Differential Diagnosis

• Single most likely diagnosis
  • Cerebral salt wasting (CSW) syndrome: The patient's symptoms of nausea, anorexia, and hyponatremia (Na 128) following the removal of a meningioma and the improvement with dexamethasone, followed by a resurgence of symptoms after tapering, are highly suggestive of CSW syndrome. The syndrome is characterized by hyponatremia and volume depletion due to renal sodium wasting, often seen in patients with intracranial disorders.
• Other Likely diagnoses
  • Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): Given the patient's hyponatremia and the presence of a meningioma, which could potentially disrupt normal ADH regulation, SIADH is a plausible diagnosis. However, the improvement with dexamethasone and the subsequent worsening after its taper, along with the presence of volume depletion signs (weight loss, possible dehydration), makes CSW more likely.
  • Brain abscess: Although the MRI suggests a possible brain abscess versus post-operative change, and the patient has symptoms that could be consistent with an abscess (such as severe headache, nausea, and altered mental status after fainting), the lack of fever, specific focal neurological deficits, and a negative Gram stain makes this less likely at this moment but still a consideration.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Subarachnoid hemorrhage (SAH): The patient's fainting episode and subsequent head strike, along with the xanthochromia and pleocytosis on LP, could suggest a SAH, especially if the initial trauma or the surgical procedure itself caused a vascular injury. Although the MRI and clinical context suggest other diagnoses, missing a SAH could be catastrophic.
  • Meningitis: Despite the negative Gram stain, meningitis (bacterial, viral, or fungal) must be considered, especially given the recent surgical intervention and the presence of pleocytosis and xanthochromia. The clinical presentation and CSF findings warrant broad coverage until more specific diagnoses can be ruled out.
• Rare diagnoses
  • Adrenal insufficiency: Although less likely, given the patient's recent use of dexamethasone and the clinical context, adrenal insufficiency could present with hyponatremia, weight loss, and nausea. This diagnosis would be considered if other more likely causes are ruled out.
  • Paraneoplastic syndrome: Given the patient's history of meningioma, although rare, paraneoplastic syndromes affecting electrolyte balance or causing neurological symptoms could be considered if no other explanation is found for the patient's symptoms.