Differential Diagnosis

• Single most likely diagnosis
  • Allergic reaction (e.g., to horse fly bite): The patient's symptoms of swelling and discomfort, which improved with Benadryl, suggest an allergic reaction. The temporal relationship between the horse fly bite and the onset of symptoms further supports this diagnosis.
• Other Likely diagnoses
  • Contact dermatitis: Direct contact with an allergen or irritant could cause a localized reaction, leading to swelling and discomfort on the eyelid.
  • Insect bite reaction (not necessarily allergic): Even without an allergic component, insect bites can cause localized reactions, including swelling and redness.
  • Blepharitis: Although less likely given the acute onset, blepharitis could cause eyelid swelling and discomfort, especially if the patient has a history of the condition.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Orbital cellulitis: Although the patient denies pain, vision changes, or other systemic symptoms, orbital cellulitis is a serious condition that requires prompt attention. It can present with eyelid swelling and could be a complication of an insect bite.
  • Angioedema: A more severe allergic reaction could lead to angioedema, which, although rare, can be life-threatening if it involves the airway.
• Rare diagnoses
  • Herpes zoster (shingles): Although the patient does not report pain or a rash, herpes zoster could present with eyelid swelling and should be considered, especially if the patient is immunocompromised or has a history of shingles.
  • Chalazion or hordeolum (stye): These conditions could cause localized swelling on the eyelid but typically are associated with more pain and possibly a visible lump or discharge.