Dr. Simon Prince, FACP, FASNAssistant Professor of Medicine NYU School of MedicineNorth Shore Nephrology

Hyponatremia

Sodium

The problems with sodium has little to do with direct effects of the ion. Disregulation of sodium causes changes in cell volume.                 WATER PROBLEM

OSMOSIS

Why we care about osmolality

Alterations in cell size disrupt tissue function.

Sodium is an indicator of osmolality

The clinically important

variable is   

tonicity.

Tonicity vs. Osmolality

• OsmolalityoTotal concentration

of all particles• Tonicity

oOnly impermeable particles contribute to tonicity.

Only impermeable particles cause changes in cell volume.

Why are we interested in TONICITY?

 o When elevated, water leaves the cells causing

cell shrinkage and dysfunction.o When decreased water moves into the cells

causing cellular swelling and dysfunction. • We are interested in sodium because it

usually tells us the plasma tonicity.

Pseudohyponatremia: high osmolality

• Elevated glucose  raise plasma tonicity which draws water from the intracellular compartment diluting plasma sodium.

Hillier TA, Abbott RD, Barrett EJ. Am J Med 1999; 106: 399-403.

Pseudohyponatremia: high osmolality

• Correcting the sodium for hyperglycemia. 

o Add 1.6 to the sodium for every 100 mg/dL the glucose is over 100.

o Example: Na = 126 mEq/L. Glucose = 600 mg/dL: 600 - 100 = 500. So the glucose is five 100’s over 100 5 x 1.6 = 8 126 + 8 =134 True sodium equals 134 mEq/L To remember 1.6 think “Sweet 16”

• If a person drinks more water than the kidney is capable of clearing the excess water will dilute the plasma.

Causes of hyponatremia: Increased fluid intake

• To exceed the maximal renal clearance of water an adult needs to drink about 18 liters a day.

True hyponatremia

• Hyponatremia does not occur when sodium excretion exceeds sodium intake.

Negative salt balance causes hypovolemia

Causes of Hyponatremia:Defect in Free H2O clearance 

Etiology of Hyponatremia: 3 steps to generating dilute urine

1.Delivery of water to the diluting segments of the nephron.

2.Functional diluting segments.

3.Collecting tubule impermeable to water (lack of ADH)

1400

285

100 50

Failure to Generate dilute urine

Lack of water delivery to the diluting segments.

• Renal Failure• Volume deficiency• Cirrhosis• Heart failure• Nephrotic syndrome

Failure to Generate dilute urine

• Ineffective solute reabsorption diluting segments:o Thick ascending limb

of the loop of Henle (TALH)o Distal convoluted

tubule. Diuretics Non-oliguric ATN

Failure to Generate dilute urine

Permeable collecting ducts (ADH)

o Volume related ADHo SIADH

Drug induced Paraneoplastic CNS Pulmonary disease

o Adrenal insufficiencyo Hypothyroidism

 

  ADds Hydration to the body.

ADHOsmolality

ADH is normally used to regulate osmolality

We start with an increase in the plasma osmolalityThis is detected by the brainThe brain releases ADHADH acts on the kidneyThe kidney reacts by retaining water and producing a small amount of concentrated urine.

The retained watergoes here

not here

Clinical Approach

 

What Studies Are Needed?

 

Tests to send...

oUA, Urine: Na and OsmolalityoBMPoSerum osmolality, TSH, uric acid, BNP, cortisol

oCXRoHead CT

What is the Volume Status?

1.Hypovolemic2.Euvolemic

3.Hypervolemic

Hypovolemic Hyponatremia    

Volume expansion with SALINE

Hypervolemic Hyponatremia

•Fluid restrict•Diurese

EUVOLEMIC HYPONATREMIA

•Excess intake in Free Water

•Defect in Free Water Clearance

•ADH problem

ADH Should NOT Be Present When...1.Euvolemic / Hypervolemic states2.Serum Osmolality is low - normal range

If ADH is elevated... that would be INAPPROPRIATE

Diagnostic Criteria for SIADH

1.Hypoosmolar hyponatremia2.Euvolemic3.Urine Na >254.Urine Osmolality elevated 

o >350 mOsmo >200 higher than Serum Osmolality

Causes of SIADH

• Neurological:o Meningitiso Tumorso Traumao SAH

• Pulmonary disease:o Asthmao Mechanical ventilationo Pneumoniao TB

• Stresso Paino Vomitingo Post-surgical

• Medicationo Antipsychoticso SSRIo First generation sulfonylureaso Pitocin/Oxytocino Narcoticso Cyclophosphamideo Ecstasy 

• AIDS

TREATMENT    

Conservative vs. Aggressive

Who should get treated and why?

Symptomatic Hyponatremia

oMental status changesoNauseaoVomittingoHead acheoMovement abnormalitiesoSeizuresoHypoxia / respiratory failure

Symptomatic vs. Asymptomatic

Symptomatic    HYPERTONIC SALINE

Asymptomatic    Conservative approach is best

Acute symptomatic hyponatremia

• In patients with neurologic symptoms due to hyponatremia: 3%.

• Increase sodium until symptoms abate or 6 mmol/L, which ever comes first.

• Increase Na < 24 mEq/L in the first 24 hours.

• Goal is not more than 0.5 mEq/L/hour

The problem with compensation

The starting point is after compensation has reduced the amount of intracellular solute and the ICP

Now, an over-eager intern sees the low sodium and starts an infusion of 3% NaCl to raise the sodium to normal.

Sodium

108Sodium

134

The sodium draws water from the inside of the cells causing the brain to shrivel.

Central Pontine Myelinolysis

• Brain Shrinkage o Quadriplegiao Respiratory paralysiso Mental status changeso Usually fatal within three to five

weeks• Risk factors:

o Hyponatremia for > 24 hourso Over-correction of

hyponatremia (> 24 mEq/L/day)o Rapid correction (greater than

1–2 meq/hr)o Alcoholismo Malnutritiono Liver disease

Damned if you do. Damned if you don’t

• Without treatment patients have cerebral edema.

• With mistreatment patients are at risk of CPM.

TAKE HOME POINTS

• Hyponatremia is a WATER problem, not sodium problem

• In general best strategy in ER if not symptomatic... DO NOTHING (Primum non nocere) ... including holding NS unless dehydrated

• Repeat blood tests to confirm and watch for psuedohyponatremia, send off urine studies

• Careful hypertonic saline (3%) if symptomatico rule of thumb start hourly rate @0.5 LBM (kg)

CASE REPORTS