Patients visiting the Eye and Laser Institute for a comprehensive eye exam may undergo one or more of the following tests or hear some of these terms.
6/6 or 20/20
Used to describe “normal vision”. If you can read the 6/12 or 20/40 line, that means that at 20 feet or 6 meters, you can see what a person with normal vision can see at 40 feet or 12 meters.
A simple screening test used to assess central vision, at the macula. The Amsler grid looks like graph paper – a series of small squares – with a dot in the centre of the grid. The patient is instructed to stare at the dot and notice if any lines appear wavy or missing.
Produces detailed information about the curvature of the cornea. Computer software is used to measure and analyze the surface of the cornea and generate a colour map from the data. The data is helpful to evaluate and plan to correct astigmatism, monitor corneal disease and detect irregularities in the shape of the cornea. Accurate measurement of astigmatism is important for refractive surgery, contact lens fitting and calculating the power of intraocular lenses.
Is useful for evaluating various disease effects on the retina. The test requires an injectable dye (fluorescein), a specialised camera with filters, and timing. The dye is injected into the patient’s arm; within seconds, the dye travels to the blood vessels inside the eye. Photographs are taken to document any fluid leakage as the dye circulates through the eye. A digital camera is often used which allows the ophthalmologist to interpret the results immediately.
This may be ordered by your ophthalmologist to document the status of the optic nerve, macula, retina, blood vessels and vitreous. Specialised cameras are used to document the progression of diseases such a macular degeneration, glaucoma and diabetic retinopathy.
Is used to examine the anterior chamber angle, the portion of the eye that directs aqueous humour (the fluid produced by the eye). Gonioscopy is a standard part of a glaucoma evaluation. A special mirrored contact lens is used during this evaluation.
Indocyanine Green Dye (ICG)
Is used to evaluate the choroidal circulatory system, just behind the retina. ICG reacts to light with a longer wavelength than fluorescein dye, allowing the ophthalmologist to determine exactly where, and if, the vessels are leaking. The ICG dye takes about 15 to 20 seconds to travel through the bloodstream to the eye. The digital photos are interpreted by the ophthalmologist. If treatment is needed, the photos indicate where the laser treatment needs to be directed.
The measurements of the steepest and the flattest surfaces of the cornea. Accurate measurements are easily obtained from most eyes with a keratometer. Corneal topography is used to obtain more corneal details, or when reliable keratometry measurements cannot be obtained. Corneal measurements are an important component in formulas used to calculate and determine the power of intraocular lenses and in the fitting of contact lenses.
Is performed with an ophthalmoscope and allows the ophthalmologist to examine the retina and vitreous. Ophthalmoscopy is usually performed with dilated pupils, to allow the best view inside the eye.
Two types of ophthalmoscopes may be used: direct and indirect.
The direct ophthalmoscope has a battery-powered light source and is hand-held. Multiple lenses may be selected with a dial, which allows the doctor to focus on and view the optic nerve and the central retina. The periphery, or entire retina, may be viewed by using an indirect ophthalmoscope, which is worn on the doctor’s head. A lens is placed in front of the patient’s eye while the doctor looks through the magnifying glasses in the headgear. The combination of the instrument and the lens allows a good view of the retina.
Optic Coherence Tomography
The technique of optical coherence tomography (OCT) produces high resolution, high speed, non-invasive, cross-sectional images of body tissue. The technology is best compared to ultrasound, except that it employs light rather than sound and thereby achieves clearer, sharper resolution.
Non-invasive OCT examinations produce real-time cross-sectional images of retinal tissue, in ophthalmic applications, and are usually accomplished in less than 10 minutes. Thanks to OCT’s high resolution, which is 10 times greater than magnetic resonance imaging (MRI) or ultrasound, microscopic early signs of disruption in tissue can be detected and treated.
The ophthalmic applications have already benefited patients with glaucoma, retinal and macular diseases and those considering corneal and refractive surgery.
Phoropter (or refractor)
The device is placed in front of a patient’s eyes, as they are asked to read an eye chart on the wall. The phoropter contains various lenses that are changed by the examiner until the best-corrected vision is determined. The eye chart on the wall used to be placed 20 feet or 6 meters in front of the patient, which meant the room had to be at least 20 feet or 6 meters long. Today, the distance has been modified by using mirrors to reflect the images, but they still appear to be 20 feet away.
Slit Lamp Examination
Is performed using a slit lamp, which is an instrument with a high-intensity light source that can be focused and narrowed. The structures in the front section of the eye are examined: the eyelids, the cornea, the anterior chamber, the surface of the sclera, the iris and the crystalline lens.
A tonometer or a tonopen may be used to measure the intraocular pressure (pressure within the eye). It is used to screen for glaucoma. It is a good idea to have the intraocular pressure checked routinely after the age of 40, or if you are at high risk for developing glaucoma.
Is used in ophthalmology when a cloudy cornea, a dense cataract or blood in the vitreous obscures a view into the eye with an ophthalmoscope.
Standardized ultrasound also may be used to evaluate the extent and location of a retinal detachment, to determine the exact borders and height of intraocular or orbital tumours; to determine the axial eye length (the distance from the outer cornea to the macula), and the thickness of extraocular muscles. B-scan ultrasound produces a two-dimensional, cross-sectional image; A-scan produces a one-dimensional view.
It is appropriate to obtain an A-scan on patients before cataract surgery to determine the appropriate power of the intraocular lens implant.
Visual Acuity Testing
Although it is a simple exam, it is an important component of a comprehensive eye exam. An eye exam measures a person’s ability to see an object at a specified distance.
The results of an eye exam determine if a person has normal vision or if a prescription for glasses or contact lenses would help put things into proper focus.
Visual Field Testing
This monitors peripheral vision. Visual fields are obtained to monitor visual changes that may be caused by specific eye diseases, such as glaucoma, as well as the neurological function of the retina, optic nerve and brain.
There are different types of visual field exams, usually, they require the patient to focus on one spot and respond to flashing lights by pressing a button.