 | ||||||||||||||||
 | ||||||||||||||||
Heidelberg Spectralis - Ocular Coherence Tomography (OCT) [ top ]OCT is a new diagnostic tool that provides images of the back of the eye of unprecedented quality. This information is particularly helpful for diagnosing and monitoring many cases of retinal disease, as well as glaucoma, which previously had mystified even the most experienced clinicians.OCT uses a principle akin to ultrasound. A beam of light is split in two, with one beam 'the reference' bouncing off a mirror a known distance away while the second passes through the pupil and is reflected off the various layers of the retina. OCT is 10 times more sensitive than ultrasound, with the ability to display objects as small as one hundredth of a millimetre wide. Unlike fluorescein angioography, previously the standard test for retinal disease injections which required the injection of a contrast dye and a very bright light, OCT is much easier to have done. OCT is helpful not just for diagnosis but also for monitoring responses to treatment. It is, for example, invaluable in assessing both the suitability and the response to treatment of patients with diabetic retinopathy. OCT is now almost mandatory for the management of "wet" macular degeneration with the new "anti-VEGF" drugs Lucentis and Avastin which are given by injection into the eye every 4-6 weeks. These treatments are very effective but we try to limit their use because we suspect they may have long term side effects if they are given too often. OCT is used to confirm that treatment has been effective by making sure there no is bleeding or swelling left. Once treatment is stopped after the macula has settled, OCT examinations are performed regularly to check for recurrence. The Spectralis is the newest generation of OCT machines and gives Eye Associates doctors the most detailed images of retinal pathology available in a manner that is comfortable for the patient, quick and non invasive.
Heidelberg Retinal Tomograph (HRT) [ top ]The HRT (Heidelberg Engineering, Heidelberg, Germany) is a scanning laser ophthalmoscope. This means that it uses a laser light beam to scan across the back of the eye and take measurements using the light that is reflected back to its sensors, much the same as a laser scanner reads a bar code. The HRT rapidly projects a laser grid across the retina and records with multiple layers of focal depth. From this the HRT is able to generate a 3 dimensional image of the optic disc and surrounding retina, using 16 to 64 consecutive images each with 384 x 384 picture elements. The accuracy of the height measurements is around 20 micrometers. The scan itself takes only 1-2 seconds to capture the information. Usually 3 quick scans are made and an average calculated. It is easy to use and has good patient acceptance.
Visual field testing - Perimetry [ top ]Most patients complain that they do not like doing field tests, but they are important for monitoring the disease. The shape of glaucoma blind spots tends to be curved or "arcuate" which corresponds to the distribution of the nerve fibres that are damaged. The defects usually start in the mid periphery. Ultimately when superior and inferior arcuate defects join up the patient is left with tunnel vision. When we test distance vision using a visual acuity chart with letters in rows we are only testing central vision, a patient can have perfect central vision till very late in glaucoma.Our conventional approach of looking for visual field defects is with subjective white-on-whiteautomated perimetry where the patient is asked to detect small light targets projected into their peripheral field of view. The Humphrey field machine is the standard and Eye Associates has 3 of these available. It is limited by the fact that a large proportion of the nerve fibres can be lost before an initial defect is seen. While some subjects are very good at field tests, we recognise that they are difficult and performance can be variable. This has to be taken into consideration when interpreting the result. Pachymeter [ top ]It has recently been determined that the thickness of an individual's cornea can influence the pressure reading and lead to under or over-estimates. For this reason we now perform Ultrasound Corneal Pachymetry on all our patients as a baseline, to determine corneal thickness. This test is simple, painless and takes a few seconds. Please let us know if you haven't had it done - it is very important for determining an accurate pressure. We are also currently assessing a new pressure measuring system called Dynamic Contour Tonometry.
Frequency Doubling Technology (FDT) and FDT Matrix [ top ]Frequency Doubling Technology works by flickering a coarse pattern of vertical dark and bright bars at a very high frequency. This produces the appearance of twice as many bars than are physically present. The advantages of the FDT are that it is a compact, transportable perimeter with tolerance to refractive errors and rapid test times (less than 4 minutes per eye). In some cases it shows defects before they are seen on standard tests. It is very useful for visual field screening in the community. It is not as useful for long term monitoring.The development of the new FDT Matrix with smaller targets spaced over narrower intervals improves the ability of the FDT to determine the spatial extent of visual field defects, making it more suitable for staging and monitoring the progression of disease. It gives an assessment similar to the Humphrey standard field.
