The eyelids of domestic animals have many important functions. They are important in protecting the globe, contributing to
and spreading the tear film, and preventing the buildup of ocular pathogens and environmental debris. This article will review
the basics of eyelid anatomy and function as well as discuss common problems associated with these structures
The eyelids near the margin are thought to be composed of four primary layers: the conjunctiva, the tarsus and stroma, the
orbicularis oculi muscle, and the dermis/epidermis. The margin is a mucocutaneous junction that connects the conjunctiva on
the bulbar surface with the haired skin of the external surface. The primary muscle involved in eyelid closure is the circumferential
orbicularis oculi muscle, innervated by the facial nerve (CN VII). The primary muscle involved in opening of the eyelid is
the levator palpebrae superioris innervated by the oculomotor nerve (CN III). The tarsus (stroma) is comprised of connective
tissue, levator muscles, and the tarsal or meibomian glands. These glands secrete the lipid component of the tear film and
the openings for the secretory ducts are visible along the eyelid margin. The palpebral conjunctiva is the smooth, inner lining
of the eyelid. It is continuous with the bulbar conjunctiva and contains lymphoid follicles, goblet cells (secrete the mucous
portion of the tear film) and lymphatics.
Congenital abnormalities of the eyelids are occasionally seen in both dogs and cats. Physiologic ankyloblepharon is typically
present until 10-14 days after birth. Premature opening of the palpebral fissure can result in severe ulceration since the
lacrimal tissue is not fully developed. Ophthalmia neonatorum is a condition where a bacterium, often a staphylococcus, gains
access to the conjunctival sac and results in purulent material building up beneath the closed eyelids. The eyelids must be
opened in this case, by gentle massage if possible or scissors if necessary. The globe and conjunctiva should be aggressively
flushed and topical antibiotics and ocular lubricants used frequently. Eyelid aplasia (agenesis) is a condition where a portion
of the eyelid margin fails to form. This is most common in cats and typically involves the lateral dorsal eyelid. Other congenital
anomalies are often associated with this condition. Various reconstructive surgical techniques have been described, depending
on the severity of the defect. Dermoids are ectopic islands of skin that develop at abnormal locations. They can be present
along the eyelid margin, palpebral or bulbar conjunctiva, or the cornea. Surgical excision is curative.
Eyelid cilia and aberrant hair abnormalities are common in dogs. Distichia are hairs emerging from the meibomian gland openings
and are oriented parallel to the corneal surface. They arise from undifferentiated portions of the meibomian glands. The clinical
significance varies. Many dogs have multiple distichia that cause no clinical signs. The vast majority of cocker spaniels
have multiple fine distichia, but treatment is rarely needed. Distichia that result in clinical signs are typically shorter,
stiffer hairs. These commonly result in excessive lacrimation and epiphora, blepharospasm, and corneal ulceration. Various
treatment options are available. Manual epilation can be used in cases of few distichia to determine if the clinical signs
are actually due to the hairs. The hairs will regrow in most cases necessitating a more permanent option. Common techniques
include electrolysis, electrocautery, cryotherapy, CO2 laser ablation, and surgical resection of the distal tarsal plate. Of these, I prefer cryotherapy, especially in cases of
multiple hairs. This technique is effective at destroying the hair follicle without damaging the eyelid margin. Some temporary
swelling and depigmentation of the margin is common however. Laser ablation and electrolysis are useful where there are few
offending hairs. Ectopic cilia are a completely different matter. In most cases, this involves a single cilium emerging from
the palpebral conjunctiva and oriented perpendicular to the cornea. These are typically seen in young dogs (rarely over 2
years old) are cause severe blepharospasm and lacrimation. A corneal ulcer that is unresponsive to medical therapy is often
present. They are most commonly present in the center of the dorsal eyelid, several millimeters from the margin. They are
most damaging and painful when they first emerge from the conjunctiva and act like little spikes rubbing on the cornea each
time the dog blinks. Unfortunately, this is also when they are the most difficult to find, and often cannot be seen without
magnification. Treatment involves removal of the cilia, either through direct excision or laser ablation, followed by cyrotherapy
of the eyelid to destroy the follicle. 
