eye health

Umor Vitreo

What is Vitreous Mood?

The vitreous humor is a connective tissue of gelatinous consistency, colorless and transparent, which occupies the cavity of the eyeball between the posterior surface of the crystalline lens and the retina (this cavity is called the vitreous chamber).

This mass helps to maintain the shape of the eye (fills the bulb), promotes the diffusion of nutrients and protects against micro-traumas coming from the outside (cushions the shocks). Furthermore, being transparent, the vitreous humor represents a means of refraction and, as such, allows the unimpeded transmission of light up to the retina (dioptric function).

The most common pathology is the degeneration of the vitreous gel microstructure, closely related to age and to particular pathological processes (such as myopia, phlogosis and retinopathy). In practice, there is a consequentiality of clinical events ranging from sinchysis (fluidification of the vitreous gel), to syneresis (thickening of collagen fibers), up to detachment of the vitreous.

Relations with neighboring ocular structures

The eyeball is a closed, spherical anatomical structure, hollow inside.

The vitreous humor (also called vitreous or simply vitreous body ) occupies the posterior ocular cavity (note: the anterior segment between cornea and iris contains, instead, aqueous humor).

Anteriorly, the vitreous gel is in contact with the posterior capsule of the crystalline lens, the zonular fibers of Zinn and the ciliary body; laterally and posteriorly, instead, it adheres perfectly to the retina (through a membrane, called internal limiting ). The vitreous humor constitutes, therefore, about four-fifths of the volume of the entire eyeball (weight: 3.9 grams; volume: 3.8 ml).

In an antero-posterior sense (from the papilla of the optic nerve to the posterior pole of the crystalline lens), the vitreous humor is crossed by the hyaloid canal, which is filled with liquid and has a diameter of about 2 mm. In the prenatal period, on the other hand, this structure is traversed axially by an artery, which irrigates the crystalline lens (note: before birth, the vitreous humor allows the development and growth of the eyeball).

MACRO

The vitreous body is covered by a transparent membrane, called hyaloid .

Anatomically, we can distinguish three vitreous regions:

  • Vitreous base : it is an annular area, about 2-4 mm wide, located posteriorly to the serrated hour (most peripheral and anterior part of the retina), where there is a firm adhesion with the pars plana;
  • Posterior vitreous cortex : covers the entire retina behind the closed hour;
  • Anterior vitreous bark : anterior to the closed hour.

Furthermore, in the front there is a central depression, called patellar dimple (or hyaloid fossa), in which the posterior surface of the crystalline lens is housed. The vitreous body is neither vascularized nor innervated.

Vitreo-retinal adhesions

The vitreous humor has some areas of particular adherence to the retina:

  • At the level of the tightened hour, between the base of the vitreous and the pars plana (it is the most tenacious area of ​​adherence);
  • At the level of the macula and optic papilla (at the posterior pole of the eye), in the area of ​​Martegiani (adherence is significant in children and young people, but gradually becomes less in patients over the age of forty and in the myopia over three diopters );
  • Along the main arterial and venous vessels.

Microstructure

The vitreous humor is a substance with a gelatinous, viscous, colorless and transparent consistency (so to speak, it is similar to the egg white of the newborn), consisting mainly of water (about 98.4%). The remaining approximately 1% is represented by various substances that make up the scaffolding; the components are essentially four: collagen fibers, cells, mucopolysaccharides and other proteins.

From a microscopic point of view, crossed bundles of thin collagen fibers (type II and IX) constitute a sort of highly ordered scaffolding, which allows to perform a mechanical support function. Their arrangement in an antero-posterior sense also guarantees the transparency of the vitreous humor and allows the transmission of light to the retina with minimal dispersion. The collagen filaments are immersed in an extracellular matrix composed of hyaluronic acid (lattice stabilizer) and proteoglycans (they keep the fibers parallel and at the right distance).

Variations in the number of collagen fibers and their organization make the vitreous more or less dense. In particular, the mass is more fluid centrally, while at the level of the base the fibers adhere closely to the retina, forming a structure similar to Velcro.

In the vitreous gel there is, then, a specific cellular population, the hyalocytes, relatively more abundant in the peripheral portion and at the level of the contour of the hyaloid canal (they lack, instead, near the optic disc and the macula). These cells perform metabolic functions, as they have the ability to regenerate (they possess the enzymatic structure capable of producing hyaluronic acid) and clean up (vitiate the waste products) the vitreous. Among the other constituents of the vitreous, soluble proteins (albumin and globulin), sugars (such as glucose, galactose and fructose) and ascorbic acid must also be mentioned. Inside the aqueous component, however, there are numerous electrolytes, including sodium, potassium, chlorides and bicarbonates.

