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Retina

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Retina

Definition and pathologies: retinal detachment and age-related macular degeneration or ARMD

The human eye works like a camera. The retina, which is a thin membrane that entirely covers the eye fundus, is the film. Its task is to transform the visual stimuli into nerve pulses that are sent to the brain. Under the retina there is a vascular layer called the choroid, which nourishes the deep layers that are formed by millions of light-sensitive cells (photoreceptors). There is a difference between photoreceptors placed in the central retina or the macula (cones), which is responsible for daylight vision, the decoding of particulars and colour perception, and photoreceptors placed in the periphery (rods), which are responsible for peripheral vision and night vision. The photoreceptors turn light into nerve pulses that are processed by many other retinal cells, arranged on layers. The extensions of the most superficial cells act as optical fibres that congregate in the optical nerve to send visual information to the brain. In practical terms, all our actions crucially depend on one retinal area, the fovea (the centre of the macula) where both the thickness and the diameter are lower than half a millimetre.

Pathology

The epiretinal membrane (also called the macular pucker), the macular hole and age-related macular degeneration are diseases that hit the central part of the retina (macular). The macular pucker and the macular hole are caused by alterations of the vitreous, a transparent and gelatinous substance that fills the interior of the eye, behind the crystalline lens and in front of the retina. Over time the vitreous can suffer from liquefaction and degeneration, which culminate with its separation from the retina. In age-related macular degeneration (ARMD) the macula cells responsible for central vision degenerate and in the end die off and are replaced by scar tissue which is devoid of any functionality. The macula is responsible for more than 35% of the visual field; therefore severe ARMD influences ordinary daily activities which affect the patient's psychological state. Peripheral vision is, however, intact and not at risk. ARMD does not cause total blindness and it is possible to take up some activities again in an autonomous and productive way by resorting to remedies and rehabilitation techniques (poor vision). There are two forms of ARMD: wet and dry. About 10% of people with ARMD develop the wet form. In this case some blood vessels develop in an abnormal way in the retinal wall under the macula. This can lead to rapid deterioration of vision if the accumulation of blood and liquid inside the eye is not treated. Early treatment allows for damage limitation, even if a permanent scar will remain once the oedema is treated. About 90% of patients with ARMD have the dry form: in this case the damage is rather stable.




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