Macular pathology of surgical interest
It is a thin and sensitive membrane with an extremely complex internal structure, composed of ten layers each with specialized functions. In the first layers there are light-sensitive cells, photoreceptors. There are light-sensitive photoreceptors and others that are sensitive to different colors. Each photoreceptor reads one point of the image (brightness, color or both) and generates electrical impulses that it transmits to the next layers, where they are encoded for transmission through the optic nerve. Photoreceptor distribution is not uniform. They have the highest concentration in the central part of the retina, called macula; as you move towards the periphery of the retina, the density of the photoreceptors decreases and these are more distant from each other. There is a precise reason for this: the macula is used for separate vision and reading, the periphery (the "tail of the eye") is more sensitive to moving things. In order to function and nourish itself, the retina must be adherent, namely attached at all points to the pigmented epithelium (which is the layer of cells immediately below). There is no "glue" at the base of the retinal adhesion, but a mechanism of aspiration by the pigmented epithelium, for which it is "swallowed up" towards the wall.
The vitreous (also called vitreous body, or vitreous gel) is a transparent, gelatinous tissue that occupies the inside of the eye, making up 2/3 of the entire volume. It is made up of 99% water and 1% collagen and hyaluronan fibrils which form the "scaffolding".
The network of collagen fibrils forms a solid structure that is supported by hydrophilic hyaluronate, creating the true vitreal morphology. Comparing the vitreous to a wet sponge, the fibrous component is the real sponge in whose interior spaces the watery part is trapped. The result is a compact and perfectly transparent gelatine.
With time, however, this balance tends to alter and thickenings and gaps can be formed, resulting in the loss of perfect transparency. Moving bodies can thus begin to appear in the field of vision, particularly visible in full brightness conditions and/or on a light background. Thanks to its high viscosity, the vitreous is intended to cushion the ocular trauma, protecting the surrounding eye tissues; its elasticity allows the anterior-posterior movements of the crystalline lens.
The vitreous detachment occurs due to a sudden breakage of the balance between the fibrous and aqueous components, therefore they separate from each other. If we take the sponge of the previous example, we wring it out and let it dry, this will shrink considerably. Likewise, the fibrous component of the glass vitreous, separating itself from the aqueous component, undergoes a marked reduction in volume. Since the volume of the eye does not change, the consequence of this phenomenon is the detachment of the vitreous fibrous component from the inner surface of the eye, namely from the retina.
The vitreous detachment usually begins at the top of the eye and continues downwards. At the end of the process, the vitreous (it would be more correct to say: the fibrous component) remains adherent to the retina only in the anterior part of the eye, where the vitreous-retinal connection is more strong. Although in most cases it starts without serious consequences, the detachment of the vitreous is a critical event for the eye, because it could produce retinal lesions: from the (harmless) rupture of a small capillary to the production of a laceration on the retina, the first reason for the detachment of the retina itself.
The symptoms of the detachment of the vitreous are usually rather impressive: one can suddenly notice numerous moving bodies accompanied by luminous sensations and flashes, the latter linked to the mechanical stress of the retina. It is difficult to overlook these symptoms: most people go to the emergency room or to an urgent specialist visit, and this is of the utmost importance because, as we will see, an initial retinal lesion can still be repaired at this stage without surgery.
EXTRAMACULAR RETINAL ALTERATIONS
It is important to point out that a retina break does not equate to the detachment of the retina, however it is often its precursor. A part from rare exceptions, the retina breaks are always caused by the vitreous and in most cases occur coinciding with the detachment of the vitreous; less often they accompany less important vitreous modifications.
There are various types of retinal fractures, which differ in shape, location and size. They range from retinal (round) holes, to horses hole breaks (from the characteristic U-shaped shape) to breakages or giant tears, which affect entire sectors of the retinal periphery. Symptoms of retinal breaks are usually confused with those of the detachment of the vitreous that caused it; only a specialist visit can resolve the doubt.
Hemovitreus consists of a blood spill inside the posterior portion of the eyeball, namely at the level of the vitreous. The bloodsheding of this jelly that fills the eye has important visual consequences.
The frequency of hemovitreus is proportional to that of the diseases associated with it, therefore the most frequent causes are represented by advanced diabetic retinopathy (31-54% of cases in the United States, depending on studies), retinal breaks (11-44%), traumas (12-19%), posterior vitreous detachment (3.7-11.7%) and macular degeneration (0.6-4.3%).
