Causes of Hyponatremia in Dialysis Patients
Hyponatremia in dialysis patients is primarily caused by excessive free water intake relative to sodium intake, often exacerbated by high dialysate sodium concentrations that stimulate thirst and promote fluid consumption between dialysis sessions. 1
Primary Mechanisms
Excessive Water Intake Relative to Sodium
- The fundamental cause is positive water balance exceeding sodium balance, leading to dilutional hyponatremia 1
- Excessive water intake typically accompanies inadequate sodium restriction, as high sodium intake stimulates thirst through increased extracellular fluid osmolality 1
- When patients consume excess sodium (often exceeding the recommended 5g sodium chloride or 85 mmol daily limit), the resulting osmotic gradient triggers thirst and water consumption, creating isotonic fluid gain that can become hyponatremic if water intake exceeds sodium 1
High Dialysate Sodium Concentration
- Dialysate sodium concentrations increased from 135 mmol/L in the 1960s to 140+ mmol/L by the 1990s to facilitate ultrafiltration during shorter treatment times 1
- High dialysate sodium (140-155 mmol/L) and sodium profiling techniques aggravate thirst, leading to excessive interdialytic fluid gain 1
- This creates a vicious cycle: high dialysate sodium → increased thirst → excessive water intake → potential hyponatremia if water intake disproportionately exceeds sodium 1
Inadequate Ultrafiltration
- When ultrafiltration is insufficient to remove accumulated fluid, patients may develop volume overload with dilutional effects 1
- Attempts to accelerate ultrafiltration can precipitate hypotension, leading to normal saline administration that further expands extracellular volume and potentially dilutes sodium 1
Contributing Factors
Residual Kidney Function Loss
- Volume expansion from sodium and water retention is especially marked in dialysis patients with poor residual kidney function 1
- Loss of the kidney's ability to excrete free water impairs compensation for excessive water intake 2
Other Stimuli for Water Consumption
- Hyperglycemia stimulates thirst independent of sodium intake 1
- Elevated blood angiotensin levels can promote drinking 1
- Certain medications (e.g., clonidine) increase water consumption 1
Iatrogenic Causes
- Normal saline administration during dialysis to treat hypotension can expand volume without proportionate sodium correction 1
- Overly aggressive ultrafiltration followed by saline infusion creates net volume expansion 1
Clinical Pitfalls
A critical mistake is advising water restriction without concurrent sodium restriction 1. This approach causes unnecessary suffering from thirst without addressing the root cause, as excessive sodium intake will continue to stimulate water consumption through osmotic mechanisms 1.
The solution requires simultaneous sodium restriction (≤5g sodium chloride or 85 mmol daily) and avoidance of high dialysate sodium concentrations 1. When sodium intake is properly controlled, water restriction becomes largely unnecessary as thirst naturally decreases 1.
For severe symptomatic hyponatremia in dialysis patients, continuous venovenous hemodialysis with specially prepared low-sodium dialysate allows controlled correction while avoiding osmotic demyelination syndrome 3, 4. Conventional intermittent hemodialysis risks overly rapid correction in these patients 3, 4.