Classification by origin
To properly understand the nature of the disease, it is first necessary to examine its sources. In pathophysiology, it is customary to distinguish two most common varieties of the syndrome of inappropriate antidiuretic hormone secretion, classified specifically by their origin. These include:
- Neurohypophyseal form — directly associated with disorders in the brain structures responsible for the production and storage of the hormone.
- Ectopic form — occurs when the hormone begins to be produced in tissues where this process should not normally occur.
Pathogenesis of SIADH: initial links
Two main, or initial, links play a leading role in the development of the pathological process. Each of them triggers its own cascade of reactions leading to an excess of antidiuretic hormone.
1. Centrogenic (neurohypophyseal) link
This mechanism is based on the excessive stimulation of central structures. It is characterized by pronounced neurogenic cortical-subcortical stimulation. As a result of this process, enhanced formation of antidiuretic hormone occurs in the neurons of the hypothalamus. Excessive synthesis is followed by an equally active transport of the formed hormone directly to the neurohypophysis, where it accumulates before release.
2. Primary glandular link
This link of pathogenesis combines two different variants of the development of excessive secretion:
- Hypothalamic variant: consists in excessive production and subsequent enhanced neurosecretion of the antidiuretic hormone by the neurons of the hypothalamus themselves.
- Ectopic variant: represents the synthesis of the hormone by so-called non-endocrine tissues. This means that cells not belonging to the endocrine system acquire the ability to produce the hormone. In clinical practice, the most common cause of this phenomenon is lung tumor cells.
Key clinical and laboratory manifestations
The excessive effect of antidiuretic hormone on the body naturally leads to a number of specific changes. According to classical schemes, the main manifestations of SIADH include the following pathological shifts:
- Oliguria — a significant decrease in the volume of excreted urine, which is a direct consequence of the antidiuretic effect.
- Weight gain — occurs against the background of fluid retention in the body.
- Hyponatremia — a decrease in the concentration of sodium ions in blood plasma.
- Increase in urine $Na^+$ content — paradoxical renal sodium loss against the background of its systemic deficiency.
- Psychoneurological disorders — a complex of neurological disorders that completes the clinical picture of the syndrome.
Briefly: answers to topical questions
What is the mechanism of development of hyponatremia in SIADH?
Hyponatremia in the syndrome of inappropriate antidiuretic hormone secretion develops due to a combination of two factors.
- Blood dilution — occurs as a result of water retention (hypervolemia) caused by the activation of fluid reabsorption in the distal renal tubules.
- Sodium loss — excessive excretion of sodium ions from the body.
What causes the increase in urine sodium content in SIADH?
The increase in sodium content in the urine in the syndrome of inappropriate antidiuretic hormone secretion is a consequence of compensatory electrolyte loss. This process is caused by a decrease in aldosterone synthesis in response to hypervolemia. Hypervolemia, in turn, is formed due to an increase in extracellular fluid volume as a result of the activation of water reabsorption in the distal renal tubules.
Why do psychoneurological disorders develop in SIADH?
The development of psychoneurological disorders in SIADH occurs due to the swelling of brain neurons. This condition is described as "water intoxication". Patients may exhibit the following symptoms:
- Apathy
- Lethargy
- Impaired consciousness
- Seizures
To easily remember the manifestations of SIADH, use the acronym: Oliguria, Weight (gain), Hyponatremia, Sodium in urine (elevated), Psychoneurological disorders.
Class and exam questions
How many varieties of SIADH by origin are most commonly distinguished?
There are two main varieties: neurohypophyseal and ectopic.
What is the most common cause of ectopic antidiuretic hormone secretion?
The most common cause of ectopic hormone synthesis by non-endocrine tissues is lung tumor cells.
How does the sodium level change in this syndrome?
The syndrome is characterized by the development of hyponatremia (decrease in blood sodium) with a simultaneous increase in the content of sodium ions ($Na^+$) in the urine.
What else to study on this topic
- Mechanisms of neurogenic cortical-subcortical stimulation of the hypothalamus
- Features of hormone transport from the hypothalamus to the neurohypophysis
- Molecular basis of ectopic hormone synthesis by lung tumor cells
- Relationship between hyponatremia and the development of psychoneurological disorders
- Pathophysiological rationale for urinary sodium loss during oliguria
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