Functions

The functions of the vitreous humor are multiple:

  • It helps to stabilize the shape of the eye (in practice, it fills the eyeball) and maintains the balance of eye tension (morphostatic function).
  • It acts as a support to the posterior portion of the crystalline lens (via the Wiegert ligament) and to the retina, pressing the neural layer against the pigmented layer.
  • It is part of the dioptric apparatus of the eye: being transparent, it is crossed by light and allows the complex formed by cornea, aqueous and crystalline humor to make the light rays converge on the retina. The refractive index of the vitreous humor is equal to 1.3349, so it is similar to that of water (1.3336). Furthermore, it absorbs part of the ultraviolet rays, as it transmits 90% of the light rays between 300 and 1400 millimicrons.
  • It also performs a metabolic function towards the crystalline lens, favoring the diffusion of nutrients coming from the ciliary bodies. Furthermore, it allows the transport of substances to and from the retina.
  • Thanks to its viscoelastic properties, the vitreous is able to absorb shocks and mechanical stresses affecting the eyeball (even if compressed, it returns to its original shape).
  • On the other hand, its elasticity facilitates the anteroposterior displacements of the crystalline lens, amplifying the accommodative action of the ciliary muscle with a spring effect ".
  • It performs a protective function on particularly delicate structures, such as the retina and the lens (just consider the rapidity of the eye movements and the tractions exerted by the eye muscles to which they are subjected).

Age-related vitreous degeneration

In young people, the vitreous humor completely fills the vitreous chamber and is dense, homogeneous and compact. Over the years, however, the vitreous gel appears rarefied due to the reduction in its volume: hyaluronic acid progressively loses its ability to retain water and undergoes a gradual depolymerization, responsible for the "fluidization" of the vitreous. This phenomenon, called sinchisi, begins in the back and subsequently extends to the entire vitreous body. The result is the formation of vitreous gaps (optically empty cavities).

The evolution of this process leads to the coarctation of collagen fibers, which float freely within the lacunar cavities or aggregate forming filamentous clusters ( vitreous syneresis ). This phase makes it possible to perceive vitreous mobile bodies (or myosopsias), often described as "flying flies" or "cobwebs" (in practice, their vision is due to the shadow that the vitreous accumulations project onto the retina).

The vitreous moving bodies are perceived above all in conditions of strong luminosity or looking at a white surface: the shadows move rapidly with the movements of the eye, then slowly return to the original site.

Returning to the degenerative process, the coalescence of the liquefaction zones leads to the fall of the scaffold of the collagen fibers and the vitreous begins to detach; in this phase, in addition to the moving bodies, phosphenes can be perceived (visual flashes due to discontinuous vitreous traction on the retina).

The next step is the separation of the vitreous cortex from the posterior retina, which leads to posterior vitreous detachment . This event can be very rapid or can occur within a few months; it appears evident due to the appearance of an annular opacity in the center of the visual field (Weiss ring).

Vitreous degenerations increase with age: they begin during the second-third decade of life and, for the patient, become manifest after the age of 40. However, the contraction of the collagen fibers following the liquefaction of the vitreous gel (syneresis) does not only depend on aging. Myopia beyond the three diopters, inflammatory pathologies, retinopathies and ocular traumas, in fact, can make this process premature and anticipate clinical signs.

Biochemical mechanisms underlying vitreous aging

The vitreous fluidification or liquefaction is triggered by the activity of metalloproteases (MMPs), enzymes present in the extracellular matrices of the human body that have a fundamental role in the degradation of connective tissues and in structures containing collagen. As a rule, the action of these enzymes is balanced by inhibitory enzymes (called TIMPs).

Due to the increase in age or certain pathologies that damage the retinal pigment epithelium, there is a reduction in the activity of the TIMPs enzymes and the consequent increase in that of the MMPs, which attack and degrade the collagen fibrils of the humor vitreous.

Vitreous detachment: other causes

In addition to occurrence due to an age-related phenomenon of degeneration, the separation of the vitreous from the retinal surface can occur following strong blows to the head (traumas and accidents) and dehydration (the onset of symptoms is more frequent in warm months, in patients taking few fluids or being treated with diuretics and antihypertensive drugs).

The posterior detachment of the vitreous can also result from myopia beyond the three diopters, vascular retinal pathologies, inflammatory processes (uveitis, retinitis, etc.), aphakia (lack of the crystalline lens), vitreous hemorrhages, hereditary vitreo-retinal syndromes (eg Marfan syndrome ) and reduction of the synthesis of hyaluronic acid, following the decrease in estrogens in the post-menopausal period.

Furthermore, there is a correlation between retinal rupture and posterior vitreous detachment.

Other vitreous diseases

The vitreous humor is subject to numerous other pathologies, schematically summarized in:

  • Congenital malformations (eg persistence of the hyaloid vascular system);
  • Vitreous prolapse in anterior chamber or outside the eyeball;
  • Inflammations (generally, they originate from the neighboring structures);
  • Haemorrhages (emovitreo caused by retinal bleeding secondary to trauma, diabetic retinopathy, neovascular glaucoma, blood disorders or surgery).

Other vitreous degenerations are:

  • Asteroid hyalosis : caused by the formation of small spherical opacities of crystalline appearance, composed mainly of fats and calcium salts;
  • Sparkling sinchisi : due to the presence of intravitreal cholesterol crystals (cholesterolosis of the bulb).