When a hemovitreus occurs, the associated disorders depend on the extent of the haemorrhage: in lighter cases there is a "smoky" vision; as the amount of blood increases one perceives moving bodies, black stripes, increasingly severe clouding until the uncertain perception of light.
The diagnosis is made following an ophthalmological examination with examination of the bottom of the eye; in less serious cases, it is possible to glimpse the retina and optic nerve and structures of vitreous, in the most serious cases, neither the ophthalmologist nor the patient can see anything, therefore it is necessary to resort to bulbar ultrasound that is the fundamental examination to understand the severity of the situation and allows to set therapeutic options.
THERAPY OF EXTRAMACULAR RETINAL ALTERATIONS
THERAPY OF RETINAL BREAKS
Early diagnosis is of great importance, since breaks without detachment are suitable for outpatient treatment with the laser, which can prevent surgery.
The principle of laser treatment of retinal break is that it forms a barrier around it, isolating it from the rest of the retina. By means of the laser, a series of close-up "spots" are produced. These spots are nothing but small burns of the retina, pigmented epithelium and the underlying choroid.
These spots are designed to draw a barrier of two, three and sometimes four rows around the retinal breaks. In the days following the treatment, these microscopic burns result in a healing process which is a permanent "glue" and a definitive barrier around the breaks.
In practice, the laser treatment of retinal breaks is an extremely simple ambulatory procedure: as in the ophthalmology examination, the patient is placed in front of the instrument, some drops are instilled for a surface anesthesia (more than enough for the purpose) and a special contact lens is applied to the eye. When the treatment begins, the patient perceives brief blue-green flashes (if the laser is in visible light), each of which corresponds to a spot on the retina. Normally this treatment is not painful.
PREVENTIVE THERAPY OF RETINAL DETACHMENT
The prevention of retinal detachment is based on the mitigation or elimination of risk factors. Degenerative areas of the retina can be surrounded by a laser barrier, in accordance with the same treatment used for simple retinal rupture: isolating the dangerous area. The suitability of this treatment is the subject of extensive debate, as it has possible collateral effects, and it is not infrequent for specialists to have differing opinions: the result is a confused and disoriented patient. The state of affairs can be summed up as follows:
- Not all degenerative areas are equally dangerous: some aren’t at all, others are dangerous proportionately to their extent, and some are dangerous in absolute terms;
- The potential danger of a degenerative area also depends on its context: high myopia, the presence of cataracts (which may require surgery), the presence of vitreous detachment, the age and lifestyle of the patient, and so forth;
- Laser treatment can also have negative effects, due mostly to the overheating of the vitreous humour;
- In other words, one has to weigh both the negative and positive effects, taking into consideration previous experience and the opinion of the operator. One must also keep in mind that many degenerative areas tend to scar spontaneously, which can help explain the disparity of opinions over time.
Many people ask whether the correction of myopia may help prevent retinal detachment. Unfortunately, the answer is no, since laser treatment (PRK or Lasik) corrects the curvature of the cornea and adapts it to a longer eye bulb, but it does not modify the internal conditions of the eye bulb. From this point of view, the removal or substitution of the crystalline lens that many specialists use to treat high myopia is very counterproductive: it is the equivalent of cataract surgery and it entails the same risk for the patient as those described above. A series of hygienic and behavioural norms can limit the action of general risk factors. People with a confirmed predisposition towards retinal detachment (such as potentially dangerous areas of retinal degeneration) should avoid exposure to high temperatures during heat waves, while increasing consumption of water and mineral salts, and should avoid head trauma and concussions: this is especially true for athletes (headers in football, diving).
SURGICAL THERAPY OF RETINAL DETACHMENT
There are basically two techniques for the surgical treatment of retinal detachment: the "ab-extero" technique, also called episcleral, and the "ab-interno" technique, called vitrectomy.
The term "episcleral" means "above the sclera", that is to say above that thick white membrane that constitutes the outer shell; it is the classic surgery for the detachment of the retina, developed (with numerous variants) from several decades but still current. The principle of this surgery is to resolve the detachment by pushing the wall of the eye towards the detached retina, in order to close the retinal breaks and neutralize the vitreal tractions. In practice, appropriate implants of biocompatible materials are sutured to the sclera in such a way that they are pushed towards the center of the eye, deforming its wall and causing a sort of rigid and permanent impression.
