Situated between the lens and the retina of the eye is a jelly-like substance known as the vitreous humour. Consisting of dissolved salts, sugars and proteins (mainly collagen Type II), the vitreous humour together with the more fluid aqueous humour which is situated in front of the lens, provide a clear medium through which light can pass to produce an image on the retina. They also exert an outward pressure which maintains the shape of the eye.
Although the vitreous humour is in contact with the retina and helps to keep it in place by pressing it against the choroid - the layer of tissue lining the inside rear of the eye - it only adheres to the retina in three places: around the anterior border of the retina; at the fovea, the tiny spot in the retina which provides us with our 'detail' of colour and vision; and at the optic nerve exit (or 'blind spot') where the eye is connected to the brain by about 1.2 million nerve fibres (axons).
Unlike the fluid in the aqueous humour, which is continuously replenished, the vitreous humour is stagnant. Therefore, if blood, cells or other debris gets into the vitreous humour, they will remain there unless removed surgically. When light hits these pieces of tissue, they cast shadows onto the retina which appear to float across our field of vision, and hence are known as 'floaters'.
As we age the vitreous humour gradually becomes more fluid and, as it does so, the vitreous mass gradually shrinks and falls away from the retina. This is known as 'posterior vitreous detachment' (PVD) and commonly occurs after the age of 40. Indeed, it is often said that the percent chance of this occurring is the same as one's age in years.
Commonly, a person experiencing PVD will report seeing flashing lights and/or an increase in the number of floaters in his field of vision. The flashes of light occur as the vitreous humour separates from the sensory layer of the retina. The mechanical force of the vitreous pulling on the retina is transformed into nerve signals, which are then interpreted as light by the brain.
In short-sighted (myopic) people, where the shape of the eyeball is elongated from front to back, thus putting more strain on the vitreous,
a PVD may occur at a younger age than normal.
A PVD can also occur after injuries to the eye or inflammation in the eye (uveitis) and is also more likely to occur following a cataract operation.
In most cases, the vitreous humour makes a clean separation as it pulls away from the retina. Occasionally, however, the vitreous humour will cause a tear in the retina at those places where it is adhering. This will enable fluid to seep between the retina and the choroid thus causing a retinal detachment. This may be accompanied by vitreous haemmorrhage (bleed) and is a very serious, sight-threatening condition.
The symptoms that may indicate a retinal detachment include:
- Sudden (slight) impairment of vision along with light flashes and increase in the number of floaters.
- A veil or curtain, that crosses the eye from any direction, that obstructs all or part of your field of vision1.
Usually, no treatment is required for uncomplicated PVD (floaters) since these are relatively 'massive' and, over time, will tend to sink below the line of sight and so become invisible. This happened to the present author as he first experienced floaters some 4 years before the retinal detachment. In the intervening period he was not aware of any floaters. In some cases the floaters may become visible after rapid eye movements or after lying flat for some time, but they generally aren't a problem.
When the floaters do seriously affect one's sight2, or in cases of retinal detachment, then it becomes necessary to surgically remove the vitreous humour and to use pressure of an injected fluid to push the retina back against the choroid. This operation is known as a 'vitrectomy'.
On the night before the operation this author was required to instil one drop of cyclopentolate ('dilating drops') to keep the pupil of the operated eye large and, on admission to the hospital, a nurse regularly applied such drops until the time of the operation.
It should be noted that cyclopentolate eye drops can cause blurred vision which may last several hours, and so one is advised not to drive or operate machinery unless the vision is clear.
How is the Vitreous Jelly removed?
Nowadays, surgery is usually performed under a local anaesthetic although the surgeon may opt to use a general anaesthetic if he deems it preferable or necessary.
The patient is permitted to wear his own clothes and lies on an articulated table. A cloth is used to cover both eyes, although there is a hole over the eye destined for surgery.
Three very small incisions are made through the sclera - the white of the eye to accomodate a fibre-optic light source, a cutting/suction device to cut through and removes the vitreous humour, and an infusion port to facilitate introduction of a fluid to maintain pressure within the eye, both during the procedure and after the operation.
Laser surgery or cryotherapy (freezing) is then often used to seal retinal tears and prevent retinal detachment. *(This author received cryotherapy).
As only a local anaesthetic is used, under the bright lights of the operating theatre, it is possible to see approaching instruments through the face-cloth. This author found it preferable to focus at a distance so as not to see the instruments!
Once the vitreous humour has been removed the retina is repaired if necessary, any foreign bodies removed and, in the case of diabetic patients, any leaking blood vessels are sealed. The procedure usually takes 1-2 hours to perform, although in this author's case, the procedure took 45 minutes.
Since the local anaesthesic causes numbing of the lids and temporarily prevents blinking, an eye patch is applied immediately after the surgery. It is most important to keep this eye patch on until you are able to blink the eye normally. Begin using drops after the patch has been removed.
