Figure 4.

Potential mechanisms by which cases with diabetes mellitus could have comparable glycemic control but lower serum [Na⁺] levels than do their matched controls. For each model, diamonds represent controls, and circles represent cases. Shapes without fill represent cases or controls before exposure to the forces causing [Na⁺] depression. Solid-filled shapes represent cases or controls after exposure to the forces depressing serum [Na⁺] levels. (A) Patients could be both hyperglycemic and have an additional disorder of water homeostasis (such as nonosmotic secretion of AVP caused by a medication) that induces a hypo-osmolar hyponatremia. V₁ represents the iso-osmolar component of [Na⁺] depression caused by the translocation of fluid from the intracellular space to the extracellular space. V₂ represents the component of [Na⁺] depression caused by nonosmotic forces, such as exposure to inappropriately high levels of AVP in SIADH, which cause a hypo-osmolar hyponatremia. V₃ represents the combined forces (V₁ + V₂) that depress the [Na⁺] level of the case lower than the sodium level of the control. (B) Some patients could have physiology that depresses serum [Na⁺] levels lower than would be expected for a given degree of hyperglycemia. Although V₄ and V₅ represent forces that cause an equal change in serum glucose concentration [glucose], the case serum [Na⁺] is depressed by a force with a greater slope than the control. (C) Some patients could have low-normal baseline serum [Na⁺] levels when euglycemic. With the same delta changes in serum glucose and subsequently the same delta changes in serum [Na⁺] caused by translocation of fluid from the intracellular space to the extracellular space (V₆= V₇), patients with lower baseline serum [Na⁺] levels when euglycemic (a lower homeostatic [Na⁺] level) would also have lower serum [Na⁺] levels when hyperglycemic compared with patients with high baseline serum [Na⁺] levels.