Hyponatremia is a plasma sodium concentration below the normal range. In most cases it is a disorder of water rather than of salt. Why water rather than salt?
What single idea holds the topic together?
Plasma sodium is a concentration, and a concentration is a ratio: the amount of sodium on top, the water that holds it underneath. Plasma sodium reflects two quantities held in balance, extracellular water content and extracellular sodium content. The number moves whenever either one moves, so it follows the water as well as the sodium.
The same ratio pushed the other way gives hypernatremia. The two disorders are mirror images, and most cases of both are caused by a change in water, not a change in salt.
There is a catch. Different pairs of quantities can give the same ratio. The same low value can arise from too much water, too little sodium, or a shift of water between compartments, and the number alone cannot say which one moved. So hyponatremia is read as a question about water: how the body measures and holds its concentration, what can upset that balance, and what the upset does to the patient. The first step is to see what each side of the ratio does.
What do too much or too little water do?
Excess water dilutes the sodium already present, so the ratio falls. A water deficit concentrates the sodium, so the ratio rises. These are the two halves of the same ratio, and the water side is what the satellite notes explain.
How is the water in the body measured?
Body fluids are described with three concentration terms: osmolality, osmolarity and tonicity. Osmolality rather than osmolarity is used clinically, and total osmolality is not the same as tonicity. Osmolality, Osmolarity and Tonicity sets out the three. Once the water is measured, the next question is where it sits.
Where does that water sit?
Total body water is divided between the intracellular and extracellular compartments. Extracellular fluid is itself divided into further compartments, and the main ions are distributed between these compartments. Total Body Water and Its Compartments describes them. Knowing where the water sits raises the question of how the body holds it.
How does the body hold its water?
Extracellular fluid tonicity is held within a narrow range by thirst and by water retention or excretion driven by ADH, also called vasopressin. Osmosensors and baroreceptors control the release of ADH. ADH and the Regulation of Plasma Tonicity explains this control. Holding water can fail in specific ways, and the causes come next.
What lowers the number?
The causes of hyponatremia group into three: intravascular volume depletion, hypotonic fluid intake exceeding the kidney’s water excretion, and the syndrome of inappropriate antidiuresis (SIAD). Causes of SIAD include central salt wasting, and in nephrogenic SIAD the abnormality lies in the kidney. Causes of Hyponatremia takes each group in turn. A low number has consequences for the patient.
What does a low sodium do to the patient?
Hyponatremia ranges from an incidental laboratory finding to a medical emergency, depending on how low the sodium is and how quickly it developed. The rate of the fall matters as much as its depth. Hyponatremia has neurological complications, both from the disorder and from its treatment. Clinical Presentation and Complications of Hyponatremia explains this. Finding the cause and treating it safely is the next question.
How is the cause found, and what is done?
The causes are narrowed down using the history, the examination and laboratory tests, and the initial management is the first steps of correction. Diagnosis and Initial Management of Hyponatremia covers both. Where to begin comes next.
Where should a student start?
The topic is a question about water, so start with how water is measured, in Osmolality, Osmolarity and Tonicity, and then read the notes in the order above. That first note answers a question every later one depends on: how do you count the particles that hold water in the first place?