The eyelids are thin, mobile muscle-skin folds that can completely cover the front of the eyeball.
Like other accessory eye formations, the eyelids perform protective functions against external agents and contribute to bulbar support. Furthermore, frequent blinking allows the continuous distribution of the tear film on the ocular surface.
Appearance and structure
The eyelids are accessory formations of the eye: placed in front of the eyeball, they represent the continuation of the skin. The upper eyelid is bordered at the top with the line of the eyebrow and is more developed, broad and mobile with respect to the lower one; contains the anterior part of the levator muscle.
Anatomy of the Eyelids. Modified from the site: //www.anatomyatlases.org/firstaid/Eye.shtml
Provided internally with a fibro-muscular skeleton ( palpebral tarsus ), each of these structures has two faces: an anterior cutaneous and a posterior covered by the palpebral conjunctiva. The free margins of the upper and lower eyelids are separated by a transverse opening called rhyme (or slit) of the eyelid ; they join, however, at the ends, in the cantus medialis (lacrimal) and lateral (ciliary). The palpebral fissure varies in amplitude with the winks.
In the lateral portion of the free margin, the eyelids are provided with eyelashes, sebaceous and sweat glands. At the medial angle of the free margin, on the other hand, the eyelids have a relief, the lacrimal papilla, where the entrance of the nasolacrimal canal is present.
Furthermore, at the level of the cantus mediale, a rosy relief is evident, the lacrimal caruncle, where the conjunctiva and epidermis meet. The lacrimal caruncle contains glands that develop a dense secret that contributes to the substance that can be found, sometimes congealed, at morning awakening.
The visible outer surface of the eyelids is covered by a thin layer of compound paved epithelium. Below the subcutaneous layer, the eyelids are supported and reinforced by broad connective laminae, called the tarsal plate.
The inner surface of the eyelids is covered by the conjunctiva, a mucous membrane covered by a specialized compound paved epithelium. The goblet cells of the epithelium contribute, with the accessory glands, to the production of a lubricating fluid, which is placed on the surface of the eyeball, keeping it moist and clean. This also avoids the friction with the ocular (or bulbar) conjunctiva that covers the front surface of the eye. Below the skin, the eyelids have a muscular and a fibro-cartilaginous layer.
Eyelashes, sebaceous glands and sweat
The eyelid margin has two or three rows of robust and curved hairs (the eyelashes ), longer and more numerous at the level of the upper eyelid than the lower one.
The eyelashes are controlled by a plexus of the piliferous root, so their displacement initiates an intermittent reflex. This movement helps to prevent foreign particles from reaching the ocular surface.
Each hair follicle has a Zeis gland that produces sebum. In the same area, near the base of the eyelashes, there are some modified sweat glands, called Moll's glands .
Along the inner edge, instead of the cilia, the meibomian glands (or tarsal glands) secrete a substance rich in lipids that prevents the eyelids from sealing one against the other. This organization involves the palpebral margin except for the medial portion (which represents about the eighth internal part of the fissure), which presents the lacrimal dots, which coincide with the beginning of the lacrimal runoff routes.
All accessory glands of the eyelids are subject to occasional bacterial invasions. From the infection of a meibomian gland, a calazion can form. The infectious process of a sebaceous gland of an eyelash, of a tarsal gland or of one of the accessory lacrimal glands that open on the surface between the eyelid follicles, on the other hand, causes a localized painful swelling, known as stye .
Muscular system
The orbicularis muscles of the eye and upper elevator of the eyelid are located between the tarsal plate and the skin. The orbicular muscle is responsible for the blinking and involuntary closure of the eyelids. The action of the upper levator muscle of the eyelid, however, consists in raising the upper eyelid.
Functions
With eyebrows, superficial epithelium of the eye and structures responsible for the production, secretion and removal of tears, the eyelids assist the visual function and defend the eye in its anterior portion from external agents and excessive light.
The eyelids work just like a windshield wiper: their intermittent movements (on average a wink every ten seconds) keep the surface lubricated and free from dust, impurities and other particles. Furthermore, they can close completely by reflex action in response to external stimuli, in order to protect the delicate surface of the eye (automatic blinking).
Eyelid Diseases
The eyelids can be affected by different pathological processes and by anomalies of shape, position or altered movement.
The most frequent disorders include allergic reactions, inflammation (blepharitis, chalazion, stye and conjunctivitis), traumatic lesions and eyelid ptosis. The eyelid skin is also the site of the onset of benign and malignant tumors.
Dermatological diseases
The eyelid skin can be affected by many of the morbid conditions affecting the skin, including eczematous dermatitis and chemical or heat burns.
