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The lacrimal system and adnexa

Summary
Function of the eyelids
The glands of the eyelashes
Movement of the eyelids
Clinical correlates
The lacrimal system
Function of tears
The structure of tears
The lipid layer
The aqueous layer
The mucin layer
Radiology in Focus
Ultrasound
CT
MRI
Fluorescein angiography
References
Author(s)

Summary

The eyelids play an important role in protecting the globe and the adnexal structures are crucial in producing clear vision.

Function of the eyelids

The skin covering the eyelids is the thinnest on the body, measuring approximately 0.5 mm thick and lacking subcutaneous fat. The eyelids serve multiple functions:

Protection: the eyelids protect the globe from environmental hazards.
Lubrication: each blink spreads tear film across the ocular surface, which helps to lubricate the eyes.
Tear drainage: eyelids contain puncta, which are small openings that allow tears to drain into the lacrimal system.

The eyelids also contain meibomian glands which provide the lipid component of tear film, prevent the eyes from drying by blinking.

The inner structure of the eyelids is supported by tarsal plates. These are fibrous layers which gives the eyelids its shape and serve as attachment points for the eyelid muscles.

There are two tarsal plates: one on the upper eyelid called the superior tarsus and one on the lower eyelid called the inferior tarsus.

Diagram showing the superior tarsus and associated adnexal structures.

At the medial end of each tarsus, the medial palpebral ligament is formed (see diagram). This ligament crosses over the lacrimal sac (which we will discuss later), to attach to the maxilla (which we will also discuss later).

The medial palpebral ligament is also sometimes called the tendo oculi and is around 4 mm in length and 2 mm in width.

The glands of the eyelashes

Diagram showing structures associated with the eyelids. Image courtesy of Lecturio

The eyelashes contain two types of glands:

Glands of Zeis: sebaceous (oil) glands
Glands of Moll: apocrine (sweat) glands

The secretions produced by these two glands protect the surface of the eyelid.

Additionally, the meibomian glands (also known as tarsal glands) are located within the tarsal plate, between the eyelashes and the bulbar conjunctiva.

There are approximately 20 to 30 meibomian glands on the lower lid and 40 to 50 on the upper lid.

The meibomian glands secrete a mixture of oil and fat onto the surface of the eye. The secretions produced by the meibomian glands functions to decrease the evaporation of the tear film.

Movement of the eyelids

The movements of the eyelids are controlled by two muscles which attach onto the superior tarsus.

Levator palpebrae superioris: this muscle elevates the eyelid and is supplied by cranial nerve 3.
Orbicularis oculi: this muscle surrounds the eyes and closes the eyelid. It is innervated by cranial nerve 7.

The edge of the medial palpebral ligament can be seen arising through the orbicularis oculi muscle.

The orbicularis oculi muscle has a palpebral part and an orbital part as shown in the diagram.

There is also another muscle worth mentioning, the Müller’s muscle (also known as superior tarsal muscle). The Müller’s muscle arises from the deep surface of the levator palpebrae superioris muscle and inserts onto the superior tarsus (see diagram), hence the name.

Just like the levator palpebrae superioris, the Müller’s muscle also functions to open the eyelid. However, unlike the levator palpebrae superioris, the Müller’s muscle is a smooth muscle innervated by the sympathetic nervous system.

So, the two muscles that elevate or open the eyelid are the levator palpebrae superioris (skeletal muscle supplied by CN 3) and the Müller’s muscle (smooth muscle supplied by the sympathetic nervous system).

Clinical correlates

Ptosis: damage to the nerves supplying either of the two eyelid elevator muscles can cause drooping of the upper eyelid. This is known as ptosis. A CN 3 lesion may cause complete ptosis, while a sympathetic lesion affecting Müller’s muscle may result in partial ptosis. Ptosis can also occur in old age as the part of the levator palpebrae superioris which inserts onto the tarsus (aponeurosis) weakens with age.
Bell’s palsy: a lesion in CN 7 due to Bell’s Palsy may affect the orbicularis oculi muscle which closes the eyelid. This means that the patient may have trouble closing their eye.

The lacrimal system

The lacrimal system describes the pathway through which tears drain. Tears are produced by the lacrimal gland, located above each eyeball. The tears then drain through the following structures:

Diagram showing the lacrimal system.

The tear drainage system consists of:

Lacrimal puncta (small openings in the medial margin of each lid)
Lacrimal canaliculi
Common canaliculi
Valve of Rosenmuller
Lacrimal sac
Nasolacrimal duct
Valve of Hasner

Finally, the tears drain into the inferior nasal meatus. Each blink helps to pump tears through this tear drainage system.

