science

Syndrome of inappropriate antidiuretic hormone secretion

Syndrome of inappropriate antidiuretic hormone secretion is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Syndrome of inappropriate antidiuretic hormone secretion rather than just read about it. In short: Syndrome of inappropriate antidiuretic hormone secretion (SIADH), also known as the syndrome of inappropriate antidiuresis (SIAD), is characterized by a physiologically inappropriate release of antidiuretic hormone (ADH) either from the posterior pituitary gland, or an ectopic non-pituitary source, such as an ADH-secreting tumor in the lung. Unsuppressed ADH causes a physiologically inappropriate increase in solute…

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Reference excerpt

Syndrome of inappropriate antidiuretic hormone secretion (SIADH), also known as the syndrome of inappropriate antidiuresis (SIAD), is characterized by a physiologically inappropriate release of antidiuretic hormone (ADH) either from the posterior pituitary gland, or an ectopic non-pituitary source, such as an ADH-secreting tumor in the lung. Unsuppressed ADH causes a physiologically inappropriate increase in solute-free water being reabsorbed by the tubules of the kidney to the venous circulation leading to hypotonic hyponatremia (a low plasma osmolality and low sodium levels). The causes of SIADH are commonly grouped into categories including: central nervous system diseases that directly stimulate the hypothalamus to release ADH, various cancers that synthesize and secrete ectopic ADH, various lung diseases, numerous drugs (carbamazepine, cyclophosphamide, SSRIs) that may stimulate the release of ADH, vasopressin release, desmopressin release, oxytocin, or stimulation of vasopressin receptor 2 on the kidney (the site of ADH action). Inappropriate antidiuresis may also be due to acute stressors such as exercise, pain, severe nausea or during the post-operative state. In 17–60% of people, the cause of inappropriate antidiuresis is never found. ADH is derived from a preprohormone precursor that is synthesized in cells in the hypothalamus and stored in vesicles in the posterior pituitary. Appropriate ADH secretion is regulated by osmoreceptors on the hypothalamic cells that synthesize and store ADH. In appropriate ADH secretion, plasma hypertonicity activates these osmoreceptors, ADH is released into the blood stream, the kidneys increase solute-free water reabsorption, and the hypertonicity is alleviated. A decrease in the effective circulating volume of blood (the volume of arterial blood effectively perfusing tissues) also stimulates an appropriate, physiologic release of ADH. Inappropriate ADH secretion causes physiologically high water reabsorption by the kidneys, causing elevated fluid retention. This causes the extracellular fluid (ECF) space to become hypoosmolar and hyponatremic (low sodium). In the intracellular space, cells swell as intracellular volume increases as water moves from an area of low solute concentration (extracellular space) to an area of high solute concentration (the cells' interior). In severe or acute hypoosmolar hyponatremia, swelling of brain cells causes various neurological abnormalities, which in severe or acute cases can result in convulsions, coma, and death. The symptoms of chronic syndrome of inappropriate antidiuresis are more vague, and may include cognitive impairment, gait abnormalities, or osteoporosis. The main treatment of inappropriate antidiuresis is to identify and treat the underlying cause, if possible. This usually causes plasma osmolality and sodium levels to return to normal in several days. In those in which an underlying cause cannot be found, or is untreatable, treatments are targeted to alleviating correcting the hypoosmolality and hyponatremia. These include restriction of fluid intake, using salt tablets (sometimes with diuretics), urea supplements, intravenous saline, or increasing protein intake. The vasopressin receptor 2 antagonists, tolvaptan or conivaptan, may also be used. The presence of cerebral edema, or other moderate to severe symptoms, may necessitate intravenous hypertonic saline administration with close monitoring of the serum sodium levels to avoid overcorrection. SIADH was originally described in 1957 in two people with small-cell carcinoma of the lung.

Signs and symptoms

Gastro-intestinal Anorexia (loss of appetite) Nausea

Musculoskeletal Muscle aches Generalized muscle weakness

Neuro-muscular Myoclonus Decreased reflexes Ataxia Pathological reflexes Tremor Asterixis

Respiratory Cheyne-Stokes respiration

Neurological Dysarthria Lethargy Confusion Delirium Seizures Coma (from brain swelling) Death

Causes Causes of SIADH include conditions that dysregulate ADH secretion in the central nervous system, tumors that secrete ADH, drugs that increase ADH secretion, among other causes. Cancer accounts for an estimated 24% of cases of SIADH, with 25% of those causes due to small cell lung cancer. Medications or drugs are responsible for 18% of cases of SIADH. This is due to a variety of mechanisms including: stimulation of ADH release (opiates, ifosfamide, vincristine, platinum-based antineoplastics and MDMA (also known as ecstasy)); enhancers of ADH effect (non-steroidal anti-inflammatories); ADH analogues (desmopressin, oxytocin); and vasopressin receptor 2 activators (selective serotonin reuptake inhibitors (SSRIs), haloperidol, carbamazepine, cyclophosphamide, and chlorpropamide). Of the causes of medication induced syndrome of inappropriate antidiuresis, antidepressants (especially SSRIs) are the most common culprit. Central nervous system (CNS) disorders or conditions may cause SIADH in 9% of cases, this includes subarachnoid hemorrhage (56% of CNS causes), pituitary surgery (35% of CNS causes), brain cancer, infections, stroke and head trauma. No cause of inappropriate antidiuresis is initially found in 17–60% of cases. A list of common causes is below:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Syndrome of inappropriate antidiuretic hormone secretion

Start with the simplest possible case. Write down what Syndrome of inappropriate antidiuretic hormone secretion claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Syndrome of inappropriate antidiuretic hormone secretion before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Syndrome of inappropriate antidiuretic hormone secretion ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Syndrome of inappropriate antidiuretic hormone secretion

In research: Syndrome of inappropriate antidiuretic hormone secretion appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Syndrome of inappropriate antidiuretic hormone secretion in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Syndrome of inappropriate antidiuretic hormone secretion is common in secondary-school and first-year university syllabi. It links to neighbouring topics Oncological emergencies, Pituitary disorders, Syndromes, so understanding it makes those chapters shorter.
In everyday life: Look for Syndrome of inappropriate antidiuretic hormone secretion outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

What is Syndrome of inappropriate antidiuretic hormone secretion in simple terms?

Why does Syndrome of inappropriate antidiuretic hormone secretion matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Syndrome of inappropriate antidiuretic hormone secretion?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Syndrome of inappropriate antidiuretic hormone secretion.