Differential Diagnosis for a Bruise with Redness in the Arm that Spreads

• Single most likely diagnosis:
  • Cellulitis: This is a common skin infection that can cause redness, swelling, and warmth in the affected area. The spreading nature of the bruise with redness is consistent with the typical presentation of cellulitis, which often starts as a small area of redness that expands over time.
• Other Likely diagnoses:
  • Phlebitis: Inflammation of a vein can cause redness, swelling, and warmth along the path of the vein, which might resemble a spreading bruise.
  • Hematoma: A collection of blood outside of blood vessels can cause a bruise, and if it becomes infected or if there is ongoing bleeding, it might appear to spread.
  • Erythema migrans (Lyme disease): Although less common, the initial stage of Lyme disease can present with a spreading rash (erythema migrans) that might be mistaken for a bruise with redness.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
  • Necrotizing fasciitis: A severe infection that destroys tissue under the skin, including fat and the tissue covering the muscles (fascia). It requires immediate medical attention and can be fatal if not treated promptly.
  • Sepsis: If the infection spreads to the bloodstream, it can lead to sepsis, a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs.
  • Deep vein thrombosis (DVT): Although DVT typically presents with swelling and pain in one leg, in rare cases, it can occur in the arms. If a DVT dislodges, it can lead to a pulmonary embolism, which is life-threatening.
• Rare diagnoses:
  • Purpura fulminans: A rare, life-threatening condition characterized by the rapid progression of hemorrhagic necrosis of the skin, often associated with severe infections or other critical illnesses.
  • Vasculitis: Inflammation of the blood vessels can cause a variety of symptoms, including skin rashes that might resemble a spreading bruise. Certain types of vasculitis can be serious and require prompt treatment.