Hyponatremia

#med #disease #y2023

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Presentation :

Pathophysiology :

ADH Physiology:

ADH is the primary hormone responsible for tonicity homeostasis
ADH is stored in neurons within the hypothalamus which express osmoreceptors responsive to changes in serum osmolarity
- Elevations in osmolarity result in the secretion of ADH to retain water and thus normalize osms
ADH secretion may also be caused by detection of hypovolemia by baroreceptors in L atrium, carotid artery, and aortic arch
- Signaling is via the vagus nerve, which directly stimulates the release of ADH
ADH principally exerts its effects by binding to the kidneys principal cells within the late distal tubule and collecting ducts. ADH binds to the V receptor which ultimately phosphorylates intracellular aquaporin-2 (AQP2) storage vesicles, causing their movement and insertion into the cell membrane.
- AQP2 is a water channel that allows water to move passively into the cell guided by the osmotic gradient established by NaCl and urea, and thus promotes reabsorption of water in the kidney.
ADH also binds to V receptors on vascular smooth muscle, causing a release of intracellular calcium from the endoplasmic reticulum and contraction of vascular smooth muscle leading to increases in total peripheral resistance and thus increases in blood pressure.

RAAS Physiology:

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Diagnostic Testing:

Prior to treatment, send:

Serum osmolality
Urine osmolality and electrolytes (Na, Cr, and optionally Urea)

Verify true hyponatremia by checking serum osmolality less than ~285.
- Normal or elevated serum osms suggests unmeasured solute:
  - Severe hyperglycemia.
  - Severe uremia
  - Triglyceride level >1,500 mg/dL.
  - High protein level (multiple myeloma, IVIG).
  - Exogenous osmoles:
    - Contrast dye.
    - Mannitol administration.
    - Maltose (from IVIG).
    - Sorbitol/glycine (used for surgical irrigation).
    - Alcohols (ethanol, methanol, etc)
Check urine osmolality (HIGH if >300)
- Low suggests body is detecting decreased serum osm and trying to correct by dumping water in urine
  - Causes include either increased free water intake OR decreased solute intake
    - Psychogenic polydipsia (free water up)
    - Tea-Toast Syndrome (solute down)
    - Beer Potomania (both free water up and solute down)
  - Alternatively, may represent recovery phase from any other cause of hyponatremia (e.g., the patient initially had hypovolemic hyponatremia, received volume resuscitation prior to urinalysis, and is currently auto-correcting their own sodium levels.)
  - Secretion of dilute urine will cause the patient's sodium to rise – so these patients may correct their own sodium levels.
  - Production of large volumes of dilute urine is often a sign that the sodium is about to over-correct.
- High suggests body is detecting elevated serum osm and trying to correct by retaining water (OR is over-producing ADH)
  - Concentrated urine indicates that the kidney is still retaining water. This indicates that the patient is not going to auto-correct their sodium.
  - A very concentrated urine suggests that giving isotonic fluid could potentially worsen the hyponatremia (because the kidney may respond by retaining water and excreting sodium, a process termed “desalination”).
Check clinical volume status and urine Na
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