At the same time, the subretinal fluid is released outside through a small hole in the sclera itself. Once retinal adhesion is obtained, it is consolidated by the application of cryogenic treatments (freezing), which have the same meaning as laser applications: to cause burns (in this case burns caused by cold) that will stimulate the subsequent healing, namely welding.
The main advantages of episcleral surgery are represented by its simplicity and the minor trauma that it provokes to retina and intraocular structures in general, since it does not provide for entry into the eye bulb. For this reason, it still represents the technique of choice for the treatment of uncomplicated forms of retinal detachment. Among the disadvantages, there is the short-sightedness of the operated eye (on average 2-3 diopters) and the possibility that anomalies of ocular motility may develop, linked to silicone implants that can interfere with the normal activity of muscles responsible for eye movements. The rejection of implanted material is very rare.
The 1970s saw the progressive development of vitrectomy and endobulbare surgery techniques. The principle is, in this case, to enter the eye through tiny small holes with microsurgery instruments and to intervene directly on the retina and on the vitreous. There are pincers, scissors, spatulas, cannulae and optical fibers that can pass through a hole of 1 millimeter diameter. The term "vitrectomy" indicates the removal of the vitreous, which is in fact only one of the steps in this type of operation; however, its use is used to indicate the whole procedure. Vitrectomy successfully addresses complicated forms of retinal detachment: old retina detachments, forms accompanied by endo-ocular hemorrhagic effusion, forms accompanied by fibrous membranes on the retina (as in diabetic retinopathy) and macular pucker and so on. There are many cases in which it is associated with episcleral surgery. The main disadvantage is represented by the greater trauma for the inner structures of the eye; manipulated directly and dazzled, the retina - which is still a very delicate nervous tissue - often shows signs of post-operative suffering and starts to work more slowly. Vitrectomy can also promote the development of cataract. The increased complexity and the need for sophisticated equipment means that it can only be carried out in highly qualified centers. Let's sum up briefly in a nutshell: both “ab-externo” and “ab-interno” techniques have advantages and disadvantages. These are different and complementary procedures, each with its own indications.
The therapy depends on the cause and extent of hemorrhage. In the least serious cases, it can be expected, usually up to one month, that the vitreous will lighten spontaneously. If this is not the case, or if there are tractions that cause retinal lifting, surgical therapy, represented by vitrectomy with correction of triggering causes, must be used.
MACULAR RETINAL ALTERATIONS
The surgically treatable macular pathologies are mainly represented by those forms of alteration of the macular region caused by mechanical phenomena. At the level of this retinal region, membranes can be formed at the area in contact with the vitreous body. The vitreous body is delimited by an external film called vitreous cortical. Close to the macula there is a region in which, due to specific anatomical peculiarities, the formation of fine abnormal membranes is possible, sometimes due to residues of the vitreous cortex, sometimes from the retina itself. These membranes, called "epiretinal" membranes, contract relations with the macular retina, adhering to the inner retina. If the cells composing the membranes are not contractile in nature, or they do not tend to exert traction on the contact points, visual disturbances caused by epiretinal membranes are moderate.
The disturbances in these cases are generally limited to distortion of the images, due to a modest wrinkling of the macular region subject to traction by the membrane (Figures 2,3). If, on the other hand, the tractive phenomena are more marked (fig. 4), the retinal tissue enters into suffering due to the strong deformation caused by the membrane and the visual lowering becomes more marked, resulting in serious reductions of vision if surgery is not carried out.
Pathologies associated with macular epiretinal membranes
The main macular pathologies related to the presence of epiretinal membranes are the main macular pathologies:
the macular pucker (that means: "wrinkle") (figures 2,3,4,5), characterized by the presence of an epiretinal membrane attached to the macular region and contracted (figures), therefore associated with distortion of the images and, in the most serious cases, the formation of central stains of non-vision (scotoma).
The surgical intervention can lead to a resolution, not always total, of the visual disturbances, and above all to a reduction of the image deformation.
The vitreous-macular traction (figs. 6,7,8), in which the vitreous cortex remains attached to the central part of the macula, called fovea. The resulting traction on such a small area can cause considerable visual reduction.
The macular hole, characterized by the formation of an opening in the central portion of the macular region, which mainly in eyes with elevated myopia may be associated with a posterior retinal detachment.
