Scanning laser ophthalmoscopy (SLO) is a diagnostic imaging technique used to examine the back of the eye. This type of eye examination produces detailed images of structures at the back of the eye, such as the retina, optic nerve and blood vessels. During scanning laser ophthalmoscopy, a laser moves across the back of the eye and the reflected light is used to form an image. A sequence of images can be used to show changes happening in the eye in real time, or scans of the eye taken at different depths can be used to build up a three-dimensional picture.
Due to the brightness of the laser light, scanning laser ophthalmoscopy provides clearer images of the eye than conventional photography. The laser scans across the back of the eye in what is called a raster pattern, moving from left to right and gradually shifting vertically to thoroughly map the area. Blurring is minimized by shining the reflected light through a pinhole. The technique is similar to one known as confocal laser scanning microscopy, which is used to study biological specimens. Scanning laser ophthalmoscopy is slightly different because the eye's lens takes the place of the objective lens which would be found inside a microscope.
Doctors can use scanning laser ophthalmoscopy to view the anatomy of the back of the eye in great detail, so that areas of subtle damage can be detected. These can represent the early signs of diseases such as glaucoma, a condition where high pressure inside the eye damages the optic nerve and causes sight loss. If it is diagnosed early, treatment can prevent progression and save a person's remaining vision.
Scanning laser ophthalmoscopy is sometimes used in combination with two diagnostic eye procedures known as indocyanine green (ICG) and fluoroscein angiography. These tests involve injecting dyes into the bloodstream. Fluorescein dye glows in response to light in the visible spectrum while ICG glows in infrared light.
Both fluorescein and ICG dyes are carried in the circulation to blood vessels at the back of the eye. Using scanning laser ophthalmoscopy, the time taken for the dyes to travel a specific distance through the eye's blood vessels can be observed and measured. This allows doctors to assess blood flow in the retina and the choroid tissue beneath, helping them to diagnose eye diseases such as diabetic retinopathy or macular degeneration. In diabetic retinopathy, damaged blood vessels develop in the eyes of patients with diabetes and laser surgery may become necessary. Macular degeneration, which is associated with aging, involves the loss of detailed vision, and there is no effective treatment for the most common form of this disease.