Which manifestation is expected with syndrome of inappropriate antidiuretic hormone (SIADH)?
Oliguria.
Weight loss.
Increased thirst.
Hypernatremia.
The Correct Answer is A
Choice A rationale
Syndrome of inappropriate antidiuretic hormone involves the excessive release of ADH, leading to significant water reabsorption in the renal collecting ducts. This process results in the production of highly concentrated urine and a marked decrease in total urine volume, known as oliguria. Normal urine output is typically ≥ 0.5 mL/kg/hr. In SIADH, the kidneys continue to retain water inappropriately despite the body being in a state of fluid volume excess, leading to concentrated urine.
Choice B rationale
Patients with SIADH typically experience rapid weight gain rather than weight loss. This weight gain is the direct result of excessive free water retention caused by the high levels of circulating antidiuretic hormone. The retained water expands the extracellular and intracellular fluid compartments. Unlike heart failure or renal failure, this fluid accumulation usually does not present with visible peripheral edema because the water is distributed evenly throughout all body fluid compartments.
Choice C rationale
Increased thirst is generally suppressed in SIADH because the patient is already in a state of fluid overload and has low serum osmolality. Thirst is a physiological response usually triggered by dehydration or high serum sodium levels. In SIADH, the serum is diluted by excess water, dropping the sodium concentration often below 135 mEq/L. Consequently, the brain's thirst center is not stimulated, and patients must often be restricted to less than 800 mL of fluid daily.
Choice D rationale
SIADH is characterized by dilutional hyponatremia, not hypernatremia. The excessive retention of free water dilutes the total amount of sodium in the extracellular fluid, resulting in serum sodium levels that are lower than the normal range of 135 to 145 mEq/L. Hypernatremia would involve a sodium concentration > 145 mEq/L and is typically seen in conditions where water is lost in excess of solute, such as diabetes insipidus, which is the physiological opposite of SIADH.
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Correct Answer is A
Explanation
Choice A rationale
SIADH involves the excessive release of antidiuretic hormone, which causes the kidneys to reabsorb too much water back into the bloodstream. This leads to an increase in total body water, which dilutes the concentration of sodium in the blood. Consequently, serum sodium levels drop below the normal range of 135 to 145 mEq/L, resulting in dilutional hyponatremia. This shift can cause cellular swelling, particularly in the brain, leading to neurologic symptoms.
Choice B rationale
Polyuria, or the production of large volumes of dilute urine, is the hallmark of diabetes insipidus, which is the opposite of SIADH. In SIADH, the body retains water, leading to very low urine output. The urine that is produced is highly concentrated because the kidneys are reclaiming as much water as possible. Therefore, a patient with SIADH will experience oliguria, not polyuria, with a high urine specific gravity typically exceeding the 1.030 mark.
Choice C rationale
SIADH causes fluid retention and an expansion of the extracellular fluid volume. This increase in intravascular volume typically leads to normovolemic or hypervolemic states, which usually results in normal blood pressure or hypertension. Hypotension is more commonly associated with conditions of fluid loss or dehydration, such as diabetes insipidus or adrenal insufficiency. In SIADH, the volume expansion prevents the blood pressure from dropping unless there are other unrelated co-morbidities present.
Choice D rationale
Polydipsia, or excessive thirst, is a common symptom of diabetes mellitus or diabetes insipidus, where the body is losing large amounts of fluid. In SIADH, the body is already overloaded with water due to excessive reabsorption. While some patients may still feel thirsty, it is not a primary manifestation of the hormone's action. Clinical management of SIADH actually requires strict fluid restriction to prevent further dilution of serum sodium levels and worsening of the hyponatremia.
Correct Answer is B
Explanation
Choice A rationale
Elevated blood pressure is a symptom of autonomic dysreflexia, not a trigger. When a noxious stimulus occurs below the level of a spinal cord injury, the sympathetic nervous system overreacts, causing widespread vasoconstriction and a sudden, severe spike in blood pressure. This hypertension can exceed 200/100 mmHg and is a medical emergency. While the high blood pressure is what makes the condition dangerous, it is the result of the autonomic imbalance, not the initiating cause.
Choice B rationale
Bladder distention is the most common trigger for autonomic dysreflexia in individuals with spinal cord injuries at or above the T6 level. A full bladder or a blocked urinary catheter sends sensory signals to the spinal cord. Because the signals cannot reach the brain due to the injury, the spinal cord initiates a massive, uncoordinated sympathetic reflex. This leads to severe hypertension above the injury and requires immediate drainage of the bladder to resolve the crisis.
Choice C rationale
A severe, throbbing headache is a primary symptom of autonomic dysreflexia, caused by the rapid increase in intracranial pressure from the sudden onset of hypertension. It is not the trigger. When the body's baroreceptors sense the high blood pressure, the parasympathetic nervous system tries to compensate by dilating blood vessels and slowing the heart rate, but these signals cannot pass below the injury site. The resulting headache serves as a critical warning sign to find the trigger.
Choice D rationale
Nasal congestion is a symptom of autonomic dysreflexia, often occurring alongside facial flushing and sweating above the level of the spinal cord injury. These signs are caused by the parasympathetic nervous system's attempt to lower blood pressure through vasodilation in the upper body. While uncomfortable, nasal stuffiness is a secondary effect of the autonomic storm and does not cause the dysreflexia itself. Identifying and removing the underlying stimulus, such as a full bladder, is the only way to reverse it.
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