Short Wave Automated Perimetry (SWAP or Blue/yellow Perimetry) [ top ]Short Wavelength Automated Perimetry is similar to standard automated perimetry except it utilizes a blue light stimulus projected onto a bright yellow background. As with FDT above, the concept is to isolate a smaller subpopulation of ganglion cells so it is therefore easier to detect defects. Previous investigations have established that SWAP is a more sensitive indicator of early damage than standard automated perimetry. Clinically, it is recommended for those with early disease or who are at high risk. Unfortunately, it is harder to perform than standard fields, takes 12-15 minutes per eye to perform, the results are more susceptible to the effects of cataracts, and many older patients find the discrimination of colour between the stimuli and background a further difficulty, increasing fatigue. Despite this, SWAP has demonstrated the ability to predict the onset and progression of glaucomatous visual field deficits. It is better suited to younger patients. A newer version of the test will soon be on trial at Eye Associates which has a much faster test time (around 5-6 minutes).New Structural tests - Imaging of the optic disc and nerve fibre layer [ top ]The advantage of these new technologies is that they move beyond optic disc photographs as a means of documenting what a nerve looks like for future comparisons. They not only gives an image of the nerve's appearance, but actual measurements of its structure which can be compared with future scans to detect changes over time. The retinal nerve fibre layer (RNFL) thickness is measured in the different techniques. In glaucoma, the retinal nerve fibre layer becomes thinner. This correlates with the visual field defects: thinner inferiorly with superior visual field defects and thinner superiorly with inferior field defects. By measuring the RNFL the test can detect abnormal regions in the optic disc and look for disease progression.When assessing an individual's optic nerve, there is such great variability among the normal population that we cannot always be certain that strangely shaped optic discs or thin nerves are not just normal variants. Therefore, while a scan can tell us whether an individual's nerve is thinner than average, it cannot say for certain whether the patient has definite glaucoma - only highlight it as abnormal. However the scanning techniques are excellent for follow up over time so repeat scans done at a later date can be directly compared to look for change. High Resolution Digital Imaging [ top ]The High Resolution Anterior Segment Imaging System is a fully digital system for photography of the anterior segment, or front of the eye. Images taken with an 8 megapixel digital camera attached to the slit lamp microscope. This system allows for objective documentation of eye disease and importantly allows monitoring of changes in ocular conditions over time. The imaging system is also useful for demonstrating diseases for teaching purposes and to the patient, for a greater understanding of ocular problems. Problems that can be easily seen on the digital imaging system include cataracts, pterygium, corneal diseases such as corneal ulcers and dystrophies.
Corneal Topography [ top ]Corneal topography is a method of corneal curvature examination assisted by computer analysis.A corneal topographer (below) projects a series of illuminated rings onto the corneal surface, which are reflected back into the instrument . The reflected rings of light are analysed by the computer and a topographical map of the cornea is generated. The topographical map and computerised analysis reveals any distortions of the cornea, such as is keratoconus or corneal scarring, as well as the corneal curvature and meridians of astigmatism. This diagnostic procedure is essential for patients being considered for refractive surgical procedures (such as LASIK) and may even be necessary in the follow-up of some patients who have undergone refractive surgical procedures.
Retinal Camera [ top ]The Canon CF60-UVi is a state of the art retinal camera used for colour photography of the retina and optic discs as well as Fluorescein angiography (which is the standard investigation for macular disease).
Laser Therapy for Glaucoma [ top ]There are now several possible applications for the use of lasers in the management of glaucoma. Since the thought of laser treatment may be frightening for many patients we will try to explain the principles and rationale for their use, and also what to expect if you are scheduled for laser.What is a Laser? L.A.S.E.R. is actually an acronym for Light Amplification by Stimulated Emission of Radiation, and it is essentially a concentration of light energy into a narrow well focussed beam. What this beam does to the objects it comes into contact with depends on the "wavelength" of the light source. A wide variety of laser types exist, ranging from visible to invisible, and they are capable of heating, burning, cutting or disrupting, depending on the type and power combination selected. Their advantage in medicine is that they are extremely fine and can be accurately targeted. What are lasers used for in glaucoma? The most common laser procedure in glaucoma is known as a laser trabeculoplasty (ALT). In this treatment the argon laser is used to make multiple microscopic burns in the drainage meshwork (the outflow filter) of the eye. The purpose is to enhance the escape of fluid from the eye through the meshwork into the normal drainage system and thereby lower the intraocular pressure. Laser trabeculoplasty is suitable for open angle glaucoma, which is the most common type.
Another type of procedure in glaucoma is laser iridotomy, which uses the laser to create a channel through the iris (the coloured part of the eye). Either the Argon laser or the YAG laser (in the Tango laser) can be used for this. The purpose of the channel is to prevent the sudden onset form of glaucoma, known as angle closure glaucoma, which can occur in certain anatomically pre-disposed eyes. It provides a longterm protection against this type of glaucoma attack by creating a bypass for the fluid within the eye. If the attack is already established it is an effective emergency treatment that saves the patient from surgery. It usually does not serve to lower the pressure on its own in the more common forms of glaucoma. Occasionally it is necessary to perform both iridotomy and argon trabeculoplasty if a patient is at risk of both forms of glaucoma, but they are usually done at separate sessions.
