Mounier-Kuhn Syndrome (MKS) is a rare clinical-radiologic entity characterized by marked dilation of the trachea and mainstem bronchi. This occurs due to the congenital atrophy or absence of elastic fibers and smooth muscle cells within the tracheobronchial wall, leading to redundant connective tissue and “pouching” between the tracheal rings.
Diagnostic Snapshot
- The Hallmark: Tracheal diameter exceeding upper limits of normal.
- Key Symptom: “Goose-honk” cough with recurrent lower respiratory tract infections.
- The “Gold Standard” Imaging: Multidetector CT (MDCT) with expiratory phases.
1. Clinical Presentation & Pathogenesis
MKS typically manifests in the 3rd or 4th decade of life, though it is often misdiagnosed for years as COPD or asthma. The weakened airway walls result in:
- Tracheobronchomegaly: Massive dilation during inspiration.
- Tracheobronchomalacia: Dynamic collapse during expiration.
- Tracheal Diverticulosis: Herniation of mucosa through the weakened wall, often leading to secretions and infection.
2. Radiologic Diagnostic Criteria
To rank for “Mounier-Kuhn measurements,” providing the specific thresholds is critical.
Chest Radiograph (CXR)
While difficult to diagnose on a standard PA view, the lateral view may show a caliber of the trachea exceeding the diameter of the vertebral bodies.
Computed Tomography (CT) – The Diagnostic Pillar
On a standard thoracic CT, the diagnosis is made by measuring the internal diameters of the airways 2 cm above the aortic arch:
| Airway Segment | Adult Male Threshold | Adult Female Threshold |
| Trachea (Transverse) | > 25 mm | > 21 mm |
| Trachea (Sagittal) | > 27 mm | > 23 mm |
| Right Main Bronchus | > 18 mm | > 15 mm |
| Left Main Bronchus | > 15 mm | > 13 mm |
3. Advanced Imaging: Dynamic Expiratory CT
A static inspiratory CT might miss the tracheobronchomalacia associated with MKS.
- Expiratory Phase: Shows a collapse of the airway lumen by >50%.
- Tracheal Diverticula: Best visualized on CT as small, air-filled outpouchings along the posterior or lateral tracheal wall.
- Associated Findings: Look for “saber-sheath” appearance (if chronic) and lower lobe bronchiectasis or “tree-in-bud” opacities from chronic infection.
4. Differential Diagnosis
Radiologists must distinguish MKS from other causes of large airways:
- Williams-Campbell Syndrome: Bronchomalacia limited to the 4th–6th order bronchi (trachea is usually normal).
- Tracheomalacia (Isolated): Dynamic collapse without the massive inspiratory dilation.
- Chronic Obstructive Pulmonary Disease (COPD): Can cause “Saber-sheath trachea,” but the coronal diameter is narrowed, whereas in MKS, all diameters are increased.
5. Classification of Mounier-Kuhn
- Type 1: Subtle, symmetric dilation of the trachea and main bronchi.
- Type 2: Distinctive dilation and diverticula.
- Type 3: Dilation and diverticula extending to the distal bronchi.
