Euvolemic Hypotonic Hyponatremia

by Amy Sussman, MD

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    00:01 Our next category is Euvolemic Hypotonic Hyponatremia.

    00:05 This means that patients have a primary water gain with the normal extracellular fluid volume.

    00:10 And the patients are euvolemic on exam.

    00:13 Some of the causes that we think about are the syndrome of inappropriate ADH secretion or SIADH.

    00:20 This can be acquired because of some of the drugs that people take.

    00:23 Antiepileptic drugs, some of the type of the psychotropic medications like SSRIs, chronic pain, CNS disorders and malignancies, pulmonary disorders, and patients who are in the postoperative state.

    00:36 We can also have hereditary issues causing the syndrome of inappropriate antidiuresis and this is if patients have a gain of function, mutation, and the V2 receptor so that they have aquaporins inserted at all times and they're reabsorbing water, or problems in the hypothalamic sensing receptors.

    00:53 So in these situations, urine Osm will be high due to the fact that ADH is concentrating the urine.

    00:59 Urine sodium interestingly will also be elevated typically greater than 40 mEq/L and that's because you have a small amount of volume expansion that's induced by water.

    01:08 That's going to suppress renin and actually activate some of the ANP or Atrial natriuretic peptide.

    01:15 So we see sodium in the urine.

    01:18 Another cause of euvolemic hypotonic hyponatremia is primary polydipsia.

    01:24 That's excessive water intake that overwhelms the excretory capacity of the kidney.

    01:30 In this situations, people are driven to drink.

    01:33 Sometimes we refer to this also as psychogenic polydipsia.

    01:36 Oftentimes, patients will have a coexistence psychiatric disorder, they might be on medications that cause excessive dry mouth.

    01:43 That's one of the things that stimulates them to drink.

    01:46 But it's not uncommon to see these patients drink about 16 liters of water per day.

    01:52 That's about four gallons, and they don't produce that much solute.

    01:56 So that's why they end up having a primary water gain.

    02:00 Now, it will be easy to distinguish these patients from SIADH patients because their urine Osm is going to be low.

    02:06 ADH here is appropriately suppressed.

    02:11 Another cause of euvolemic hypotonic hyponatremia is reduced solute intake.

    02:16 Something that we affectionately refer to as beer drinker's potomania, or the tea and toast diet.

    02:23 Here, again, water intake will exceed the daily osmolar load.

    02:28 Let's look at the example.

    02:30 If a patient's daily omolar generation and excretion is only 240 mOsmol of solute per day, and the urine can maximally dilute at let's say 60 mOsmol/L, then it would take four liters of urine to excrete that daily osmolar load.

    02:49 So, if one drinks in excess of four liters, then hyponatremia will result.

    02:56 Here, once again, the urine Osm is going to be low and that will help to distinguish this disorder from SIADH.

    03:05 Hypothyroidism is another cause of euvolemic hypotonic hyponatremia.

    03:10 Now, the mechanism behind that is not completely understood, and patients have to be significantly hypothyroid for this to occur.

    03:16 But the thought is perhaps there is reduced cardiac contractility and then appropriate ADH stimulation from there.

    03:23 Thiazide diuretics are another category that we need to think about with euvolemic hypotonic hyponatremia.

    03:28 We see this in our elderly population or a malnourished patients.

    03:32 The mechanism in part is due to appropriate ADH stimulation because remember, these patients are losing or inducing sodium loss from their thiazide diuretic.

    03:42 But there are other ideologies involved and perhaps there is a direct action on aquaporins stimulation.

    About the Lecture

    The lecture Euvolemic Hypotonic Hyponatremia by Amy Sussman, MD is from the course Water Balance: Hypo- and Hypernatremia.

    Included Quiz Questions

    1. Pneumonia
    2. Congestive heart failure
    3. Liver cirrhosis
    4. Hypothyroidism
    1. Urine-to-plasma osmolality of <1.0
    2. Urine sodium of >20 mEq/L
    3. Lower limb edema
    4. Orthostatic hypotension

    Author of lecture Euvolemic Hypotonic Hyponatremia

     Amy Sussman, MD

    Amy Sussman, MD

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