Primary bilateral micronodular hyperplasia is characterized by the presence in the adrenal cortex of one or more yellow nodules visible to the naked eye and 2 to 3 cm in diameter. Its pathogenesis is unknown. Whether this type of hyperplasia is of primary adrenal origin or the result of a combined mechanism by which the pituitary predominates initially but the adrenal eventually becomes autonomous, is not clear.26 Increased adrenocortical cell sensitivity to ACTH has also been suggested.27

Histologically, the micronodules consist of clear cells arranged in acini and cords, and are occasionally encapsulated and surrounded by simple or micronodular cortical hyperplasia. The frequency of mitotic figures is low. In one form of ACTH-independent micronodular adrenal disease, darkly pigmented micronodules are seen in the presence of atrophy of the perinodular adrenal tissue, disorganization of normal zonation of the cortex, and small glands28 (Fig. 75-3). Microscopically, the nodules are
composed predominantly of large globular cortical cells with granular eosinophilic cytoplasm that often contains lipofuscin.