Scanning laser polarimetry is an advanced retinal imaging technique which provides objective measurements of the retinal nerve fiber layer (RNFL) thickness. It is a sophisticated technology that uses polarizing electromagnetic waves and particularly polarized light. SLP consists of a confocal scanning laser ophthalmoscope equipped with an ellipsometer to analyze retardation.
The technology is rooted in the principle that a polarized beam of light traveling through the birefringent RNFL goes through a measurable phase shift called retardation. It is linearly related to micro anatomically measured RNFL thickness. Polarized light is projected into the eye and reflected by the retina. All the eye structures, including the cornea, lens and retina, are treated as linear retarders. The RNFL thickness is determined by measuring the aggregate retardation in the reflected light beam. The RNFL has two different refractive indicators, so the two elements of the laser beam change phases relative to each other. The detector captures the waveform and converts it into tissue thickness. The measurements are objective and highly reproducible.
Also Known As
The scanning laser polarimetry exam can be done at the eye clinic or hospital by:
- A technician
- An optometrist
- An ophthalmologist
It is useful in the assessment of glaucoma, macular degeneration and other retinal abnormalities. Glaucoma is an acquired increasing optic neuropathy which is denoted by changes in the RNFL and optic nerve head. SLP is of vital importance in the diagnosis of glaucoma because the structural RNFL changes often lead up to alterations in the functional visual field. The combination of RNFL measures can improve the doctor’s ability to distinguish between healthy eyes and those with an early and moderate visual field loss caused by glaucoma. Therefore, SLP is essential in the early diagnosis, treatment and management of the disease.
Preparation & Expectation
There is no special preparation required for a scanning laser polarimetry test. Patients who wear glasses or contact lenses will have to remove them before the test. There is no pain or discomfort.
The scanning laser polarimetry exam is a non-invasive and non-contact test. The patient will sit in front of the scanning machine and place their head on a support to maintain it in a steady position. The doctor will instruct the patient to look at a light source in the device. The scanner will project a polarized light beam into the patient’s eye. As the laser light travels through the eye structures, it changes and slows.
The detector will measure the change in the polarization of the radiation reflected by the retina and translate it into tissue thickness units (microns). The results are sampled, digitized and graphically displayed on the computer screen. It can also be printed or stored by a computer. The test will take five to ten minutes.
The results for a healthy eye will have yellow and red color in the superior and inferior at the nerve fiber layer (NFL) areas.
Abnormal results may indicate glaucoma. The image will not have red and yellow colors in the NFL regions. Images showing a superiorly and inferiorly uniform blue color may mean an advanced stage of the disease.
The nerve fiber indicator (NFI) may show the probability that the eye has glaucoma. The machine provides the NFI value. An NFI value:
- Below 30 indicates a low risk of glaucoma
- Between 30 and 50 shows a glaucoma suspect
- Above 50 means a high of probability glaucoma
Risks & Complications
Scanning laser polarimetry has no risks or complications.