Recognizing tracheobronchomalacia

Tracheobronchomalacia is a form of expiratory central airway collapse characterized by softening of the airway wall cartilaginous structures. Symptoms often mimic asthma and chronic obstructive pulmonary disease. Pulmonary function test results may suggest a diagnosis, but findings are neither sensitive nor specific. Bronchoscopy and novel dynamic radiographic studies contribute to the diagnosis and help differentiate true malacia from other forms of expiratory central airway collapse. Treatment options include medication; noninvasive ventilatory support; interventional bronchoscopy with airway stent insertion; and open surgical procedures, such as tracheostomy, tracheal resection, and tracheoplasty. (J Respir Dis. 2006;27(8):327-335) Tracheobronchomalacia (TBM) is a disease of the central airways characterized by weakness of the tracheal and bronchial walls caused by the softening of supporting cartilages.1 As a result, the airways lose their stiffness and the airway walls move closer together, especially during expiration. The congenital form of TBM may be associated with other conditions, some of which allow survival of affected children into adulthood. Congenital syndromes and anomalies associated with TBM include Ehlers-Danlos syndrome, trisomy 21 syndrome, tracheoesophageal fistula, double aortic arch, and pectus excavatum. Pediatric TBM has been recently reviewed.2The adult forms of TBM are either idiopathic or acquired during the course of other illnesses.1,2 TBM has also been described as tracheobronchial collapse,3,4 expiratory tracheobronchial collapse, expiratory tracheobronchial stenosis,5 and tracheobronchial dyskinesia.6 More recently, it seems that true malacia should be regarded as a form of expiratory central airway collapse.7 The incidence of TBM is unknown; however, it appears to be more common in middle-aged and elderly men who are smokers than in other populations.8,9 In this article, we review the signs and symptoms of TBM and its known causes, and we describe diagnosis and treatment.PATHOGENESIS The malacic airway has a crescent-shaped or "saber-sheath" appearance on bronchoscopic or radiographic examination, depending on whether the anterior or lateral walls of the airway, respectively, are weakened (Figure 1). The abnormality is circumferential when a combination of crescent-type and saber-sheath-type narrowing occurs. This abnormality is often seen in patients with relapsing polychondritis. The normal tracheal cartilage-to-soft tissue ratio is approximately 4.5:1. In patients with TBM, however, this ratio is often as low as 2:1. The cartilaginous rings may be destroyed or thinned out, limiting airflow. Atrophy of elastic fibers in the air-ways of these patients has also been described, but this probably characterizes the entity referred to as excessive dynamic airway collapse. Although excessive dynamic airway collapse is seen mainly in patients who have emphysema and asthma, it can also be idiopathic.9 TBM is a form of dynamic airway obstruction that may cause hyperinflation and air trapping. During normal expiration, an increase in intrathoracic pressure causes intrathoracic airway narrowing, which is counteracted by tracheobronchial rigidity, elevated intraluminal pressures, and structural attachments. In the malacic intrathoracic airway, however, expiratory collapse occurs when intrathoracic pressure exceeds intratracheal pressure. This event limits airflow in patients who have malacia, and it may eventu-ally contribute to dyspnea, difficulty in clearing secretions, recurrent infections, and even respiratory failure.ETIOLOGY The causes of TBM include the following: • Indwelling tracheotomy and endotracheal intubation with inflatable cuffs are considered to be common causes of acquired TBM, most likely resulting from pressure necrosis, decreased blood supply, infection, and friction of tubes on the airway mucosa.10,11 The malacic areas are focal and are usually seen at the site of the inflatable cuff, adjacent to the tracheostomy stoma, or at the point of impingement of the tip of the tracheostomy or endotracheal tube on the tracheal wall.10 • Long-term ventilation has been described in patients with Duchenne dystrophy who have had many years of positive pressure ventilation via tracheostomy.12 It is not clear whether the myopathic