Differential Diagnosis

The patient's presentation of dehydration, somnolence, and recent initiation of mirtazapine raises several potential diagnoses. These can be categorized as follows:

• Single Most Likely Diagnosis
  • Dehydration: The patient's history of not eating or drinking for the last few days is a strong indicator of dehydration, which can cause somnolence and is a common issue in elderly patients, especially those with dementia who may have difficulty communicating their needs.
• Other Likely Diagnoses
  • Medication side effect (mirtazapine): The patient was given mirtazapine recently, and while it's less likely to cause severe somnolence at a low dose of 7.5 mg, it's possible, especially in elderly patients who may be more sensitive to medication side effects.
  • Urinary Tract Infection (UTI): Elderly patients, particularly those with dementia, may not exhibit typical symptoms of UTI, such as dysuria or frequency, and instead may present with altered mental status or somnolence.
  • Electrolyte imbalance: Dehydration can lead to electrolyte imbalances (e.g., hyponatremia, hypernatremia), which can cause somnolence and other neurological symptoms.
• Do Not Miss Diagnoses
  • Sepsis: Although less likely given the information, sepsis can present with non-specific symptoms in the elderly, including somnolence and dehydration. It's crucial to consider and rule out sepsis due to its high mortality rate if untreated.
  • Stroke or Transient Ischemic Attack (TIA): These conditions can cause sudden changes in mental status, including somnolence, and are critical to diagnose promptly for appropriate management.
  • Hypoglycemia: Low blood sugar can cause somnolence and is a medical emergency. Given the patient's lack of eating, hypoglycemia is a possibility that should not be missed.
• Rare Diagnoses
  • Adverse reaction to mirtazapine (e.g., serotonin syndrome): While rare, especially at a low dose, it's a potential side effect that could explain the patient's symptoms.
  • Other infections (e.g., pneumonia): Elderly patients may not exhibit typical symptoms of infection, making these diagnoses potentially more challenging to identify.

Labs and Imaging

Given the differential diagnoses, the following labs and imaging should be considered:

• Complete Blood Count (CBC) to evaluate for infection or other abnormalities
• Basic Metabolic Panel (BMP) to assess electrolyte balance and renal function
• Blood glucose to rule out hypoglycemia
• Urinalysis to check for UTI
• Blood cultures if sepsis is suspected
• Imaging such as a non-contrast CT of the head if stroke or TIA is considered, especially if there are focal neurological deficits
• Chest X-ray if pneumonia is a concern

These tests will help narrow down the differential diagnosis and guide appropriate management.