Eosinophilic granuloma, also known as pulmonary histiocytosis X (PHX) or pulmonary Langerhans cell histiocytosis X (PLCH), is an uncommon interstitial lung disease that is epidemiologically related to tobacco smoking. It chiefly affects young adults, primarily occurring in the third or fourth decades of life. 
See also Imaging in Eosinophilic Granuloma of the Skeleton and Langerhans Cell Histiocytosis Imaging.
Pulmonary Langerhans cell histiocytosis X (PLCH) is histologically characterized by parenchymal infiltration of the lungs by activated Langerhans cells. Langerhans cells are differentiated cells of the dendritic cell system and are closely related to the monocyte-macrophage line. These antigen-presenting cells are normally found in the skin, reticuloendothelial system, heart, pleura, and lungs. They may be identified by immunohistochemical staining or by the presence of Birbeck granules via electron microscopy.
PLCH is similar to pediatric histiocytic disorders (Letterer-Siwe disease and Hand-Sch ü ller-Christian disease). However, in contrast to pediatric histiocytoses, which involve multiple organs, PLCH usually manifests in a single organ—the lung. About 4-20% of patients with PLCH also have cystic lesions in the bones. Other organ systems are only rarely affected. 
The accumulation of Langerhans cells in the lungs is hypothesized to occur in response to exposure to cigarette smoke. Supporting this hypothesis is the finding that the initial histologic and radiographic findings are peribronchiolar. In addition, the disease is most prominent in the upper and middle lung zones, as seen in other smoking-related lung diseases. The granulomatous infiltrates seen in PLCH are composed of Langerhans cells, eosinophils, lymphocytes, macrophages, plasma cells, and fibroblasts, which form nodules centered on the terminal and respiratory bronchioles, causing destruction of the airway walls. In late stages of the disease, fibrotic stellate scarring occurs, and end-stage PLCH is characterized by this scarring along with cystic spaces and honeycombing.
Pulmonary Langerhans cell histiocytosis X (PLCH) is a rare disorder and the true prevalence is unknown. At 1 specialty referral center in the United States, PLCH was identified in less than 5% of patients who underwent lung biopsy for the diagnosis of interstitial lung disease.  At another center, 15 cases of PLCH were found after lung biopsy, compared with 274 cases of sarcoidosis. 
In Belgium, 3% of patients evaluated at 20 pulmonary referral centers were diagnosed with PLCH.  A large Japanese study estimated the prevalence of pulmonary Langerhans cell histiocytosis X (PLCH) at 0.27 males and 0.07 females per 100,000 population based on hospital discharge diagnoses over a 1-year period.  Scant epidemiologic data are available regarding this disease in the developing world.
Because of the rarity of pulmonary Langerhans cell histiocytosis X (PLCH) , no definitive epidemiologic data related to racial background are available.
No sex predilection is recognized for pulmonary Langerhans cell histiocytosis X (PLCH).
The peak incidence of pulmonary Langerhans cell histiocytosis X (PLCH) occurs in the 20- to 40-year age bracket.
The prognosis for pulmonary Langerhans cell histiocytosis X (PLCH) varies and is related to smoking cessation. Most patients who continue to smoke experience disease progression, but for those who successfully quit smoking, the disease often stabilizes or regresses.
Retrospective studies suggest that the following factors are associated with adverse outcomes in PLCH:
Extremes of age
Extensive cysts and honeycombing radiographically
Prolonged corticosteroid therapy
Abnormal pulmonary function, including reduced gas exchange, as measured by diffusing capacity for carbon monoxide; obstructive ventilatory defect (reduced ratio of forced expiratory volume in 1 second (FEV 1) to forced vital capacity [FVC], or FEV 1/FVC); and/or evidence of airtrapping (high residual volume/total lung capacity)
Young men who have diabetes insipidus have the worst prognosis.
Favorable signs include the radiographic finding of sparing of the costophrenic angles and a cellular, nonfibrotic biopsy specimen.
Pulmonary Langerhans cell histiocytosis X (PLCH) has a highly variable course. Some patients have spontaneous remissions, especially when they stop cigarette smoking, whereas others progress to end-stage fibrotic lung disease.
Factors associated with a poorer prognosis include multisystem involvement other than bone (including diabetes insipidus related to pituitary involvement), recurrent pneumothorax, severe pulmonary artery hypertension, older age at diagnosis, severe pulmonary function test abnormalities, and more widespread cystic changes on imaging studies.
Cigarette smoking is important to disease activity. Smoking worsens morbidity and mortality. Smoking cessation frequently stabilizes the disease and sometimes leads to its regression.
The public must be educated about the likely etiologic role of cigarette smoking. Pulmonary Langerhans cell histiocytosis X (PLCH)is thought to be largely preventable through smoking cessation.
Instruct patients to promptly report the development of hemoptysis. This symptom may indicate malignancy or superimposed bacterial/fungal infection, such as Aspergillus species infection.
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