Syndrome of inappropriate antidiuretic hormone secretion: Clinical sciences

Syndrome of inappropriate antidiuretic hormone secretion, or SIADH for short, is when too much antidiuretic hormone, also called ADH, or arginine vasopressin, is secreted.

Normally, ADH is secreted when there’s too little water and too much sodium in the body, and it works by increasing water reabsorption in the renal tubule, restoring the water-sodium balance.

However, with SIADH, there’s increased ADH secretion in the absence of fluid depletion, and this causes excessive water reabsorption, diluting the blood to the point of euvolemic hyponatremia.

SIADH most often occurs as a secondary response to another condition, and is commonly seen in patients with pulmonary disease like pneumonia or lung cancer, as well as central nervous system disorders, like meningitis and head trauma.

Now, if your patient presents with chief concerns suggesting SIADH, you should first perform an ABCDE assessment to determine if they are unstable or stable.

If the patient is unstable, stabilize the airway, breathing, and circulation. Next, obtain IV access and put your patient on continuous vital sign monitoring, including blood pressure, heart rate, and pulse oximetry. Don’t forget to monitor the patient’s urine output closely! Finally, if needed, provide supplemental oxygen.

Now, here’s a high-yield fact! Unstable patients often have severe symptoms like seizures or even respiratory arrest! In severe hyponatremia, sodium levels can fall below 125 milliequivalents per liter, and the hyponatremia is frequently acute, meaning it developed within a 24- to 48-hour period.

Symptomatic hyponatremia indicates the presence of cerebral edema, so if your patient presents with severe symptoms, obtain a serum sodium level to confirm hyponatremia, and administer intravenous 3% hypertonic saline to treat the cerebral edema and correct serum sodium. Make sure not to give 0.9% isotonic saline, because this can dilute the serum sodium even more!

Also, keep in mind that, in patients with severe hyponatremia, raising the serum sodium too rapidly can lead to osmotic demyelination syndrome, a potentially fatal condition associated with lasting neurologic dysfunction! This is why, generally speaking, the goal is to correct the serum sodium concentration by raising no more than 4 to 6 milliequivalents per liter in a 24-hour period.

Now, let’s go back to the ABCDE assessment and look at stable patients.

In this case, obtain a focused history and physical examination and order a basic metabolic panel or BMP.

History findings typically include headache, nausea, vomiting, and confusion. But bear in mind that the vast majority of patients with SIADH can be asymptomatic!

Now, patients with SIADHare often elderly and may have a history of pulmonary conditions, like pneumonia or small cell lung cancer; as well as central nervous system conditions, such as meningitis, CNS tumors, or traumatic brain injury. They might also report use of medications like NSAIDs, antidepressants, or antipsychotics.

Alright, now, let’s move on to physical exam findings. These patients usually have normal blood pressure and heart rate, and appear well hydrated, with moist mucous membranes and normal skin turgor. Signs of hypervolemia, like ascites and edema, are absent.

Finally, labs reveal a sodium level lower than 135 milliequivalents per liter, as well as a normal BUN and creatinine.

If your patient has this spectrum of findings, you should suspect SIADH and check plasma osmolality.

If the plasma osmolality is greater than 280 milliosmoles per kilogram, then you should consider alternative diagnoses, such as pseudohyponatremia caused by hyperlipidemia or hyperproteinemia.

Let’s take a look at when the plasma osmolality is lower than 280 milliosmoles per kilogram.