35.2.1 Eyelash Cilia

The normal eyelash follicles are located 1.5 to 2.5 mm deep to the lid margin epithelium either just anterior to the tarsal plate or, less commonly, embedded in the anterior surface. ^{2 ,​ 3} There are approximately 100 eyelashes on the upper eyelid and each bulb measures just under 200 μm. ³ The orbicularis oculi is located anterior to the follicles and the muscle of Riolan is typically just posterior. ³ Eyelashes grow in a 4- to 9-month cycle and compared to other hairs in the body, the anagen phase is shorter (30 days) and the telogen phase is longer (3–8 months). ^{1 ,​ 3} The normal lashes curve away from the globe in a parallel manner. ⁴ There is no arrectores pilorum to affect the eyelash position or curvature and lash position is entirely dependent on the surrounding sturctures. ⁴

Trichiasis and entropion represent pathology wherein the eyelid margin or the eyelash follicle is abnormally directed toward the globe. For the purposes of this discussion, these entities differ from eyelash ptosis or downward pointing lashes without disruptive misdirection of the eyelash follicle or eyelid margin rotation.

Trichomegaly refers to an increase in length or thickness of lashes. It can be congenital (Oliver-McFarlane syndrome, Cornelia de Lange syndrome, familial trichomegaly, cone-rod dystrophy, Goldstein-Hutt syndrome), drug-related (epidermal growth factor receptor inhibitors, prostaglandin analogues, interferon, cyclosporin), or acquired (human immunodeficiency virus [HIV] infection). ¹ The heaviness of the long, thick lashes can result in eyelash ptosis and disruption of the follicle support.

Floppy eyelid syndrome is a condition characterized clinically by lax, easily evertable upper eyelids, and tarsal expansion. ⁵ In addition to the rubbery, loose tarsus and reactive changes on the posterior conjunctiva from mechanical irritation there is eyelash ptosis in the majority of patients. ⁴ There is upregulation of elastolytic enzymes and elastin degradation in the tarsal plate and the connective tissue around the eyelash roots and eyelash ptosis results. ⁵

Other causes of cilia root disruption included ocular leprosy and congenital lamellar ichthyosis. ⁶

35.2.2 Levator Aponeurosis and Eyelid Crease

A similar phenomenon of eyelash ptosis can be seen in patients with alterations in the levator aponeurosis. The normal levator aponeurosis is well described elsewhere. It inserts onto the anterior surface of the tarsal plate and into the pretarsal skin. There is a normal zone of fusion with the septum superior to the tarsal plate for several millimeters. The insertion through the orbicularis oculi fibers into the subdermal tissue and the strength of this entire apparatus results in the location and definition of the lid crease.

In studies of congenital ptosis, eyelash ptosis of some degree has been identified in more than 90% of patients. ⁶ It is theorized that this occurs from dysgenesis of these aponeurosis fibers and the resultant absence of their strengthening of the connecting tissue around the eyelash root. ⁶ It can also be found in cases of more significant acquired blepharoptosis, particularly with associated loss of lid crease definition.

As described elsewhere, the Asian eyelid has a variation in the location and extent of the lid crease and associated eyelid fold. This occurs because of variation in the zone of fusion between the septum and levator aponeurosis, often with little to no fusion and an insertion point well below the superior tarsal border. ⁷ Lash ptosis of a mild degree has been observed in the non-ptotic eyelid of Asian patients as well as with increasing frequency in patients with acquired blepharoptosis. ⁷ This is likely related to similar loss of support from a less robust levator aponeurosis insertion and indeed the degree of lash ptosis seems to be worse in patients without a history of a double eyelid fold. ⁷