A cataract describes the lens inside the eye when it loses its clarity and prevents light from focusing on the retina at the back of the eye. A common misconception is that it is a film growing over the surface of the eye. Cataract can be congenital (born with it) or acquired. The discussion here will be about acquired cataract.
- Cataract is the most common cause of blindness throughout the world.
- It is typically a disease of older age.
- It is more common in diabetics.
- Drugs such as steroids, and chemotherapy agents as well as radiotherapy can cause cataract.
- Cataract may occur in certain disorders such as myotonic dystrophy, retinitis pigmentosa and neurofibromatosis.
- Secondary cataract can occur following uveitis.
- Cataract occurs in almost all patients following vitrectomy surgery.
The actual cause for the loss of lens clarity is denaturation of the lens proteins, just like the changes you see as the white of an egg changes as it is cooks. There are many biochemical causes for this, which are beyond the scope of this discussion. Typically the whole lens goes cloudy, so called nuclear cataract. Other types are cortical lens opacity and posterior sub-capsular opacity. Mature cataract occurs when the whole lens turns white, often fairly rapidly.
Examples of different types of cataract. You can see how the mature cataract on the
top right would lead to blindness.
The time to remove a cataract is when the vision is impaired, and the impairment is interfering with daily activities. Typically this will be driving. Clearly the visual requirements of an airline pilot will be different from those of an elderly person who does not drive. There is no hard and fast rule as to when to do surgery; in the past it was felt that the cataract had to mature before considering surgery. Sometimes cataract surgery will be considered when the lens changes mean that glasses are having to be changed frequently. These days the decision is made between the patient and the surgeon. The most important part of this discussion is the risks as well as the benefits of the procedure.
Removing the cataract means removing the lens. There are options regarding the intended refractive outcome – e.g. the requirement for glasses after the procedure.
The most common scenario is that a lens power for the new intraocular lens is chosen such that the patient will have good distance vision without glasses. If you have astigmatism – eye shaped like a rugby ball – a toric lens can be chosen which will eliminate this. It should be pointed out that for people with big refractive errors pre operatively, there may be some difficulty balancing the two eyes after surgery. Generally in these circumstances both eyes are operated upon within a month so that this problem is only temporary. With this option you will require glasses for reading. Mostly people do not require glasses for distance, but this is not guaranteed.
Spectacle freedom (i.e. no glasses for distance or near)
There are two further options, which may suit people who want to be spectacle free: True spectacle freedom with excellent vision for both distance and near is not possible to achieve. A compromise has to be struck for spectacle freedom.
Monovision is a situation where one eye is set up for distance vision, usually the dominant eye, and the other is set up for reading. This is achieved by leaving the non-dominant eye slightly myopic or shortsighted. You will need to have this demonstrated before you can decide if it is for you. This is demonstrated by wearing a trial contact lens for a short period of time.
With this set up the distance vision will not be perfect as the non-dominant eye is slightly blurred. Many people do like this combination and accept that they may need distance glasses for a long drive or the cinema.
Multifocal lenses are intraocular lenses that correct both distance and near vision simultaneously. There are various clever designs that achieve this. They do as stated on the packet, namely offer distance and near vision. The downside is, however, that the vision has poor contrast, particularly in poorly illuminated situations. Most people accept this as a trade-off for being spectacle free, however I would not advocate it for people who are expecting perfect near and distance vision without glasses.
In order to choose an appropriate powered lens to implant in the eye – there is a great variation in lens powers in different eyes – measurements of the cornea are made and the axial length of the eye is measured. The length is measured with a scanning laser. Using these measurements different calculations can be made, including the ability to eliminate astigmatism.
Cataract surgery is the most commonly performed operation in the world. Great strides have been made in the past decades. Surgery is performed through a tiny incision, which is less than 2 mm. The natural lens is removed by peeling its surrounding membrane, then breaking the lens into pieces. The fragments are sucked up with an ultrasonic probe. A new artificial lens is then inserted into the eye. Generally this procedure is performed without stitches. Operating time is less than half an hour.
A vibrating probe breaks up the lens and sucks it out. A plastic lens is placed in the natural capsular bag.
The vast majority of people elect for surgery under local anaesthesia. This can be an injection behind the eye, which has the advantage of immobilizing the eye, oranaesthetic drops can be used. Your surgeon and anaesthetist will discuss the risks, benefits and advantages between the two types of local anaesthesia.
General anaesthesia is still available for those who get claustrophobic under drapes, or those who have pain and cannot keep still.
Generally cataract surgery is one of the safest procedures known. There can however, be rare complications which can affect the visual outcome.
• Capsule rupture (1-2%) – the membrane surrounding the natural lens can break and the vitreous jelly comes forward. This can usually be dealt with at the time of surgery. It may mean that a different lens type has to be inserted. Cystoid macular oedema (see below) is more common, as well as infection in the eye – endophthalmitis. Retinal detachment is more common following this complication.
• Dislocated lens fragments (0.2% or 1:500). If the capsule ruptures or becomes free, the lens or lens fragments can disappear into the back of the eye. This usually means that a further surgery will be required at a later date to remove the vitreous jelly and the fragments. Visual rehabilitation will be delayed.
• Cystoid macular oedema CMO (1%) represents swelling at the macula. Typically the vision is excellent for the first day or two, and then deteriorates as the swelling occurs. Generally this settles after 6-8 weeks but intervention may be required.
• Retinal Detachment may occur at any time after surgery. Generally rates are around 0.5% but increase with surgical complications or in people with high myopia.
• Endophthalmitis (0.001% or 1:1000) – this can be a devastating event that may lead to blindness. Rates have reduced markedly over recent years with improved surgical technique and the use of injected antibiotics.
• Corneal decompensation in vulnerable eyes – the cornea, which is the front window of the eye, may become waterlogged. Generally this can be predicted at pre op assessment. It usually settles after a few weeks. Rarely a corneal graft is required.
• Post-operative uveitis can occur but is generally controlled with added steroid eye drops.
• Lens disclocation. The lens may shift and further surgery may be required to reposition it.
• Capsular opacification 5-20%. The lens bag can go cloudy reducing vision. This can easily be remedied with a YAG laser capsulotomy which clears the haze away. This is a quick in-house procedure with low risk.
This discussion does not cover the whole cataract surgery experience. Before undergoing surgery, you will be asked to sign a consent form. You will have been given a handout explaining the surgery and attendant risks. What to expect and Do’s and don’ts will be explained to you by nursing staff. You will be given follow-up appointments with your doctor.
The capsule is cloudy on the left, and a hole has been cleared with the laser right.
Cataract surgery is advancing at a rapid pace – a possible way of doing this in the near future will be almost entirely by Laser. This allows the surgeon to remove the cloudy lens and insert regular, toric and bifocal or multifocal lenses, in addition to touching up the refraction by doing laser refractive surgeryu at the same time. This is all very exciting.
You may like to download an information booklet. Click below:
Download Treatment Consent Form – Laser Capsulotomy