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Mounier-Kühn syndrome, also known as idiopathic tracheobronchomegaly, is a congenital disorder characterized by marked dilatation of the trachea and proximal bronchi that leads to impaired airway secretion clearance and recurrent lower respiratory tract infections.
It is a rare underdiagnosed condition of unknown prevalence. About 300 cases have been reported to date. The condition is more frequent in men.
The disorder may manifest at various ages but is usually diagnosed in young adults with recurrent bronchopulmonary infections (e.g. bronchitis, pneumonia), and tracheobronchial irritation of varying degrees of severity causing chronic dry or productive cough with purulent sputum production, dyspnea (occasionally on exertion) and hemoptysis. Progressive hoarseness has also been observed. Bronchial crackles and/or wheezing and finger clubbing are common. In mild cases, patients are asymptomatic or present only with chronic cough and have normal pulmonary function. In the rare severe cases, patients may suffer from pulmonary obstructive diseases such as bronchiectasis or bullous emphysema, and airflow obstruction that can lead to chronic respiratory failure.
Mounier-Kühn syndrome results from the atrophy of elastic fibers in the trachea and main bronchi which lead to thinning of the smooth muscle layer and subsequent tracheobroncheal flaccidity, dilatation and collapse. The etiologic mechanism remains unknown. Irritants like cigarette smoke and air pollution could act as irritating factors. The disease is sometimes associated with connective tissue diseases such as Ehlers-Danlos syndrome, Marfan syndrome, and cutis laxa (see these terms) and might have a genetic origin; however no gene mutation has been identified to date.
Mounier-Kühn syndrome is diagnosed when the coronal and the sagittal diameters of the trachea, measured on plain chest radiography or computed tomography (CT), are greater than 25 mm and 27 mm, respectively, in males, and greater than 21 mm and 23 mm in females. Fiberoptic bronchoscopy shows dilatation of the trachea and proximal bronchi, expiratory collapse in patients with tracheomalacia, and possible presence of diverticula on the posterior wall. Biopsies of broncheal and tracheal wall tissue show loss of elastic fibers but are usually unnecessary. Pulmonary function tests can be normal or show varying degrees of airflow obstruction and increased residual volume.
Differential diagnosis includes lower respiratory tract infections like bronchitis and chronic obstructive pulmonary disease, ankylosing spondylitis, and Williams-Campbell syndrome (see these terms).
The disease is sporadic in most cases, although several familial cases have been described.
Management and treatment
Treatment is symptomatic. Chest physiotherapy can be proposed to improve mucociliary clearance and antibiotics are administered for treatment of pulmonary infections. Some patients may benefit from non-invasive positive-pressure ventilation, also needed at night in severe cases. Insertion of a tracheal or tracheobronchial stent, with or without surgical tracheobronchoplasty, was shown to reduce airway collapsibility in some cases with tracheomalacia. Double lung transplantation has been successful in one case.
The impact of this condition on life expectancy has not been evaluated. Complications include respiratory infections, tracheomalacia, pulmonary obstructive diseases, pneumothorax, and lung fibrosis that can lead to chronic respiratory failure.