What causes swelling and pain in the left upper eyelid?

Swelling and pain of the left upper eyelid may result from trauma, conjunctivitis, or dacryoadenitis.

1. Trauma

Direct impact to the left eye can cause rupture of small subcutaneous blood vessels, leading to swelling, pain, and conjunctival injection of the left eyelid. Additionally, prolonged prone sleeping positions may compress the eye, impairing local blood circulation and potentially causing left eyelid swelling and discomfort.

2. Conjunctivitis

Conjunctivitis may develop due to spread of inflammation from adjacent ocular tissues, exposure to external allergens, or infection by pathogenic microorganisms—including bacteria, viruses, or chlamydia. Symptoms may include left eyelid swelling and pain, conjunctival pallor or redness, pruritus, foreign-body sensation, and a gritty or sandy feeling in the eye.

3. Dacryoadenitis

Dacryoadenitis is commonly caused by infection with pathogens such as *Staphylococcus aureus* or *Neisseria gonorrhoeae*, but may also be associated with autoimmune responses or systemic diseases including sarcoidosis, leukemia, or lymphoma. Clinical manifestations include left eyelid swelling and pain, conjunctival hyperemia, and lacrimal gland enlargement and congestion; some patients may also experience headache and fever.