The eyelid is the first and foremost defense of the eye, covering the cornea and also distributing and eliminating tears. Understanding of the structures of the eyelid margin area allows easier diagnosis of periocular disorders (Figure 29.1). Anatomically, the eyelid is composed of skin, the orbicularis oculi muscle (innervated by the seventh cranial nerve), and tarsus and conjunctiva. The levator muscle (supplied by the third cranial nerve) and Müller's muscle (sympathetically innervated) open the upper lid. The eyelashes themselves can be affected in an isolated fashion, for example, or the effect may extend to the meibomian glands within the tarsus. Both the nasolacrimal duct and the lacrimal sac can become obstructed, producing dacryocystitis and canaliculitis, respectively. The orbital septum, contiguous with the tarsal plates both superiorly and inferiorly, serves a barrier between the eyelid and posterior orbital structures. An infection that is anterior to this septum is known as preseptal (or periorbital) cellulitis; postseptal infections are known as orbital cellulitis.
BLEPHARITIS AND HORDEOLA
Blepharitis (Figure 29.2) and hordeola (Figure 29.3), both infections of the eyelids or eyelashes, are often confused with each other. Blepharitis is inflammation of the eyelids and/or eyelash follicles and is a relatively common ocular disorder. It is usually bilateral. The mean age is approximately 40–50 years old, affecting women more than men, and is more common in those with fair skin.