Welcome to PICU Doc On Call, A Podcast Dedicated to Current and Aspiring Intensivists.
I'm Pradip Kamat and I'm Rahul Damania. We are coming to you from Children's Healthcare of Atlanta - Emory University School of Medicine.
Here's the case:
A 6-year-old child with a known h/o craniopharyngioma who has been endocrinologically intact with exception of needing thyroid replacement was admitted to the PICU prior to craniotomy to proceed with further tumor resection as well as the removal of a secondary cyst impacting his brainstem. The patient is receiving Keppra for seizures and per mother, he has recently been significantly more sleepy at school.
On POD Op day 5: the PICU the bedside nurse notices increased urine output (6cc/kg/hr to as high as 10cc/kg/hr). Initially, there was an increase in Na to 157mEq/L within 48-72 hours the serum Na dropped to 128mEq/L
To summarize key elements from this case, this patient has:
Increase UOP
Rapidly increasing Na initially followed by a drop
All of which brings up a concern for Na abnormality post craniotomy

In today’s episode, we will be breaking down all things Sodium & the Brain. We will discuss diagnostic & management frameworks related to three pathologies:
Central Diabetes Insipidus
Syndrome of inappropriate Anti-Diuretic Hormone or SIADH
Cerebral Salt Wasting

These diagnoses can certainly be seen individually inpatients or as a spectrum of diseases — as we go through each of these diagnoses, pay particular attention to patient characteristics and lab abnormalities. Namely, serum sodium, serum osm, and urine osm.
To build the fundamentals, lets first start with classic nephrology saying: Serum Na represents Hydration
This takes us into a brief review of normal physiology — talking about three important hormones:
ADH
Aldosterone
Atrial Natriuretic Peptide (ANP)

Let’s go through a quick multiple-choice question.
A patient is recently started on DDAVP for pan-hypopituitarism. The medication acts similarly to a hormone that is physiologically synthesized in which of the following from which are in the body?
A. Paraventricular Nucleus of the Hypothalamus
B. Supraoptic Nucleus of the Hypothalamus
C. Anterior Pituitary
D. Vascular Endothelium
The correct answer here is B the Supraoptic Nucleus of the Hypothalamus. Remember that ADH is synthesized in the hypothalamus and released from the posterior pituitary.
What are the physiologic actions of ADH?
ADH Increases H2O permeability by directing the insertion of aquaporin 2 (AQP2) H2O channels in the luminal membrane of the principal cells. Thus, as we will see with Central Diabetes insipidus, in the absence of ADH, the principal cells are virtually impermeable to water.
Let's talk about our next hormone, aldosterone. What are the important physiologic considerations?
Aldosterone is secreted from the adrenal cortex as a byproduct of the RAAS.
Aldosterone increases Na+ reabsorption by the renal distal tubule, thereby increasing extracellular fluid (ECF) volume, blood volume, and arterial pressure.
It also helps in secreting K and H. This physiology is applied directly at the bedside when we have patients in the ICU who have a contraction alkalosis secondary to diuretics. The increase in aldosterone as these patients lose free water from their Lasix administration results in hypokalemia and metabolic alkalosis.

Alright, what about the third hormone, ANP?
Atrial natriuretic peptide (ANP) is released from the atria in response to an increase in blood volume and atrial pressure.
ANP causes relaxation of vascular smooth muscle, dilation of arterioles, and decreased TPR.
causes increased excretion of Na+ and water by the kidney, which reduces blood volume and attempts to bring arterial pressure down to normal.

As ANP causes natriuresis, diuresis, and inhibition of renin, you can consider this hormone as having a complementary & opposite effect to ADH and aldosterone.
Alright, now that we...