The eye is constructed as a camera. The front transparent cap is called cornea, the coloured (blue, green, or brown) iris has a hole in its centre, called the pupil. Like in the camera, the pupil regulates the amount of light entering the inside of the eye. Behind the pupil, the lens helps focus the picture, i.e. to see sharp images of objects at different distances. The most important part for vision is the film in this camera, called retina, a thin membrane lining the inside of the eyewall in the back. The retina captures the light. Transforms it into electric signals, and sends these through the cable, called optic nerve, to the centre of vision in the brain where the visual sensation, or the image, is finalized. The small central area of the retina is called macula. Macula enables sharp vision of details, thus enables us to read, recognize faces, or drive.
Various diseases of the eye can endanger vision. The eye can be affected by inflammations, by vascular diseases, by degenerative processes, by inherited diseases, by injuries, by tumors, or by mechanical complications of the interaction between the vitreous gel and the retina.
Most often, the vision is decreased due to the turbidity of optical media, i.e. the cornea, the lens, or the vitreous body. Especially the turbidity, or the opacification of the lens is a very common cause of decrease of vision. This loss of transparency of the lens is called cataract and the cataract surgery is the most common surgery on the eye (also on the human body). During this procedure, which is typically performed in topical anaesthesia using eyedrops, the opacified lens is removed and replaced with an artificial lens, placed behind the pupil. This surgery usually takes ten to fifteen minutes and the visual results are perfect, unless there is another disease, worsening vision.
Opacification of the cornea of various origin can be the reason for corneal transplantation during which either the whole cornea, or only some of its layers, is replaced with a donor tissue. This surgery, often performed in general anaesthesia, takes one hour or more. The healing takes several months and the functional benefit may be limited, depending on the ability of the eye to accept the transplant and to keep it clear.
The inside of the eye, unlike the camera, is filled with a semiliquid gel substance called vitreous body. This gel consists of many collagen fibres which are originally perfectly transparent, thus we cannot detect them. The young vitreous body behaves as an inflated balloon, completely filling the space inside the eye, lightly affixed to the retina. The aging vitreous body is less perfectly transparent; due to biochemical changes, the collagen fibres may be visible as hair, little threads, strands, cobwebs, dots, or rings, perceived as floating objects – floaters. Later, the inflated balloon partly deflates, i.e. the gel shrinks. Most of the gel is now no more affixed to the retina, however, the adhesion remains in the periphery.
During the process of the detachment of the vitreous body from the retina, various complications can happen. Movements of the shrunken gel can cause traction on the retinal periphery. The mechanical irritation of the retina surface can produce visual phenomena like flashes of light, glowing wire, or sparks. If the pull of the affixed vitreous on the retina is too strong, the retina can be torn. The tear in the peripheral retina can cause retinal detachment. In other cases, the interaction between the detaching vitreous and macula, the central area of the retina, can cause vitreomacular traction, macular hole, or epimacular membrane (see below).
When the fragile retina is torn by the traction from the vitreous, it can detach from the eyewall like a wallpaper separating from the wall. This leads to loss of vision, firstly in a shape of a dark curtain coming from the periphery of the visual field, later resulting in a total loss of vision. Visual symptoms that warn us before the retinal detachment include flashes and floaters; the curtain signals a detachment.
Flashes of light indicate that the vitreous is pulling the retina. They can be visible mainly in the dark, induced by eye movements. These flashes usually last several weeks, sometimes months, and gradually disappear when no retinal complications happen. Flashes of light are a warning that there is a danger of retinal tear and warn that the patient should undergo a meticulous examination by a retinal specialist.
Soot, or hundreds of small floaters, looking like black snowfall, suddenly appearing in the visual field, indicate that the retina has already been torn and red blood cells are falling from a torn blood vessel into the vitreous gel. Therefore, this condition needs urgent intervention, which usually means a special laser treatment with the aim to fix and glue the retinal tear before the retina detaches. This treatment with a laser beam is performed in the ophthalmologist´s office and lasts only a few minutes.
A curtain, sometimes black, sometimes grey or brown, growing from the periphery of the visual field towards the centre, is a signal that the retina is already detaching. The longer the detachment lasts, the smaller is the chance of gaining vision back, therefore, this condition needs urgent surgical intervention, performed in the operating room under local anaesthesia, sometimes under general anaesthesia.
