The vitreous humour
The vitreous humour is a transparent gel that fills the interior of the eye. It should not be confused with the crystalline lens: the vitreous is a compact gel (at least in young subjects), whose consistency is comparable to that of egg whites. Without entering into a complex discussion of biochemistry, we can consider the vitreous humour to be the outcome of the perfect balance between two components: a fibrous component and an aqueous component. If we like the vitreous humour to a wet sponge, the fibrous component is the actual sponge, which traps the aqueous component. The result is a compact, perfectly transparent gel. However, over time this balance tends to alter, leading to areas of clumping or liquefaction, resulting in the loss of perfect transparency. This can cause the appearance of floaters in the visual field, which are particularly apparent in bright light and/or against a pale background. These are minor and harmless symptoms, quite unlike the symptoms of vitreous detachment, which we will discus later on.
The retina is a thin, light-sensitive membrane with an extremely complex internal structure comprising ten layers, each with specialized functions. In the accompanying illustration it is depicted in orange. The upper layers host light-sensitive cells, the photoreceptors. Some photoreceptors are sensitive to light, while others are sensitive to various colours. Each photoreceptor reads one point of the image (brightness, colour, or both) and generates electrical impulses which it transmits to the successive layers, where the impulses are codified for transmission through the optical nerve. The distribution of photoreceptors is not uniform. The highest concentration is in the central part of the retina, known as the macula; the density of photoreceptors decreases gradually towards the periphery, where the distance between them is greater. There is a precise reason for this: the macular is used for sharp vision and reading, while the periphery (the “tail of the eye”) is more sensitive to movement. In order to function and nourish itself, the retina must be fully adherent to the pigmented epithelium (the layer of cells immediately underneath it). The retina does not adhere to the pigmented epithelium by a “glue”; instead, it is a suction mechanism on the part of the pigmented epithelium that keeps the retina attached to it.
Macular pathologies of surgical interest
We have already described the macula: it is the noblest part of the retina, which allows us to perceive detail in the images we see. As such, macular pathologies have an immediate negative effect on vision. There are essentially two macular pathologies of surgical interest: macular holes and macular pucker. A macular hole is, as the term indicates, a small perforation in the central retina; it can be followed by retinal detachment, in particular in eyes affected by high myopia. There are multiple causes of this pathology, and not all are perfectly understood; in all likelihood, the traction on the macula on the part of vitreous fibres comes into play. For this reason, many cases can be treated with a vitrectomy, which in this case aims to eliminate these microscopic tractions and stabilize the retina; nevertheless, the recovery of vision is often disappointing. A macular pucker indicates the formation of a translucid membrane on the macula which contracts and curls up, causing the deformation of the underlying retina. The membrane is made up of a layer of condensed vitreous humour and can be removed surgically with a vitrectomy: in this case, results are generally satisfactory, as is functional recovery.
Diabetic retinopathy is an insidious disease caused by compromised circulation brought about by diabetes, leading to the reduced oxygenation of tissues. Non-proliferating diabetic retinopathy manifests itself with small haemorrhages, deposits of fluid known as exudates, and other anomalies in retinal circulation. In this phase, fluoroangiography plays a vital role, since it allows for these anomalies, which must be treated with argon laser, to be viewed in detail. In more advanced phases, diabetic retinopathy becomes proliferating. This means than membranes containing anomalous vessels appear inside the eye. The membranes originate from the retina and the optic nerve and they spread like vines, causing retinal detachment and massive haemorrhages. The only possible remedy is vitrectomy, with the goal of removing the membranes and re-attaching the retina; nevertheless, results are often disappointing, because once retinopathy becomes proliferating the situation is highly compromised, and visual recovery extremely limited.
Vitreous detachment takes place after a sudden shift in the equilibrium between the fibrous and aqueous components, causing the two to separate. Going back to our example of the sponge, if we squeeze the water out of it and let it dry, it will shrink considerably. By the same token, by separating from the aqueous component, the fibrous component of the vitreous humour will shrink significantly. Since the volume of the eye remains the same, the consequence of this phenomenon is the detachment of the fibrous component of the vitreous humour from the interior surface of the eye, namely the retina. Vitreous detachment generally begins in the upper portion of the eye and proceeds downwards. At the end of the process, the vitreous humour (more precisely, its fibrous component) remains attached to the retina only in the lower part of the eye, where the vitreous-retinal connection is stronger. Although most cases of vitreous detachment have no serious consequences, it is nevertheless a critical event for the eye, since it could lead to retinal lesions: from the rupture of a minor capillary vessel (harmless) to a retinal laceration, which could eventually lead to retinal detachment. The symptoms of vitreous detachment are generally quite obvious: patients suddenly notice a great increase in the number floaters accompanied by flashes of light, the latter being tied to the mechanical stimulation of the retina. These symptoms are hard to ignore, and most people visit the emergency room for an urgent ophthalmological visit. This is extremely important since, as we shall see, a retinal lesion at this early stage can still be repaired without recourse to surgery.
It is important to specify that retinal rupture is not the same thing as retinal detachment, although the former is often the precursor to the latter. Except for a few rare exceptions, retinal rupture is caused by the vitreous humour, and in most cases it takes place concurrently with vitreous detachment. Less often, retinal rupture accompanies less significant changes in the vitreous humour.
There are various types of retinal rupture, which vary in terms of shape, location, and size. They range from retinal holes (round) to horseshoe (U-shaped) ruptures and giant ruptures or lacerations, which affect entire sectors of the retinal periphery. The symptoms of retinal rupture are generally difficult to distinguish from those of the vitreous detachment that provoked it; only a specialized exam can remove all doubts. Early diagnosis is extremely important, since retinal rupture without detachment can be treated on an outpatient basis with lasers, thus avoiding surgery.
