Glaucoma tests – Part 2
Pachymetry measures the thickness of the window of the eye, the cornea. It is measured in all new patients at their first appointment. It does not need to be repeated unless the patient has problems with the cornea. We are all differing height and shoe size and so the cornea comes in different widths or thickness. The average cornea measures 550 microns. If your cornea is thinner or thicker than this it does not make any difference to your vision or to you, but it does influence the interpretation of the IOP.
A common scenario is someone who has a cornea that is thicker than average (>600 microns). The air puff test carried out by the optician will over read the IOP significantly, suggesting there may be a problem when if fact none exists. The puff of air has to overcome the extra thickness of the cornea before it reads the true pressure in the eye hence the reading is falsely high.
The gold standard test (GAT) reduces this measurement error significantly. When you come to the clinic and we find that your corneas are thicker than average and, with this taken into account, the IOP is normal and there are no signs of glaucoma, we can discharge you back to the care of your optometrist. But all further IOP should be measured with the GAT and not air puff. Otherwise, you will keep being referred back erroneously each time you have an eye test.
Conversely, if the cornea is thinner than average, glaucoma treatment will be altered appropriately.
Visual Field Tests
Visual field tests, also known as HVF- Humphrey Visual Field testing, are an important part of glaucoma care. They test how well you can see in the periphery of your vision. This is the part of your vision that is first affected by glaucoma. You will not generally notice a loss of the outside part of your vision, especially if it only affects one eye as the other eye can compensate for this. However, as glaucoma gets worse the more central parts of the vision are affected. So by the time you notice it the glaucoma is very advanced and this vision loss is not reversible.
Visual field testing is designed to find the level of brightness of a spot that you can only just see. Patients often find the test frustrating as there are times when you are not sure whether you have seen the spot or not but this is how the test is designed. Try to relax and only press the button when you actually see a light spot. You cannot ‘fail’ this test. It gives personalised information about your peripheral vision. Ms Crawley will show and discuss the results and what they mean with you. There is a learning curve in doing this test and often the first few tests are not as accurate as subsequent tests. It is important to bear this in mind and not become anxious in doing this test.
Optical Coherence Tomography (OCT) is a test that scans the nerve at the back of the eye using light waves. It is quick and easy to do and you usually do not have to have dilating drops to get a good scan. The picture generated does more than a colour photograph. It analyses the size and thickness of the fibres that make up the optic nerve and displays these measurements in a colour coded map. This digital record of the structure of the nerve makes it easier to look for changes in the nerve over time, suggesting that the glaucoma is getting worse. It means that changes can be picked up early and treatment adjusted. You will have this test each time you come for a consultation.
More about Laura Crawley
Ms Laura Crawley is a Consultant Ophthalmologist at Clinica London. Her special expertise is in treating glaucoma patients as well as patients with glaucoma and cataracts. She has a lot of experience in treating glaucoma and has published extensively in scientific journals and on medical education. She still does a lot of emergency operations at the emergency department at the Charing Cross and Western Eye Hospitals for the NHS. At Clinica London, she is responsible for glaucoma patients and glaucoma patients with cataracts. She also sees patients with general eye problems.