Differential Diagnosis for Sudden Onset Swelling and Itchiness

• Single most likely diagnosis
  • Angioedema: The patient's sudden onset of swelling to the face, neck, and mouth, along with a history of itchiness and recent Benadryl use, suggests angioedema, possibly an allergic reaction. The fact that the patient denies shortness of breath (SOB) or dysphagia reduces the likelihood of a more severe respiratory compromise but does not rule out angioedema.
• Other Likely diagnoses
  • Allergic Reaction: Given the patient's itchiness over the last two days and the sudden swelling, an allergic reaction to an unknown substance is possible. The use of Benadryl suggests an attempt to manage allergic symptoms.
  • Urticaria: The patient's description of itchiness throughout the body could indicate urticaria (hives), which often accompanies angioedema in allergic reactions.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Anaphylaxis: Although the patient denies SOB or dysphagia, anaphylaxis is a life-threatening condition that can progress rapidly. The presence of swelling and itchiness, even without respiratory symptoms at the moment, necessitates consideration of anaphylaxis.
  • Superior Vena Cava Syndrome: Swelling in the face and neck could, in rare cases, be indicative of a more serious condition like superior vena cava syndrome, especially if there's an obstruction of the superior vena cava.
• Rare diagnoses
  • Hereditary Angioedema: This is a rare genetic disorder that causes recurrent episodes of severe swelling. The absence of a clear allergen and the severity of the swelling might suggest this, though it's less likely without a family history.
  • Acute Allergic Contact Dermatitis: While less likely given the description, an acute and severe allergic reaction to something the patient came into contact with could cause widespread itchiness and localized swelling.