Trichiasis is essentially an acquired condition of a cicatricial nature,⁴ and any inflammatory process involving the eyelid margin can deform the eyelashes.⁴ The list of etiologies is extensive and includes infectious causes like trachoma, leprosy, measles, or herpes zoster; chronic inflammation of the eyelid margin including blepharitis or meibomianitis; skin diseases including actinic elastosis, eczema, or atopic diseases; diseases that involve the conjunctiva like Stevens-Johnson syndrome, ocular pemphigoid, or vernal keratoconjunctivitis; and chemical or physical burn injuries to the eyelid margin or eyelid margin scars related to trauma or surgery.³,⁵,⁶,⁷,⁸,⁹,¹⁰ All these conditions fundamentally share a common final endpoint, which is inflammation and scarring of the eyelash follicles.³

In general, trichiasis is rarely seen before the 3rd decade,³ may be unilateral or bilateral, and could affect the central or the lateral portions of the upper or lower eyelids. In patients with trachoma, trichiasis is more frequent in women and predominantly involves the upper eyelids.³,⁵,⁶,⁷ Trichiasis is frequently classified according to severity. Minor trichiasis involves the aberrant orientation of five or fewer eyelashes in any one eyelid (Figure 42.1), and major trichiasis involves six or more eyelashes (Figures 42.2 and 42.3).⁵,⁷,¹¹ Patients usually present with symptoms of ocular irritation including photophobia, tearing, dryness, burning pain, blepharospasm, or conjunctival congestion with discharge.³ Diagnosis of trichiasis is easy and should not be hard to miss. However, it is important to exclude associated conditions that may alter the treatment strategy, including eyelid margin malpositions (entropion), symblepharon formation, or tarsal conjunctival scarring (trachoma).³ All misdirected eyelashes should be mapped in the patient’s chart, preferably with photographic documentation or a hand-drawn illustration.³ Up to 70% of patients epilate their lashes²; therefore, they should specifically be asked about any recent episode of self-epilation. Clinically, evidence of epilation is recognized by the presence of broken or newly growing lashes, which are usually short and sharp, with or without areas of absent lashes.²

The major confusion concerning trichiasis may be nomenclatorial.³ The terms trichiasis, metaplastic lashes, or distichiasis are frequently used interchangeably. Distichiasis is specifically defined as an abnormal growth of one or several accessory rows of lashes behind the normal one and arising from the orifices of the meibomian glands (see Chapter 30). Eyelash metaplasia is a more general term loosely defined as any lash originating from a follicle that is in an abnormal location, and the term is more frequently used in reference to abnormally located eyelashes in trachoma.⁷ Dystrichiasis is an archaic term that is still confusingly used in the literature, albeit infrequently,¹² and is an all-encompassing term that includes under its umbrella misdirected lashes of both varieties, the trichiatic and the aberrant ones.