Normovolaemic, normal Na, slightly increased water

Causes of normovolaemic hyponatraemia include:

• SIADH
  • tumours
    • chest/mediastinum e.g. mesothelioma, thymoma,bronchogenic carcinoma
    • nonchest e.g. duodenal carcinoma, pancreatic carcinoma, nasopharyngeal carcinoma, ureteral/prostate carcinoma, uterine carcinoma,leukaemia
  • CNS disorders
    • mass lesions (tumours, abscesses, subdural hematoma)
    • inflammatory diseases (encephalitis, meningitis, multiple sclerosis, systemic lupus, acute intermittent porphyria,)
    • degenerative/demyelinative diseases (Guillain-Barré syndrome; spinal cord lesions)
    • miscellaneous (head trauma, subarachnoid hemorrhage, acute psychosis, delirium tremens, pituitary stalk section, transphenoidal adenomectomy, hydrocephalus)
  • drug induced
    • stimulated AVP release (nicotine, phenothiazines, tricyclics)
    • direct renal effects and/or potentiation of AVP antidiuretic effects (DDAVP, oxytocin, prostaglandin synthesis inhibitors)
    • mixed or uncertain actions (e.g. ACE inhibitors, chlorpropamide, clofibrate, carbamazepine, clozapine, cyclophosphamide, 3,4-methylenedioxymethamphetamine ['Ecstasy'], omeprazole, serotonin reuptake inhibitors, vincristine)
  • pulmonary diseases
    • infections (tuberculosis, acute bacterial and viral pneumonia, aspergillosis, empyema)
    • mechanical/ventilatory (acute respiratory failure, COPD, positive pressure ventilation)
• prolonged strenuous exercise (marathon, triathalon, ultramarathon, hot-weather hiking)
• idiopathic
• glucocorticoid deficiency
• hypothyroidism
• water intoxication – primary polydipsia, excess intravenous hypotonic fluids (2)

Reference:

• (1) hyponatraemia Treatment Guidelines 2007: Expert Panel Recommendations. American Journal of Medicine 2007; 120 (11);S1:S1-S21.
• (2) Reynolds RM, Seckl JR. Hyponatraemia for the clinical endocrinologist. Clin Endocrinol (Oxf). 2005;63(4):366-74