Intraocular pressure (IOP) is a key piece of information in both the diagnosis and management of glaucoma. Intraocular pressure is not something that can be felt, unless it is extremely high, and needs to be measured to tell what it is.
IOP is always measured indirectly and through the cornea. There are a number of different techniques for doing so, but the most common one that we use is the Goldmann Applanation Tonometer. These are calibrated devices that are used on slit lamps in the office, and represent the “gold standard” for measuring pressure.
We also have a handheld device, known as the Perkins tonometer, as well as a Rebound tonometer, known as the “ICare”.
We also have a home tonometer system (a version of the ICare) which we use on certain patients with very variable intraocular pressures.
In special circumstances, we will use the Pascal dynamic contour tonometer, which can be useful with coexisting corneal disease.
All four methods of measuring IOP are available in our offices.
All intraocular pressure measurements are a one point in time and need to be considered in the overall scheme and risk. Intraocular pressure measurements are quite variable in the individual and across populations, so a pressure may be fine for one person and not for another.
Treatment in glaucoma is aimed at lowering the intraocular pressure, and thus lowering the stress on the nerve at the back of the eye.
Central Corneal Thickness (CCT)
Central corneal thickness, or CCT, is a piece of information sometimes used to help calibrate intraocular pressure measurements. As intraocular pressure measurements are taken through the cornea, a thickened cornea can sometimes cause over-reading, and a thin cornea under-reading of intraocular pressures.
There are various different calibration charts, none of which have great utility, but we do take note of central corneal thickness when we are establishing risk.
It is measured in two ways: one is a contact system using ultrasound, and the other is a non-contact system using the swept-scan anterior segment OCT (Casia), which has now become the standard. Both are available in our offices.
An A-scan is a complicated set of measurements done to establish the curvature, position of objects in the globe, as well as the length of the eye. These are all combined together into a formula, which can predict what intraocular lens power is used at the time of surgery to improve the outcome of cataract surgery in terms of glasses.
A-scans have improved dramatically over the years and we use a Haag-Streit scanner, which has multiple corneal points and a laser-based scan, and has excellent accuracy and consistency.
Astigmatism is further investigated using swept-scan anterior segment OCT (Casia) to establish the extent of corneal astigmatism, and this is added to calculations. Astigmatic intraocular lenses are a great advance and can improve unaided vision post-cataract. We use astigmatic lenses where there is a reasonable chance that they will offer better vision postoperatively.
Anterior Segment Imaging
The front part of the eye, known as the anterior segment, has many structures in it, but often is the cause of concern and problems.
The cornea, the very front surface, can become opaque or scarred, and significant alterations in curvature, such as astigmatism, can be very destructive to vision. Corneal ectasias, such as keratoconus, are resolved easily now with swept-scan anterior segment OCT (Casia).
Going deeper into the front of the eye, angle closure is a condition where the iris opposes the trabecular meshwork and obstructs outflow. Angle closure is a common form of glaucoma and can be mostly prevented with YAG laser iridotomy. In order to establish and manage risk, anterior segment OCT is a valuable device.
The development of peripheral anterior synechiae (PAS), or the results of trauma, such as cyclodialysis clefts, can be picked up with swept-scan anterior segment OCT
Objective imaging for Angle Closure Risk
The swept can anterior segment OC gives objective and precise measurements of the structures of the front of the eye. This is particularly useful in angle closure glaucoma, or where there is a risk of this disease. Clinical assessment of the front of the eye has been quite subjective, and the assessment of risk glaucoma developing possibly more so. Significant risk of angle closure suggests that intervention (laser or cataract surgery or both) may be useful - less risk may just need monitoring. We are not great at predicting what risks the individual eye runs, and hence who needs what. But these new 3D scanners have very significantly improved the data and made the process much more objective.
Posterior Segment OCT
Ocular coherence tomography (OCT) has been an extraordinary advance in both retinal disease and glaucoma management. Current OCTs, such as the NIDEK that we use, have resolutions down to one micron and tell change in structures at the back of the eye, sometimes with great accuracy. There is still some variation between tests, but the advantage of the NIDEK is it takes three complete data sets and so it is much less prone to single point errors.
The OCT is used for retinal disease, such as diabetic retinopathy or macular degeneration, but also to check for structural changes in glaucoma, serial OCT is a very valuable way to tell change in glaucoma. We use the OCT for assessing structural changes in glaucoma.
Sometimes there is a role for plain photographs of the optic nerve or some structure at the back of the eye, and we use a Kowa non-mydriatic camera, whose capacity for clarity, even through an undilated pupil, is unrivalled. Simultaneous stereo photographs allow us to give a three-dimensional rendition, particularly of the optic nerve head, that which is affected in glaucoma.
Although we do not routinely take serial photographs, we often take photographs at the beginning of a course of treatment, which allows us to follow against these photographs with time.
Slit lamp images
We are capable of taking high resolution video and still images off the slit lamp of structures on the surface and within the eye. This can be particularly useful to follow evolving lesions and to be able to review changes, particularly where they are longstanding.