Conjunctiva

The conjunctiva (tunica conjunctiva) is a pale pink mucous membrane that lines the back of the eyelids and extends onto the eyeball to the cornea, thus connecting the eyelid to the eyeball. When the palpebral fissure is closed, the conjunctiva forms a closed cavity, the conjunctival sac, which is a narrow slit-like space between the eyelids and the eyeball.

The mucous membrane covering the back of the eyelids is called the palpebral conjunctiva (tunica conjunctiva palpebrarum), and the one covering the sclera is called the conjunctiva of the eyeball (tunica conjunctiva bulbaris) or sclera. The part of the palpebral conjunctiva that passes onto the sclera to form vaults is called the conjunctiva of the transitional folds or the vault. Accordingly, there are superior and inferior conjunctival vaults (fornix conjunctiva superior et inferior). At the inner corner of the eye, in the area of the rudiment of the third eyelid, the conjunctiva forms a vertical semilunar fold and the lacrimal caruncle.

The entire space in front of the eyeball, limited by the conjunctiva, is called the conjunctival sac (saccus conjunctivalis), which closes when the eyelids close. The lateral angle of the eye (angulus oculi lateralis) is sharper, the medial (angulus oculi medialis) is rounded and on the medial side limits the depression - the lacrimal lake (lacus lacrimalis). Here, at the medial angle of the eye, there is a small elevation - the lacrimal caruncle (caruncula lacrimalis), and lateral to it - the semilunar fold of the conjunctiva (plica semilunaris conjunctivae) - a remnant of the nictitating (third) eyelid of lower vertebrates. On the free edge of the upper and lower eyelids, near the medial angle of the eye, outside the lacrimal lake, an elevation is noticeable - the lacrimal papilla (papilla lacrimalis). At the top of the papilla there is an opening - the lacrimal point (punctum lacrimale), which is the beginning of the lacrimal canaliculus.

The conjunctiva has two layers - epithelial and subepithelial. The conjunctiva of the eyelids is tightly fused with the cartilaginous plate. The epithelium of the conjunctiva is multilayered, cylindrical with a large number of goblet cells. The conjunctiva of the eyelids is smooth, shiny, pale pink, through which the yellowish columns of the meibomian glands passing through the thickness of the cartilage are visible. Even with a normal state of the mucous membrane at the outer and inner corners of the eyelids, the conjunctiva covering them looks slightly hyperemic and velvety due to the presence of small papillae.

The following are distinguished:

• The conjunctival epithelium is 2 to 5 cell layers thick. Basal cuboidal cells transition to flat polyhedral cells reaching the surface. With chronic exposure and drying, the epithelium can become keratinized.
• The stroma (substantia propria) consists of highly vascularized connective tissue separated from the epithelium by the basilar membrane. The adenoid superficial layer does not develop until approximately 3 months after birth. This is responsible for the absence of a follicular conjunctival reaction in the neonate. The deeper, thicker fibrous layer is associated with the tarsal plates and represents subconjunctival tissue rather than conjunctiva proper.

Conjunctival glands

Mucin secreting cells

• Goblet cells are located within the epithelium, with the highest density in the lower nasal region;
• Henle's crypts are located in the upper third of the superior and lower third of the inferior tarsal conjunctiva;
• The Manz glands surround the limbus.

NB: Destructive processes in the conjunctiva (eg, cicatricial pemphigoid) usually cause a disturbance in mucin secretion, whereas chronic inflammation is associated with an increase in the number of goblet cells.

The accessory lacrimal glands of Krause and Wolfring are located deep within the lamina propria.

The conjunctiva of the transitional folds is loosely connected to the underlying tissue and forms folds that allow the eyeball to move freely. The conjunctiva of the fornices is covered with stratified squamous epithelium with a small number of goblet cells. The subepithelial layer is represented by loose connective tissue with inclusions of adenoid elements and accumulations of lymphoid cells in the form of follicles. The conjunctiva contains a large number of additional lacrimal glands of Krause.

The conjunctiva of the sclera is delicate, loosely connected to the episcleral tissue. The multilayered flat epithelium of the conjunctiva of the sclera smoothly passes onto the cornea.

The conjunctiva borders on the skin of the eyelid margins and on the other side - on the corneal epithelium. Diseases of the skin and cornea can spread to the conjunctiva, and diseases of the conjunctiva - to the skin of the eyelids (blepharoconjunctivitis) and cornea (keratoconjunctivitis). Through the lacrimal punctum and lacrimal canal, the conjunctiva is also connected to the mucous membrane of the lacrimal sac and nose.

The conjunctiva is abundantly supplied with blood from the arterial branches of the eyelids, as well as from the anterior ciliary vessels. Any inflammation and irritation of the mucous membrane is accompanied by bright hyperemia of the vessels of the conjunctiva of the eyelids and fornices, the intensity of which decreases towards the limbus.

Due to the dense network of nerve endings of the first and second branches of the trigeminal nerve, the conjunctiva acts as a covering sensitive epithelium.

