Hyponatremia with elevated tonicity can occur with high blood sugar, causing an shift of excess free water into the serum. Pathophysiology
The causes of and treatments for hyponatremia can only be understood by having a grasp of the size of the body fluid compartments and subcompartments and their regulation; how under normal circumstances the body is able to maintain the sodium concentration within a narrow range (homeostasis of body fluid osmolality); conditions can cause that feedback system to malfunction (pathophysiology); and the consequences of the malfunction of that system on the size and solute concentration of the fluid compartments. Normal homeostasis
There is a hypothalamic-kidney feedback system which normally maintains the concentration of the serum sodium within a narrow range. This system operates as follows: in some of the cells of the hypothalamus, there are osmoreceptors which respond to an elevated serum sodium in body fluids by signalling the posterior pituitary gland to secrete antidiuretic hormone (ADH) (vasopressin). ADH then enters the bloodstream and signals the kidney to bring back sufficient solute-free water from the fluid in the kidney tubules to dilute the serum sodium back to normal, and this turns off the osmoreceptors in the hypothalamus. Also, thirst is stimulated. Normally, when mild hyponatremia begins to occur, that is, the serum sodium begins to fall below 135 mEq/L, there is no secretion of ADH, and the kidney stops returning water to the body from the kidney tubule. Also, no thirst is experienced. These two act in concert to raise the serum sodium to the normal range.
This is because charged sodium ions attract around them up to 25 water molecules, thereby creating a large polar structure too large to pass through the cell membrane: "channels" or "pumps" are required. Cell swelling also produces activation of volume-regulated anion channels which is related to the release of taurine and glutamate from astrocytes. Diagnosis
The history, physical exam, and laboratory testing are required to determine the underlying cause of hyponatremia. A blood test demonstrating a serum sodium less than 135 mmol/L is diagnostic for hyponatremia. The history and physical exam are necessary to help determine if the person is hypovolemic, euvolemic, or hypervolemic, which has important implications in determining the underlying cause. An assessment is also made to determine if the person is experiencing symptoms from their hyponatremia. These include assessments of alertness, concentration, and orientation. False hyponatremia
False hyponatremia, also known as spurious, pseudo, hypertonic, or artifactual hyponatremia is when the lab tests read low sodium levels but there is no hypotonicity. In hypertonic hyponatremia, resorption of water by molecules such as glucose (hyperglycemia or diabetes) or mannitol (hypertonic infusion) occurs. In isotonic hyponatremia a measurement error due to high blood triglyceride level (most common) or paraproteinemia occurs. It occurs when using techniques that measure the amount of sodium in a specified volume of serum/plasma, or that dilute the sample before analysis. True hyponatremia
True hyponatremia, also known as hypotonic hyponatremia, is the most common type. It is often simply referred to as "hyponatremia."