atopy: eczema, allergic rhinitis, family history
Allergic (atopic) mechanism
Allergen → Th2: IL-4 (IgE switch), IL-5 (eosinophils), IL-13 (mucus).
Early phase: IgE cross-linking on mast cells → histamine, leukotrienes · late phase: eosinophils, major basic protein
Type I hypersensitivity
Other triggers
Viral infection, exercise, cold air, aspirin (AERD), occupational agents, GERD, β-blockers.
Aspirin blocks COX → arachidonic acid shunted to leukotrienes
Asthma + nasal polyps + aspirin reaction
Pathology
Mucus plugs, eosinophils, smooth muscle hypertrophy, thickened basement membrane, goblet cell hyperplasia.
Curschmann spirals (shed epithelium), Charcot-Leyden crystals (eosinophil protein)
Chronic remodeling → partly fixed obstruction
Testing
Obstructive PFTs reversing with bronchodilator (FEV1 ↑ ≥ 12% and 200 mL).
Normal baseline? → methacholine challenge
DLCO normal or ↑
Status asthmaticus
Severe attack not responding to treatment; pulsus paradoxus.
Tiring muscles → CO₂ climbs back to "normal" or high
Normal/↑ PaCO₂ during an attack = impending failure
Treatment
SABA rescue; ICS controller; LABA only with ICS; montelukast; biologics: omalizumab (IgE), mepolizumab (IL-5), dupilumab (IL-4Rα).
Target the Th2 cascade
Eosinophilic severe asthma → anti-IL-5
Bronchiectasis — definition
Permanent bronchial dilation after necrotizing inflammation destroys muscle and elastic tissue.
Chronic infection + impaired clearance → vicious cycle
Copious foul purulent sputum, hemoptysis, clubbing; tram-track / signet-ring sign on CT
Bronchiectasis — causes
Cystic fibrosis, primary ciliary dyskinesia (Kartagener: situs inversus, sinusitis, infertility), post-infection (TB, measles, pertussis), obstruction (tumor, foreign body), ABPA, immunodeficiency.
Kartagener: dynein arm defect · ABPA: hypersensitivity to Aspergillus in asthma/CF
Complications: AA amyloidosis, cor pulmonale

Mechanism — allergic asthma

AllergenDendritic cell
→
Th2IL-4, IL-5, IL-13
→
IgE on mast cellsSensitized
→
Early phaseHistamine, LTs → bronchospasm
→
Late phaseEosinophils → inflammation
→
RemodelingSmooth muscle, BM, goblet cells

Signature findings

FINDING
MEANING
Curschmann spirals
Coiled mucus casts of shed epithelium — asthma
Charcot-Leyden crystals
Crystallized eosinophil protein (galectin-10) — asthma, parasites
Reid index > 0.4
Mucous gland hyperplasia — chronic bronchitis
Tram-track airways
Dilated, thick-walled bronchi — bronchiectasis
Situs inversus + sinusitis + infertility
Kartagener syndrome
↑ IgE + eosinophilia + central bronchiectasis
ABPA

How it's tested

COMMON CONFUSIONAsthma obstruction reverses with a bronchodilator. COPD obstruction largely does not, although the two can overlap.
THE TRAPA wheezing patient early in an attack has a low PaCO₂ from hyperventilation. If PaCO₂ is "normal" while still severely wheezy, the patient is tiring and may need intubation.
YOUR APPROACHEpisodic + reversible → asthma → trigger and phenotype (allergic, AERD, eosinophilic). Chronic purulent sputum + dilated airways → bronchiectasis → find the cause.

If you see… think…

Child with nocturnal cough, wheeze, eczema, eosinophilia → allergic asthma
Coiled mucus casts in sputum → Curschmann spirals
Needle-shaped crystals from eosinophils → Charcot-Leyden crystals
FEV1 rises 15% after albuterol → reversible obstruction (asthma)
Symptoms but normal spirometry → methacholine challenge
Severe attack with PaCO₂ 44 → respiratory fatigue — prepare to intubate
Asthma + nasal polyps + aspirin sensitivity → AERD
Copious foul sputum, clubbing, tram-track CT → bronchiectasis
Situs inversus, chronic sinusitis, male infertility → Kartagener syndrome
Asthmatic with fleeting infiltrates, very high IgE, central bronchiectasis → ABPA