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Functions

The main physiological function of the conjunctiva is to protect the eye: when a foreign body gets in, the eye becomes irritated, the secretion of lacrimal fluid increases, blinking movements become more frequent, as a result of which the foreign body is mechanically removed from the conjunctival cavity. The secretion of the conjunctival sac constantly moistens the surface of the eyeball, reduces friction during its movements, and helps maintain the transparency of the moistened cornea. This secretion is rich in protective elements: immunoglobulins, lysozyme, lactoferrin. The protective role of the conjunctiva is also ensured by the abundance of lymphocytes, plasma cells, neutrophils, mast cells and the presence of immunoglobulins of all five classes.

Clinical features characteristic for the diagnosis of conjunctival diseases are: complaints, discharge, conjunctival reaction, films, lymphadenopathy.

Symptoms of conjunctival diseases

Non-specific symptoms: lacrimation, irritation, pain, burning sensation and photophobia.

1. Pain and foreign body sensation suggest corneal involvement.
2. Itching is a sign of allergic conjunctivitis, although it can occur with blepharitis and dry keratoconjunctivitis.

Separable

Consists of exudate that filters through the conjunctival epithelium from dilated blood vessels. The surface of the conjunctiva contains products of the breakdown of epithelial cells, mucus and tears. The discharge may vary from watery, mucopurulent to pronounced purulent.

1. Watery discharge consists of serous exudate and excessive amounts of reflexively secreted tears. It is typical of acute viral and allergic inflammations.
2. Mucous discharge is typical for vernal conjunctivitis and keratoconjunctivitis sicca.
3. Purulent discharge occurs in severe acute bacterial infections.
4. Mucopurulent discharge occurs in both mild bacterial and chlamydial infections.

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Conjunctival reaction

• Conjunctival injection is most pronounced in the fornices. Velvety, bright red conjunctiva indicates bacterial etiology.
• Subconjunctival hemorrhages usually occur with viral infections, although they can also occur with bacterial infections caused by Strep. pneumoniae and N. aegypticus.
• Edema (chemosis) occurs with acute inflammation of the conjunctiva. The translucent swelling is due to exudation of protein-rich fluid through the walls of inflamed blood vessels. Large, excessive folds may form in the fornix and, in severe cases, the edematous conjunctiva may extend beyond the closed eyelids.
• Scarring may occur with trachoma, ocular cicatricial pemphigus, atopic conjunctivitis, or with long-term use of topical medications.

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Follicular conjunctival reaction

Compound

• Follicles are subepithelial foci of hyperplastic lymphoid tissue within the stroma with additional vascularization;

Symptoms

• Numerous, separate, slightly elevated formations, reminiscent of small grains of rice, are most prominent in the vaults.
• Each follicle is surrounded by a tiny blood vessel. The size of each formation can range from 0.5 to 5 mm, which indicates the severity and duration of the inflammation.
• The follicles increase in size, so the vessel accompanying them moves to the periphery, resulting in the formation of a vascular capsule, which forms the basis of the follicle.

Reasons

• Causes may include viral and chlamydial infections, Parinaud syndrome and hypersensitivity to local treatment.

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Papillary conjunctival reaction

The papillary reaction of the conjunctiva is nonspecific and therefore has less diagnostic value than the follicular reaction.

Compound

• Hyperplastic conjunctival epithelium arranged in numerous folds or ridges with a central vessel, diffuse infiltrate of inflammatory cells including lymphocytes, plasma cells and eosinophils.
• Papillae can form only in the palpebral and bulbar conjunctiva in the limbal region, where the conjunctival epithelium is connected by fibrous septa to the underlying structures.

Symptoms

• Papillae are the most common finding on the upper eyelid conjunctiva and appear as a delicate mosaic-like structure with raised polygonal hypersmetic areas separated by paler grooves.
• The central fibrovascular core of the papilla secretes a secretion onto its surface.
• With prolonged inflammation, the fibrous septa that attach the papillae to the underlying tissues can rupture, causing them to fuse and increase in size.
• Recent changes include superficial stromal hyalinization and formation of crypts containing goblet cells between the papillae;

With a normal upper edge of the tarsal plate (when the lower one is everted), the papillae can imitate follicles, which cannot be considered a clinical sign.

Reasons

Chronic blepharitis, allergic and bacterial conjunctivitis, contact lens wear, superior limbal keratoconjunctivitis and sleeping eyelid syndrome.

Films

1. Pseudomembranes consist of coagulated exudate attached to inflamed conjunctival epithelium. They are easily removed, leaving the epithelium intact (a characteristic feature). Causes may include severe adenovirus and gonococcal infections, fibrous conjunctivitis, and Stevens-Johnson syndrome.
2. True membranes permeate the superficial layers of the conjunctival epithelium. Attempts to remove the membrane may be accompanied by epithelial rupture and bleeding. The main causes are infections caused by Strep, pyogenes, and diphtheria.

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Lymphadenopathy

Lymph drainage from the conjunctiva goes to the parotid and submandibular nodes (as does lymph drainage from the eyelids). The main causes of lymphadenopathy are viral, chlamydial, gonococcal infections and Parinaud's syndrome.