The eyelids are muscle-cutaneous folds, thin and mobile, able to completely cover the anterior part of the eyeball.
Like other accessory formations of the eye, the eyelids perform protective functions from external agents and contribute to bulbar support. Frequent blinking also 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 borders at the top with the eyebrow line and is more developed, broad and mobile than the lower one; it contains the anterior part of the levator muscle.
Anatomy of the Eyelids. Modified from the site: http://www.anatomyatlases.org/firstaid/Eye.shtml
Internally equipped with a fibro-muscular skeleton (palpebral tarsus), each of these structures has two faces: an anterior cutaneous and a posterior covered by the eyelid conjunctiva. The free margins of the upper and lower eyelids are separated by a transverse opening called the palpebral rim (or fissure); however, they join at the ends, in the medial (lacrimal) and lateral (ciliary) cantus. The eyelid rim varies in width with the winks.
In the lateral portion of the free margin, the eyelids are provided with eyelashes, sebaceous and sweat glands. At the medial corner of the free margin, on the other hand, the eyelids present a relief, the lacrimal papilla, where the entrance to the nasolacrimal canal is present.
At the level of the medial cantus, moreover, a pinkish relief is evident, the lacrimal caruncle, where the conjunctiva and the epidermis meet. The lacrimal caruncle contains glands that process a dense secretion that contributes to the substance that can be found, sometimes congealed, upon awakening in the morning.
The visible outer surface of the eyelids is lined with a thin layer of compound paving epithelium. Below the subcutaneous layer, the eyelids are supported and reinforced by wide connective laminae, collectively called the tarsal plate.
The inner surface of the eyelids is lined by the conjunctiva, a mucous membrane lined with specialized compound paving epithelium. The goblet cells of the epithelium contribute, with the accessory glands, to the production of a lubricating fluid, which is arranged on the surface of the eyeball, keeping it moist and clean. This also avoids friction with the ocular (or bulbar) conjunctiva. which covers the anterior surface of the eye. Below the skin, the eyelids have a muscular and a fibro-cartilaginous layer.
Eyelashes, sebaceous and sweat glands
The eyelid margin has two or three rows of strong and curved hairs (the eyelashes), longer and more numerous at the level of the upper eyelid than the lower one.
The cilia are controlled by a plexus of the hair root, whereby their displacement initiates an intermittent reflex. This movement helps prevent foreign particles from reaching the ocular surface.
Each lash follicle has a Zeis gland, which 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 margin, at the emergence of the eyelashes, however, the meibomian glands (or tarsal glands) secrete a substance rich in lipids which prevents the eyelids from sealing themselves against each other. This organization affects the eyelid margin except of the medial portion (which represents approximately the internal eighth of the fissure), which presents the lacrimal dots, which coincide with the beginning of the lacrimal outflow ducts.
All the accessory glands of the eyelids are subject to occasional bacterial invasion. From the infection of a meibomian gland, a chalazion can form. The infectious process of a sebaceous gland of an eyelash, a tarsal gland or one of the accessory lacrimal glands that open on the surface between the follicles of the eyelid, on the other hand, causes a localized painful swelling known as a sty.
Muscular system
The orbicularis muscles of the eye and superior levator eyelid are located between the tarsal plate and the skin. The orbicularis muscle is responsible for involuntary blinking and closing of the eyelids. The action of the levator superior muscle of the eyelid, on the other hand, consists in elevating the upper eyelid.
Functions
With eyebrows, the superficial epithelium of the eye and the 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 blink 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 blink).
Diseases of the Eyelids
The eyelids can be affected by various pathological processes and by anomalies of shape, position or altered movement.
The most frequent disorders include allergic reactions, inflammations (blepharitis, chalazion, sty and conjunctivitis), traumatic lesions and eyelid ptosis. The eyelid skin is also the site of the onset of benign and malignant tumors.
Dermatological affections
The eyelid skin can be affected by many of the morbid conditions that occur in the skin, including eczematous dermatitis and chemical or heat burns.
