Histiocytosis X (eosinophilic granulomatosis, EG, pulmonary histiocytosis X, PHX, pulmonary Langerhans cell granulomatosis)

It is an uncommon idiopathic disease characterized in its early stages by granulomatous nodules containing Langerhans histiocytes and eosinophils, which are primarily peribronchial. The majority of patients with pulmonary histiocytosis X are young or middle-aged adults presenting with nonspecific symptoms of cough and dyspnea. Over 90% of patients are smokers, and a causal relationship with smoking is likely. Classically, there is involvement of the skeletal system with a characteristic lytic bone lesion (young children) or a more acute disseminated form which occurs in infants.

The combination of nodules and thin walled cysts on HRCT are virtually diagnostic of this condition. Nodules are peribronchial, appearing centrilobular on HRCT. Cysts are variable in size, can be upto 10 mm, particularly in the upper lobes. The walls of the cysts are usually thin (1mm or less), but can be variable, and the cysts are not necessarily round (they may be bilobed cloverleaf-shaped or branching). Associated pneumothorax is found in 15-25% of cases. In the late stages, the disorder may be indistinguishable from lymphangiomyomatosis- but sparing of the costophrenic angles suggests the diagnosis of histiocytosis X.