Trichiasis, a very common lid abnormality, is defined as the misdirection of eyelashes toward the globe. The misdirected lashes may be diffuse across the entire lid or in a small segmental distribution.
Trichiasis has numerous causes, and the strategies to correct this problem are dictated by the anatomic abnormality causing the lash misdirection. See the image below.
The primary causes of trichiasis are involutional changes, posterior lamellae scarring (superior or inferior), epiblepharon, and distichiasis.
Trachoma is relatively uncommon in the United States. Exact numbers on the frequency of trichiasis are unknown. Simple trichiasis involving only a few lashes is relatively common. Diffuse trichiasis involving the entire lid margin is much less common, and it is seen primarily in countries where trachoma is endemic. 
The primary morbidity associated with trichiasis is corneal abrasion, corneal scarring, and microbial keratitis. This condition can be vision threatening.
No known racial predilection is evident.
No known sexual predilection is evident.
Trichiasis can occur in all ages; however, it is seen most commonly in the adult years.
Epiblepharon, one of the common causes of trichiasis, is found primarily in children.