The skin around the eyes is extremely sensitive and can react to even the slightest exposure to 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 wrinkle. the eyes. The eyelid skin may be dry and peeling. Possible triggers include eye cosmetics (eye shadows, mascara and face creams), hair spray, nail polish, pollen, cat and dog hair, dust mites and molds.
The skin of the eyelid can be affected by febrile herpes (herpes simplex) and by reactivation of the varicella-zoster virus (ophthalmic herpes zoster) infection. The anterior surface of the eyelids is also home to skin manifestations secondary to syphilis, Chagas disease and various forms of tuberculosis.
Entropion
Entropion consists in the inward rotation of the free lid margin. This condition can be present at birth (congenital) or occur later in life (acquired). Over time, the edge of the eyelid and the (abnormally positioned) eyelashes rub against the front of the eye with each blink, causing redness and irritation. If the patient does not resort to proper treatment, entropion can lead to the development of abrasions. and corneal ulcers.
The disorder is most commonly observed in elderly people due to the hyperlaxity of the tissues linked to the aging process. Entropion can also occur due to trauma, previous surgery, muscle alterations (eg paralysis), post-infectious outcomes (eg chronic conjunctivitis) and blepharospasm. The most effective correction of the disorder involves surgery.
Ectropion
The ectropion consists in the rotation of the edge of the eyelid towards the outside. This condition can affect both eyelids (upper and lower), but the lower one is most affected.The extent of the ectropion is variable: in the most severe cases, there is a complete eversion of the eyelid (with exposure of the conjunctiva up to the fornix), while when it is slight only a small segment of the eyelid rim can move away from the eyeball.
Ectropion can lead to changes in lacrimation (epiphora), eye irritation, dryness and redness. The most serious complications are abrasion and ulceration of the cornea.
The ectropion is often due to the loss of tone of the orbicular muscle, but it can also depend on inflammatory corneal or conjunctival processes, facial paralysis and scar retraction (trauma, post-surgical outcomes and dermatological affections). 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 "drooping eyelid" can interfere with vision and cause other disorders, such as amblyopia (from occlusion).
Eyelid ptosis can be congenital or acquired. The most common cause is weakening, paralysis or injury of the muscles and nerves normally responsible for the movement of the eyelid. In adults, the condition is often a consequence of aging (senile or age-related ptosis).
Ptosis also arises as a complication of trauma (fractures of the eye socket or eyelid wounds), neurological disorders (such as stroke, oculomotor nerve palsy and multiple sclerosis), muscle disorders (eg myasthenia gravis), severe inflammatory conjunctival processes and, in rare cases cases, tumors of the eye socket. Surgical correction can be an effective treatment to improve both vision and aesthetic appearance.
Blepharocalase
Blepharocalasis is a senile laxity of the epidermis of the upper eyelid, associated with the fall of the upper eyelid and, for this reason, often confused with ptosis.
Blepharospasm
Blepharospasm is the forced and persistent contraction of the orbicular muscle of the eye that causes involuntary blinking and closing of the eyelids; in severe cases the patient is unable to open the eye. It can be secondary to ophthalmic disorders that cause irritation, including: trichiasis, corneal foreign bodies, inflammatory processes of the iris or ciliary body and keratoconjunctivitis sicca. In other cases, it is a consequence of systemic spasmogenic neurological diseases (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 rosacea and seborrheic dermatitis. Chronic blepharitis, on the other hand, can be caused by an "altered secretion of the meibomian glands. Symptoms, common to all forms of blepharitis, include itching and burning of the eyelid margin, conjunctival irritation with redness, tearing, sensitivity to light and sensation. sticky secretions and scabs may be present near the root of the eyelashes.
Chalazion and sty
Chalazion 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 of a meibomian gland on a non-infectious basis, while the sty is an acute inflammation on an infectious basis. Both conditions begin with redness, edema, swelling, and eyelid pain. Over time, the chalazion tends to become a small indolent lump in the center of the eyelid, while the sty persists as a painful lump on the eyelid margin.
