Differential Diagnosis for Unilateral Swollen Eye

Single Most Likely Diagnosis

• Allergic Conjunctivitis: This is often the most common cause of a unilateral swollen eye, especially if the swelling is accompanied by itching, redness, and discharge. Allergic reactions can be triggered by a variety of factors including pollen, dust, or makeup.

Other Likely Diagnoses

• Blepharitis: Inflammation of the eyelids can cause swelling, redness, and sometimes a gritty sensation in the eye. It's often associated with poor eyelid hygiene or conditions like rosacea.
• Conjunctivitis (Viral or Bacterial): Both viral and bacterial conjunctivitis can cause a unilateral swollen eye, along with redness, itching, and discharge. The nature of the discharge can sometimes help differentiate between viral (more watery) and bacterial (thicker and pus-like).
• Hordeolum (Stye): A localized infection or inflammation of the eyelid margin involving hair follicles of the eyelashes (external hordeolum) or meibomian glands (internal hordeolum). It presents as a painful, swollen nodule on the eyelid.
• Chalazion: A small bump on the eyelid due to a blocked oil gland. It can cause swelling but is usually painless and may not necessarily cause the entire eye to swell.

Do Not Miss Diagnoses

• Orbital Cellulitis: A serious infection of the tissues surrounding the eye, which can lead to severe complications if not promptly treated. Symptoms include swelling, redness, pain, and sometimes fever. It's crucial to differentiate it from preseptal cellulitis, which is less severe.
• Angle-Closure Glaucoma: A medical emergency that can cause unilateral eye swelling, along with severe eye pain, blurred vision, and sometimes nausea and vomiting. It's caused by increased pressure in the eye.
• Herpes Zoster Ophthalmicus: Shingles affecting the eye, which can cause a swollen, painful eye, along with a rash on the forehead. It requires prompt antiviral treatment to prevent complications.

Rare Diagnoses

• Graves' Ophthalmopathy: Associated with Graves' disease, it can cause bulging of the eyes (exophthalmos) and swelling of the tissues around the eye.
• Sarcoidosis: A condition that can affect multiple parts of the body, including the eyes, causing uveitis, conjunctivitis, or swelling of the lacrimal gland.
• Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A rare autoimmune disorder that can affect various parts of the body, including the eyes, causing inflammation and damage.
• Tumors (e.g., Lacrimal Gland Tumors, Orbital Tumors): Though rare, tumors in or around the eye can cause swelling and should be considered in the differential diagnosis, especially if other symptoms or signs are present.

Each of these diagnoses has distinct features and requires a tailored approach to diagnosis and management. A thorough history, physical examination, and sometimes additional tests (like imaging or laboratory tests) are necessary to determine the cause of a unilateral swollen eye.