Epiretinal membranes as a disease were first described by Iwanoff in 1865. They can be associated with other ocular problems, such as posterior vitreous detachment, retinal breaks, retinal detachment, inflammatory and vascular eye diseases and vitreous hemorrhage. In most cases, however, they can also occur without any association with other diseases, in which case they are called idiopathic membranes. These membranes are a rather frequent disease because from large-scale studies they were previously present in 7% of the population, with increasing frequency as age rises, but with the arrival of new diagnostic techniques (optical consistency tomography, OCT) in a 2016 study it was found out that the prevalence in the population can reach 34.1%.
The disorders caused by the disease depend on the characteristics of the membranes, such as thickness, position and contractile capacity, as mentioned above. Initially they cause little disturbes; when they increase, they cause vague disorders which the patient can’t describe. The most frequent disorder is image distortion, followed by visual fogging and reduced visual acuity. In a 2016 study it was observed that in 5 years 29% of epiretinal membranes progress, 26% regress and 39% remain stable.
A key role in the treatment of these diseases affecting the macular region is played by Optical Consistency Tomography (OCT), a new method of visualization of the macular region developed in the mid-1990s. This technique uses a light beam with low optical coherence, not monochromatic, that "brushes" the macular region with a series of several parallel lines; the reflected light is picked up by a detector that analyzes it and establishes through a computer the optical density of each retinal point, reconstructing a three-dimensional map of the tissue area analyzed. The result are high-resolution 3D images of the macular region and the vitreous in front of it, which make it possible to visualize the shape of the membranes, the relationships they have with the retina and the damage they are causing. OCT technique has considerably evolved over the years: initially a technology called "Time Domain" was used, namely the analysis of the signal was carried out in the time domain, with a mirror that moved to follow the depth of the different points analyzed. Afterwards another method called "Spectral Domain" was developed, in which there are no moving parts and therefore the accuracy of the analysis is increased. The last and most accurate system is called "Swept Source", in which the precision is increased by the fact that the light source consists of a modulated laser, with a wavelength in the infrared field, which not only allows an extreme detail production, but also a deeper penetration in the tissues, so that even the deepest structures are displayed.
Diabetic retinopathy is a subtle and insidious disease caused by circulatory impairment caused by diabetes itself, followed by reduced oxygenation of the tissues.
Non-proliferative diabetic retinopathy occurs with small hemorrhages, deposits and other anomalies of the retinal circulation.
Fluoroangiography is fundamental at this stage, which allows to visualize in detail these anomalies, that must be treated with the laser.
In the most advanced phases, diabetic retinopathy becomes proliferative. This term means that membranes with abnormal vessels are formed within the eye.
The membranes arise from the retina and optic nerve and increase as climbing plants, causing retinal detachments and massive hemorrhages.
TREATMENT OF MACULAR DISEASES
The most frequently used surgical technique is vitrectomy with removal of epiretinal membranes. In these cases, this is a complex operation because it involves an action on extremely delicate ocular structures. It should be considered that the entire macular region is 1.5 mm large and consists of a tissue with high-density nerve cells with a critical disposition for the maintenance of visual acuity, so it should be done only after a careful morphological and functional evaluation of the patient to determine whether the benefit/risk is favorable. Vitrectomy surgery has also proved to be very efficient in the treatment of the most serious cases of retinopathy occurring in diabetic patients. Diabetes, in fact, in the most advanced cases, which nowadays, thanks to the improvement of preventive treatments, have become increasingly rare, may hesitate in intraocular hemorrhages and in retinal detachments with tractions even at the macular level. The vitrectomy surgery, by removing the blood-filled vitreous and the tractions that this exerts on the retina, allows the resolution of cases that in the past led to blindness.
In the case of tractions localized on the macula, the solution, as well as the surgical one, in which the vitreous-macular traction and the vitreous are removed, can be easier thanks to the introduction of new therapies that, in selected cases, solve the traction on the retina. Enzymes are used which, injected from the ocular globe, are able to "digest" the adhesion point, separating the vitreous cortex from the retina and resolving the traction without having to resort to more invasive surgical procedures.
Figure 3. Normal situation (right eye). Map in false colors that highlights the thickness of the macular retina; warm colors indicate a greater thickness, cold ones a lower thickness. Scan with Swept-Source OCT. The normal profile of the macular region (right) and the terminal portion of the optic nerve (left) can be observed. It is also evident the profile of the main blood vessels that depart from the optical nerve and surround part of the macula.