Anatomy Of The Eyelid And Its Role In Trichiasis

Imagine waking up every morning with a gritty, scratchy feeling in your eye, as if a tiny object is constantly poking you. For many with trichiasis, this is a daily reality — an irritation that stems from the misdirected growth of eyelashes due to subtle anatomical (structural) changes in the eyelid. Have you ever wondered why a single misdirected eyelash can cause persistent eye irritation? The answer lies in the intricate structure of our eyelids and how they normally keep our lashes in perfect alignment to protect our eyes. To understand trichiasis, it’s helpful to know a bit about the anatomy of the eyelid and how it works.

Anatomy of the eyelids

The eyelids are intricate structures that serve several important functions in protecting and maintaining eye health. They act as a barrier to prevent injury, assist in tear drainage, and help distribute the tear film over the ocular (eye) surface. By blinking 20–30 times per minute, the eyelids keep the cornea and conjunctiva moist. They also help propel tears toward the puncta lacrimalia, where the tears enter the lacrimal drainage system. The eyelashes, which grow along the edge of the eyelid, protect the eye from debris by trapping airborne particles before they touch the eye surface and serve as a warning mechanism for nearby objects, like insects.^1,3

Did you know the skin of the eyelid is the thinnest in the entire body (about 1 mm thick) allowing for mobility? The eyelid has very little fat underneath the skin, which leads to the fine wrinkles that appear as we age in the soft skin around the eyes.¹

Layers of the eyelids

Structurally, each eyelid comprises two layers: the anterior layer and the posterior layer.

The anterior layer: This superficial layer consists of skin, subcutaneous fat, sweat glands, sebaceous glands, and muscle fibres, including the orbicularis oculi muscle which is the main muscle which helps close the eyelid, and the levator palpebrae muscle, which assists in opening it. 

Posterior layer: This deeper layer contains the tarsal plate (which provides firmness and shape to the eyelid), the Meibomian glands, conjunctiva (a thin mucous membrane) and the smooth muscle of the levator palpebrae and Müller muscle (tarsal muscle), which regulate the width of the eyelid opening.¹

Conjunctiva: a type of tissue made up of flat, scale-like cells that do not produce keratin, a tough protein found in the skin that lines the inner eyelid surface, covers the front part of your eyes and continues at the corneal edge. The conjunctiva contains mucin-secreting goblet cells and accessory lacrimal glands that help lubricate the ocular surface. ²

The glands 

• Meibomian glands: Located within the tarsal plates of both lids, these sebaceous glands secrete lipids essential for maintaining the tear film’s stability. This is achieved by preventing tear evaporation by spreading over the eye surface with each blink
• Sebaceous glands of zeis: These glands open into each eyelash follicle, contributing to the lubrication of the lashes
• Ciliary glands of moll: Positioned behind and between the follicles, these modified sweat glands open either into the follicle or directly onto the eyelid margin. When their openings become obstructed, they can lead to the formation of hordeola (styes) and are also potential sites for sebaceous cell carcinoma^1,3

Eyelashes (Cilia)

The upper eyelid typically contains five to six rows of eyelashes with around 100 to 150 lashes. While the lower eyelid usually has three to four rows with approximately 50 to 75 lashes. Each eyelash grows from a follicle (a special type of cell that makes proteins that form eyelashes) located about 2 mm beneath the skin, following a cyclic growth pattern — anagen (growth phase), catagen (transition phase), and telogen (resting phase). 

The anagen phase lasts 20 to 55 days, with a complete cycle taking roughly three months. Every day an eyelash grows by about 120 to 130 micrometres (µm). An individual eyelash lasts around five months before being shed; however, provided the follicle remains intact, a new lash will continuously replace the old one. 

Unlike other hair, eyelashes do not turn grey with age, although acquired whitening (poliosis) may signal conditions like blepharitis or Vogt-Koyanagi-Harada syndrome. ³ Also, the eyelids have a rich blood supply, lymphatic drainage, and sensory nerve innervation from the trigeminal nerve, supporting their protective and functional roles.¹

Eyelash orientation

The lashes on the upper lid naturally curl outward and upward, while those on the lower lid curl outward and downward to prevent them from interlacing during blinking. Abnormal lash orientation, known as lash ptosis, occurs when the upper eyelid lashes point downward instead of upward.³

Although the upper and lower eyelids are similar in many ways, they differ mainly in structure and movement. The upper eyelid is larger, has a distinct crease and is responsible for about 90% of eyelid movement. In contrast, the lower eyelid, which sits just below the eye socket, is less mobile, has a less distinct crease, and features a simpler structure.¹

How eyelid anatomy relates to trichiasis

What is trichiasis?

Trichiasis is a condition that affects the edge (margin) of the eyelid. It is a common acquired condition where the eyelashes grow in the wrong direction — towards the eye rather than away from it. This misdirection causes the lashes to rub against the cornea (the clear front part of the eye), leading to irritation, tearing, and redness. Over time, this irritation can result in significant eye health problems (ocular morbidity).^1,4 

What causes trichiasis?

