Question 21

College answer

Syllabus topic/section:

2.1.7    Renal intensive care: Renal failure, renal replacement therapy: L1
2.1.7    Renal intensive care: Acid-base and electrolyte disorders: L1

Discussion: 

Risk factors for demyelination syndrome were well recognised.

Candidates attracted more marks when they approached the discussion in part b) by starting with the underlying principles. Better answers addressed all the information in the stem (seizure, unknown duration of hyponatraemia and renal failure). By recognising the important risks of over correction of hyponatraemia in parts
b) and c), it was expected that candidates could address safe target levels of sodium along with a safe, specific plan for achieving this correction. There are many acceptable approaches to correction of hyponatraemia. All approaches were given marks based on the depth and specificity of their answer and not on any single preferred approach.
 

Discussion

a) Risk factors for osmotic demyelination syndrome:

• Chronic (vs acute) hyponatremia
• Rapid sodium correction
• Alcoholism
• Chronic liver disease
• Malnutrition
• Other electrolyte disturbances (esp. hypokalemia)
• Use of diuretics
• Liver transplantation

b) Approach to correcting the hyponatraemia:

The "discuss your approach" answer should probably be regarded somewhat differently to the "outline your management" answer. The concept of the "outline" is that short notes are added but that the rest remains mostly as a protocol, almost as if one is writing a plan of action for some junior staff to follow (including some rationale in the list of tasks, mainly for their education). The "discuss" answer is expected to be more thoughtful, making space for the possibility of doing the same thing using several possible options. And because this is worth four marks, one would have to communicate this thoughtfulness in 80-100 words. That would probably look something like this:

• Immediately: 150 ml of 3% saline over 20 min,
  • to raise sodium by 2-4 mmol/L
  • Repeat the infusion if goal is not met
  • Rationale: seizures (European and US guidelines)
• Slow correction of sodium by 8-10 mmol/L/day 
  • Rationale:  avoid osmotic central myelinolysis
  • This risk may be overestimated in the guidelines
• Method of correction: Hypertonic saline
  • Via CVC (irritant/vesicant)
  • Dose can be calculated, eg:  
  • = 0.6 ×body weight × (desired Na - current Na)
  • Or: adjust rate and test blood regularly, eg. every 2-4 hrs
• Overcorrection
  • Add 5% dextrose or nasogastric H₂O

c) How ongoing hyponatraemia alters your approach to renal replacement therapy:

• Delay dialysis while correcting sodium. Each 24 hrs period brings the patient 8-10 mmol closer to a low risk threshold of ~ 125-130 mmol/L.
• Modify dialysis technique if dialysis becomes necessary. Several options exist:
  • Use low dose dialysis (eg. daily, but short; or continuous with reduced dialysate flow and ultrafiltration)
  • Replace water as dextrose, systemically or via the circuit
  • Add sterile water to the dialysate (CRRT)
  • Adjust the sodium concentration controls in the reverse osmosis water (SLED)