Cellulitis is an infection of soft tissues (like fat and muscle tissue). It can occur in any soft tissue, anywhere in the body.
Periorbital cellulitis is an infection of the soft tissues around the eye. (The orbit is the eye socket itself.) It is usually caused by an infection spreading from the membrane covering the outer part of the eye -- conjunctivitis (pink eye) (the eyeball itself is rarely the site of an infection) -- or from the nose and throat (a bacterial infection of the nose or the sinuses; the latter is called sinusitis).
Usually a child with periorbital cellulitis starts out with a sinusitis or a pink eye. Most often, the sinusitis or pink eye doesn't spread beyond the sinuses or the outside of the eyes. However, if you see redness or (especially) swelling around the eye (which may or may not also include the eyelids), then periorbital cellulitis is likely beginning.
There are actually two different kinds of periorbital cellulitis, depending on exactly where the infected soft tissues are. Preseptal cellulitis involves the eyelids and/or the soft tissues around the eyes, but not the eyeball or anything else in the orbit (like the muscles that move the eye so you can keep them fixed on something you're looking at). Although the infection can spread to other parts of the face, the orbital septum (a membrane that separates the orbit from the soft tissues of the face) will help keep the orbit itself from being infected. Preseptal periorbital cellulitis is less serious, since it does not affect the eye and socket, but it can spread to the orbit. It is seen often in small children (usually under 5 years; I once treated a 1-month-old baby for it). Several different bacteria can be responsible, including Haemophilus influenzae (which all children should be immunized against by age 2 at the latest). Although some milder cases of periorbital cellulitis can be treated at home with oral antibiotics, I almost always start treatment in the hospital with IV antibiotics, and send children home on oral medicines when they have improved. I always draw blood cultures before starting these children on antibiotics: even though the cultures rarely show bacteria, if there are bacteria in the culture the lab will be able to tell me what the best antibiotic is for that child.
I usually admit children with all but the mildest cases of periorbital cellulitis to the hospital, to make sure that they are not developing orbital cellulitis -- infection of the soft tissues in the eye socket. Usually this involves the muscles of the eye, and the fatty tissue surrounding the eyeball which cushions the eyeball and helps it move easily. A child with orbital cellulitis usually has trouble moving the eyes, pain on trying to move the eyes, and trouble seeing (like double vision, because she can't move the eyes easily enough to keep both eyes fixed on one object. In more severe cases, the swelling in the orbit may be so bad that the eye is actually pushed part-way out of the socket. Although it is difficult for the eye itself to be infected, it can happen with an orbital cellulitis, and a doctor looking at the eye with an ophthalmoscope may see swelling inside the eyeball, especially at the "optic disc" (the spot on the retina where the nerves and blood vessels enter the eye). To be sure that the orbit is or is not infected, we usually do a CT scan of the head: this shows swelling in the eye socket if it is there. Besides the possible damage to the eye itself, another big danger of orbital cellulitis is that the infection may spread along the eye's nerves and blood vessels to the brain and its surroundings, producing meningitis. Since it is so dangerous, orbital cellulitis, even if only suspected, needs to be treated in the hospital with IV antibiotics.
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