What is the protective shutter of the eye? What is another term for palpebrae? Today’s topic is all about Eyelid Anatomy (External and Internal), Parts, Layers, and Function. Stay connected.
An eyelid is defined as a mobile, flexible, multilamellar structure that covers and protects the anterior parts of the eyeball. The eyelids play an important role to distribute tears over the anterior surface of the eyeball (cornea and conjunctiva) and contribute to the drainage of the tear. These act as shutters thereby providing protection to the eye from excessive light, foreign particles, chemicals, and injuries.
Medical Term for Eyelids
The common medical term for eyelid is palpebra. The medical term for eyelids is palpebrae (plural of palpebra). Any medical words related to eyelid has a prefix palpebra-, e.g., palpebral fissure, levator palpebrae superioris, palpebral glands.
Embryology or Development of Eyelid
The eyelids are derived from surface ectoderm. The reduplication of surface ectoderm above and below cornea takes place in the second month of gestation. The folds enlarge and margins fuse with each other. The folds thus formed contain some mesoderm which forms the muscles of the lid and tarsal plate. Then, eyelids adhesion breaks down during the 5-6th month and the lids separate after the 7th month of gestation.
- The connective tissue and the tarsal plate develop from the mesenchymal core
- The orbicularis oculi muscle arises from the mesenchyme of the second pharyngeal arch at 12 weeks of gestation.
- Eyelashes develop as epithelial buds from surface ectoderm. The eyelashes arise first in the upper eyelid.
- The embryogenesis of meibomian glands occurs from columns of ectodermal cells in the lid margin.
- The glands of Moll and Zeis develop from ciliary follicles.
Function of Eyelid
- The major function of eyelids is to act as shutters to protect the inner content of the eye from excessive light, heat, and injuries.
- It also helps to spread the tear over the cornea and conjunctiva and facilitate tear drainage through blinking, thereby preventing dry eyes and associated eye conditions.
- Eyelids contribute to the facial identity and beauty of the person.
- The eyelids provide information on whether a person is sleeping or awake and attentive.
External Eyelid Anatomy
Eyelid Creases and Folds
Based on the anatomical position, there are 2 eyelids in an eye: the upper eyelid and the lower eyelid. The superior palpebral sulcus divides the upper lid into a tarsal plate and orbital plate.
The insertion of the aponeurotic fibers from the Levator Palpebrae Superioris (LPS) muscle creates a superior palpebral sulcus or superior lid crease. Likewise, the adhesion between the skin and orbicularis oculi gives rise to the inferior palpebra sulcus or inferior led crease.
Additional folds or sulcus in the lower lid such as nasojugal sulcus or fold (medial) and malar sulcus or fold (lateral) are formed with age. They are the junction between the skin of the eyelid and the denser tissue of the cheek. This sulcus limits the spread of blood downward from lids to cheek.
In the Asian population, the superior lid crease is usually 2-5 mm above the upper lid margin, whereas it is usually 8-12 mm above the upper lid margin in Europeans.
Extend and position of Lids
In primary gaze position, the lower lid just touches the cornea, and the upper lid covers one-sixth of the cornea. The upper eyelid extends from the superior boundary of the palpebral fissure to the eyebrow. Similarly, the lower eyelid extends from the inferior boundary of the palpebral fissure to merge into the cheek.
The upper and lower eyelids meet each other at an angle of about 60 degrees at medial (nasal)and lateral (temporal) canthi. The position of the lateral canthus is about 5-7 mm medial to the lateral orbit margin. The tear lake (lacus lacrimalis) separates the nasal canthus from the globe. Caruncle (small pinkish elevation) and plica semilunaris (semilunar fold) are situated in this area. The racial variations in eyelids position and angles are mongoloid slant and antimongoloid slant.
Each eyelid margin is 2 mm wide and is divided into the lacrimal portion and the ciliary portion by the lacrimal papilla. The lacrimal portion covers a one-sixth area of margin medial to the punctum while the ciliary portion covers the five-sixth part of the lid margin, lateral to the punctum.
The lacrimal part has a round border and lacks eyelashes and glands. Lacrimal canaliculi lie in this part of the lid margin. Likewise, the ciliary part has a round anterior border and a sharp posterior border.
The upper eyelid contains approximately 100-150 cilia (eyelashes) and the lower eyelid has around 50-70 cilia that are arranged in 2 to 3 rows. Eyelashes in the upper eyelid are directed forward, upward, and backward. Similarly, the eyelashes in the lower eyelid are directed forward, downward, and backward. The lifespan of eyelashes is around 3-4 months.
