A 60-year-old man with small cell carcinoma of lung is rushed to emergency room in coma after suffering a clonic-tonic seizure. Patient's temperature is 37degC , blood pressure 100/50 mm Hg and pulse is 88 per minute. Laboratory studies show a serum sodium of 103 mmol/L, normal serum levels of BUN & creatinine and a dilute but otherwise normal urine. A CT scan of the head is normal. Which of the following is the most likely cause of seizures in this patient?

- Given clinical features suggests the diagnosis of syndrome of inappropriate ADH (SIADH) secretion. SIADH - Associated with ADH excess, which causes resorption of excessive amount of free water, resulting in hyponatremia. - Causes of SIADH Secretion of ectopic ADH by malignant neoplasms (small-cell carcinomas of lung, intracranial neoplasms); Non-neoplastic diseases of lung Local injury to hypothalamus / neurohypophysis. - C/F- Hyponatremia, cerebral edema & neurologic dysfunction. - Cardinal features- Marked | in both serum sodium & osmolality | Urine osmolality. - Although total body water is increased, blood volume remains normal & peripheral edema does not develop. Diabetes mellitus may cause polyuria, glucosuria and does not cause severe hyponatremia. Diabetes insipidus - ADH deficiency causes diabetes insipidus, a condition characterized by excessive urination (polyuria) due to an inability of the kidney to resorb water properly from the urine, can occur in a variety of conditions, including head trauma, CNS tumors, inflammatory disorders of the hypothalamus and pituitary, and surgical complications.