Allergic Reaction: This is often the most common cause of bilateral periorbital swelling, typically resulting from an allergic response to environmental factors, cosmetics, or certain foods. The swelling is usually accompanied by other symptoms like redness, itching, and tearing.

Fluid Retention (Edema): This can be due to various factors such as hormonal changes, high salt intake, or standing/sitting for long periods, leading to fluid accumulation in the tissues around the eyes.
Sinusitis: Inflammation of the sinuses can cause swelling and puffiness around the eyes due to the close proximity of the sinuses to the orbital area.
Dermatitis (Contact or Atopic): Skin conditions like eczema or contact dermatitis can lead to swelling, redness, and itching around the eyes.

Thyroid Eye Disease: Although less common, thyroid disorders can cause significant swelling around the eyes due to inflammation of the tissues around the eye, including the eyelids, eye socket, and lacrimal gland. Missing this diagnosis could lead to serious complications, including vision loss.
Cellulitis: A bacterial infection of the skin and tissues beneath, which can rapidly progress and lead to serious consequences if not promptly treated with antibiotics.
Angioedema: A condition characterized by the rapid swelling of the dermis, subcutaneous tissue, mucosa, and submucosal tissues, which can be life-threatening if it involves the airways.

Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A rare autoimmune disorder that can cause inflammation of the blood vessels, leading to a variety of symptoms including swelling around the eyes.
Sarcoidosis: An inflammatory disease that can affect multiple parts of the body, including the eyes, leading to swelling and other ocular symptoms.
Lymphoma: Although extremely rare, certain types of lymphoma can manifest with swelling around the eyes due to the involvement of the orbital tissues.