Astrid M. Libman, Julio Libman

This syndrome is made up of a constellation of symptoms and signs that result from the continuous secretion of antidiuretic hormone or arginine-vasopressin (AVP), which is inappropriate in the presence of low plasma osmolarity.

It is characterized by a state of normovolemia with hyponatremia and submaximal dilution of urine due to inadequate suppression of AVP whose secretion is not regulated by osmotic or other stimuli.

Pathophysiology

Notwithstanding the great variability in the intake of water and solutes, osmolarity ranges within narrow limits between 282 and 298 mOsm / kg. The maintenance of this constant depends on multiple factors, including the function of osmoreceptors, the production of AVP and neurophysins by neurons in the paraventricular area, the presence of functional renal receptors for AVP, renal tubular cells capable of producing aquuporin, and maintenance of adequate osmolarity in the renal medulla.

The regulation of the body volume of water and its osmolarity depends on delicate adjustment mechanisms in the ingestion and excretion of the same. Ingestion is controlled by thirst centers located in the anterior and ventromedial portions of the hypothalamus. Renal excretion of free water is regulated by the action of AVP on the distal nephron and collecting tubes. Urine can be considered as composed of two parts: one, constituted by the liquid volume excreted by the osmotic action of solutes, such as Na and urea; another, made up mainly of free water, that is, the liquid volume not linked to the effect of solutes. It is the ability to excrete this free water not dependent on solutes that indicates the action of AVP on the kidney, which is mediated by cAMP. High AVP levels decrease free water clearance, conversely, low levels increase free water clearance. AVP increases the permeability of distal tubules to water, allowing their absorption by an osmotic gradient as urine passes through the renal medulla. The result is the production of hyperosmolar urine. AVP, made basically in the supraoptic nuclei, passes through axonal transport linked to a transporter protein, neurophysin, to the nerve endings of the infundibular stem and the neurohypophysis, being released by decreases in intracellular volume and mainly increases in extracellular osmolarity . The inhibition of its secretion is a product, under physiological conditions,

The cardiac ventricles, the aortic arch, and the carotid sinuses have pressure receptors that, under physiological circumstances, tonicly inhibit the release of AVP via the vagus and glossopharyngeal nerves. Different circumstances of decreased arterial fullness. Different circumstances of decreased arterial filling, such as hemorrhages, decrease in systemic vascular resistance or development of a third space, determine the loss of tonic inhibition and produce water retention. However, this system is less sensitive than that of osmoreceptors and requires a decrease of approximately 10% in blood volume for its activation.

In the renal collecting tubules, AVP binds to a V2 subtype of receptors in the collecting tubes and increases the concentration of urine, increasing their permeability to water through a family of water channels called aquoporins.

The syndrome of inappropriate AVP secretion can present in different circumstances, which are detailed in Table 62-1.

Oat cell bronchial carcinoma is responsible for 80% of tumor-induced cases of the syndrome, and in these circumstances the hormone is produced by neoplastic tissue. Antidiuretic activity has also been identified in tuberculous lung tissue, but not in healthy tissue or in Koch bacilli suspensions. Loss of hypothalamic neurons has been demonstrated in acute intermittent porphyria. The different drugs capable of inducing this condition can act by stimulating the release and / or increasing renal sensitivity to AVP. Except in cases of ectopic production of AVP, in other circumstances, due to the action of the underlying disorder or drugs, the corresponding hypothalamic receptor cells become insensitive to variations in plasma osmolarity,

As a consequence of excess AVP, there is a decrease in free water clearance with greater tubular reabsorption of water, inability to excrete maximally diluted urine and serum hypo-osmolarity. Hyponatremia associated with increased natriuresis would be mediated, among other mechanisms, by increased levels of atrial natriuretic peptides. Hyponatremia can be discreet and asymptomatic, or it can be marked and accompanied by signs of water intoxication, the clinical manifestations being greater the faster the development of the picture.

Symptoms and signs

Anorexia, which is followed by nausea and vomiting, is the earliest symptom. Subsequently, a series of manifestations related to central nervous system dysfunction and attributable to the development of cerebral edema appear, such as headache, irritability and personality changes. When serum Na is below 125 mEq / L, there may be Babinski's sign, deep tendon areflexia, muscle weakness, mental confusion, seizures, and coma. The severity of the signs and symptoms depends not only on the absolute level of plasma Na, but also on the speed with which hyponatremia develops. It is important to emphasize the lack of edema and the absence of clinical evidence of hypovolemia and dehydration. The presence of signs suggestive of extracellular volume depletion, such as orthostatic hypotension,

Likewise, manifestations of hypervolemia such as subcutaneous edema and ascites tend to exclude this syndrome.

Study methodology

It comprises two parts: firstly, determining its existence, and secondly, its etiology.

Plasma osmolarity . There is hypoosmolarity,

It is necessary to exclude the presence of other potential causes of normovolemic hypoosmolarity such as hypothyroidism, adrenal insufficiency, and the use of diuretics.

Plasma sodium and urea . They are below normal values.

Urinary sodium . It is elevated compared to low natraemia, indicating increased renal loss.

Urinary osmolarity . There is an inappropriately concentrated urine. This does not mean that urinary osmolarity is always greater than that of plasma, even though the urine is generally hypertonic with respect to it. A urine that does not present a maximum dilution (50 mOsm / kg) in the presence of plasma hypoosmolarity clearly indicates a deficiency in the excretion of free water, even when in absolute terms its osmolarity is lower than that of plasma.

Determination of AVP in plasma . It is inappropriately increased relative to osmolarity.

Once the diagnosis of the syndrome has been made, its cause must be determined by guiding the complementary investigations according to the questioning and the clinical picture.