Nelson syndrome is an infrequent clinical entity that represents the development of skin pigmentation, an expanding and often aggressive pituitary mass, and marked elevations of adrenocorticotropic hormone (ACTH) after total bilateral adrenalectomy as a therapeutic modality for Cushing's disease. Visual field deficits are also reported as a result of direct compression of the optic chiasm. Clinical criteria, magnetic resonance imaging (MRI) of the endocranium, and a positive history for adrenalectomy are crucial findings during the diagnostic workup.
Nelson syndrome is a phenomenon described in patients who undergo total bilateral adrenalectomy (TBA), mainly in the attempt to treat Cushing's disease    . Studies report a variable incidence rate of this potentially life-threatening syndrome, ranging from 8-38% in most of the reports , but some authors have established that Nelson syndrome appears in up to 47% of patients in whom TBA was performed  . The exact pathogenesis of Nelson syndrome remains unknown, as well as why initial signs and symptoms may appear within months or after a few decades . One of the main characteristics of Nelson syndrome is a rapidly expanding mass in the sellar region (sometimes termed corticotrophinoma) that directly compresses the optic chiasm or optic tract, resulting in visual field deficits, palsies of different cranial nerves, ophthalmoplegia, and headaches   . Hypersecretion of adrenocorticotropic hormone (ACTH) that persists despite TBA is the second cardinal feature and excessive stimulation of melanocyte production is the reason why hyperpigmentation of the skin (often appearing in a generalized fashion) is a constitutive feature of Nelson syndrome   . Furthermore, hyperpigmentation of scars, the buccal mucosa, and even tongue might be observed . In rare cases, paraovarian or paratesticular tumors can develop due to stimulation of gonadal tissues by ACTH, whereas hypopituitarism, oligospermia accompanied by testicular pain, and diabetes insipidus have also been reported .
Many reports have stressed the role of close monitoring of patients who undergo TBA because of the life-threatening nature of Nelson syndrome . For this reason, the first piece of information that points to Nelson syndrome is a positive history for this surgical procedure. During the physical examination, physicians must conduct a meticulous neurological and ophthalmological assessment, which will unequivocally show visual field deficits, and together with pigmentation of the skin, help to make a presumptive diagnosis. Laboratory studies must include ACTH testing with reference to the last glucocorticoid dose that patients take for Cushing's disease - One sample should be obtained at 8 AM (values of > 500 ng/L are considered to be high), 20 hours after the last dose, and in the early morning before the next glucocorticoid dose  . An increase in > 30% of physiological values on three separate occasions has been proposed as a diagnostic criterion for Nelson syndrome, but in addition to laboratory criteria, and imaging procedures must be used to confirm clinical suspicion . MRI of the endocranium is a highly useful study that identifies a sellar mass  . In fact, some authors advocate the regular use of MRI every 6-12 months after TBA in order to detect the hypophyseal tumor as early as possible .