Diffuse idiopathic pulmonary neuroendocrine cell hyperplasia (DIPNECH) is defined by the World Health Organization (WHO) as “a generalized proliferation of scattered single cells, small nodules (neuroendocrine bodies), or linear proliferations of pulmonary neuroendocrine cells (PNCs) that may be confined to the bronchial and bronchiolar epithelium.” Neuroendocrine hyperplasia has been known for years to occur as a secondary phenomenon in chronic lung disease, but its recognition as a primary pathologic process, independent of other lung abnormalities, has occurred only within the past couple decades. In patients with DIPNECH, peribronchiolar fibrosis, believed to be driven by PNC production of peptides that affect surrounding cells, may obliterate small airways, leading to a severe, progressive obstructive ventilatory defect. The amount of fibrosis varies, with greater amounts seen in tissue that also contains Neuroendocrine tumorlets. In one report of 25 patients with typical Neuroendocrine tumors, 76% had PNC hyperplasia associated with Neuroendocrine, and 32% had constrictive bronchiolitis associated with foci of PNC hyperplasia.
DIPNECH falls within the spectrum of PNC hyperplasia that ranges from individual cells to Neuroendocrine tumors. When the proliferation extends beyond the bronchiolar epithelium basement membrane or beyond the confines of the airway wall, the term “tumorlet” is appropriate. If tumorlets exceed 0.5 cm, they are designated as Neuroendocrine tumors; thus, DIPNECH is believed to be a premalignant condition. Although most reported cases of DIPNECH-associated Neuroendocrine tumors are typical bronchial NETs, atypical lung NETs with metastasis to hilar lymph nodes and distant organs also have been reported. The molecular underpinnings of this transformation are not known.
DIPNECH affects women more often than men, although the reason for this sex bias is not clear. Symptoms include chronic cough and exertional dyspnea that may be present for many years prior to diagnosis. High-resolution CT (HRCT) scans show air trapping (demonstrated by mosaic attenuation accentuated on expiratory images) accompanied by multiple nodules and airway wall thickening. The largest study to date reported 19 patients with DIPNECH and described treatment in seven. The 5-year survival was 83% with varied therapeutic approaches. Although the literature contains several additional case reports and small case series, the pathogenesis, clinical course, and treatment options for DIPNECH are not well described.