During the surgery, the surgeon fixes (plugs, glues) the retinal tear, or multiple tears, using either an external procedure (sewing a silicone buckle on the eyewall from the outside), or using an internal procedure, called vitrectomy. During vitrectomy, the surgeon removes all vitreous gel from the eye, fixes the tear using a laser beam or cryopexy (freezing) and uses an inner eye tamponade. For short term inner tamponade, a special gas is used, which remains in the eye for several days or weeks, keeping the retina stable and enabling the tear to be glued; this gas gradually reabsorbs spontaneously. A long-term tamponade is achieved using silicone oil, which remains in the eye for several months and is later removed from the eye via another surgery - provided the retina is successfully healed. Vitrectomy lasts from thirty minutes to one hour or more, depending on the complexity of the case. After the surgery, the patient may be asked to minimize all body and head movements and to maintain a certain position (sometimes even including an uncomfortable face-down and gaze down position) for one, two, or even more weeks, to enable the retina to settle and glued back to the eyewall. The vision begins to slowly restore after the surgery. Through the gas bubble, the patient can only see the light and hand movement, but after the reabsorption of the gas, the patient regains vision – how much vision is dependant on the extent of detachment and the time gap before the surgery. Some eyes are necessary to operate on several times; the detached retina is always a difficult challenge and in ten to thirty per cent it can redetach, even being complicated by a scarring process. This scarring process makes the following surgery even more difficult. In some cases, alas, the scarring retina is resistant to all surgical attempts and only remnants of vision can be saved, using extremely complex surgical procedures.
Retinal blood vessels can bleed into the vitreous body under various pathologic conditions. The most common causes of vitreous bleeding apart from retinal tears, mentioned above, are damages of retinal vessels caused by diabetes or hypertension. These diseases cause narrowing, or even closure, of the retinal vessels, resulting in ischemia and lack of oxygen in the tissue. The retina tissue reacts in the attempt to renew the oxygen supply, producing a biochemical factor, stimulating new vessel growth. However, this is an erroneous correction on the body’s part; the new vessels do not grow in the retina to be able to supply the retina with oxygen, but they behave as enemies, like a weed in the garden, growing in front of the retina and causing bleeding into the vitreous body. This causes decrease of vision from dense floaters to dark fog and blindness. Vitreous bleeding, if it does not reabsorb within weeks, should be operated using vitrectomy similar to the vitrectomy in retinal detachment. The surgery lasts from twenty minutes to one hour or more, depending on the cause of bleeding and on the potential complications. The final vision is logically also dependent on the severity of the disease. It can return to hundred per cent but may also be very poor when the retina suffers from serious ischemia.
In some cases of the detachment of the vitreous gel from the retina, a thin layer of the vitreous is left on the surface of the macula. It may undergo a cell remodelling during which fibrous cells grow in this layer and they form a membrane, looking like a cellophane. This epimacular membrane can contract, i. e. shrink, causing a contraction and distortion of the retina, resulting in vision distortion. In some cases of the epimacular membrane, the patients suffer from blurred vision, in some cases they even notice wavy or curly lines and deformed images. Epimacular membranes can be removed during vitrectomy (see above). During vitrectomy lasting from ten to thirty minutes, the surgeon removes the vitreous gel, stains the membrane using a beautiful blue stain, and then he peels the membrane from the retinal surface. The healing of the macula after the procedure takes a long time; the cell reconstruction of the distorted macula takes six months or even one year, after which the picture improves; again, how much, depends on the severity of the case and the duration of the pathology. The surgery at least stops further worsening of vision.
In some cases of the detachment of the vitreous gel from the retina, the balloon does not detach completely but a small adhesion in the central area remains, causing pulling on the macula, resulting in blurred or distorted vision. The surgery of this vitreomacular traction syndrome is similar to the epimacular membrane: the adhesion is relieved and the remaining vitreous layer is peeled off the macula. Also the functional results of the surgery are similar.
In middle-aged patients, more frequently in women, a round hole in the centre of the macula can appear, causing loss of the central vision. The exact cause of this condition is not well known; physicians reckon that surface tension and traction of the vitreous on the surface of the macula play a role in this process. The hole is small in the beginning, e. g. 0.1 mm in diameter, causing only small defects in the image, such as not seeing a dot above “i”, however the hole usually grows up to 1 mm or more, causing inability to read at all. The treatment is again surgical: during vitrectomy, the surgeon peels a very tiny membrane from the retinal surface, often using it as a flap, or a patch, over the hole, and uses gas tamponade, enabling the hole to close within several days after the surgery. Vision starts to rehabilitate several weeks several after the surgery, gaining the maximum after six or twelve months, depending on the diameter and chronicity of the hole. Macular hole surgery is very successful: most patients gain back sixty to eighty per cent of the visual acuity.