One eyelid suddenly swollen: common causes and when it is urgent

One eyelid is puffy, and the other is perfectly normal. In Malay it is "mata bengkak sebelah", and it sends many people searching late at night. Many causes are minor and pass in days. A few are not, and they can be told apart by a short list of signs. This piece gives the signs first, then the usual causes, so that you know whether to wait or to go today.

When is a swollen eyelid an emergency?

Go the same day if the swelling comes with pain in or behind the eye, an eye that does not move fully, a change in vision, or a high fever. The NHS lists an eyelid that is red, hot, painful, tender or blistered, a lid that suddenly droops or will not open, severe redness of the eye, vision change, or a high fever as reasons to get help urgently. The reason is one condition in particular: orbital cellulitis, an infection of the tissue behind the eyelid.

The signs that separate it from a simple lid infection are proptosis, which means the eye pushed forward, limited eye movement, pain, and reduced vision, with redness and swelling of the lid on top. A review in the Community Eye Health Journal says orbital cellulitis is an emergency because infection can easily spread into the brain, and that untreated it can lead to vision loss, cavernous sinus thrombosis, intracranial abscess and meningitis. Treatment is intravenous antibiotics, a CT scan to look for an abscess, and sometimes surgery. This is why the action is a hospital emergency department, not a wait-and-see week.

If the eyelid swelling comes with swelling of the lips, mouth or throat, or difficulty breathing, that is a different emergency. The NHS says to call 999 for it.

Could it be a stye or a chalazion?

Yes, the NHS lists both among the causes of a swollen lid. A stye is a small, painful lump on or inside the eyelid, and the skin around it may be swollen and red. The NHS says a stye should go away within a week and is rarely a sign of anything serious. Its self-care advice is a clean flannel soaked in warm water, held against the closed eye for 5 to 10 minutes, 2 to 4 times a day. It also says not to try to burst a stye, because that can spread the infection.

A chalazion is a related lump on the lid, covered in my guide to the chalazion, including when it needs a doctor. The NHS advice on when to go further is an urgent appointment if the lid is very painful or swollen, the vision is affected, there is pus, or the infection seems to be spreading to the eye or lid. A stye that has not improved within a few weeks also needs a GP. Little Eyes 101's Dr Chan Li Yen writes about styes in children,.

Could it be an allergy or an insect bite?

It could. The NHS lists allergy, blepharitis or conjunctivitis, and insect bite, injury or surgery among the causes of a swollen lid, and says swelling from a bite or injury resolves within about a week. The NHS pairs allergy, blepharitis and conjunctivitis with a swollen lid that is itchy, flaky or sticky. A reaction to cosmetics, lash glue or an eye product is a common trigger; it is covered in allergic eyelid reactions to lash extensions and cosmetics.

The word to hold on to is "one side", because a bite or a product can affect a single lid. But the NHS lists a hot, painful, swollen lid under cellulitis, not allergy. A lid that is getting hotter and more painful is a reason to move to the signs in the first section, not to keep waiting for an allergy to settle.

Injury and surgery are on the NHS list as well. Swelling from an insect bite, an injury or surgery is described as resolving within about a week. A knock to the eye is a different matter, and is covered in orbital fracture after a knock to the eye.

What is the difference between a lid infection and one behind the eye?

The difference is the orbital septum, a thin sheet of tissue that separates the eyelid from the eye socket. Preseptal cellulitis is infection in front of it, in the lid. Orbital cellulitis is infection behind it. Moorfields, in guidance written for children, describes the mild form as lid swelling and redness, or a chalazion, insect bite or scratch, with no fever and normal vision.

The same guidance lists the red flags that change the plan: a raised temperature, looking unwell, limited eye movements and blurred vision. It says suspected orbital cellulitis needs urgent referral for intravenous antibiotics. It is a paediatric page, so I use it only for the signs the two age groups share. The Community Eye Health review adds that orbital cellulitis usually starts as infection spreading from the sinuses, and it can also start from a lid infection or spread through the blood. My article on orbital cellulitis and abscess explains the operation side. For a child, the parent's version is a swollen eyelid in children.

What if the swelling is at the inner corner, or the outer upper lid?

Where the swelling sits gives a clue. Swelling below the inner corner of the eye, with pain and redness, can be an infected tear sac. A case report and review of acute dacryocystitis describes it as characterised by pain, previous epiphora (a watering eye), erythema and swelling, and notes that inflammation can extend to the orbit. The case was treated with oral antibiotics, with a DCR operation reserved for recurrent cases. My article on dacryocystitis covers this.

Swelling at the outer part of the upper lid, where the eyelid seems to sag in an S shape, can be the tear gland. A report in the Indian Journal of Ophthalmology describes dacryoadenitis as the most common cause of painful lacrimal gland enlargement, commonly viral or autoimmune, and describes an S-shaped ptosis in one patient. That report is two cases, so it shows the picture, not how common it is. I co-authored a poster on a case of acute dacryoadenitis, presented at Pahang Research Day 2023.

A slower pattern needs a different question. Puffiness that comes and goes over months, or that has changed how one eye looks, is a different question; the article on why one eye looks different covers it, and morning puffiness in both lids is in puffy eyelids in the morning.

What should I do tonight, and what can wait?

Look for the signs in the first section and act on them. If none are present and the lid is mildly swollen, itchy or has a small tender lump, a warm compress and a day or two of watching is what the NHS advice supports for a stye. Do not squeeze or burst anything, as the NHS advises for a stye.

See a doctor if the swelling is not settling. If it is getting bigger, hotter or more painful, go the same day, and go the same day too, to an emergency department, for pain in or behind the eye, an eye that will not move fully, blurred vision, a high fever, an eye that looks pushed forward, or a lid that will not open. You do not need to be sure it is orbital cellulitis to go. The wider background is in the guide to what makes eyes look tired, and the other lid conditions are under eyelid conditions.