Glands of Moll and Zeis open into each hair follicle. The dense plexus of blood vessels and nerves are situated around the follicles that are responsible for tactile sensibility.
Palpebral Fissure or Aperture
Palpebral fissure or aperture is defined as the space between the upper and lower eyelid margin. The vertical height of palpebral aperture is 8 mm in newborns and 9-11 mm in adults. Similarly, the horizontal distance of the palpebral fissure is about 18-21 mm in newborns and 28-30 mm in adults.
Microanatomy (Layers) or Parts of Eyelid
Eyelid skin is the thinnest in the human body. The folds and elastic nature of the skin help easy and speedy mobility of the upper eyelid. The lateral part of the skin consists of many sweat glands, while the medial part has numerous sebaceous glands. Due to the presence of sebaceous glands, xanthelasma develops on the nasal side of lids.
Microscopically, this layer of the eyelid is made of epidermis and dermis. The epidermis has 6-7 layers of stratified squamous epithelium and the dermis is a layer of dense connective tissue. The dermis has elastic fibers, blood vessels, melanocytes, lymphatics, and nerves.
Layer of Subcutaneous Tissue
the layer of subcutaneous tissue has a loose connective tissue arrangement with elastic fibers. No fat is found in this layer of the eyelid. In pathological conditions with edema or hemorrhage, the fluid rapidly swells the loose subcutaneous eyelid tissue.
Layer of Striated Muscles
The layer of striated muscles consists of orbicularis muscle (oval sheet) across the eyelids. It is divided into three portions, the orbital, palpebral, and lacrimal. The main function is to close the eyelids. It is supplied by the zygomatic branch of the facial nerve. Hence, lagophthalmos occurs in paralysis of facial nerves.
The layer of striated muscles in the upper eyelid contains levator palpebrae superioris muscle (LPS) which is supplied by a branch of the oculomotor nerve. The main function of LPS is to raise the upper eyelid.
Submuscular Connective Tissue
The submuscular layer contains loose connective tissue present between the orbicularis muscle and the fibrous layer. It communicates with the subaponeurotic stratum of the scalp. Nerves and blood vessels of the eyelid lie in this layer. So, to anesthetize the lid, an injection is made in this plane. This layer divides the eyelid into the anterior lamina and posterior lamina and can be easily approached via a grey line.
The fibrous layer provides a structural framework to the eyelid. It contains the central thick part (tarsal plate) and the peripheral thin part (septum orbitale).
The dense fibrous tissue of the tarsal plate forms the skeleton of the eyelid. The tarsal plate bears the meibomian glands. The lateral end of the tarsi is attached to Whitnall’s tubercle by the lateral palpebral ligament. Similarly, the medial end of the tarsi is attached to the anterior lacrimal crest and the frontal process of the maxilla by the medial palpebral ligament.
The orbital septum and Muller’s muscle are attached at the superior border of the upper tarsus. Orbital septum, capsulopalpebral fascia, and Inferior palpebral muscle are attached to the inferior border of the lower tarsus.
Septum orbitale is a thin floating membrane of connective tissue. It is thick and strong on the lateral side than on the medial side. It takes part in all movements of eyelids.
Layer of Non-Striated Muscle
This layer of eyelid contains Muller’s muscle. The Muller’s muscle lies deep to the septum orbital in both eyelids. The point of origin in the upper eyelid is the inferior terminal stratified fibers of LPS muscle in the upper eyelid and the prolongation of the inferior rectus muscle in the lower eyelid. The sympathetic nerve fibers innervate the Muller’s muscle.
The palpebral conjunctiva lines the inner surface of the eyelids. Depending on the position of the eyelids, there are upper and lower palpebral conjunctiva. The palpebral conjunctiva is subdivided into 3 parts: marginal palpebral conjunctiva, tarsal palpebral conjunctiva, and orbital palpebral conjunctiva (orbital zones).
The conjunctiva is the posterior-most layer of the eyelid. It contains openings of glands of Krause and Wolfring.
Blood Supply of Eyelids
The internal carotid artery supplies arterial blood to the eyelids via the ophthalmic artery and its supraorbital and lacrimal branches. The external carotid artery also supplies blood through arteries of the face (angular and temporal).
The anastomosis of the lateral palpebral artery and medial palpebral artery results in the marginal arterial arcade that supplies arterial blood to both the upper and lower eyelid. The superior arterial arcade which is formed by the branches of the medial palpebral artery supplies only the upper eyelid. The lateral palpebral artery is the branch of the lacrimal artery, whereas the medial palpebral artery is the direct branch of the ophthalmic artery.
