Sodium Disorders Flashcards
Manifestations of HyperNa and HypoNa
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
General approach to abnormal values
Is the result real? Assess patient Diagnosis Treatment Monitor response and complications
HypoNa approach
- Rule out spurious hypoNa e.g. drip arm with D5
- Check patient’s condition
- Determine tonicity and osmolar gap
- -> DDx for hypertonic hypoNa and isotonic hypoNa - 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)
Hypovolemic HypoNa
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
Hypervolemic HypoNa
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
Euvolemic HypoNa
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
SIADH diagnosis and DDx
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)
Treatment of HypoNa
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!
Approach to HyperNa
- Rule out spurious hyperNa (rare)
e. g. Anticoagulant contamination from blue tube (sodium citrate) which has additional Na - Assess patient
- Rule out pseudohyperNa (isotonic, rare)
e. g. severe hypoproteinemia in electrolyte exclusion effect - Assess hydration status to form DDx of hypertonic hyperNa
- dehydration, diuresis and DI most common!
Hypervolemic HyperNa (Rare!!)
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)
Euvolemic/ Hypovolemic HyperNa
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
DI and water deprivation test
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
Treatment of HyperNa
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!
Drugs associated with dysnatremia
HypoNa
- diuretics – hypovolemic hypoNa
- SSRI, carbamazepine – SIADH
HyperNa
- NaHCO3, hypertonic saline – hypervolemic HyperNa
- osmotic diuretics e.g. glucose – hypovolemic HyperNa
- lithium – DI
