Differential Diagnosis

The patient's symptoms of confusion, tremulousness, and unsteadiness following a bout of gastroenteritis and considering his medications, lead to the following differential diagnoses:

• Single Most Likely Diagnosis

  • Lithium: The patient's symptoms are highly suggestive of lithium toxicity, especially given the recent gastroenteritis which could lead to dehydration and decreased renal function, increasing lithium levels. Lithium has a narrow therapeutic index, and its levels can easily become toxic, especially in elderly patients or those with decreased renal function.

• Other Likely Diagnoses

  • Donepezil (Aricept): While less likely than lithium toxicity, donepezil can cause or exacerbate tremors and confusion, especially if the patient has been taking it for a while and has had a recent increase in dose or has started another medication that interacts with donepezil.
  • Zofran: Though not as directly implicated as lithium, ondansetron (Zofran) can cause confusion and sedation, particularly in elderly patients. However, it's less commonly associated with tremors and unsteadiness compared to the other options.

• Do Not Miss Diagnoses

  • Medication Interaction or Overdose: It's crucial not to miss the possibility of an interaction between the patient's current medications and any recently prescribed or over-the-counter drugs. An overdose or a significant interaction could lead to severe and potentially life-threatening complications.
  • Dehydration and Electrolyte Imbalance: Given the recent gastroenteritis, dehydration and electrolyte imbalances (such as hyponatremia or hypernatremia) could significantly contribute to the patient's confusion, tremulousness, and unsteadiness. These conditions are potentially life-threatening and require immediate attention.

• Rare Diagnoses

  • Levadopa: While levodopa can cause confusion and dyskinesias, it's less likely to be the primary cause of the patient's symptoms given the context. Levodopa is used in the treatment of Parkinson's disease, and its side effects are generally more movement-related.
  • Other Neurological Conditions: Rarely, the patient's symptoms could be the onset of a new neurological condition. However, given the acute presentation following gastroenteritis and the potential for medication-related causes, these would be less likely initial considerations.