Hyponatremia

Definition: < 134 mEq/L; severe < 120
Manifestations: anorexia, apathy, restlessness-->confusion, muscle cramping, lethargy, anorexia, nausea --> seizure, coma

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1. Check Osmolarity:

• Isotonic hyponatremia: pesudohyponatremia: hypertriglyceridemia, hyperproteinemia; isotonic infusion (glucose, mannitol
• Hypertonic hyponatremia: hyperglycemia (each 100mg/dl rise in serum glucose decreases plasma Na by 1.6 mEq/L
• Hypotonic hyponatremia --> measure urine osmolality

2. If hypotonic hyponatremia, check urine osmolality

• Low Urine Osmolality:  primary water intoxication (psychogenic polydipsia, beer drinkers potomania)
• High (>100) Urine Osmolaity--> check volume status

3. If Hypotonic, Hyperosmolar urine, check volume status:

• Hypovolemic:
  • Urine Na < 10: vomiting Diarrhea, Third Spacing
  • Urine Na > 20: Adrenal insufficiency, salt-wasting syndrome, diuretics
• Euvolemic: SIADH, renal insufficiency, thiazides, reset osmostat, hypothyroidism, glucocorticoid deficiency
• Hypervolemic:  CHF, nephrotic syndrome, cirrhosis

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Quick Workup:

• Plasma: sodium, osmolality, BUN, creatinine, K, random cortisol, TSH
• Urine: sodium, osmolality

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SIADH:

• low plasma osmolality
• urine osmolality > 300
• urine sodium > 40
• low BUN
• low Uric Acid
• Normal Creatinine, K
• Normal Adrenal, Thyroid Function

Causes of SIADH

• enhanced hypthalamic ADH production, infections, SAH, CVA, temporal arteritis, drugs, pulmonary disease, postoperative state, severe nausea, idiopathic, ectopic ADH (small cell carcinoma of lung), ocytocin during labor, prolactinoma, walderstroms, Head trauma, Shy-Drager, Delierium tremens

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Acute Management

If symptoms have developed acutely (risk of cerebral edema >> risk of rapid correction):

• Target correction rate 1.5-2 meq/L/hr for 3-4 Hours until symptoms resolve)
• 3% saline with IV furosemide
• check sodium q2H
• consider ICU placement

Chronic development

• treatment is controversial, especially if patient is only mildly symptomatic
• Goal hourly correction rate is 0.5 mmol/hr; not to exceed a serum value of 126-130; no more than 25 mmol/L per 48H
• restrict free water

Unknown duration

• assess clinical symptoms; consider imaging to evaluate for edema
• if symptoms severe, goal hourly correction rate is 102 mmol/L for 3-4H

Volume Depletion

• recommend isotonic saline for replacement
• hypertonic saline is reserved for patients with symptoms
• dilute solutions have no role

Hypokalemia

• replete K
• it is an effective osmole, and as it shifts between intracellular and extracellular spaces, sodium will also shift

Note

• if plasma sodium rises too quickly, water can be given to reduce the risk of demyelinating disease

SIADH: free water restrict
do not correct faster than 1 meq/Hr

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Osmolality Formulas:

Calculated Osmolality:    2([Na] +[K]) + (Glc/18) + (BUN/2.8))

Effective Osmolality: 2[Na] + (g/c/18)

Osmolal gap: measured osmolality - calculated osmolality

Corrected [Na] = measured Na + 1.6 x (glucose -100)/100

Total Body Water (TBW) = 0.6 (men) or 0.5 (women) x body weight (kg)

Sodium Deficit = TBW x (Desired [Na] - Plasma [Na]