Preseptal cellulitis (sometimes called periorbital cellulitis) is an infection of the anterior portion of the eyelid, not involving the orbit or other ocular structures. In contrast, orbital cellulitis is an infection involving the contents of the orbit (fat and ocular muscles) but not the globe. 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, diagnosis, and treatment of preseptal cellulitis will be reviewed here. Orbital cellulitis and its complications, such as subperiosteal abscess and orbital abscess, are 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 elsewhere. (See "Orbital 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). Neither infection involves the globe. (See 'Anatomy' below.)
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.
Basic familiarity with the anatomy of the eye is fundamental to understanding the pathogenesis, clinical manifestations, and complications of preseptal and orbital cellulitis. The orbit is a cone-shaped structure, lying horizontally, with its apex in the skull. It is surrounded by paranasal sinuses, namely, the frontal (lying superior), ethmoid (medial) and maxillary (inferior) sinuses (figure 2). The orbit is lined by periosteum. The ethmoid sinuses are separated from the orbit by a paper-thin layer called the lamina papyracea, which contains many perforations for nerves and blood vessels as well as some natural fenestrations termed Zuckerkandl’s dehiscences. The most common route of infection of the orbit is by extension from the ethmoid sinuses, presumably through these perforations.