The best-known form of scanning laser polarimetry is GDx scanning laser polarimetry, which is used in ophthalmology to diagnose and monitor cataracts and allows this disease to be diagnosed up to five years earlier than any previous measurement method. Polarimetry makes use of the polarization property of light by means of a laser scanner and thus determines the layer thickness of the optically transparent retina, so that signs of decay of the transparent material are made visible. The thickness of the retina determined in each case is color-coded and compared by the ophthalmologist with a series of standard values, so that the physician can possibly make a glaucoma diagnosis after the measurement and initiate measures for therapy at an early stage, which ideally still averts impending visual field impairments.
What is scanning laser polarimetry?
The best-known form of scanning laser polarimetry is GDx scanning laser polarimetry, which is used in ophthalmology to diagnose and monitor cataracts. Scanning laser polarimetry is an objective method used by physicians to determine the layer thickness of optically transparent materials. The measurement is performed using a laser scanner. The method makes use of the polarization property of light. The measuring beam of the laser scanner first passes through a layer, where it is reflected and divided into two polarization states. These two partial states move on at different speeds, creating a delay. This delay between polarizations allows conclusions to be drawn about the thickness of the layers. The method is most commonly used in the form of GDx scanning laser polarimetry, which is used in ophthalmology to assess the deterioration of the transparent retina. For this purpose, the method records the three-dimensional profile of the optic disc. In addition, the thickness of the nerve fiber layers covering the retina near the optic nerve is determined.
Function, effect, and goals
Scanning laser polarimetry is used primarily in ophthalmology, where it is used for early detection and observation of glaucoma. In this disease, high eye pressure initially develops. This unnaturally high pressure ratio causes the fibers of the retina to perish bit by bit and can ultimately cause the eye to go blind. Due to the transparency of the retina, the damage of such a process can only be recognized when more than half of all retinal fibers have died and severe visual field impairments are present. Since retinal fibers do not regenerate, retinal damage cannot be revised at such a late diagnosis. With scanning laser polarimetry, the ophthalmologist can assess and observe any retinal fiber damage much earlier. Even the smallest changes in the retina, which remain invisible with other methods, are visible to the ophthalmologist. In polarimetry, the ophthalmologist first illuminates a single point on the retina with the laser scanner and measures the intensity of the reflectivity. This principle is eventually applied to up to 100,000 different points on the retina, which takes about two seconds per eye. The laser scanner creates a fundus image from the polarimetry measurement data. This fundus image color-codes the reflectivity of the individual layers. Yellow highlighting indicates high reflectivity, while dark brown highlighting codes low reflectivity. All intermediate levels are each recorded in shades of red. The ophthalmologist then evaluates the fundus image created in this way. He compares the respective data with a reference value, which corresponds to a culturally independent average value. The results of this comparison provide information about the layer thickness and the physician records them in a deviation chart using the standard values. Often he also creates a so-called TSNIT diagram on this basis. This shows the thickness of the layer in a circular path that starts in the temporal sector and runs through the upper, nasal and lower sectors back to the starting point. The standard values of the layer thickness are shaded in this diagram, which makes deviating measured values recognizable as exiting the shaded area.
Risks, side effects and dangers
The objective procedure of scanning laser polarimetry is entirely harmless and painless. It can be performed on an outpatient basis and is completed in seconds. Medications are not given beforehand, nor afterward. The patient is thus spared even the dilation of the pupil by drops, which many people find unpleasant. Vision is also not further impaired by the measurement. The patient can therefore drive machines and vehicles without hesitation on the same day. Normally, the ophthalmologist schedules two separate appointments for scanning laser polarimetry of the retina, at least one year apart. With smaller time gaps between the two appointments, it is difficult to assess actual decay via the procedure. Ultimately, the scanning laser polarimetry method allows cataracts to be diagnosed up to five years earlier. With therapy immediately following, impairing visual field loss can often be avoided with such early diagnosis, so polarimetry has taken on a revolutionary role in the field of glaucoma treatment. Since the method is a rather newer procedure, public health insurances do not usually cover the costs of the treatment. Private health insurances, on the other hand, usually bear a large part of the treatment costs or even completely cover the amount incurred. Because polarimetry, as an objective measurement method, does not require any cooperation from the patient and is independent of the patient’s own impressions, the procedure can also be used on unwilling patients, mentally impaired patients or children with unchanged meaningful results.
