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Mounier-Kuhn syndrome
Mounier-Kuhn syndrome (also called tracheobronchomegaly) is a rare disorder of the lungs characterized by atrophy or an absence of elastic fibres, and a thinning of the smooth muscle layer in the trachea and main bronchi. Due to the decreased muscle and elasticity, the airways become flaccid. The thinning of the smooth muscle and lack or decrease in elastic fibres means that the airways are unable to hold themselves open, and as a result of this, during inhalation they dilate, and during exhalation they collapse. This is due to the normal changes in air pressure inside the lungs during breathing. In 1897, an autopsy by Cyhlarz noted an abnormal dilation of the trachea and proximal bronchi, but it was not until 1932 that a connection was drawn between tracheobronchomegaly and the clinical syndrome of recurrent chronic respiratory tract infections by Mounier-Kuhn. There are a number of other names that Mounier-Kuhn syndrome has been called including tracheal diverticulitis, bronchial diverticulitis, tracheobronchiectasis, tracheocele, tracheomalacia, trachiectasis, megatrachea, and tracheobronchiopathia malacia, but it was recognized in 1962 by Katz et al. that all of these terms referred to the same diagnosis, and coined the term tracheobronchomegaly.

There have been fewer than 100 cases reported in the medical literature since 1932 when the disease was first described, but there has been identification of patients who present few to no symptoms which suggests that the incidence of the disease may be higher than suspected.

There are three sub-types of Mounier-Kuhn syndrome. Type 1 involves a slight symmetric dilation of the trachea and main bronchi. In type 2, the dilation of the trachea and main bronchi becomes distinct, and diverticula develop. In type 3, the diverticula and saccular structures which develop extend to the distal bronchi. One of the main problems associated with these diverticula and saccular structures is the fact that they impair the activity is cilia along the respiratory tract that are responsible for aiding in the expulsion of mucous. The result of this is increased mucous in the respiratory tract which both traps pathogens and provides an environment for them to grow and reproduce, but also decreases the ability to cough up mucous which contains those pathogens. This ultimately leads to increased incidence of respiratory tract infections.

Clinical Manifestations
Mounier-Kuhn syndrome often presents when a person is in their 30s or 40s, but can range from 18 months to 76 years. Occasionally, it can occur and a patient will present with no signs or symptoms of the disease, and therefore it will appear that there is also no progression of the disease. When the disease manifests, however, it often has non-specific signs and symptoms that are, for the most part, indistinguishable from the signs and symptoms of chronic bronchitis or bronchiectasis. For most, the disease will follow a regular course of chronic, recurrent pulmonary infections including recurring pneumonia that can develop into a harsh, productive cough, purulent sputum, occasional hemoptysis, and progressive shortness of breath. It is rare, but if the case is more severe, a patient can experience a spontaneous pneumothorax or clubbing of the fingers.

Causes
The primary cause of Mounier-Kuhn syndrome is largely unknown. There is some evidence to support this being a congenital genetic defect, while other evidence suggests that it is an acquired disease process. Some research suggests a genetic link has been identified between siblings with Mounier-Kuhn syndrome as well as adults who suffer from Ehlers-Danlos syndrome and children with cutis laxa. Although this link of Mounier-Kuhn with connective tissue disorders has been proven in some cases, there is also evidence indicating that those who developed it later in life did not have any evidence of suffering from any other connective tissue disorders. There is some speculation that barotrauma to neonates due to intensive ventilatory therapy and oxygen support is a pathophysiologic factor in the development of Mounier-Kuhn syndrome. In adults, some speculation exists to link chronic inhalation of irritants such as cigarette smoke or air pollution can be important in the development of Mounier-Kuhn syndrome.

Diagnosis
There are many other disorders and diseases that can be mistaken for Mounier-Kuhn syndrome including other congenital disorders such as Ehlers-Danlos syndrome and Williams-Campbell syndrome. Disorders which cause a widening of the airway and lead to tracheal retraction including sarcoidosis, pneumonia, cystic fibrosis, severe fibrosis of the upper lobes can be confused with Mounier-Kuhn syndrome as well as upper airway inflammatory processes such as allergic bronchopulmonary aspergillosis. It is important that a careful evaluation of the airway is done in order to accurately diagnose a patient with Mounier-Kuhn syndrome as well as prescribe the appropriate antibiotic therapy.

Through the use of an X-ray, the diameter of the trachea can be determined which is necessary in diagnosing Mounier-Kuhn syndrome. In males, the following measurements (in millimeters) must be met in order to diagnose a patient with Mounier-Kuhn syndrome : Trachea (transverse/sagittal): 25/27 Left main bronchus: 18 Right main bronchus: 21 In females, the following measurements (in millimeters) must be met in order to diagnose a patient with Mounier-Kuhn syndrome : Trachea (transverse/sagittal): 21/23 Left main bronchus: 17.4 Right main bronchus: 19.8

Not only is it important to rule out other diseases when diagnosing a patient with Mounier-Kuhn syndrome, it is also important to rule out Mounier-Kuhn for many patients. There are many cardio-pulmonary diseases that can manifest from Mounier-Kuhn syndrome, but in order to appropriately treat the patient, utilization of advanced imaging techniques should be utilized in order to rule out the possibility of Mounier-Kuhn.

Management
There are currently no specific treatments for Mounier-Kuhn syndrome, but antibiotics are often used in order to treat the respiratory tract infections that arise due to the disease. Often antibiotic therapy is used as a form of respiratory therapy and also to eliminate the excessive secretions in the respiratory tract. In very advanced cases, prostheses that are permanent may be used to hold the trachea and bronchi in place, but there are no precise indications for this practice.