Two sets of venous plexuses, pretarsal venous plexus and post tarsal venous plexus, drains impure deoxygenated blood from various structures of the eyelid.
Pretarsal venous plexus drains from structures anterior to tarsal plate into subcutaneous veins namely angular vein (at medial side via a medial palpebral vein) and superficial temporal vein (at lateral side via a lateral palpebral vein). The angular vein then drains into the internal jugular vein, and the superficial vein then drains into the external jugular vein.
The deoxygenated blood from structures posterior to the tarsal plate is drained into ophthalmic veins by post tarsal venous plexus.
The lymphatic drainage of the lower lid and medial portion goes into the submandibular lymph node. The upper lid and lateral portion drains into the superficial preauricular lymph nodes, and then to deeper cervical nodes.
Nerve Supply of Eyelids
The motor nerves supply the striated muscles and non-striated muscles of the eyelid. Orbicularis muscle gets nerve supplies from the temporal and zygomatic branches of the facial nerve. Levator palpebrae superioris gets innervation from the superior division of the oculomotor nerve. Similarly, the sympathetic fibers of the superior cervical ganglion (autonomic nerve innervation) supply the Muller’s muscles (superior and inferior tarsal muscle).
The eyelid receives sensory nerve supply from the branches of the first and second divisions (the ophthalmic and maxillary divisions) of the trigeminal nerve. supraorbital, supratrochlear, infratrochlear, and lacrimal nerves supply the upper eyelid. Likewise, infraorbital, infratrochlear, and lacrimal nerves supply the lower eyelid.
It is the rare congenital anomaly of the eyelid that occurs due to failure of lid development in the fetus. Here, the skin passes continuously from cheek to eyebrow, and the eyeball is hiding inside the skin.
In congenital coloboma of the eyelid, there will be a full-thickness triangular gap in eyelid tissue. The nasal side of the upper lid is more frequently affected than the lower lid and lateral side. The plastic repair of the coloboma region is the only treatment option available.
Epicanthus is a semicircular fold of skin seen in the medial canthus of most kids that disappears with the development of the nose. It persists in Mongolian faces even in adults. If you find epicanthus problematic, the plastic repair is the surgical option available.
Microblepharon and Ablepharon
In rare eye conditions such as microphthalmos and anophthalmos, there are abnormally small eyelids. This condition is known as microphthalmos. If the eyelid is absent, the condition is called ablepharon.
Congenital ptosis is the abnormal drooping of eyelids that occurs in newborns. Surgery is the only option for the treatment of congenital ptosis.
Blepharitis is the chronic inflammation of eyelids, which gives rise to scaly, dandruff-like particles on the eyelid margins and eyelash bases.
Also known as posterior blepharitis, meibomitis is the inflammation of the meibomian glands.
A chalazion is referred to as chronic non-infective lipogranulomatous inflammation of the meibomian gland.
A stye is the acute suppurative inflammation of the gland of Zeis or Moll.
An internal hordeolum is the suppurative inflammation of the meibomian gland associated with blockage of the duct.
Problems of Eyelid Margin Position
Entropion is the inward rolling or turning of the lid margin towards the globe due to aging and other pathological conditions of the eyelid.
Ectropion is the outward turning of the lid margin due to aging and other pathological conditions of the eyelid.
The inability to close eyelids voluntarily is known as lagophthalmos.
Sometimes, the adhesion between the palpebral and bulbar conjunctiva forces the eyelid to get attached to the eyeball, the condition is symblepharon.
In ankyloblepharon, there is adhesion between the margins of the lower and upper eyelids.
Blepharospasm is an involuntary, sustained, and forceful closure of eyelids.
Anomalies of Eyelashes
Although the position of the eyelid is normal, sometimes, the eyelashes misdirect inward instead of normal upward (in the upper eyelid) and downward (in the lower eyelid) directions. The condition is known as trichiasis. Pseudotrichiasis is seen in entropion.
If the extra row of eyelashes covers the position of meibomian glands, it is called distichiasis.
Partial or complete loss shedding or loss of eyelashes is known as madarosis. There is a decrease in the number of eyelashes.
Poliosis refers to the eyelid condition in which there is premature greying or whitening of the eyelashes and eyebrows.
Trichomegaly is the condition in which there is excessive growth of the eyelashes.
Papilloma, sebaceous adenoma, xanthelasma, angioma, naevus, hemangioma, and neurofibroma are benign tumors of the eyelid.
Basal-cell carcinoma, squamous cell carcinoma, sebaceous gland carcinoma, malignant melanoma are malignant tumors of the eyelid.
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