Differential Diagnosis for Sudden Unilateral Facial and Upper Back Swelling

Single Most Likely Diagnosis

• Allergic Reaction: A sudden allergic reaction, possibly to an insect bite, food, or medication, could cause localized swelling on one side of the face and upper back. This is a common and relatively benign condition, but it can be severe and life-threatening if it progresses to anaphylaxis.

Other Likely Diagnoses

• Cellulitis: A bacterial infection of the skin and subcutaneous tissues, which can cause swelling, redness, and warmth. It often occurs on one side of the body and can be triggered by a break in the skin.
• Angioedema: Similar to an allergic reaction but can be hereditary or caused by certain medications. It involves deeper layers of the skin and can be more severe.
• Insect Bite or Sting: Certain insects can cause a localized reaction that leads to swelling, especially if the individual has an allergy to the insect's venom.

Do Not Miss Diagnoses

• Deep Vein Thrombosis (DVT): Although less common in the upper body, a DVT could potentially cause swelling, especially if it leads to a pulmonary embolism. This is a life-threatening condition that requires immediate medical attention.
• Lymphoma or Other Malignancies: In rare cases, sudden swelling could be a sign of a malignancy affecting the lymph nodes or other tissues in the face and upper back.
• Infective Endocarditis: An infection of the heart valves, which can lead to embolic phenomena, including localized swelling due to septic emboli.

Rare Diagnoses

• Sjögren's Syndrome: An autoimmune disorder that can cause swelling of the salivary and lacrimal glands, potentially leading to facial asymmetry.
• Relapsing Polychondritis: A rare autoimmune disorder that can cause cartilage inflammation, leading to swelling and deformity of the ears, nose, and other cartilaginous structures.
• Ehlers-Danlos Syndrome: A group of genetic disorders affecting the body's connective tissue, which can lead to easy bruising, poor wound healing, and potentially episodes of localized swelling due to tissue fragility.

Each of these diagnoses has a different set of implications for treatment and prognosis, emphasizing the importance of a thorough diagnostic workup to determine the underlying cause of the swelling.