What does the laser treatment involve? Laser therapy is performed on site at the Eye Associates. It is done as a day procedure and the patient can walk out afterwards. Eye drops are required both before and after the procedure. The ALT/SLT and iridotomy are virtually painless so these do not require any anaesthetic other than the eye drops. A special contact lens is placed on the eye to hold the lids open and help the surgeon focus the beam. The treatments do not involve the visual part of the eye, so apart from some initial blurring for a few hours, vision quickly returns to normal. Due to the transient blurring however, it is advisable to arrange for transport home and not to drive immediately afterwards. Each treatment only takes a few minutes to perform. The laser trabeculoplasty is commonly divided into two treatment sessions per eye. Patients occasionally state that they have spoken to others who have had laser treatments lasting 40 minutes or more with hundreds of laser shots, or laser treatments for macular degeneration. This refers to laser treatment of the retina (at the back of the eye). Retinal laser is used to treat leaky blood vessels in diabetes, retinal vein thromboses (blocked veins) and abnormal blood vessels associated with macular degeneration. It is quite different from the laser technique used in glaucoma as described in retinal diseases of the eye. When does laser treatment become necessary? In open angle glaucoma laser trabeculoplasty is usually used as a second line treatment when eye drops have either not provided sufficient effect on their own, or were not well tolerated due to side effects. Laser can also be used as a first line treatment if the ophthalmologist and patient prefer this approach. Occasionally the laser treatment lowers the intraocular pressure enough to allow the patient to reduce the number of eye drops being used. The laser can be repeated at a later date, but the second treatment has a lower chance of achieving any pressure lowering effect. What are the potential risks? In the vast majority of patients no side effects are encountered. Vision should return to normal after ALT/SLT or iridotomy. There is a small risk that the pressure may rise initially following the treatment. We use additional drops at the time to safeguard against this. Extremely rarely the pressure may rise and stay elevated requiring long term increased medication or even glaucoma surgery. Occasionally an eye may react to the laser by becoming inflamed (iritis). Steroid drops are commonly given at the time and for a few days following the laser to prevent this and should it occur these drops may need to be increased or continued for longer until the iritis settles. We have not needed to use steroid drops for SLT in most cases as there is minimal inflammation. How successful is the treatment? ALT or SLT lowers the intraocular pressure in the majority of patients, but the magnitude of the reduction achieved varies greatly between individuals and is impossible to predict. The duration of effect is also unpredictable but ranges from months to many years (up to 10 years). Most studies find that around 75% of patients will achieve a lowering of pressure, with the remainder unchanged. A-Scan [ top ]An Axial Scan or A-Scan is an ocular ultrasound that accurately measures the length of your eye along the visual axis. A-scans are performed under a topical anaesthetic using two methods, applanation and immersion.With immersion A-scans the scanner head is inserted into a holder, which is placed under the upper and lowers lids. Saline solution is then inserted into the holder covering both the patient's cornea and scanner head. This ensures a smooth passage of the sound waves that measure the length of the eye. With applanation A-scans the scanner head is placed directly onto the cornea to allow the sound waves to pass through the eye. A-scans are used in the calculation of Intra Ocular Lenses (IOL) that are inserted in cataract surgery once the patient's natural lens has been removed. It is one of two vital tests that must be carried out before surgery.
The Zeiss Humphrey IOL Master [ top ]Preoperative assessment of the eye with cataract.History of Biometry Nineteenth century attempts to measure the length of the eye were academic and relatively clumsy. With the timely advent of ultrasonic measurement, precision was at last attained. It had become essential. The completion of a cataract operation became possible for the first time in 1949 when English surgeon Harold Ridley inserted an implant to replace the power of the cloudy lens he had removed. (To that time, the wearing of extremely powerful glasses had been the necessary and unsatisfactory alternative) . For true rehabilitation of the eye, the power of the cornea, the window and principal focusing element of the eye, must be measured and related to the axial length of the eye. A suitable lens implant can then be chosen. The Zeiss-Humphrey IOL Master is the first non-contact optical coherence biometer. This safe and effective laser system offers the most accurate and precise method of calculating a person's intraocular lens power. And it allows fast, accurate measurements of eye length and surface curvature, necessary for cataract surgery. The IOL Master is very efficient because it allows technicians to take all measurements with a high level of confidence in the accuracy of the results. Also, because it is non-contact (nothing touches the eye itself) there is no need for anaesthesia and there is no potential for spread of contamination from the instruments. Eye Associates technicians use this state-of-the-art technique for preoperative evaluation of the eye. As a back-up, we have a highly accurate immersion ultrasound system which is more reliable in the presence of a dense cataract which obstructs the passage of light. The IOL Master has been shown to increase measurement accuracy up to five-fold. The information is then subjected to a range of sophisticated formulae which are gauged to best suit the individual eye so that an optimal outcome can be attained in each case.
| ||||||||||||||||
Friday 1 May 2015
Subscribe to:
Post Comments (Atom)
No comments:
Post a Comment