What tests do I need before cataract surgery?
This is a common question I get asked by many patients who know they have cataracts and want to have cataract surgery.
I wanted to provide a comprehensive list of what I think are essential tests before cataract surgery and the reasons why.
A test for your existing glasses prescription
This test can be conducted by your optometrist or ophthalmologist. Essentially this is a test of what refractive error you already have. For example short-sighted people (myopia) can only see clearly at near distances and need glasses for longer distance. Long sighted people (hypermetropia) generally need glasses for near and distance. Emmetropia describes people who don’t need glasses at all. Presbyopia refers to the natural inability to focus up near to due to reduced accommodation. Most people over the age of 45 will need glasses for reading.
A test for your existing glasses prescription is done routinely by your optometrist. It is often included in the referral to an ophthalmologist when cataract surgery is required. As well, an automated test of glasses refraction can be performed using a machine called an auto-refractor to give an estimate of your glasses prescription.
It is useful for your cataract surgeon to know your existing glasses prescription in cases where there is a high refractive error (people who are very short or long sighted), in order to plan to correct this error after cataract surgery. This can be done with the new intra-ocular lens implant. Or, we can adjust for it if patients are having just one eye cataract surgery and do no want to be left unbalanced after their cataract surgery (anisometropia).
A full ocular examination
This includes looking at
- The eyelid anatomy and inflammation. Are there co-existent conditions that could increase the risk of infection after cataract surgery?
- The state of the tear film – dry eye changes could make surgery difficult
- The presence of abnormalities in the cornea that could make visualisation of the cataract difficult during surgery. Additional steps may be required to protect the inner cell layer of the cornea during cataract surgery
- The depth of the anterior chamber of the eye – if it is shallow, access into the capsule may be more difficult
- The amount of dilation the iris undergoes with dilating drops. If small – is iris stretching required as an extra step during cataract surgery
- The type of cataract. Soft cataracts can be aspirated. Hard cataracts need more ultra-sound energy and surgical time to break up and remove. White cataracts may need trypan blue staining to visualize the capsule
- The measurement of intra-ocular pressure. With this test, we aim to exclude glaucoma and ensure optimal control of immediate pre-operative and intra-operative eye pressures often with extra eye drops that temporarily lower eye pressure.
- The dilated fundal examination to look for co-existent retinal conditions like age related macular degeneration or macular hole formation.
This is a simple pre-operative measurement. It calculates the correct power of artificial intra-ocular lens. They will be implanted into your eye once your cataract is removed.
We measure the length of the eyeball known as the axial length. Then we measure the curvature of the cornea at the front of the eye in 2 meridians. They are known as the keratometry readings.
These figures are unique in every patient and are measured automatically with a modified camera. The figures are entered into a specific formula. The biometry machine then calculates a range of artificial lens powers for a range of refractive outcomes (e.g. normal distance (emmetropia), long distance (hypermetropia) and short distance (myopia)). In most patients, the lens power used is the one for emmetropia.
The keratometry readings also allow the cataract surgeon to see whether there is astigmatism present and how we can reduce this astigmatism by modifying the position of the main corneal opening into the eye.
Very occasionally, patients with dense cataracts require a different method of measuring the axial length of their eye, as the standard machine may not be accurate. This test involves using a small painless ultrasound probe, which is placed on the eyelids and takes a few minutes to take a measurement.
If you wear contact lenses, we will ask you to stop wearing them before the biometry test to ensure your readings are accurate. This is because contact lenses can subtly change the shape of your cornea. We recommend the following:
- Soft daily disposable contact lenses to be removed 48 hours before
- Soft two weekly and monthly disposable contact lenses 48 hours before
- Extended wear soft contact lenses 1 week before
- Rigid gas permeable contact lenses 1 week before
This is a test to map out the corneal curvature in greater detail. I tend to order this extra test if the biometry readings show larger than normal differences in keratometry readings. Corneal topography can diagnose keratoconus and irregular astigmatism. Knowledge of this can alter our surgical management of patients.
Optical coherence tomography (OCT)
This test allows detailed visualisation of the macula (the central sensitive part of the retina used for fine vision). If I suspect any macula changes that may prevent a patient from visual improvement after cataract surgery, I always suggest performing optical coherence tomography. It is a relatively simple test, which involves sitting at a machine that takes pictures of the retina in cross section.
If my patients do have conditions like age related macular degeneration or macular holes, I often have to explain that their vision may not improve as much as they had expected following cataract surgery as the surgery will not correct that issue.
Blood pressure measurements
It is very important to know your blood pressure measurements before your cataract operation. The risk of having a bleed at the back of your eye during cataract surgery (supra-choroidal haemorrhage) is very small (1 in 10000). But is higher if you have uncontrolled blood pressure. My upper safe limit blood pressure readings before cataract surgery are 200 mmHg – systolic and 100 mmHg – diastolic. There are uncommon situations where I have recommended delaying cataract surgery to treat the high blood pressure and avoid the higher risk of bleeding at the back of the eye.
Blood sugar measurements in diabetic patients
Blood sugar measurements relates to my diabetic patients. Cataract surgery itself does not affect your blood sugar measurements if you are diabetic. We routinely measure blood sugars in diabetics before cataract surgery and we would recommend delaying cataract surgery if the levels were greater than 20mmol/L as the risk of post-operative infection is higher.
The majority of my patients undergoing cataract surgery have local anaesthesia for their operation. This is where the eye is put to sleep and the patient remains awake for the procedure.
You will not require tests such as blood tests, a chest X ray, ECG or urine testing before your cataract surgery if you are having a local anaesthetic.
Patients who require local anaesthetic with sedation or general anaesthesia will need to be reviewed by an anaesthetist. They will then decide which pre-operative tests are needed as intravenous medicines are used and can have side effects.
The routine tests before cataract surgery are simple, quick and painless and you should not be unduly worried by them. They are very accurate as well and good preparation before cataract surgery. They contribute to the excellent outcomes seen in my patients.
If you would like to discuss any of the tests that we do before cataract surgery in more detail or indeed would like to discuss any aspects of cataracts or cataract surgery, please feel free to call me on 020 7935 7990.
I would be more than happy to discuss your specific needs in person.