What is the conjunctiva
The conjunctiva is the thin mucous membrane that covers the anterior surface of the eyeball (with the exception of the cornea) and the inner surface of the eyelids.
Its main function is to protect the eye, thanks to the coating it provides, from foreign bodies and infections. Furthermore, this anatomical ocular structure helps maintain the tear film and facilitates the sliding of the two opposing conjunctival surfaces, thus avoiding friction in the blinking phases.
The conjunctiva can be the site of numerous pathological processes: inflammation (conjunctivitis), congenital malformations, benign or malignant neoplasms, dystrophic alterations and degenerative diseases. On the conjunctiva, moreover, general affections of the organism of various types, such as infectious diseases, allergic reactions and metabolism disorders, are reflected.
Structure
The conjunctiva is a mucous membrane, almost completely transparent, well vascularized and abundantly innervated by trigeminal filaments (called ciliary nerves).
From the histological point of view, the conjunctival tunic consists of epithelial cells (columnar and squamous epithelium), arranged in 2-5 layers, and stroma (connective tissue). In addition, a glandular system is present, consisting mainly of goblet cells that contain mucin granules and provide for the production of the mucous layer of the tear film.
The conjunctiva can be divided into 3 anatomical parts:
- Palpebral conjunctiva (or tarsal) : consisting of cylindrical epithelium, it is a thin, transparent, red or pinkish membrane. Following the skin, the conjunctival tunic begins at the free edge of the eyelids, then covers the posterior aspect of the tarsi, to which it adheres tightly.
- Bulbar (or scleral) conjunctiva : it is the part of the conjunctival tunic that is applied on the eyeball and covers the anterior surface of the sclera, with the exception of the corneal portion. Consisting of paving epithelium, the bulbar conjunctiva rests weakly on its own loose connective tissue. The bulbar conjunctival tunic is smooth, very thin and so transparent that it gives a glimpse of the white color of the sclerotic and the anterior conjunctival and ciliary vessels. In the medial position, then, the tarsal conjunctiva receives the upper and lower lacrimal dots, which represent the beginning of the lacrimal pathways.
- Conjunctiva of the fornixes : at the level of the space between the eyelids and the eyeball, the conjunctival membrane bends and coats the upper and lower fornices, allowing freedom to the movements of the bulb.
Conjunctival sac
As a whole, the conjunctiva forms a kind of "pocket", which results from the folding of the bulbar membrane (which covers the eye) and the eyelid (adhering to the inner part of the eyelid). The conjunctival sac is closed when the free margins of the eyelids come into contact during blinking, while communicating with the outside when the palpebral fissure is open.
Lacrimal caruncle and semilunar fold of the conjunctiva
At the inner corner of the eyelid fissure, there are two formations that represent rudiments of embryonic structures: the semilunar plica and the lacrimal caruncle.
The semilunar fold is a vertical folding of the bulbar conjunctiva, whose free edge is concave. It extends from the upper to the lower part of the conjunctiva, but is visible only in its middle portion, being largely hidden by the eyelids.
The lacrimal caruncle is, on the other hand, a small pinkish, roundish and raised mucous excrescence, placed between the lacrimal portions of the edges of the eyelids; contains a dense connective tissue, crossed by some bundles of smooth and striated muscle cells. The lacrimal caruncle has hair follicles with rudimentary hairs and attached sebaceous glands. It also contains accessory tear glands.
Note : in humans, the semilunar fold in the corner of the eye is considered a small vestigial residue of the nictitating membrane, ie the "third eyelid" that is observed in other animals, such as birds and reptiles.
Functions
The conjunctiva has the main function of protecting the front surface of the eye .
Moreover, it facilitates the sliding of the eyelids in the blinking phases and allows the eyeball to move, without frictions at the level of its surfaces, thanks to the secretion of the mucinous component of the tear film (a sort of viscous mucus that protects the cornea and allows the part prominent watery layer of tears).
