Cystic lung disease is an uncommon phenomenon that is seen in various conditions of autoimmune, genetic, neoplastic, and inflammatory origin. As a result, its signs and symptoms may considerably vary. Imaging studies mainly computed tomography, is the gold standard for diagnosis. Biopsy and subsequent histopathological evaluation, are also required to determine the underlying cause of the condition.
The clinical presentation of patients who develop cystic lung disease is diverse since numerous conditions have been described as potential causes. The following disorders should be noted     :
- Pulmonary Langerhans cell histiocytosis (PLCH) - Primarily diagnosed in younger adults who are smokers, PLCH is caused by peribronchial infiltration of Langerhans dendritic cells and subsequent formation of cavitating nodules that turn into cysts . Symptoms may be absent in up to 25% of patients, but a dry cough, dyspnea on exertion, fever, weight loss and night sweats are most important complaints . Chest pain, occurring due to a spontaneous pneumothorax, is an important diagnostic hallmark of PLCH, whereas extrapulmonary manifestations (diabetes insipidus presenting as polyuria and polydipsia, as well as bone and skin lesions) are rare .
- Lymphangioleiomyomatosis (LAM) - Dyspnea, recurrent pneumothoraces, and hemoptysis are the main symptoms of this disease characterized by infiltration of smooth muscle cells into the bronchial tree and the nodal systems of the thorax and abdomen . LAM is seen exclusively in women of childbearing age .
- Lymphoid interstitial pneumonia (LIP) - A benign lymphoproliferative disease affecting middle-aged women is also one of the causes of cystic lung disease, and its association with Sjögren syndrome, acquired immunodeficiency syndrome (AIDS), Castleman disease, systemic lupus erythematosus (SLE) as several other autoimmune diseases is well-established  . Dyspnea, cough, fever, weight loss and recurrent pneumothoraces are important clinical features .
- Pulmonary infections - Echinococcosis granulosus (alveolar hydatid disease), pneumocystis jirovecii pneumonia (PCP), and coccidiomycosis are important infectious causes of cystic lung disease  . PCP is an acquired immunodeficiency syndrome (AIDS)-defining illness and often presents with rapidly deteriorating respiratory failure in the absence of appropriate therapy.
- Light-chain deposition disease - Most frequently identified in patients who suffer from multiple myeloma, B-cell lymphoma or Waldenström macroglobulinemia, accumulation of immunoglobulin light chain in the lungs rarely causes symptoms, but can be responsible for respiratory failure and the need for lung transplantation in some cases .
- Birt-Hogg-Dubé syndrome (Folliculin gene-associated syndrome) - Distinguished by the presence of facial papules (known as fibrofolliculomas), acrochordae (skin tags) on the thorax and the head and neck region, colonic polyps, renal tumors, and thin-walled cysts, this autosomal-dominant genetic disease most frequently presents with a spontaneous pneumothorax .
- Metastatic disease - Sarcomas, mesenchymal tumors, and adenocarcinomas can present with cystic lung disease .
The diagnosis of cystic lung disease solely relies on imaging studies, and high-resolution computed tomography (HRCT) is considered to be the gold standard    . Observation of a circular, circumscribed space lined by an epithelial or fibrous layer of variable thickness is diagnostic of a cyst, and in most cases, it contains air, but fluid may be located in the parenchyma as well . Thickness of the cyst wall is around 2 mm, in which case it is considered (thin), but cysts of greater thickness (more than 4 mm) may be a useful diagnostic feature, and together with the distribution, number, and size, as well as presence of other lung pathologies (nodules, pleural effusion, lymphadenopathy, pneumothorax) and extrapulmonary symptoms, are criteria used to make a presumptive diagnosis  . For e.g. multiple, thin-walled and diffusely distributed cysts are typical of LAM, whereas a combination of nodules and cysts (that appear in the later stages of the disease) in the upper lobes are characteristic for PLCH . On the other hand, up to 100 cysts of variable sizes and shapes are diagnostic for Birt-Hogg-Dubé syndrome . In addition to imaging studies, biopsy of the cyst (usually through bronchoscopy) and subsequent histopathological examination is often necessary to confirm the findings from CT, and thus confirm the etiology of cystic lesions .
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