The Fluid Insertions
The vitreous humour is replaced with saline to maintain the pressure and then an air or gas3 bubble or silicone oil bubble is inserted so that it pushes against the reattached segment of retina, holding it in place while scarring and healing occur. If an air or gas bubble is inserted, this is absorbed and replaced naturally with isotonic fluid (aqueous humour) over about 2 weeks. If silicone oil used, this is not absorbed and has to be removed surgically at a future date.
Silicone Oil Insertion
Silicone oil is a clear, viscous fluid, which may be used instead of gas. Its main advantages are:
- Quicker visual recovery.
- No restriction in air travel.
- Less need for head positioning post operation.
- Longer duration of effect.
The main disadvantage, however, is that it is not absorbed and therefore has to be removed surgically. Another disadvantage is that it can be difficult to establish a stable spectacle correction for patients with silicon oil.
Risks of Vitrectomy
This procedure has a high success rate but potential complications include post-operative bleeding, serious infection4, retinal detachment and increased risk of cataracts.
A major factor determining the success of a vitrectomy operation is ones ability to comply with post-operative instructions/recommendations. This will be determined by the location of the retinal tear.
In this author's case, it was the right eye that was affected and there was a retinal tear close to the nose. Thus immediately after the operation, this author was wheeled from the operating theatre in a wheel chair, having been instructed to sit with his head down between his knees. He was then provided with a bed in a side room where he had to lie face down for 3 hours.(The provision of a radio in the room was a God-send!). On arriving home, this author was instructed to sit with his head in an upright position for 50 minutes in each hour; the remaining ten minutes being a break to walk around/sit normally. This was suggested as being the ideal time to get oneself a drink or a meal. At night, the author was instructed to sleep with his right cheek to the pillow. As this author had had his eye filled with air, this position ensured that the area of the retinal tear was kept dry thus facilitating healing.
This author was prescribed a combination of three types of drops to be used regularly after discharge. These had to be administered in the order shown with a ten minute spacing between each type of drop. These included:
- Chloramphenicol5 (antibiotic). This had to be stored in the fridge and one drop had to be instilled 4 times per day for 7 days.
- Dexamethasone (Trade Name 'Maxidex). This is a corticosteroid, used for reducing inflammation caused by irritation. Irritation causes the release of substances (cytokines6) that are important in the immune system. Such substances cause blood vessels to dilate, resulting in the affected area becoming red, swollen, itchy and painful. Dexamethasone and other such corticosteroids work by causing affected cells to decrease their release of cytokines, thereby reducing inflammation. One drop had to be instilled 4 times a day for 28 days
- Cyclopentolate, which, in addition to being a dilatant, acts as an anti-inflammatory agent. One drop had to be instilled twice daily for 7 days.
This author was given a follow-up appointment in 4 days, and was 'signed off' work for 28 days.
He was informed that he would be unable to see anything with the affected eye on the first day following the operation, and that sight would gradually improve day-by-day after that. Optimal vision may not be achieved for several months. This, indeed, is what was expoerienced. On Day 2, this author could see white light, on Day 3, colours could be discerned and by Day 4 shapes of objects could be discerned. Interestingly, as the eye re-filled with fluid, the meniscus could be seen as a black line, which was inverted due to the way in which the eye works. (Although in practice, the air bubble is above the meniscus, it appears to be below the meniscus). An emerald green spot was visible in the centre of the meniscus, with a purple band above and below. As the air bubble was absorbed, so the meniscus gradually appeared lower and lower in the eye, the vision being clearer through the fluid above the line. As the fluid within the partially-filled eyeball is very mobile, a most peculiar effect is that this sloshes around as the head is moved. This heightens ones awareness of the condition, thus providing a reminder of the need to keep ones head reasonably still during the healing process.
At the follow-up appointment, the chloramphenicol and cyclopentolate eyedrops were withdrawn, and replaced with 'Cosopt', a preparation containing two active ingredients designed to reduce pressure within the eye, by inhibiting production of aqueous humour. This had to be administered ten minutes after the Maxidex, but only twice a day.
Patients who have had an air or gas bubble inserted are advised to avoid flying, as the reduced atmospheric pressure causes the gas bubble to expand, thus raising the pressure in the eye to perhaps dangerous levels. Furthermore, the increased intraocular pressure caused by flying with an air bubble can prevent blood from reaching the retina and thus blind the recovering patient.
2Given the risks inherent in any form of surgery, the impairment from floaters would have to be fairly severe to consider victrectomy.
3The gases used are chemically unreactive and biologically compatible. Typically, either sulphur hexafluoride or perfluoropropane are used. A device known as a 'retinal tamponoid' is employed to hold the retina in place or temporarily seal off holes in the retina.
4Eye infections are sight-threatening and can spread to the brain.
5Chloamphenicol is a 'broad spectrum' antibiotic, meaning that it is effective against a wide variety of microorganisms. It was the first antibiotic to be manufactured synthetically on a large scale. Its main use is in eye drops or ointment to combat bacterial conjunctivitis.
6Cytokines are groups of at least 20 proteins, including interferons and interleukins. They are the hormones of the immune system, mediating interactions between immune cells, and having pathological as well as protective actions on infectious diseases.