In fact, the conjunctiva contains serous, muciparous glands (or mucus-secreting goblet cells) and accessory lacrimal glands (from Krause and Ciaccio). These structures pour their secretion into the conjunctival sac, thus helping to keep the ocular surface moist, clean and complete.
In addition to the physical and biological protection of tears, the conjunctiva has an immune defense system mediated by lymphatic elements, mostly located in the tarsal portion (lymphatic follicles). In fact, due to its particular anatomy, the conjunctival tissue is particularly exposed to external agents, such as dust, pollen and bacteria.
Note : the conjunctival mucosa responds to stimuli of different nature, which modify its appearance. These reactions, for example, can cause a redness due to the dilation of the blood vessels of the conjunctiva ( hyperemia ) or cause a more severe picture with abundant secretion, pain, foreign body sensation and tearing, sometimes associated with edematous swelling ( chemosis ).
Conjunctivitis
Conjunctivitis is an inflammation of the conjunctival surface. It is a frequent pathology, which can manifest itself in acute or chronic form.
The causes can be various, but the most frequent are the following:
- Eye infections (due to bacteria, viruses, fungi or parasites);
- Seasonal or perennial allergies (hypersensitivity to pollens, cosmetics, dust mites or animal dander);
- Intense irritation from foreign bodies and chemical-physical agents (caused by drugs, heat, wind, dust and atmospheric pollutants, acids, alkalis, soap, cigarette smoke and fertilizers, excessive exposure to sunlight or other forms of radiation, etc.) ).
The symptoms of conjunctivitis depend on the causes, but often include burning, itching, redness, photophobia, increased formation of tears, swelling of the eyelids and a sense of a foreign body (feeling of having sand in the eyes). In the infectious form, a catarral or mucopurulent secretion can be added to the listed manifestations (the eyes tend to "stick").
The therapy varies according to the type of conjunctivitis and is established by an ophthalmologist.
Bacterial forms can heal with a therapy based on antibiotic eye drops. In the case of an allergic conjunctivitis, antihistamine and cortisone drops are used, which can be associated with the use of artificial tears and systemic antihistamine drugs.
The viral forms often caused by adenovirus and herpes virus, have a longer and more difficult course than bacterial conjunctivitis. In general, frequent instillations of antibiotic eye drops (in order to prevent bacterial superinfection) and, with caution, topical corticosteroids (to decrease hyperaemia and conjunctival edema) are used.
Subconjunctival hemorrhage
Subconjunctival hemorrhage appears as a bright red spot, not associated with other signs of inflammation. These extravasations of blood below the conjunctiva derive from the rupture of the wall of a capillary and usually appear as a result of minor trauma, coughing and sneezing (for example, in the course of diseases of the upper respiratory tract). In some cases, subconjunctival bleeding may be accompanied by systemic arterial hypertension, blood dyscrasias and viral conjunctivitis.
The disorder tends to resolve spontaneously in about 15 days, so no treatment is required. In any case, it is advisable to consult your ophthalmologist for an assessment.
Conjunctival foreign body
The presence of a foreign body at the conjunctival level causes a unilateral symptomatology, characterized by pain, difficulty in keeping the eye open, conjunctival hyperemia, lacrimation and photophobia.
If considered to be tarsal, foreign bodies can cause corneal lesions due to the continuous rubbing of the eyelid during blinking. For this reason, they must be removed as soon as possible.
Conjunctival degeneration
Pinguecula and pterygium are benign degenerations of the conjunctiva, which appear as growths adjacent to the cornea. Both of these injuries cause redness, irritation, a sense of a foreign body and a burning sensation.
pinguecula
Pinguecula is an accumulation of degenerated collagen, located in the nasal and temporal conjunctival area.
