|Classification and external resources|
Bronchiolitis obliterans (BO), also called obliterative bronchiolitis (OB) and constrictive bronchiolitis (CB), is a rare and life-threatening form of non-reversible obstructive lung disease in which the bronchioles (small airway branches) are compressed and narrowed by fibrosis (scar tissue) and/or inflammation. Bronchiolitis obliterans is also sometimes used to refer to a particularly severe form of pediatric bronchiolitis caused by adenovirus.
Bronchiolitis means inflammation of the bronchioles and obliterans refers to the inflammation or fibrosis of the bronchioles partially or completely obliterating the airways. Bronchiolitis obliterans has always been thought to be irreversible and progressive,  but a new study that uses inhaled cyclosporine has been shown to increase the FEV1% while patients are able to decrease their systemic steroid doses. 
Signs and symptoms
Bronchiolitis obliterans is a lung disease characterized by fixed airway obstruction. Inflammation and scarring occur in the airways of the lung, resulting in severe shortness of breath and dry cough.
FEV1 (forced expiratory volume in 1 second) should be above 80% of predicted values to be considered normal. Bronchiolitis obliterans reduces this to 16% to 21%.
- dry cough
- shortness of breath
Bronchiolitis obliterans has many possible causes, including: collagen vascular disease, transplant rejection in organ transplant patients, viral infection (respiratory syncytial virus, adenovirus, HIV, cytomegalovirus), Stevens-Johnson Syndrome, Pneumocystis pneumonia, drug reaction, aspiration and complications of prematurity (bronchopulmonary dysplasia), and exposure to toxic fumes, including: diacetyl, sulfur dioxide, nitrogen dioxide, ammonia, chlorine, thionyl chloride, methyl isocyanate, hydrogen fluoride, hydrogen bromide, hydrogen chloride, hydrogen sulfide, phosgene, polyamide-amine dyes, mustard gas and ozone. It can also be present in patients with rheumatoid arthritis. Certain orally administrated emergency medications, such as activated charcoal, have been known to cause it when aspirated. Additionally, the disorder may be idiopathic (without known cause).
There are many industrial inhalants that are known to cause various types of bronchiolitis, including bronchiolitis obliterans. 
Industrial workers who have presented with bronchiolitis:
- nylon-flock workers 
- workers who spray prints onto textiles with polyamide-amine dyes 
- battery workers who are exposed to thionyl chloride fumes
- workers at plants that use or manufacture flavorings, e.g. diacetyl butter-like flavoring 
Diacetyl (popcorn worker's lung)
In rare instances, bronchiolitis obliterans may be caused by inhalation of airborne diacetyl, a chemical used to produce the artificial butter flavoring in many foods such as candy and microwave popcorn and occurring naturally in wines. This first came to public attention when eight former employees of the Gilster-Mary Lee popcorn plant in Jasper, Missouri, developed bronchiolitis obliterans. In 2000, the Missouri Department of Health called in NIOSH to make a determination of the cause, and to recommend safety measures. After surveying the plant and each patient's medical history, NIOSH recommended respiratory protection for all workers in microwave popcorn production. Due to this event, bronchiolitis obliterans began to be referred to in the popular media as "Popcorn Lung" or "Popcorn Workers Lung". Bronchiolitis obliterans caused by diacetyl inhalation begins with severe cough and dyspnea (shortness of breath), and can progress very rapidly or slowly. CT images show bronchial wall thickening and trapped air. Non-smokers may be at higher risk for this form of bronchiolitis obliterans. The Occupational Safety and Health Administration (OSHA) recommended that diacetyl manufacturing companies regularly sample air in work environments, provide air purifying respirators, and engage in medical surveillance of at-risk workers. In 2011, National Institute for Occupational Safety and Health proposed a recommended short-term exposure limit of 25 parts per billion (ppb) and a time weighted average exposure of 5 ppb.
In 2007 a heavy consumer of microwaved popcorn was diagnosed by a doctor in Denver with "popcorn lung," the first known case involving a consumer. On 16 January 2008, it was announced that Wayne Watson, the Denver consumer who developed "popcorn lung" after inhaling fumes from microwaved popcorn, was suing the Kroger grocery store chain and its affiliates. In the lawsuit, filed in U.S. District Court, Watson's attorney claimed that the companies "failed to warn that preparing microwave popcorn in a microwave oven as intended and smelling the buttery aroma could expose the consumer to an inhalation hazard and a risk of lung injury." On September 19, 2012 a jury in U.S. District Court in Denver awarded $2.3 million in actual damages and $5 million in punitive damages to Wayne Watson of Centennial, Colorado who habitually consumed microwave popcorn for a decade. Defendants included Glister-Mary Lee, the manufacturer; Kroger, and Kroger's subsidiary Dillons, owners of King Soopers & City Market, a Colorado regional supermarket chain.
In September 2007, Dr. Cecile Rose, pulmonary specialist at Denver's National Jewish Medical and Research Center, warned federal agencies that consumers, not just flavoring or food factory workers, may be in danger of contracting bronchiolitis obliterans. David Michaels, of the George Washington University School of Public Health, first published Rose's warning letter on his blog.
On 4 September 2007, the Flavor and Extract Manufacturers recommended reduction of diacetyl in butter-like flavorings. The next day ConAgra Foods announced that it would soon remove diacetyl from its popcorn products.
Diacetyl is approved by the Food and Drug Administration as a safe flavor ingredient, but there is evidence to suggest that inhalation in large amounts is dangerous. There are currently no warnings from federal regulators about diacetyl.
A NIOSH peer-reviewed publication documents that acute inhalation exposures to acetyl propionyl(2,3-pentanedione) cause airway epithelial damage that is as harmful as diacetyl in laboratory studies.
A new form of constrictive bronchiolitis is starting to present in Iraq and Afghanistan Veterans. It has been attributed to Veterans being exposed to burn pits in these theaters to get rid of trash. Veterans present with shortness of breath and other asthma like symptoms. The only way to diagnose this condition in Veterans is by doing a lung biopsy as chest x-rays and CT scans come back as normal. The government still denies that there is any correlation between burn pits and health problems but the government has started an "Airborne Hazards and Open Burn Pit Registry" to begin tracking the health of Veterans who were exposed to burn pits to see if there is a connection.
Diagnosis may include the following tests:
- Chest X-rays tests.
- Diffusing capacity of the lung (DLCO) tests are usually normal.
- Spirometry tests show fixed airway obstructions and sometimes restriction. FEV1/FVC may therefore be <75%.
- Lung volume tests may show hyperinflation (excessive air in lungs caused by air trapping).
- High-resolution computerized tomography scans of the chest at full inspiration and expiration may reveal heterogeneous air trapping on the expiratory view as well as haziness and thickened airway walls.
- Lung biopsies may reveal evidence of constrictive bronchiolitis obliterans (i.e., severe narrowing or complete obstruction of the small airways). An open lung biopsy, such as by thoracoscopy, is more likely to be diagnostic than a transbronchial biopsy. Special processing, staining, and review of multiple tissue sections may be necessary for a diagnosis.
This disease is irreversible and severe cases often require a lung transplant. Transplant recipients are at risk for re-developing the disease, as bronchiolitis obliterans is a common complication of chronic rejection. Evaluation of interventions to prevent bronchiolitis obliterans relies on early detection of abnormal spirometry results or unusual decreases in repeated measurements.
A multi-center study has shown the combination of inhaled Fluticasone propionate, oral Montelukast, and oral Azithromycin may be able to stabilize the disease and slow disease progression. This has only been studied in patients who previously underwent hematopoietic stem cell transplantation.
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