Mounier-Kuhn Syndrome

Mounier-Kuhn syndrome is marked dilatation of the trachea and main bronchi (tracheobronchomegaly) from thinning of the elastic and smooth muscle layers. It is rare, found mostly in men aged 30 to 50 years, and often leads to recurrent lower respiratory infection and bronchiectasis.

A tracheal diameter over 3.0 cm in men or 2.5 cm in women (measured 2 cm above the aortic arch), with a right main bronchus over 2.4 cm and a left over 2.3 cm, defines the disorder on CT.

Pathophysiology. Atrophy of longitudinal elastic fibres and smooth muscle leads to a flaccid airway that dilates on inspiration and collapses on expiration, and to tracheal diverticula between the cartilage rings. Poor cough and impaired mucociliary clearance cause retained secretions and infection.

It may be congenital or sporadic, or occur with connective tissue disorders (Ehlers-Danlos syndrome, cutis laxa, Marfan syndrome), ankylosing spondylitis and HIV.

Chronic productive cough, recurrent chest infection, dyspnoea, haemoptysis, and a loud, ineffective "honking" cough. Some are found incidentally on imaging. Complications include bronchiectasis, pneumothorax and respiratory failure.

Intubation carries a risk of tracheal injury and inadequate cuff seal, so the diagnosis must be flagged before any anaesthesia. Severe infection with respiratory failure needs urgent treatment.

• CT chest with measurement of tracheal and bronchial diameters, and tracheal diverticula, bronchiectasis and emphysema. Dynamic expiratory CT shows airway collapse of more than 50%.
• Flexible bronchoscopy (with care) showing a flaccid, dilated trachea with redundant posterior membrane and diverticula, collapsing on cough.
• Spirometry: obstructive or mixed pattern, with a flow volume loop showing a sawtooth or reduced expiratory flow. Full lung function and oximetry.
• Sputum cultures for bacteria, mycobacteria and fungi, immunoglobulins and HIV test.
• Search for connective tissue disorder: joint hypermobility, skin laxity, vascular and ocular features, with echocardiography and genetic review when suspected.
• Distinguish from acquired tracheomegaly (prolonged ventilation, fibrotic lung disease with traction), by history and the absence of such a context.

• Airway clearance: huffing, positive expiratory pressure, chest physiotherapy, and nebulised hypertonic saline 3 to 7% 4 mL twice daily after salbutamol. Carbocisteine 750 mg three times daily as a mucolytic.
• Treat exacerbations promptly with culture directed antibiotics for 10 to 14 days, empirically amoxicillin-clavulanate 625 mg three times daily or doxycycline 200 mg on day 1 then 100 mg daily, adding pseudomonal cover when previously isolated (ciprofloxacin 500 to 750 mg twice daily).
• Prophylaxis for 3 or more exacerbations a year: azithromycin 250 mg daily or 500 mg three times weekly after excluding nontuberculous mycobacteria and checking QTc.
• Vaccination against influenza, pneumococcus and COVID-19, smoking cessation and avoidance of irritants.
• Severe collapse on expiration: continuous positive airway pressure or non-invasive ventilation, a temporary silicone tracheal stent in interventional centres (complications include migration and granulation), and tracheobronchoplasty with a mesh splint in carefully selected patients.
• Anaesthesia: inform the anaesthetist, use a cuffed tube of suitable size with cuff pressure kept at the lowest to seal, since over inflation can rupture a thin trachea.
• Lung transplantation for advanced bronchiectasis with failure.

Complications and prognosis. Progressive bronchiectasis, recurrent infection, haemoptysis and respiratory failure. Outcome depends on the infection control and lung damage at diagnosis.

Monitoring. Review every 3 to 6 months with spirometry and sputum culture, and CT only when symptoms change.

Referral and escalation. Refer to an interventional pulmonologist for stenting or surgery, and to the transplant team in advanced disease.