Hyponatremia in the Patient With Subarachnoid Hemorrhage

Ellen Dooling; Chris Winkelman


J Neurosci Nurs. 2004;36(3) 

In This Article

Differentiating Between CSW and SIADH

When all other potential causes of hyponatremia have been ruled out ( Table 1 ), differentiation between CSW and SIADH should occur in the patient with SAH. Hyponatremia alone is not a reliable diagnostic indicator for either SIADH or CSW. Table 2 summarizes diagnostic criteria for SIADH and CSW.

One of the most basic and most important steps in differentiating SIADH from CSW is the physical examination. CSW is associated with a decreased fluid volume with symptoms of hypovolemia. In contrast, hyponatremia in SIADH is associated with either euvolemia or hypervolemia. In this case study, there are many "clues" to the presence of hypovolemia: C.L. is tachycardic with an elevated respiratory rate, weak peripheral pulses, dry skin, and a decreased body weight from admission. While C.L.'s weight loss is not large, with the amount of fluid that is usually administered postoperatively, a loss in body weight is not expected. Another finding is a CVP reading of 3 mm Hg. CVP readings and pulmonary capillary wedge pressure (PCWP) readings are believed to be critical in differentiating between CSW and SIADH (Suarez, 2004). In this case study, this value is within normal limits. However, with the large amount of fluid received, a higher reading would be anticipated. Not only would it be expected, but it would be desired as a preventative measure against vasospasm for which CVP levels are frequently kept above 8 mm Hg (Suarez, 2004).

The basic physical examination alone points toward the diagnosis of CSW rather than SIADH. Additional confirmatory steps include checking for orthostatic changes in blood pressure and heart rate if the patient's condition permits (Palmer, 2000). C.L.'s findings from physical examination are strongly indicative of CSW. Laboratory data are confirmatory. While nurses at the bedside are unable to order labs without a provider's order, they can utilize their knowledge by analyzing the labs that have been ordered. Serum osmolality, electrolytes, and uric acid, along with urine osmolality and electrolytes, are useful in distinguishing CSW from SIADH. Serum hematocrit, uric acid, and urine uric acid may also be helpful. In this scenario, C.L.'s hematocrit and blood urea nitrogen (BUN) in the presence of normal creatinine level, were both elevated. This is indicative of hypovolemia and thus CSW (Palmer, 2000). Potassium on the high end of normal is also consistent with a diagnosis of CSW; with SIADH an elevated potassium level is not expected (Harrigan, 2001). Plasma uric acid is in a low-to-normal range. In patients experiencing CSW, plasma uric acid levels are usually normal or low. It is important to know that plasma uric acid levels are also decreased in SIADH (Palmer, 2000). Patients with hypovolemia from causes other than CSW are more likely to exhibit elevated serum uric acid (Milionis, Liamis, & Elisaf, 2002).

Some laboratory values that would lead the clinician to suspect SIADH rather than CSW are dilute serum with a osmolality less than 280 mOsm/L, hyponatremia and low BUN with concurrent concentrated urine, and an elevated urine sodium greater than 25 mEq/L (Palmer, 2000; Woo & Kale-Pradhan, 1997). Serum hematocrit, albumin concentration, and potassium levels remain normal (Palmer, 2000). CVP and PCWP readings demonstrate hypervolemia. However, while SIADH does result in an increased fluid status, the patient is not automatically expected to have peripheral edema upon physical examination. The explanation for this is that the excess free water is equally distributed and the hypo-osmolar state causes subsequent swelling of cells (Albanese et al., 2001) preventing an overexpansion of the interstitial space (Palmer).