Orbital cellulitis is an infection involving the contents of the orbit (fat and ocular muscles). It must be distinguished from preseptal cellulitis (sometimes called periorbital cellulitis), which is an infection of the anterior portion of the eyelid. Neither infection involves the globe itself.
Although preseptal and orbital cellulitis may be confused with one another because both can cause ocular pain and eyelid swelling and erythema, they have very different clinical implications. Preseptal cellulitis is generally a mild condition that rarely leads to serious complications, whereas orbital cellulitis may cause loss of vision and even loss of life. Orbital cellulitis can usually be distinguished from preseptal cellulitis by its clinical features (ophthalmoplegia, pain with eye movements, and proptosis) and by imaging studies; in cases in which the distinction is not clear, clinicians should treat patients as though they have orbital cellulitis. Both conditions are more common in children than in adults, and preseptal cellulitis is much more common than orbital cellulitis.
The pathogenesis, microbiology, clinical manifestations, complications, diagnosis, and treatment of orbital cellulitis will be reviewed here. Preseptal cellulitis is discussed separately. Orbital infections caused by fungi, mainly the Mucorales (which cause mucormycosis) and Aspergillus spp, and, much more rarely, Mycobacterium tuberculosis are also presented separately. (See "Preseptal cellulitis" and "Mucormycosis (zygomycosis)" and "Epidemiology and clinical manifestations of invasive aspergillosis" and "Tuberculosis and the eye".)
Preseptal cellulitis and orbital cellulitis involve different anatomic sites, with preseptal cellulitis referring to infections of the soft tissues anterior to the orbital septum, and orbital cellulitis referring to infections posterior to it (figure 1). Orbital cellulitis involves the muscle and fat located within the orbit. Orbital cellulitis does not involve the globe. (See 'Anatomy' below and "Preseptal cellulitis", section on 'Anatomy'.)
There is some debate regarding the appropriate terminology for these infections. Some clinicians use the term "periorbital cellulitis" rather than "preseptal cellulitis" or use the terms interchangeably. We prefer the term "preseptal cellulitis" to make a clear distinction between this infection and the more serious infection, "orbital cellulitis." Orbital cellulitis is sometimes referred to as "postseptal cellulitis"; we favor the term "orbital cellulitis," and will use it throughout this topic.