Function of tears

Each blink coats the surface of our eyes with tear film. The tear film is replenished with each blink as tears drain through the lacrimal drainage system. Tears serve several important functions:

Smooth surface: tears smooth out the surface of our eyes so that light is refracted without distortion at the cornea, allowing us to see clearly.
Lubrication: tears lubricate our eyes by coating the surface of our eyes, as well as protecting it from external irritants.
Nutrient supply: the cornea is avascular, so oxygen and nutrients are transported to the surface cells of the eyes via tears.
Infection prevention: tears wash away foreign bodies and debris that enter our eyes. Tears also contain lysozyme, lactoferrin, defensins and immunoglobulins, particularly IgA which are antibacterial.

The structure of tears

The make-up of the tears are important since they must not evaporate as soon as we have finished blinking, otherwise our eyes would dry out pretty quickly. This property of the tear film to be maintained for a while until the next blink is called tear stability.

The tear film is composed of three different layers from the outside in to ensure tear stability and to allow the different functions of tears: lipid layer, aqueous layer and mucin layer (see diagram below).

The lipid layer

The lipid layer is secreted by the Meibomian glands. As mentioned above, these glands secrete oil, which coats the surface of our eyes and keeps the water component of our tears from evaporating.

Disruption in the function of the Meibomian glands can result in the tear film evaporating too quickly. This may happen if the openings of the Meibomian glands become blocked or the composition of the oily substance changes. This may occur with age and can result in decreased tear stability, leading to dry eye syndrome.

The aqueous layer

This layer makes up the majority of the tear film. It performs the key functions of tears mentioned above, including lubrication, nutrient supple and protection against infection.

The mucin layer

Mucin is secreted by the goblet cells on the surface of the eye, which helps distribute tears across the surface of the eye evenly and plays a crucial role in maintaining tear stability.

Radiology in Focus

In the context of ophthalmology, radiological imaging plays a significant role in diagnosing and managing conditions affecting the eyelids and surrounding structures. Here are some key imaging modalities and their applications:

Ultrasound

Ultrasound is useful for evaluating eyelid masses, cysts and assessing the integrity of meibomian glands. It can help differentiate between solid and cystic lesions and assess the vascularity of eyelid tumours.

Ultrasound showing orbital dermoid cyst. There is typically a ‘sack of marbles’ sign. Case courtesy of Ammar Haouimi, Radiopaedia.org. From the case rID: 83352.

CT

CT scans are valuable for assessing orbital anatomy, particularly in cases of trauma, infections, or tumours. They provide detailed images of the bony structures of the orbit and can reveal the extent of any pathological processes affecting the eyelids and surrounding tissues.

CT scan showing a capillary haemangioma of the orbit. Case courtesy of Mostafa Elfeky, Radiopaedia.org. From the case rID: 85870.

MRI

MRI is the preferred modality for evaluating soft tissue structures, including the eyelids, lacrimal glands, and surrounding muscles. It is particularly useful for characterising lesions, assessing inflammatory conditions, and evaluating the extent of tumours.

MRI showing intraconal schwannoma. Case courtesy of Vijay Mistry, Radiopaedia.org. From the case rID: 60621.

Fluorescein angiography

This technique is used to assess the vascular supply to the eyelids and conjunctiva, particularly in cases of suspected neoplasms or vascular malformations. It can help identify abnormal blood flow patterns and assist in surgical planning.

Late arteriovenous phase angiogram demonstrates hyperfluorescence of microaneurysms as well as some irregular and abnormal retinal vessels in the macula. Image source.

References

Mass, Mark B. Abelson, MD, CM, FRCSC, FARVO, George Ousler, Aron Shapiro and David Rimmer, Andover. The Form and Function of Meibomian Glands. https://www.reviewofophthalmology.com/article/the-form-and-function-of-meibomian-glands. Accessed 27 June 2022.

Author(s)

Dr Abhiyan Bhandari

Abhiyan is the Co-Founder and Radiology & Imaging Lead of Ophtnotes. He is a doctor who graduated from UCL Medical School in London. He scored in the top 10% of candidates who sat the Duke Elder examination and runs ophthalmology and Duke Elder revision sessions aimed at medical students. He also runs a YouTube channel aimed at medical students, covering topics ranging from study tips, productivity and vlogs of his journey through medical school.

Published: 27/11/21

Last updated: 23/1/25