Trichiasis is often a result of inflammatory damage and scarring of the eyelash follicles. In some cases, if the inflammation also affects the eyelid lamellae (layer), it can lead to the development of entropion.⁴

Primary causes of trichiasis 

• Chronic inflammation: blepharitis and meibomitis lead to persistent eyelid margin inflammation
• Skin diseases: Disorders like actinic elastosis, eczema, atopic conditions, leprosy, and herpes zoster can contribute to follicular scarring
• Conjunctival and ocular surface diseases: Diseases including cicatricial trachoma, Stevens-Johnson syndrome, ocular pemphigoid, vernal keratoconjunctivitis, and injuries from chemical or physical burns may precipitate trichiasis
• Post-surgical or traumatic scarring: Surgical interventions or trauma that affect the eyelid margin can alter normal eyelash orientation
• Trachoma, a chronic infection caused by Chlamydia trachomatis, has historically been a major cause of trichiasis. In endemic areas, trichiasis is often bilateral — affecting about 70% of cases — and tends to present more frequently in women, particularly as the incidence increases with age⁴

Classification

Trichiasis can be categorised according to the number of misdirected lashes and the extent of eyelid involvement:

• Major Trichiasis: Five or more lashes are misdirected
• Minor Trichiasis: Fewer than five lashes are misdirected

The severity of the disease can be determined by examining how much of the eyelid is affected: whether it's limited to a specific area (such as the nasal, central, or temporal parts) or involves the entire eyelid.⁴

Symptoms of trichiasis 

The most common symptoms reported by over 90% of patients include⁴

• The feeling of something in the eye
• Photophobia, also known as excessive sensibility to light 
• Tearing
• Discharge
• Dry eye sensation
• Burning
• Pain
• Blepharospasm
• Conjunctivitis

An eye examination reveals 

• One or more misdirected lashes
• Signs of superficial diseases of the cornea (keratopathy) or corneal injury
• Evidence of infection 
• The formation of blood vessels or cloudiness and scarring of the cornea
• Vision loss⁴

Treatment 

Trichiasis management aims to eliminate the misdirected lashes and alleviate the patient’s symptoms. Treatment modalities can be broadly categorised into non-surgical (temporary) measures and definitive surgical approaches.⁴

Non-surgical measures

These treatments are typically employed to provide symptomatic relief or as interim solutions while addressing active inflammation:

• Eye Lubricants and Contact Lenses: These can help protect the ocular surface
• Mechanical Epilation: Removal of lashes using forceps is a simple and inexpensive method. However, this approach is temporary since recurrences are common within 2 to 6 weeks, and broken lash stubs may exacerbate irritation

Surgical and ablative treatments

Ablative techniques

• Bipolar Electrolysis: This Involves inserting a fine needle into the lash follicle and applying an electrical current to destroy the follicle. Although relatively simple, it is associated with high recurrence rates (around 60%)
• Radiofrequency Ablation is similar to electrolysis but uses radiofrequency waves to destroy the follicles. This method offers higher success rates with fewer complications; many patients achieve a cure after one or two sessions
• Cryotherapy utilises freezing temperatures (typically between –20°C and –30°C) to destroy the hair follicles. Success rates improve with repeated applications, reaching between 70% and 90% after two sessions, though complications such as skin depigmentation or lid notching can occur
• Laser ablation: Argon laser, and more recently diode, YAG, alexandrite, and ruby lasers, have been employed for lash ablation. Laser treatment, especially with diode lasers (with deeper penetration), can be performed under topical anaesthesia and tends to produce less inflammation, However, it requires a precise application to avoid complications like lid notching or hypopigmentation

Surgical procedures

Surgical interventions are considered when non-surgical measures are insufficient or for more severe cases. Although many procedures show promising initial success, long-term recurrence remains a challenge, particularly in patients with Chlamydia trachomatis infection — where adjunctive azithromycin may significantly reduce recurrence rates.

Various surgical techniques available include:

• Eyelash trephination (Excision of Trichiatic Eyelash Bulbs): This procedure uses a specialised micro trephine to extract the abnormal lash follicles. It is a quick and cost-effective method with low morbidity
• Full-thickness block resection: This technique is suitable for localised trichiasis affecting less than one-third of the eyelid, and it involves excising a pentagon-shaped block of tissue with subsequent multi-layered suturing to ensure proper alignment
• Partial excision of the anterior lamella: This approach is reserved for cases with extensive involvement, particularly in older patients or those with multiple recurrences, where function is prioritised over cosmetic appearance
• Anterior layer repositioning: A blepharoplasty-like technique is used to reposition the anterior layer, thereby rotating the lashes away from the ocular surface
• Mucocutaneous graft: A barrier is created between the eyelashes and the eye by splitting the eyelid into anterior and posterior lamellas and interposing a graft (usually harvested from the lip or upper lid)
• Tarsal rotation procedures: In cases of combined entropion and trichiasis, horizontal fracturing of the tarsal plate followed by rotation of the eyelid margin (as in the lamellar or bilamellar tarsal rotation procedures) is effective. The World Health Organisation endorses these techniques based on studies showing success rates of approximately 77%

Summary 

The eyelids are complex, layered structures that protect the eyes by distributing and draining tears while shielding against debris. They consist of an anterior layer (skin, muscles, and glands) and a posterior layer (tarsal plate, Meibomian glands, and conjunctiva), with eyelashes that help trap particles. Abnormal eyelash growth (trichiasis) can lead to eye irritation and damage and is managed through non-surgical and surgical treatments.