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Also, persons medicated with diuretics may be dehydrated prior to initiating exercise. If sodium chloride deficits occur then the extracellular fluid volume will contract and cause salt depletion dehydration. A sodium chloride deficit usually occurs due to sweat sodium losses combined with excessive water consumption, but a sodium deficit can also occur without excessive water intake owing to high sweat sodium losses.
Both of these scenarios produce sodium dilution more commonly known as hyponatremia or water intoxication. This chapter reviews the physiology, needs, and assessment of human water and electrolyte balance. The extent to which water and electrolyte imbalances affect temperature regulation and exercise performance are also considered.
In these patients, however, vasopressin secretion and urinary dilution are usually preserved. Hyponatremia develops because renal water excretion is limited by the reduced glomerular filtration rate, leading to a decrease in maximal water excretion by the kidney.
In patients in whom hyponatremia develops in the absence of volume-stimulated vasopressin secretion, the hyponatremia results from water ingestion exceeding maximal renal water excretion. This may occur as a result of disordered vasopressin secretion or in the setting of normal suppression of vasopressin secretion in the setting of hypotonicity.
In primary psychogenic polydipsia, water intake exceeds the normal maximal rate of renal water excretion, leading to progressive water intoxication.
In these patients, the maximal daily water excretion will be severely limited and water intoxication may develop despite only modest water intake.
Euvolemic hyponatremia as the result of elevated vasopressin secretion despite prevailing hypotonicity is the hallmark of the syndrome of inappropriate antidiuresis SIAD, also known as the syndrome of inappropriate antidiuretic hormone, SIADH.
In addition, glucocorticoid deficiency and severe hypothyroidism are associated with impaired suppression of vasopressin secretion. The specific criteria to make a diagnosis of the syndrome of inappropriate antidiuresis SIAD are:. SIAD may be the result of a variety of etiologies including pain, nausea, stress, malignancy most commonly bronchogenic carcinoma, mesothelioma, pancreatic cancer, duodenal cancer, prostate cancer, endometrial carcinoma, thymoma, leukemia and lymphoma , pulmonary disease tuberculosis, pneumonia, bronchiectasis, cystic fibrosis, asthma, positive pressure ventilation , CNS disease head trauma, subdural hematoma, subarachnoid hemorrhage, stroke, primary or secondary brain tumors, encephalitis, meningitis and medications.
Hypernatremia develops when water ingestion is insufficient to match ongoing water losses. Since normal individuals are able to compensate for even massive water loss though thirst-stimulated drinking, the development of hypernatremia generally implies the presence of impaired thirst or restricted access to water.
The etiology of hypernatremia may therefore be broken down into defects in thirst in the absence of increased water losses and states associated with increased water losses. In addition, in a small percentage of patients, hypernatremia may result from the administration of hypertonic sodium solutions or salt ingestion. The prognosis in both hyponatremia and hypernatremia depends on the etiology, initial sodium, and clinical course.
Symptoms of hyponatremia may be non-specific, including nausea and vomiting, headache and gait instability, and may be difficult to differentiate from symptoms associated with underlying disease. Mental status change, seizures, coma and respiratory arrest are associated with more severe hyponatremia, with severity of symptoms correlated with the acuity of onset.
The symptoms of hyponatremia, termed hyponatremic encephalopathy, primarily reflect the development of cerebral edema. Cerebral pontine myelinolysis CPM is an irreversible neurologic disorder that has been associated with overly rapid correction of the serum sodium concentration.
As with hyponatremia, the symptoms of hypernatremia are non-specific and range from lethargy, nausea and vomiting to impaired mental status, seizures and coma. In severe hypernatremia, particularly in children, intra-cerebral bleeding may occur as the result of traction on bridging veins as the result of osmotic shrinkage of the brain. Although CPM may develop as a result of rapid onset of hypernatremia, it does not appear to be a consequence of treatment.
The result is too much water and not enough sodium. Thus, overhydration generally results in low sodium levels in the blood Hyponatremia Low Level of Sodium in the Blood In hyponatremia, the level of sodium in blood is too low. A low sodium level has many causes, including consumption of too many fluids, kidney failure, heart failure, cirrhosis, and use of diuretics However, drinking large amounts of water usually does not cause overhydration if the pituitary gland, kidneys, liver, and heart are functioning normally.
Overhydration is much more common among people whose kidneys do not excrete urine normally—for example, among people with a disorder of the heart, kidneys, or liver, or among premature infants, whose kidneys are immature. Certain drugs, such as some antidepressants, can also cause overhydration in susceptible people. Overhydration may also result from the syndrome of inappropriate antidiuretic hormone secretion Syndrome of Inappropriate Secretion of Antidiuretic Hormone SIADH The syndrome of inappropriate secretion of antidiuretic hormone develops when too much antidiuretic hormone vasopressin is released by the pituitary gland under certain inappropriate conditions In this syndrome, the pituitary gland secretes too much vasopressin also called antidiuretic hormone , stimulating the kidneys to conserve water when that is not needed.
Drinking too much water without replacing salt can be harmful, occasionally even in healthy people. Brain cells are particularly susceptible to overhydration and to low sodium levels in the blood. When overhydration occurs slowly and is mild or moderate, brain cells have time to adapt, so only mild symptoms if any like distractibility and lethargy may ensue. When overhydration occurs quickly, vomiting and trouble with balance develop.
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