Euvolemic Hypotonic Hyponatremia

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.