Risks Of Cataract Surgery

Risks In Detail

In the following section the risks are discussed more fully.

Although it is essential that each patient should have a satisfactory understanding of the risks involved in having cataract surgery it is important to keep things in proportion. The likelihood of suffering a vision threatening complication is very small.

The vast majority of people who have cataract surgery are delighted with the results. There is though always a small risk in having a surgical operation. This is true for cataract as for any other form of surgery. The possibility of experiencing a complication that results in worse vision is remote. The possibility of losing vision in the eye completely is extremely remote but it can happen. The risk is very small but it is not zero. It is rather like crossing the road or driving a car. Even with the greatest care mishaps can happen and it may be no ones fault. It is particularly important to think through the risks associated with surgery if the patient has only one useful eye.

Before having a cataract operation each patient should be made aware of the risks involved. The surgeon has a legal responsibility to do this. This is not in any way to alarm the patient or to put him or her off having the operation. Each patient should be sensibly informed of the nature of the procedure and the risks involved. This is important so that when the patient signs the consent form and agrees to have the operation they do so with an adequate understanding of the issues involved.

The main risks associated with a cataract operation are:

• Infection
• Disturbance to the retina
• Clouding of the cornea
• Haemorrhage
• Technical problems with the surgery
• Risks related to the anaesthetic

Sometimes as a consequence of one of these complications or in order to treat a complication a further operation on the eye may be necessary. These issues will be considered in turn.

Infection

In order to remove a cataract a small opening must be made in the eye. This may allow bacteria to get in side the eye. It is thought that the usual source of such bacteria is the surface of the eye itself. It is common practice therefore to wash the surface of the eye and eyelids with an antiseptic solution at the very start of the operation before the first incision is made. It may be necessary to cancel the operation if there is any evidence of infection on the surface of the eye e.g. a conjunctivitis or sticky eye.

Obviously all the instruments and solutions used during surgery are sterile. In an attempt to further prevent infections antibiotics may used before, during and after the operation.

If an infection develops within the eye after a cataract operation this is called endophthalmitis. It is rare. The incidence should for any eye unit be less than one case in every 500 operations. When it happens endophthalmitis usually occurs within the first few days following the operation and the eye becomes red and painful with dramatic loss of vision. However sometimes the condition occurs later or with few symptoms. The severity of the infection will depend on the virulence (nastiness) of the bacteria causing it. About half of cases are caused by relatively mild bacteria, although any infection within the eye is serious. If these are treated properly reasonable vision may be regained. The other half of cases involve more damaging bacteria and these may suffer profound loss of vision even with aggressive treatment.

Retinal Disturbance

The retina may be disturbed in two main ways following cataract surgery. These are retinal detachment and cystoid macular oedema. These phenomena may occur even following entirely straight forward surgery but are more likely if there have been other technical problems during the operation.

Retinal detachment: The retina is the vital seeing layer at the back of the eye. It is like the film in a camera. In a camera the image to be photographed is focused onto the film. In a similar way the visual scene is focused by the eye onto the retina. A retinal detachment is where the retina peels away from the back of the eye like wallpaper peeling from a wall. A retinal detachment is a serious threat to vision.

The reason a retinal detachment may occur following surgery is as follows.

The main cavity within the eye is filled with a jelly like substance called the vitreous. The outer surface of the vitreous is in contact with the retina. The lens of the eye sits immediately in front of the vitreous.

During a cataract operation the lens is removed from the eye. Even though the natural lens is replaced by an artificial lens there is more space within the eye after the operation. This extra room may allow the vitreous to move forwards a little. If there are any abnormal adhesions between the back surface of the vitreous jelly and the retina the vitreous may pull on the retina and tear it. Fluid may then seep through the retinal tear and lift off the retina. A retinal tear does not always progress to a retinal detachment. If only a retinal tear has occurred then this may be successfully treated with laser. Laser burns are placed around the tear to seal it off and bond down the retina around it. If a retinal detachment does occur this will need specialist retinal surgery. Eyes that are very short sighted before surgery have a somewhat higher risk of retinal detachment.