The skin around the eyes is extremely sensitive and can even react to the slightest exposure to the allergens to which the body is vulnerable. At the level of the eyelids, an allergic reaction can manifest itself with intense irritation, swelling and redness, associated with a strong desire to rub the eyes. The eyelid skin may be dry and flaking. Possible triggers include eye cosmetics (eye shadows, mascara and face creams), hairspray, nail polish, pollen, cat and dog hair, dust mites and molds.
The eyelid skin may be affected by the febrile herpes (herpes simplex) and by the reactivation of the varicella-zoster virus infection (herpes zoster ophthalmic). The anterior surface of the eyelids is also the site of secondary cutaneous manifestations of syphilis, Chagas disease and various forms of tuberculosis.
Entropion
The entropion consists in the rotation towards the inside of the free eyelid margin. This condition may be present at birth (congenital) or occur during life (acquired). Over time, the edge of the eyelid and the eyelashes (in an abnormal position) rub against every blink against the front of the eye, causing redness and irritation. If the patient does not resort to adequate treatment, entropion can lead to the development of corneal abrasions and ulcers.
The disorder is most commonly seen in older people due to the hyper-quality of tissues linked to the aging process. The entropion can also occur due to trauma, previous surgery, muscle changes (eg paralysis), post-infective outcomes (eg chronic conjunctivitis) and blepharospasm. The most effective correction of the disorder involves surgery.
Ectropion
The ectropion consists of the outward rotation of the edge of the eyelid. This condition can affect both eyelids (upper and lower), but the lower one is more affected. The extent of ectropion is variable: in the most serious cases, there is a complete eversion of the eyelid (with exposure of the conjunctiva to the fornix), whereas when it is mild only a small segment of the eyelid rhyme can move away from the eyeball.
Ectopion can lead to tear changes (epiphora), irritation, dryness and red eye. The most serious complications are abrasion and corneal ulceration.
Ectopion is often due to the loss of tonicity of the orbicular muscle, but may also depend on corneal or conjunctival inflammatory processes, facial paralysis and scar retraction (trauma, post-surgical outcomes and dermatological diseases). The therapy is surgical.
Eyelid ptosis
Eyelid ptosis is a complete or partial failure of the upper or lower eyelids. If the condition is severe enough, the "waning eyelid" may interfere with vision and cause other disorders, such as amblyopia (by occlusion).
Palpebral ptosis can be congenital or acquired. The most common cause is the weakening, paralysis or injury of muscles and nerves normally used to move the eyelid. In adults, the condition is often a consequence of aging (senile or age-related ptosis).
Ptosis also occurs as a complication of traumas (fractures of the orbit or eyelid wounds), neurological disorders (such as stroke, paralysis of the oculomotor nerve and multiple sclerosis), muscular pathologies (eg myasthenia gravis), severe conjunctival inflammatory processes and, in rare cases, tumors of the eye cavity. Surgical correction can be an effective treatment for both improving vision and aesthetic appearance.
Blefarocalasi
Blepharocalase is a laxity of the epidermis of the upper eyelid of a senile nature, associated with the fall of the upper eyelid and, therefore, often confused with ptosis.
Blepharospasm
Blepharospasm is the forced and persistent contraction of the eye's orbicular muscle which causes blinking and involuntary closure of the eyelids; in the most serious cases the patient cannot open the eye. It may be secondary to ophthalmic disorders causing irritation, including: trichiasis, corneal foreign bodies, inflammatory processes of the iris or ciliary body and dry keratoconjunctivitis. In other cases, it is a consequence of spasmogenic systemic neurological pathologies (eg Parkinson's disease).
Blepharitis
Blepharitis is an acute or chronic inflammation of the eyelid margin. The acute form can be caused by infections, seasonal or contact allergic reactions and is often associated with acne rosacea and seborrheic dermatitis. Chronic blepharitis, on the other hand, can be caused by an altered secretion of the meibomian glands. The symptoms, common to all forms of blepharitis, include itching and burning of the eyelid margin, conjunctival irritation with redness, lacrimation, sensitivity to light and a foreign body sensation. Sticky secretions and scabs may be present near the root of the eyelashes.
Calazio and stye
Calazi and styes are characterized by the sudden appearance of a focal swelling of the upper or lower eyelid. The chalazion is caused by the occlusion on a non-infectious basis of a meibomian gland, while the stye is an acute infectious inflammation. Both conditions start with redness, edema, swelling and eyelid pain. Over time, the chalazion tends to become a small indolent nodule at the center of the eyelid, while the stye persists as a painful detection on the eyelid margin.
