Hyponatremia diagnosis can be challenging as serum sodium concentration is affected by wide variety of variables. In order to understand the concept of hyponatremia, we came up with this infographic that illustrates how to diagnose the cause of hyponatremia in 5 simple steps.
Hyponatremia is defined as serum concentration of Na+ < 135 mEq/L. Patients with Na concentration of 130-135 are mildly hyponatremic while those with Na of 120-129 are moderate. Serum Na less than 120 mEq/L comes in severe category and can many times be a life threatening condition.
The first important step of hyponatremia assessment is to rule out severe hyponatremia. This is usually associated with severe neurological symptoms such as ataxia, headaches, seizure and eventual coma. These patients are best treated with hypertonic saline with close monitoring of serum Na to prevent overcorrection ideally in ICU settings.
Mild to Moderate Hyponatremia
Patients with mild to moderate hyponatremia form the majority of the patients. Most of these patients are either asymptomatic or present with a condition that is causing the drop in sodium levels. The first step in assessing the cause of hyponatremia in these patients is to calculate serum osmolality. Serum osmolality can be calculated by the following formula:
Calculated osmolarity = 2 Na + Glucose + Urea (all in mmol/L). ] + [Glucose]/18 + [ BUN ]/2.8 where [Glucose] and [BUN] are measured in mg/dL.
If serum osmolality is normal:
Normal serum osmolality ranges from 280-295 and varies by 5 points based on different sources. If serum Na concentration is low and plasma serum osmolality is normal, it means this is pseudohyponatremia which simply means false positive lab results. Most common reason is high lipid content such as triglycerides, cholesterol or other agents such as proteins. These substances are osmotically inactive and therefore occupies plasma volume. As Na is calculated by labs with denominator as plasma volume, this results in abnormally low Na concentration values while the true Na concentration is serum is normal. Patients with pseudohyponatremia should be tested for paraproteinemia disorders such as plasma cell dyscrasias (multiple myeloma) in which proteins are abnormally elevated. They should also be screened for hyperlipidemia.
If serum osmolality is low:
If serum osmolality is low this means that this is true hyponatremia and the patient needs to be assessed further to elucidate the cause. The first step after confirming low serum osmolality is to do a physical exam and assess the volume status of the patient.
Hypovolemic hyponatremia patient:
Hypovolemic patients have two things going on. They are losing Na and they are losing water. Why do they get hyponatremic if they are losing both (concentration should remain constant, no?). The reason is they lose Na relatively faster and that lost Na being osmotically active then causes loss of water with it. The key to finding the cause of this type of hyponatremia is to locate the exact location from where the body is losing Na. If patient is losing Na through their kidneys then we should see increase urine Na concentration. While, if they are losing sodium due to diarrhea or vomiting we should see decreased urine Na concentration as kidneys struggle to conserve the body’s Na concentration.
Euvolemic patients maintain their Na at normal levels but they tend to retain free water that lowers the overall concentration of serum Na. For example, in SIADH there is excessive ADH production which causes retention of water in the body. As it has no effect on Na reabsorption, the overall concentration of Na decreases in the body. This is why patients with SIADH need to be kept strictly off of fluids. Other causes include cases of primary polydipsia in which again the patient is compelled to drink free water and thus develops hyponatremia eventually.
Patients with heart failure, cirrhosis and nephrotic syndrome have a lot of extracellular shift of volume. This leads to a extracellular overload state where physiologically we have a lot of total body water and relatively increased Na content as well but less compared to the amount of TBW. In this scenario the driver of hypervolemia is fluid therefore patient develops relative hyponatremia due to excess volume. This effect is mainly driven by renin aldosterone system. Low intravascular volume means low perfusion to kidneys which results in increase secretion of renin and aldosterone. Aldosterone leads to increase Na reabsorption from the kidneys leads to increase in Na content. This is the reason we see low urine Na in patients with heart failure. On the other hand if the volume overload is due to kidney disorder then we see poor reabsorption of Na and poor excretion of free water from kidneys which again results in high TBW and increase loss of Na in the kidneys creating a perfect storm of hypervolemic hyponatremia.
If serum osmolality is high:
High serum osmolality means there is another active osmole in the blood. Most commonly elevated glucose levels compete with Na as major osmole and draws significant volume into the blood which then cause relative drop in Na concentration. The true Na content is not decreased in this case. It’s just that glucose is creating a hypertonic state where Na concentration comes back abnormally low. It’s vital to always look for high glucose levels and correct Na levels accordingly. The formula to correct Na levels for glucose is:
corrected sodium = measured sodium + [1.6 (glucose – 100) / 100].
In summary, Hyponatremia
can be challenging to diagnose but with our visual aid it will definitely be easier if you approach it in a systemic fashion. For more visual infographics related to medicine, subscribe to our blog. Check out our VT vs SVT algorithm cheat sheet! If you’re looking for clinical trials visual abstracts get our Visualmed app which is available on Google Play store and App store and contains more than 500 summaries of landmark clinical trials.
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