Serum sodium below <130mmol/L occurs in up to 30% of hospital inpatients and many older patients. However, sudden and dramatic falls in sodium to <120mmol/L can be life-threatening and requires evaluation of fluid status to determine emergency management.
A history of recent fluid loss makes hypovolaemia the most likely cause of hyponatraemia.
Dizziness on standing suggests postural hypotension.
The rapidity of onset of hyponatraemia is as important as its biochemical severity in terms of planning treatment.
Previous blood results are extremely useful in helping define the duration of hyponatraemia.
Thirst is usually a good indicator of dehydration, and may raise the possibility of new diabetes mellitus with factitious hyponatraemia due to high glucose levels. The sensation of thirst must be differentiated from that of a dry mouth.
It is useful to know if the patient tends to drink large or small volumes.
Recent surgery increases the chance that the patient is dehydrated.
Recent IV fluid use increases the chance that the patient is hypervolaemic due to inappropriate treatment.
Neurological sequellae are common with hyponatraemia and can be a guide to its duration.
Acute and deteriorating neurological signs demand HDU, very close monitoring and even consideration of IV hypertonic saline.
Virtually any intra-thoracic pathology such as pneumonia, pulmonary abscess, tuberculosis or broncogenic carcinoma may be associated with the syndrome of inappropirate anti-diuretic hormone (SIADH).
Recent neurosurgery as well as stroke, intracerebral hemorrhage, abscess or other space occupying lesions may be associated with hyponatraemia due to SIADH or occasionally cerebral salt wasting.
Alcohol excess increases the risk of developing hyponatreamia and also increases the risks associated with it.
These all raise the possibility of hypervolaemic hyponatraemia.
Diuretic use is very frequently associated with hyponatraemia and increased urinary sodium excretion.
ACE inhibitors and ARB receptor blockers are also frequently associated with hyponatraemia.
Multiple agents are also associated with the syndrome of inappropriate ADH including selective serotonin reuptake inhibitors, Carbamazepine, Desmopressin, Phenothiazines, Tricyclic antidepressants, Cyclophosphamide, Opiates, Vincristine, Non steroidal anti-inflammatory drugs, Clofibrate and Proton pump inhibitors.