Hyponatremia

Defined as a serum sodium < 135. The first step when working up hyponatremia, is to look and make sure that it is true hyponatremia by calculating the serum osmolality. 

Serum Osmolality = (2 x Na) + (glucose/18) + (BUN/2.8)

If the serum osmolality is < 290, then it is true hyponatremia

If the serum Na is > 290, there is an osmolyte in abundance causing a pseudohyponatremia

Osmolytes that would cause pseudohyponatremia include...

• Hyperglycemia
• Hypertriglyceridemia
• Hyperglobulinemia
• Mannitol

Once you have established the fact that there is true hyponatremia, assess the patient's volume status (BP, orthostatic BP, mucous membranes, I/O, peripheral edema). The causes of hyponatremia can be categorized under euvolemia, hypovolemic or hypervolemic

Euvolemia - all causes measure the urine Na < 20

• Psychogenic Polydipsia
• SIADH - the following criteria
• Serum Na < 135 and serum osmolality < 290
• Euvolemia
• Inappropriate urine osmolality - this is how you would distinguish SIADH from Psycogenic polydipsia
• Adrenal Insufficiency - cortisol has a negative feedback loop on CRH. If there is no cortisol due to adrenal insufficiency, the release of CRH is stimulated and CRH has a positive affect on the release of ADH and ACTH. The ADH will cause reabsorption of water from the kidneys. 
• Hypothyroidism
• Diuretics (TZDs)
• Extasy 
• Low dietary intake of solutes (potomania)

Hypovolemia - losing too much Na

Urine Na > 20 - kidneys are not functioning because they are not reabsorbing Na

• Diuretics
• RTAs
• Urinary obstruction
• Tubular interstial disease

Urine Na < 20 - kidneys are responding appropriately to volume depletion 

• Vomiting 
• Diarrhea
• Burns
• Excessive sweating

Hypervolemia - retaining too much Na (seems counter intuitive but think about the rule wherever Na goes, water follows which will cause a hyponatremia)

Urine Na > 20 - again, means something is wrong with the kidneys

• ESRD - pt will be anuric and will have a urine output of < 100 ml/day

Urine Na < 20 - body thinks that it is hypovolemic because you are third spacing your fluid. This this fluid is not going to the kidneys and the kidneys think that you are hypovolemic so they reabsorb as much fluid as possible

• HF
• Cirrhosis
• Nephrotic Syndrome

Treatment - Divide the categories into if the patient is symptomatic vs asymptomatic

Symptomatic - Happens when the hyponatremia is acute < 48 hours. pt is lethargic, confused, seizures

This is a situation where it is ok to give 3% saline. That can be very dangerous though so my attending told me that he will give about a 500 cc (100 cc brings it up about 1 point of serum Na) bolus of 3% saline and recheck the serum Na in 1-2 hours. The goal of correction is about 4-6 mEq but don't want to correct more than 2 meq/hr. 

Asymtomatic - Hyponatremia is usually chronic  > 48 hours. Treat according to volume status and DON'T give 3% saline. Goal of correction is about 8-12 meq 24 hours

Hypovolemia - Replete the patient with NS

Euvolemia - water restriction 1200-1600 ml/day was what we usually did

Hypervolemia - water restriction, diuretics