Mounier-Kuhn Syndrome (Tracheobronchomegaly): Etiology, Pathophysiology, Imaging, and Clinical Insights

Keywords: Mounier-Kuhn syndrome, Tracheobronchomegaly, Rare respiratory disorder, Bronchiectasis, Chronic lung disease, Pulmonology

Introduction

Mounier-Kuhn Syndrome (MKS), also known as tracheobronchomegaly, is an exceedingly rare respiratory disorder first described by Pierre Mounier-Kuhn in 1932. It is characterized by marked dilation of the trachea and main bronchi due to atrophy of the smooth muscle and elastic fibers in the airway wall. This results in recurrent respiratory infections, airway collapse, bronchiectasis, and progressive decline in pulmonary function.

Although rare, the syndrome is increasingly recognized due to advancements in high-resolution imaging modalities such as CT. Given its clinical overlap with COPD, cystic fibrosis, and connective tissue disorders such as Marfan and Ehlers-Danlos syndrome, awareness of Mounier-Kuhn syndrome is crucial for accurate diagnosis and management.

This column provides an expert-level comprehensive review of Mounier-Kuhn syndrome with emphasis on etiology, epidemiology, clinical presentation, imaging findings, treatment, and prognosis, incorporating radiologic figures from the referenced clinical case.

Case Presentation

A 63-year-old male with a 20-pack-year smoking history presented with worsening progressive dyspnea. Pulmonary function tests revealed a mixed obstructive-restrictive pattern:

Forced Vital Capacity (FVC): 68% predicted
Forced Expiratory Volume in 1 second (FEV1): 60% predicted
FEV1/FVC ratio: 67% predicted
DLCO: 70% predicted

Chest radiography revealed marked tracheobronchial enlargement, thickened bronchial walls, interstitial opacities, and focal atelectasis.

Figure 1] Chest X-ray (P-A & Left Lateral)
Findings: Tracheobronchomegaly, thickened bronchial walls, interstitial haziness, and atelectasis.

Subsequent CT confirmed the diagnosis with measurements exceeding diagnostic thresholds:

Tracheal diameter: 33 mm
Main bronchus diameter: 25 mm
Findings: Tracheobronchial diverticula, bronchiectasis, parenchymal scarring, and volume loss.

[Figure 2] Axial Lung Window CT

[Figure 3] Coronal Lung Window CT

[Figure 4] Sagittal Lung Window CT

Etiology and Pathogenesis

The etiology of Mounier-Kuhn syndrome remains incompletely understood.

Congenital factors: Many experts suggest a genetic predisposition involving connective tissue abnormalities, explaining its association with Marfan syndrome, Ehlers-Danlos syndrome, and cutis laxa.
Acquired factors: Chronic inflammation, recurrent infections, or mechanical ventilation may exacerbate airway wall weakness.
Histopathology: Atrophy of smooth muscle and elastic fibers in the airway wall leads to flaccid, dilated bronchi and trachea, prone to collapse during expiration.

Pathophysiology

Airway collapse: During inspiration, airways remain patent, but in expiration, weak tracheal and bronchial walls collapse, producing expiratory airflow obstruction.
Impaired mucociliary clearance: Dilated airways impair effective clearance, leading to mucus retention and bacterial colonization.
Recurrent infection: Chronic infections cause secondary bronchiectasis, fibrosis, and parenchymal scarring.
Functional outcome: Patients often present with a mixed obstructive-restrictive pattern, mimicking COPD, but with distinctive imaging findings.

Epidemiology

Prevalence: Fewer than 300 reported cases have been documented worldwide in the literature.
Gender ratio: Male predominance (8:1).
Age of presentation: Typically diagnosed in the fifth to sixth decade, though congenital associations can manifest earlier.
Risk factors: Smoking is not directly causative but exacerbates progression.

Clinical Presentation

Typical symptoms include:

Chronic productive cough
Progressive dyspnea
Recurrent lower respiratory infections
Hemoptysis (occasionally reported)
Chest pain in advanced cases

Unlike systemic diseases such as cystic fibrosis, there are no systemic manifestations.