Cystoid Macular Oedema

The macula is the central area of the retina and gives us our central zone of vision. Swelling of any tissue within the body caused by the accumulation of water is called oedema.

The very centre of the macula, called the fovea, has a special structure and is designed this way in order to give us our very detailed central vision. This is the part of vision we use when we look straight at something and the part you are using as you read this. The fovea is particularly thin and delicate. Many disturbances to the eye, including cataract surgery, can cause swelling of the fovea. Tiny amounts of fluid gather within the layers of the retina. Under the microscope this fluid looks like little droplets or cysts within the tissue; hence the name Cystoid Macular Oedema.

Cystoid Macular Oedema is actually quite common following surgery perhaps occurring in about 1 in 4 cases. However it is usually very mild and can only be detected by special photographs of the retina. In this form it does not affect vision and soon goes away without extra treatment. The patient will not even be aware that it happened. Occasionally though it can be more marked. It will then cause the very centre of vision to be reduced. The peripheral or side vision is not affected but it will be more difficult to read the eye examination letter chart. The patient will notice difficulty in reading print and recognising faces.

Cystoid Macular Oedema is difficult to treat.

There is no one treatment that has been proved to work in every patient. Fortunately about half of cases get better on their own with the normal eye drops that are used after a cataract operation. For the other half treatments that may help include steroids either as eye drops, injections around or into the eye or as tablets, aspirin like drugs either as eye drops or as tablets, and a drug called Diamox which is taken as a tablet.

Clouding of the Cornea (Corneal Oedema)

The cornea is the window at the front of the eye. It should be clear and transparent. Sometimes following cataract surgery it may become hazy and cloudy. This is usually temporary and clears within a few days. Occasionally it can persist and be permanent.

On the inner surface of the cornea is a single layer of cells called the endothelium. The job of the endothelium is to keep the cornea transparent. It does this by controlling the water content of the cornea. Water from the front chamber (called the anterior chamber) of the eye is constantly seeping into the cornea. If too much water accumulates in the cornea it will become hazy. The effect on vision is then like spectacles steaming up. The endothelium continually pumps water out of the cornea back into the anterior chamber.

The endothelial cells of the cornea do not replace themselves. We are born with a certain number, about 4,000 per sq mm. As we age the number of cells gradually declines. If the number of endothelial cells fall below a critical level, perhaps 800 - 1,000 per sq mm, then there are too few to keep the cornea clear. Water then swells the cornea and it becomes hazy. This is called corneal oedema. The cornea is said to have decompensated. That is it can no longer control its water content to a normal state.

During cataract surgery a significant volume of fluid is passed through the anterior chamber of the eye. This helps to irrigate the natural lens material out of the eye and cools the tiny instrument that is placed into the eye. Some of this fluid will inevitably soak into the cornea and may cause it to swell a little and loose transparency. However if the corneal endothelium remains healthy this fluid will be pumped away and the cornea will clear in the hours or days following the operation.

Cataract surgery, no matter how gently performed, will deplete the number of corneal endothelial cells. If following surgery the number of endothelial cells falls below the critical level required for corneal health then the cornea will become permanently hazy. If this significantly impairs vision then a corneal graft (transplant) may be required; i.e. a new cornea. This is not a quick fix. Vision following a corneal graft takes a minimum of a year to settle. This is mainly because the very fine sutures that hold the corneal graft in place must remain for this long and they tend to distort the focus of the eye.

Sometimes the examination of the eye before surgery reveals features that suggest that the cornea is at risk of becoming permanently hazy as a result of the surgery. If this risk is felt to be high then surgery should not be undertaken.

Haemorrhage

Haemorrhage means bleeding. Bleeding during cataract surgery is usually minor and there may be none at all.

Bruising is the result of minor bleeding within the tissues of the body. The commonest example is a bruise of the skin. This occurs when a little blood escapes from a small blood vessel within the skin, as may occur after a bump.

