The Pre-Operative Assessment

This section provides more information about the pre-operative assessment. It expands on the brief explanation given on the previous page.

To Recap:

The purpose of the pre-operative assessment is to prepare the patient for the operation both medically and personally. Each Eye Unit will have their own specific arrangements for conducting this assessment. The assessment will probably be conducted by an ophthalmic nurse or technician and includes the following:

• Biometry
• Health Checks
• Counselling and Organising

It is also a further opportunity for the patient to ask questions.

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Biometry

These are the measurement of the eye from which the power of the intra ocular lens implant is calculated.

There are two main measurements: the curvature of the cornea (the window of the eye) and the overall length of the eye. Sometimes the depth of the front chamber of the eye is also measured.

The curvature of the cornea is found by analysing reflections from its surface. Instruments that do this shine an array of lights or bright rings onto the eye and then measure the size of their reflections. Some instruments are table mounted. With these the patient places their chin and forehead against rests just in front of the instrument. Others are hand held by the examiner who positions the instrument just in front of the eye. Although the instrument may need to come close to the eye there is no need to touch the eye. These instruments are called keratometers. The corneal measurements they take are called the 'k' readings or 'k' values.

The overall length of the eye is called the 'axial length'. It is the distance from the front central point on the cornea to the centre of the retina at the back of the eye. This distance can be measured either using ultrasound or a weak laser light.

The instruments that use the ultrasound method need to touch the eye. An anaesthetic eye drop is used to numb the surface of the eye. The measurement is then taken either using a pencil like probe which lightly touches the centre of the cornea, or with a saline bath placed over the eye. An ultra sound signal is passed into the eye. This signal bounces back from each surface within the eye, rather like sonar. The axial length can be calculated from the time taken for the ultra sound signal to bounce back from the retina. The depth of the front chamber of the eye is calculated from the time taken for the signal to bounce back from the front surface of the lens of the eye.

The calculation makes certain assumptions about the density of the contents of the eye. As the density of the structures within the eye may vary a little from patient to patient there is the possibility of a small error in determining the axial length. For example the signal will travel a little faster through a very dense nuclear sclerotic cataract compared to a mild cortical cataract. The signal may then take less time to return to the instrument sensor. This may cause the length of the eye to be slightly under estimated. In turn this can lead to an over estimation of the power of intra ocular lens required for that eye. Instruments that use laser light to determine the axial length do not suffer from this potential error. They may therefore be more accurate but are considerably more expensive. However they may not be able to obtain a measurement from an eye with a particularly opaque cataract as this can block the laser signal. These instruments do not need to touch the eye.

Once the corneal curvature and the length of the eye are known formulae are used to calculate the intra ocular lens implant power required for the desired post-operative focus of the eye. Although these formulae are complex the principle is simple. To see clearly an image must be in focus on the retina at the back of the eye. Most of the focusing of the eye (about 3/4) is done by the cornea. If this amount is known (from the corneal curvature) then the additional focus needed can be calculated. This remaining part of the focus of the eye must be provided by the intra ocular lens implant after the cataract has been removed. The precise power of the lens implant will depend on its exact position within the eye. This can vary with different lens designs and types of surgery. All of these factors are taken into account when calculating which is the most appropriate lens power to use in any individual patient. Because it is a complex calculation there is always the possibility that the focus of the eye after surgery may not be exactly what was aimed for. However in the great majority of patients the result is close to that intended. The errors and uncertainty tends to be greater for eyes that were either very long or very short sighted before the operation.

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Health Checks

At the pre-operative assessment it is important to check that the patient is fit for surgery. This is especially so as many of the patients are elderly and may have other health problems. Common conditions like high blood pressure and diabetes need to be adequately controlled before the cataract operation can safely proceed. If the patient is taking an anticoagulant (e.g.Warfarin) the blood clotting may need to be checked and the dose of the drug adjusted; in liaison with the patient's General Practitioner or Haematologist.

If the operation is to be under local anaesthetic the assessor should check that the patient can lie reasonable flat and still without undue distress for as long as the surgery will take; typically about 15 - 20 minutes.

If a general anaesthetic is planned then some additional investigations may be needed, e.g. an ECG (electrocardiogram) and perhaps some simple blood tests, and these can be organised at the pre-operative assessment. If the patient has significant heart or breathing problems then the anaesthetist may wish to examine the patient before the day of surgery.

As a general rule a cataract operation, even under a local anaesthetic, should not be performed within 3 months of a heart attack or stroke. This is because even though the surgery itself may not unduly disturb the general health of the patient it is nonetheless a stressful experience for some. For example the worry and anxiety of the ordeal may increase blood pressure which could place an undue strain on a recently damaged heart or circulatory system. Opinions on the length of rehabilitation before surgery may differ a little between specialists and some may feel that a longer period is wise, particularly if a general anaesthetic is contemplated.

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Counselling and Health Checks

The pre-operative assessment provides the opportunity to discuss with the patient the necessary arrangements for the day of surgery. The staff should find out about any particular needs the patient may have. It should be decided who will come with the patient on the day of surgery and who will escort them home afterwards; also whether special transport arrangements will be required. There may be a need for help with instillation of eye drops after surgery. If so a district nurse or relative should be organised to assist with this. The patient should also be briefed on what they will need to do on the day. For example what to wear, what to bring with them, when and where to check in, and whether any drops should be instilled beforehand. The discussion should be tailored to the patient's individual circumstances and the issues gone through in a logical and unhurried manner. If the patient knows in advance what to expect he or she is more able to take the operation day in their stride.

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An Opprotunity To Ask Questions

Finally but very importantly the pre-operative assessment gives the patient a further opportunity to ask questions. Of course questions may be asked at any time and particularly during the consultation with the eye specialist. It is though all too easy to become mesmerized by the medical complexities of the whole ordeal. The pre-operative assessment is often conducted in a relaxed environment where the patient may feel more able and confident to ask questions. If there is something that is important to you that you do not understand the time with the pre-op' nurse provides a good opportunity to clarify things. Often the assessment is an interactive time and as the various components of the work up are gone through the patient gains an insight into the nature of modern cataract surgery. When the time comes to sign the consent form the patient should be able to do so in an informed way with an appropriate knowledge and understanding of what the cataract operation they are about to have involves for them.

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