The basic principle behind laser treatment of a retinal rupture is the erection of a barrier around the rupture, isolating it from the rest of the retina. The laser produces a series of closely-spaced spots, which spots are small burns in the retina, the pigmented epithelium, and the choroid underneath. The spots are produced in such a way as to erect a barrier of two, three, and sometimes four rows around the retinal rupture. On the days following treatment, these microscopic burns create a scar, which serves as a permanent “glue” and establishes a definitive barrier around the rupture. In practical terms, laser treatment of retinal ruptures is an extremely simple inpatient procedure. Much like in an ophthalmological visit, the patient is placed in front of the laser instruments, eye drops are administered for superficial anesthesia (which is more than sufficient for this purpose) and a special contact lens is placed on the eye. When treatment begins, the patient perceives brief flashes of blue-green light, each of which corresponds to one spot on the retina. Treatment is generally not painful.
Retinal detachment takes place when, after one or more retinal ruptures, a certain quantity of vitreous fluid works its way underneath the retina and begins to detach it. A retinal detachment can remain limited to a small area, but it generally extends to increasingly large portions of the retina. At this point a dark curtain appears over the field of vision. If detachment continues until it affects the macula, there is a sudden loss of vision. The speed of progression of retinal detachment is very variable: in some cases complete detachment occurs after a few hours, while in other cases detachment remains limited to a relatively small area. This is influenced by numerous factors, one of the most important of which is rest: for this reason, retinal detachment patients must stay in bed before surgery. Laser treatment is not applicable to retinal detachment: any “glue” requires that the parts to be attached be perfectly adherent, and this is not the case here. Unfortunately, retinal detachment requires surgery. There are many reasons why surgery for retinal detachment is rather urgent. First of all, if left untreated retinal detachment tends to extent, requiring more complex treatment; secondly, the detached retina gradually loses vitality. Furthermore, the detached retina tends to stiffen and curl up, making surgery more difficult and reducing the likelihood of success.
There are a series of risk factors than can increase the likelihood of retinal detachment. Local risk factors (those related to eye) include so-called “peripheral degenerative areas”, areas of localized anomalies that crop up spontaneously in one or more peripheral sectors of the retina. Myopia, especially high myopia, is a condition in which the eye bulb appears elongated (as opposed to spherical) and comprises various risk factors: a thinner retina, vitreous alterations, and the more frequent occurrence of degenerative areas. Also among local factors, it is well known that cataract surgery, however well performed, significantly increases the risk of retinal detachment, especially in young subjects. The higher incidence of retinal detachment in summer suggests that heat, by favouring dehydration, can facilitate or precipitate vitreous detachment. Direct and indirect trauma can also play a significant role. The former includes direct trauma to the eye bulb (such as a punch); the latter includes concussions (such as a head butt) and counterstrokes in general (traffic accidents are classic example). These cause the vitreous fluid inside the bulb to move, causing traction on the retina in correspondence with the area of the trauma.
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).
There are two basic options for the surgical treatment of retinal pathologies: the ab-externo option, also known as episcleral surgery, and the ab-interno option, also known as vitrectomy.
The term “episcleral” means “above the sclera”, the thick white membrane that comprises the exterior envelope of the eye. Episcleral surgery is the classic treatment for retinal detachment; it was developed (with numerous variants) several decades ago but it remains current. The principle behind this surgery is to treat the detachment by pushing the wall of the eye towards the detached retina, in order to close retinal ruptures and neutralize vitreal tractions. In practical terms, implants made with biocompatible materials are sutured to the sclera in such a way as to be pushed towards the centre of the eye, thus deforming its wall and producing a sort of rigid, permanent print. At the same time, subretinal fluid is pushed out through a tiny hole in the sclera. Once retinal adherence is obtained, it is consolidated through cryogenic treatment (freezing), which has the same function as laser treatments: provoking burns (in this case, freezer burns) that will stimulate scarring, thus completing the suturing process.
The main advantages of episcleral surgery are its simplicity and the reduced trauma it inflicts on the retina and, more generally, on the eye’s structure, since it does not invade the eye bulb. For this reason, it remains the preferred technique for the treatment of non-complicated forms of retinal detachment. Disadvantages include the myopization of the operated eye (2-3 dioptres on average) and the possibility of anomalies in ocular motility due to the use of silicone implants, which can interfere with the normal activity of the muscles responsible for eye movements. Rejection of the implanted material is very rare.
The 1970s saw the gradual development of vitrectomy and endobulbar surgical techniques. In this case, the technique involves entering the eye with microsurgical instruments through tiny holes, and to operate directly on the retina and the vitreous humour. There are tweezers, scissors, spatulas, pipers and optic fibres that can fit through a 1 mm hole. The term “vitrectomy” indicates the removal of the vitreous humour, which is actually only one of the steps in these types of operations; nevertheless, it is used to describe the entire procedure. Vitrectomy makes it possible to successfully treat complex forms of retinal detachment: retinal detachments that have been left untreated for some time, retinal detachment accompanied by endo-ocular haemorrhages or by fibrous membranes on the retina (such as in diabetic retinopathy) or macular pucker and so forth. In many cases it is performed along with episcleral surgery.
The main disadvantage is the higher amount of trauma it places on the eye’s internal structures. Since the retina is handled directly and exposed to strong light, and in light of the fact that it is a very delicate nervous tissue, it often shows signs of post-surgical stress and resumes its functions more slowly. Additionally, vitrectomy can favour the development of cataracts. The greater complexity of this procedure and the need for sophisticated equipment mean that it can only be performed in highly qualified centres. To summarize briefly: both ab-externo and ab-interno techniques have advantages and disadvantages. They are different, complementary techniques, each with its own indications.