Some bruising of the eyelids is very common when local anaesthetic is injected to numb the eye before the operation. The local anaesthetic is injected just next to the eyeball. This bruising is usually minor and will disappear after a week or two with no long term effects. Sometimes there may be bruising on the surface of the eye itself. This can look quite alarming as the blood is seen against the white of the eye and makes the eye look very red. This bruise is called a subconjunctival haemorrhage. All that this name means is that the bruise is located beneath the conjunctiva which is the loose skin on the surface of the white of the eye. Again this will clear with no adverse effects.

Bleeding can occur within the eye. The most come source is the iris. This may be accidentally nicked by the instruments during the operation especially if it has not been possible to fully dilate the pupil. Also it is sometimes necessary to make a small cut or cuts within the iris or to remove part of it. Blood can also trickle into the eye from the incision.

Tiny amounts of blood within the eye can have a profound effect upon vision. This is usually temporary and vision returns as the blood is cleared by the normal fluid turn over within the eye. If necessary blood can be removed by a further operation.

Blood within the anterior chamber of the eye is called hyphaema. Blood within the jelly that fills the main cavity of the eye is called vitreous haemorrhage.

Two forms of haemorrhage are more serious though both are rare. They are a retrobulbar haemorrhage and an expulsive haemorrhage.

A retrobulbar haemorrhage is a haemorrhage that occurs within the orbit (the eye socket) behind the eye. It is an occasional complication of a sharp needle technique of local anaesthetic injection. Patients with blood clotting disorders or those taking Warfarin may be at higher risk of retrobulbar haemorrhage.

In severe cases an enlarging pool or clot of blood forms behind the eye which pushes the eye forwards. This is one feature which may alert the doctor to a developing retrobulbar haemorrhage. In the restricted space behind the eye the blood may press on the nerve of the eye (the optic nerve) and threaten vision. In such cases it may be necessary to cut the upper and lower lid apart at the outer aspect of the eye in order to release the pressure on the optic nerve. This is called a lateral canthotomy. If a retrobulbar haemorrhage is suspected the operation should be cancelled. The operation may be performed some weeks or months later once the haemorrhage has all cleared.

An expulsive haemorrhage is the most serious form of bleeding that can occur during cataract surgery. Fortunately it is very rare especially with modern surgical techniques. It is caused by bleeding from within the eye. This bleeding occurs spontaneously within the choroid. This is the layer beneath the retina and it contains many blood vessels. A pool or clot of blood develops beneath the retina which may grow in size. If the eye has an open incision the haemorrhage can start to push the contents of the eye out through this incision; hence the name expulsive haemorrhage. In severe cases this can be impossible to control and the eye may be lost.

Most cataract operations are performed through tiny self sealing incisions using a technique called phacoemulsification. During this procedure the fluid pressure within the eye is maintained. In effect the eye remains a sealed unit. In this setting a full blown expulsive haemorrhage is almost unheard of. It is though occasionally necessary to use larger incisions.

Technical problems

Modern small incision phacoemulsification surgery, as used for cataracts, offers the promise of excellent visual results with rapid visual rehabilitation. It is though a highly skilled procedure. The level of surgical expertise required is far greater than that for previous or other forms of cataract type surgery. It is not surprising therefore that in a small minority of patients there is some technical imperfection with the surgery. Often this does not affect the successful outcome of the operation, but it can do.

The problems that all eye surgeons are mindful of are:

Posterior capsule rupture

Zonular dialysis

Vitreous loss

Dropped nucleus

Posterior Capsule Rupture (PCR)

The posterior capsule is an extremely thin delicate membrane like structure. In anatomical terms it is the back layer of the natural lens of the eye. Once the natural lens has been removed the posterior capsule separates the front fluid filled compartment of the eye from the vitreous gel filled rear cavity of the eye. Preserving this compartmentalisation of the eye is beneficial. Sometimes during surgery a tear or rent may develop within the posterior capsule. This is called a posterior capsule rupture. When this happens a number of unhelpful consequences may follow. The vitreous gel may move forward into the front compartment of the eye. There may be problems securely placing the lens implant within the eye. There is an increased risk of cystoid macular oedema and retinal detachment.

