Differential Diagnosis
- Single most likely diagnosis
- Syndrome of inappropriate anti-diuretic hormone secretion (SIADH): This condition is likely given the patient's hyponatremia (low sodium levels), low urine sodium despite hyponatremia, and the absence of significant hypovolemia or edema. The recent head trauma could have triggered SIADH, and the patient's symptoms such as headaches and vomiting are consistent with this diagnosis. The failure of serum sodium to correct with normal saline infusion also supports SIADH, as the body retains water inappropriately.
- Other Likely diagnoses
- Hypovolemic hyponatremia: Although the patient's blood pressure and pulse rate suggest some degree of hypovolemia, the lack of significant improvement in sodium levels after fluid resuscitation and the presence of low urine sodium make this less likely. However, it cannot be entirely ruled out without further assessment of volume status.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Cerebral salt-wasting syndrome (CSWS): This condition involves excessive renal sodium loss leading to hyponatremia and volume depletion, often seen in patients with cerebral disorders. Although less common than SIADH, it's crucial to differentiate between the two because the treatment approaches are opposite. CSWS would require sodium and volume replacement, whereas SIADH involves fluid restriction.
- Rare diagnoses
- Psychogenic polydipsia: This condition, characterized by excessive water intake leading to hyponatremia, is less likely given the patient's recent head trauma and the fact that she has been tolerating only minimal fluids. However, it could be considered if other diagnoses are ruled out, especially given her history of major depressive disorder, which might predispose her to psychogenic polydipsia.