Endocrinology - Hyponatraemia

Question 1

what is the differential diagnosis for hyponatraemia? how would you differentiate between these?

Answer 1

1. Hypervolaemic causes
   i) urine Na+ <20 mmol/L: heart failure, cirrhosis, nephrotic syndrome
   ii) urine Na+ >20 mmol/L: renal failure
2. Euvolaemic causes:
   i) urine Na+ <40 mmol/L, urine osmolality <100 mOsml/L: acute water load, e.g. exercise-associated hyponatraemia
   ii) urine Na+ >40 mmol/L, urine osmolaity >100 mOsml/L: SIADH, severe hypothyroidism
3. Hypovolaemic causes
   i) urine Na+ <20 mmol/L: vomiting, diarrhoea, burns, 3rd space losses
   ii) urine Na+ >20 mmol/L renal loses, e.g.diuretics, adrenal insufficiency/crisis

Question 2

describe the possible presentation of hyponatraemia

Answer 2

1) Symptoms (often asymptomatic if mild-moderate)
- nausea and vomiting
- muscle cramps, weakness, ataxia
- headache, confusion
- lethargy

2) Signs
- decreased GCS
- seizures
- brainstem herniation (severe acute hyponatraemia): coma, fixed unilateral dilated pupil, respiratory arrest…
- signs of hypovolaemia or hypervolaemia

Question 3

how would you assess a pt with hyponatraemia?

Answer 3

1. Clinical examination: fluid status
   - BP, standing BP
   - HR
   - cardiac and resp examination (pulmonary/peripheral oedema, S3, JVP)
2. Bloods
   - serum osmolality
   - U and Es: Na (decreased), K+ (if increased, consider Addison’s)
   - LFTs: ?cirrhosis
   - TSH and T3/4: ?hypothyroidism
   - serum cortisol or ACTH stimulation test: if ?adrenal insufficiency
3. Urinalysis
   - osmolality
   - Na+
   - K+

Question 4

how would you manage a pt with acute severe hyponatraemia (<120 mEq/L) who is seizing?

Answer 4

Pre-hospital care:

1) ensure airway is open, e.g. using adjuncts.
2) O2
3) diazepam 10mg - unlikely to respond but should be administered until definitive diagnosis and treatment available
4) establish IV access

Definitive treatment:

5) 150 mL bolus 3% NaCl (aim to increase serum Na+ by 4-6 mEq/L over first 1-2 hrs)
6) repeat after 20 mins if no clinical improvement
7) re-check serum Na+ at 4, 12, 24 and 48 hrs for over-correction

Question 5

what is the maximum correction rate of hyponatraemia? explain why.

Answer 5

10 mEq/L in 24 hrs - serum Na+ should not be allowed to reach normal level within 1st 48 hrs. Especially true in chronic hyponatraemia where max. correction rate is 6-8 mEq/L.

Risk of CENTRAL PONTINE MYELINOSIS: large shifts of intracellular water cause demyelination of pons and extra-pontine sites. Symptoms occur 2-4 days later, typically with quadriplegia and pseudobulbar palsy, but can take form of mutism and paralysis (‘locked-in’ syndrome).

Question 6

explain why acute severe hyponatraemia can cause death

Answer 6

Hyponatraemic encephalopathy: hypoosmolality results in intracellular or cytotoxic cerebral oedema caused by influx of water into intracellular space down concentration gradient… parenchymal brain swelling… ↑ intracranial pressure… brain ischaemia, herniation and death (if sudden fall in sodium conc. over 24-48 hrs).

Question 7

how would you manage a hypervolaemic pt with mild-moderate hyponatraemia?

Answer 7

1. sodium and water restriction
2. treat underlying cause, e.g. HF, nephrotic syndrome, renal failure…
3. +/- IV furosemide

Question 8

how would you manage a hypovolaemic pt with mild-moderate hyponatraemia?

Answer 8

1. 0.9% NaCl infusion to restore extracellular volume.
   Monitor serum sodium levels frequently to ensure that the serum sodium level increases slowly, with a maximum rise of 6 mEq/L in the first 24 hours.
2. As euvolaemia is regained, ADH is suppressed and resulting diuresis may ↑ sodium levels overly rapidly – if so, desmopressin can be given.

Question 9

what is SIADH? suggest possible causes

Answer 9

Inappropriate ADH secretion from posterior pituitary or from ectopic source despite low serum osmolality.

Causes inc.:

• pulmonary: pneumonia, small cell lung cancer, TB…
• neurological: tumour, cerebral trauma or surgery (esp. to pituitary), intracranial haemorrhage…
• drugs: SSRIs, thiazide diuretics, carbamazepine, amitriptylline
• other: hypothyroidism, pain, idiopathic…

Question 10

which investigation results suggest SIADH?

Answer 10

Bloods:

• decreased serum osmolality (<275 mOsml/L)
• decreased serum sodium (<135 mmol/L)

Urinalysis:

• increased urine osmolalit y (increased ADH causes increase water retention but not solute retention)
• increased urine sodium (>40)

Question 11

how would you treat mild/asymptomatic SIADH? moderate SIADH? severe symptomatic SIADH?

Answer 11

1. mild:
   - adequate solute intake, inc. salt and protein
   - fluid restriction - starting at 500 ml/day with adjustments based on serum sodium levels. Long-term fluid restriction of 1,200-1,800 ml/day may maintain person asymptomatic.
2. moderate/symptomatic:
   - furosemide + 0.9% saline
3. severe:
   - 3% saline

Question 12

name medications that can be used in long-term management of SIADH

Answer 12

1. DEMECLOCYCLINE - most potent ADH inhibitor but extensive ADRs
2. UREA
3. VAPTANs - ADH receptor antagonists