Zonular Dialysis

The zonules act as the guy ropes of the natural lens of the eye. They are very fine strands that extend outwards in all directions from the edge of the circular lens holding it in place. During cataract surgery these supports can be torn or uprooted. This is called a zonular dialysis. If minor it will only affect the zonules over one or two clock hours. If it affects more than 3 clock hours (90 degrees or a quarter) of the zonules then the lens can then become unstable which may make removing the natural lens (much) more difficult. Once the natural lens has been removed the consequences of a zonular dialysis are very similar to those of a posterior capsule rupture. Occasionally a zonular dialysis may be present before surgery.

Vitreous Loss

Vitreous loss can occur if there has been either a posterior capsule rupture or zonular dialysis. Strictly speaking vitreous loss means that some of the vitreous gel has been lost from the eye. This may occur if the vitreous moves forward within the eye, up to and then out of the surgical incision.

Many surgeons though consider that they have lost vitreous if it moves forward into the front compartment of the eye even if it doesn't actually reach the incision. This is because as soon as the vitreous escapes front the posterior compartment of the eye the risks of cystoid macular oedema and retinal detachment increase. In any case vitreous within the front compartment of the eye should be surgically cleared away. This is performed with a fine instrument that sucks and chops away the offending vitreous gel.

Dropped Nucleus

The nucleus is the harder denser core of the natural lens. Younger patients have a less well defined and softer nucleus. In order to perform phacoemulsification removal of the natural lens this nucleus is loosened with fluid and then manipulated and disintegrated. Cataract surgery is usually performed with the patient lying on their back and the eye looking up at the ceiling.

Very occasionally during the operation the nucleus, or part of it, may fall backwards into the vitreous gel. This is called a dropped nucleus. Once dropped the nucleus is out of reach. Indeed any attempts by the surgeon to go chasing after it may be harmful to the eye, particularly the retina. The correct treatment is to abort the surgery and refer the patient immediately to a vitreo-retinal surgeon. These are eye surgeons with a particular expertise and available equipment to perform a second operation to retrieve the nucleus and complete the procedure. Handled in this way most cases of dropped nucleus do well.

Anaesthetic Risks

Different types of anaesthetic and anaesthetic techniques may be used in cataract surgery. Which is used will depend on surgeon and patient preference and on the medical condition of the patient and the features of the eye. In the UK nowadays the majority of cataract operations are performed using some form of local anaesthetic.

General anaesthetic: Modern general anaesthetic is safe especially for short operations like cataract surgery. However there is a small risk of general medical complications with such an anaesthetic. This risk is higher for the elderly especially if the patient already has a heart, breathing or circulatory illness. There is a very remote possibility of death following general anaesthetic.

Local anaesthetic: If a sharp needle is used to inject anaesthetic next to or behind the eye two mishaps are possible.

Firstly the needle may accidentally puncture the eye. The tip of the needle cannot be seen and so its exact position is uncertain even in the most expert hands. If the eye is punctured then further treatment or surgery may be required depending upon the damage done. The risk of this complication should be less than one occurrence in every thousand cases.

Secondly the local anaesthetic may be accidentally injected beneath the coatings that surround the optic nerve behind the eye. The drug can then pass back within the cerebrospinal fluid and affect the brain stem. If it does so the patient may stop breathing. Artificial ventilation will then be required until the anaesthetic wears off. This is an extremely unlikely event but it is advisable to have an anaesthetist in attendance when this from of local anaesthetic administration is used.

The above covers the most important risks involved in having a cataract operation. Other adverse events are possible but even more unlikely. It is worth repeating that most patients have uneventful surgery with a good result.