Sodium Disorders

Question 1

Manifestations of HyperNa and HypoNa

Answer 1

HyperNa
- thirst, lethargy, seizure, coma
- brain shrinkage
==> but rapid correction leads to cerebral oedema

HypoNa
- nausea, confusion
- mental impairment, seizure, coma if severe
- brain swelling
==> but rapid correction leads to central pontine myelinolysis

Aim to correct <12mM over 24 hr and <0.5mM/hr

Question 2

General approach to abnormal values

Answer 2

Is the result real?
Assess patient
Diagnosis
Treatment
Monitor response and complications

Question 3

HypoNa approach

Answer 3

1. Rule out spurious hypoNa e.g. drip arm with D5
2. Check patient’s condition
3. Determine tonicity and osmolar gap
   - -> DDx for hypertonic hypoNa and isotonic hypoNa
4. Assess hydration status (clinical and Ur:Cr) to stratify hyper/hypo/euvolemic hypotonic hypoNa
   - -> DDx for each based on UNa and normal kidney response (UNa <20 = normal excretion)

Question 4

Hypovolemic HypoNa

Answer 4

UNa <20

• normal aldosterone response to conserve Na
• extra-renal loss of Na (ECF depletion)
  e. g. diarrhoea, vomiting, burns, abdominal sequestration

UNa >20
- renal loss of Na (renal salt wasting)
e.g. thiazide diuretics, diuresis, cerebral salt wasting 
(also hypoaldosteronism)
- cause usually apparent

Question 5

Hypervolemic HypoNa

Answer 5

UNa <20

• Na and water retention
• kidneys responding to effective hypovolemia via RAAS and ADH activation
  e. g. CHF, cirrhosis, nephrotic syndrome, hypoalbuminemia

UNa >20

• water retention
• dysfunctioning kidneys unable to excrete water –> water retention and increase intravascular volume with subsequent decrease in aldosterone
  e. g. oliguric phase of AKI and CKD

Usually obvious clinically or by other lab measurements e.g. RLFT

Question 6

Euvolemic HypoNa

Answer 6

Uosm < Sosm (“dilute” urine), UNa <20

• appropriate water excretion in response to water intoxication
  e. g. psychogenic polydipsia, beer potomania (low solute intake), excessive hypotonic IV fluid, TURP

Uosm > Sosm (“concentrated” urine), UNa >20

• inappropriately concentrated urine with inappropriate water retention
• -> DDx: hypothyroidism, hypocortisolism (decreased ADH suppression and increased hypotension)
• -> UNa >40 with normal thyroid, adrenal and renal fx ==> SIADH

Reset osmostat

• change in plasma osmolality threshold due to alteration in osmoreceptor metabolism
• hypoNa stable despite variation in Na and water intake

Question 7

SIADH diagnosis and DDx

Answer 7

Lack of maximally diluted urine in presence of hyponatremia ==> inappropriate ADH activity (water retention and mild Na wasting in chronic cases)

• malignancy, lung pathology, brain, drugs (SSRI, carbamazepine)

DIAGNOSIS OF EXCLUSION!

• Uosm>Sosm or Uosm>100
• UNa >40 (but <100)
• euvolemic with decreased urine output
• r/o diuretic use, renal/endrocine causes (TFT, short synacthen test)
• improve with fluid restriction

DDx: cerebral salt wasting

• increase BNP due to brain injury –> excessive renal salt loss
• UNa >100
• hypovolemic with high urine output (>2L)

Question 8

Treatment of HypoNa

Answer 8

Hypovolemic
- rehydrate and replace Na and water with IV fluids (NS)

Hypervolemic

• treat underlying cause
• fluid restriction <1L/day
• diuretics

Euvolemic
- treat underlying cause
- fluid restriction <1L/day
- oral sodium tablet
(don't give NS in SIADH because body unable to dilute urine properly = even more retention of water; same for post-operative fluid replacement as stress induces ADH secretion)

Remember: correct <0.5mM/hr or <12mM/day to prevent cerebral pontine myelinolysis!

Question 9

Approach to HyperNa

Answer 9

1. Rule out spurious hyperNa (rare)
   e. g. Anticoagulant contamination from blue tube (sodium citrate) which has additional Na
2. Assess patient
3. Rule out pseudohyperNa (isotonic, rare)
   e. g. severe hypoproteinemia in electrolyte exclusion effect
4. Assess hydration status to form DDx of hypertonic hyperNa
   - dehydration, diuresis and DI most common!

Question 10

Hypervolemic HyperNa (Rare!!)

Answer 10

Normal kidney response to hyperNa is retain free water and excrete minimum volume of concentrated urine

UNa >20

• normal response to excrete Na load
• excess Na load
  e. g. hypertonic NaCl infusion, salt toxicity, NaHCO3 infusion

UNa <20
- inappropriate aldosterone effect
- mineralocorticoid excess
e.g. hyperaldosteronism, Cushing’s syndrome
(HyperNa usually not exaggerated/ high normal because water reabsorbed with Na –> mainly seen as HT)

Question 11

Euvolemic/ Hypovolemic HyperNa

Answer 11

UNa <20, Uosm>Sosm, Uosm >600-800

• extra-renal free water +/- Na loss
• normal aldosterone and ADH response to low volume
  e. g. dehydration (MC!) via GI/skin (hypovolemic), decreased intake (euvolemic)

UNa >20, Uosm Uosm <300 = DI (diagnostic)
–> Uosm 300-600 = diuresis, diruretics or DI
—–> >1000 mOsm daily solute excretion = diuresis or diuretics
—–> <1000 = exclude diuresis, consider partial DI
==> water deprivation test

Question 12

DI and water deprivation test

Answer 12

Large volume (<3L/day) of inappropriately dilute urine (<300 Osm/kg)

Cranial (primary hypothalamic, tumour, vascular, infectious) or nephrogenic (lithium, hypoK, hyperCa)

Water deprivation test

• > 750 = normal
• 300-750 = primary polydipsia or partial DI
• <300 = DI (DDAVP response to differentiate cranial or nephrogenic)

(300-750: DDAVP response >50% = partial cranial DI; <50% = partial nephrogenic DI or primary polydipsia)

• -> plasma Osm normal = primary polydipsia
• -> plasma Osm >300 = partial nephrogenic DI

Question 13

Treatment of HyperNa

Answer 13

Treat underlying cause (usually obvious from clinical context)

Hypovolemic until proven otherwise ==> give IV 5% dextrose

HyperNa usually occurs in unconscious patients/ decreased access to water (condition usually compensated by thirst reflex)

DI:

• cranial - DDAVP
• nephrogenic - thiazide diuretics, amiloride

Remember to correct <0.5mM/hr to avoid cerebral oedema!

Question 14

Drugs associated with dysnatremia

Answer 14

HypoNa

• diuretics – hypovolemic hypoNa
• SSRI, carbamazepine – SIADH

HyperNa

• NaHCO3, hypertonic saline – hypervolemic HyperNa
• osmotic diuretics e.g. glucose – hypovolemic HyperNa
• lithium – DI