Pinguecula may represent the outcome of an inflammation of the ocular surface resulting from trauma, caustic burns and peripheral corneal ulcerations.
pterygium
Pterygium is a small fibrovascular formation, shaped like a triangle, determined by an abnormal growth of the bulbar conjunctiva. This lesion progressively extends towards the cornea, until it is covered. Unlike pinguecula, in fact, pterygium has its own vessels.
This lesion typically occurs on the nasal side of the cornea and often leads to a reduction in visual acuity, due to the induction of astigmatism. In fact, pterygium can distort the corneal surface, changing the refractive power of the eye.
Reduced vision determines the need to remove the lesion surgically, even if recurrences are very frequent.
The causes at the origin of the pterygium are still partially unknown, however chronic exposure to irritative factors (in particular, to the sun and to the wind) increases the risk of developing the pathology.
Cicatricial pemphigoid
The cicatricial pemphigoid is an alteration characterized by progressive scarring and bilateral narrowing of the conjunctiva. This process is also associated with the simultaneous neovascularization, opacification and keratinization of the cornea.
The mechanism underlying cicatricial pemphigoid is autoimmune.
In the beginning, the disease manifests itself in a similar way to chronic conjunctivitis, causing hyperemia, discomfort, itching and secretion. However, the progression of the disease leads to phenomena such as simblefaron (adhesion between tarsal and bulbar conjunctiva), trichiasis (introflection of the cilia), dry keratoconjunctivitis and conjunctival keratinization. Chronic corneal lesions can lead to secondary bacterial ulceration and blindness.
The diagnosis can be confirmed by a biopsy. Treatment may require systemic immunosuppression with dapsone or cyclophosphamide.
Tumors of the conjunctiva
The conjunctiva can be the site of benign or malign neoplastic processes. In most cases, these originate from the epithelium (the most superficial cell layer) or from the melanocytes (present in the conjunctival epithelium).
Corneo-conjunctival intraepithelial neoplasia
Corneo-conjunctival intraepithelial neoplasia is the most frequent ocular surface tumor. It occurs with clinical pictures ranging from mild dysplasia to locally invasive carcinoma (rarely causes metastasis). Generally, it appears as a thickening or a whitish, translucent or gelatinous conjunctival mass, often vascularized.
The therapy involves a wide surgical excision, sometimes associated with cryotherapy and reconstruction of the conjunctival plane. The use of topical chemotherapy may also be considered.
Squamous cell carcinoma
The most common malignant tumor is squamous cell carcinoma. This can originate from scratch or derive from a previous in situ phase. The initial forms are reminiscent of a pterygium, while the less differentiated ones have a gelatinous and translucent appearance. Squamous cell carcinoma, on the other hand, takes on a vegetative appearance, occupies the interpalpebral fissure and tends to protrude outwards. A prompt and rapid surgical excision, associated with cryotherapy, radiotherapy and topical chemotherapy, is generally associated with a good prognosis.
Lymphoid tumors
Non-Hodgkin's lymphoma of the ocular adnexa is quite rare (it represents about 8% of all extra-nodal cases). Generally, these tumoral forms involve the lymphoid tissue associated with the mucous membranes, that is the so-called MALT (acronym of "mucous-associated lymphoid tissue") and can appear with the appearance of eyelid swelling or visual changes of various types.
Conjunctival melanoma
In most cases, pigmented tumors are benign, but they must always be considered as possible carriers of malignancy (evolution is similar to that leading to the formation of cutaneous melanoma).
Conjunctival melanoma is a rare neoplasm (it represents about 2% of malignant eye tumors). It can originate from scratch or derive from the transformation of benign pigmented lesions ( congenital nevus and melanosis ) or pre-cancerous lesions ( primary melanosis acquired with atypia ).
The conjunctival melanoma can determine metastasis by lymphatic diffusion to the loco-regional lymph nodes and via the blood. The therapeutic approach is guided by the size and location of melanoma. In most cases, the treatment of choice is a broad surgical excision, often associated with cryotherapy. In large or localized melanomas, however, radical surgery is indicated, which involves the removal of all orbital contents.
