Syndrome of inappropriate antidiuretic hormone associated with rectal small cell neuroendocrine carcinoma: a case report.

Neuroendocrine tumors can arise from any portion of the gastrointestinal tract including the colon and rectum. In the scope of all colon and rectal malignancies, they are a rare cause of colorectal carcinoma. Relating to their pluripotent neuroendocrine cellular origins these carcinomas can produce a variety of biologically active peptides with several resultant paraneoplastic syndromes. One of these paraneoplastic syndromes is the syndrome of inappropriate antidiuretic hormone (SIADH). In the SIADH, supraphysiologic levels of vasopressin (antidiuretic hormone, AVP) precipitates exorbitant free water retention and resultant electrolyte abnormalities, most notably hyponatremia. Herein we report a rare case of paraneoplastic SIADH from rectal small cell neuroendocrine carcinoma. To our knowledge, SIADH from rectal small cell neuroendocrine carcinoma has been reported only three times.