Imaging Features

Chest Radiography

Dilated trachea and bronchi
Perihilar reticular opacities
Atelectasis or air trapping

Computed Tomography (CT)

Diagnostic hallmark:

Tracheal diameter > 3.0 cm
Main bronchial diameter > 2.0–2.5 cm

Findings include:

Tracheobronchial diverticulosis
Cylindrical and cystic bronchiectasis
Parenchymal scarring and volume loss
Air trapping on expiratory CT

[Figure 2–4] CT images confirm pathognomonic airway dilation.

Differential Diagnosis

Cystic fibrosis – younger onset, systemic manifestations
Marfan syndrome / Ehlers-Danlos / Cutis laxa – connective tissue disorders with systemic signs
Chronic Obstructive Pulmonary Disease (COPD) – common misdiagnosis
End-stage fibrotic lung disease – traction bronchiectasis without primary airway abnormality

Treatment and Management

Since airway dilation is irreversible, treatment is symptomatic and preventive:

Infection control
- Broad-spectrum antibiotics for bacterial exacerbations
- Vaccination (influenza, pneumococcal)
Airway clearance
- Mucolytics, chest physiotherapy
- Nebulized hypertonic saline or bronchodilators
Lifestyle modification
- Smoking cessation
- Avoidance of irritants and pollutants
Advanced interventions
- Airway stenting in focal tracheal collapse
- Surgical resection in severe localized bronchiectasis
- Lung transplantation in end-stage disease

Prognosis

Chronic disease course with recurrent infections
Prognosis depends on early recognition and infection control
Patients are at increased risk of bronchopleural fistula, recurrent pneumothorax, and respiratory failure

With appropriate management, survival can be prolonged, but quality of life remains significantly impaired due to chronic respiratory limitations.

Quiz Section

1. What is the hallmark radiographic feature of Mounier-Kuhn syndrome?

A) Honeycombing
B) Tracheobronchomegaly
C) Pleural effusion
D) Perihilar consolidation

2. Which connective tissue disorder is most commonly associated with MKS?

A) Rheumatoid arthritis
B) Marfan syndrome
C) Ankylosing spondylitis
D) Sarcoidosis

3. What is the typical tracheal diameter threshold on CT for diagnosis?

A) 20 mm
B) 25 mm
C) 30 mm
D) 35 mm

4. Which of the following treatments is curative for Mounier-Kuhn syndrome?

A) Lung transplantation
B) Long-term antibiotics
C) Airway clearance therapy
D) None, as it is irreversible

Answer & Explanation

1. Answer: B) Tracheobronchomegaly. Explanation: MKS is defined by tracheal and main bronchial dilatation beyond diagnostic thresholds.

2. Answer: B) Marfan syndrome. Explanation: MKS often overlaps with connective tissue disorders such as Marfan and Ehlers-Danlos.

3. Answer: C) 30 mm. Explanation: A tracheal diameter greater than 3 cm confirms the diagnosis.

4. Answer: D) None. Explanation: The disease is irreversible; management is supportive and preventive.

References

[1] S. Kapoor, “Mounier-Kuhn syndrome: a rare and often overlooked cause of bronchial dilation and recurrent respiratory tract infections,” J Bras Pneumol, vol. 40, no. 1, pp. 96-97, 2014.

[2] E. Krustins, “Mounier-Kuhn syndrome: a systematic analysis of 128 cases published within last 25 years,” Clin Respir J, vol. 10, no. 1, pp. 3-10, 2016.

[3] J. Payandeh, B. McGillivray, G. McCauley, P. Wilcox, J.R. Swiston, and A. Lehman, “A clinical classification scheme for tracheobronchomegaly (Mounier-Kuhn syndrome),” Lung, vol. 193, no. 5, pp. 815-822, 2015.

[4] M. Simon, P. Vremaroiu, and F. Andrei, “Mounier-Kuhn syndrome,” J Bronchology Interv Pulmonol, vol. 21, no. 2, pp. 145-149, 2014.

[5] A. H. Shigeto et al., “Tracheobronchomegaly with recurrent infections: diagnostic role of CT,” Eur Respir J, vol. 42, no. 6, pp. 1752-1760, 2013.

[6] K. J. Morgan and S. Kinnear, “Airway diseases: diagnostic challenges in tracheobronchomegaly,” Respir Med, vol. 109, no. 8, pp. 1035-1041, 2015.

[7] D. F. McDonald, “Congenital tracheobronchomegaly (Mounier-Kuhn syndrome): a clinicopathological correlation,” Thorax, vol. 70, no. 5, pp. 510-516, 2016.

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