OPHTHALMIC INSTRUMENTATION
Various instruments are used to assess the health and function of elements of the visual pathway and the supporting structures. This section briefly describes some of these instruments and the structures examined.
The curvature of the cornea is one of the factors that determine the corneal refractive power. A keratometer measures the curvature of the central 3 to 4 mm of the anterior corneal surface and provides information about the power and the difference in curvature between the principle meridians at that location. The smoothness of the corneal surface can also be assessed by the pattern reflected from the cornea during the measuring process.
The automated corneal topographer maps the corneal surface and gives an indication of curvatures at selected points. This instrument is an important adjunct in the fitting of contact lenses in difficult cases.
The optometric physician can objectively determine the optical power of the eye with a set of lenses and a retinoscope. This instrument is beneficial also for assessing the accommodative function of the lens.
The inside of the eye, called the fundus, is examined using an ophthalmoscope, which illuminates the interior with a bright light. The retina, optic nerve head, and blood vessels can be assessed and information about ocular and systemic health obtained. This is the only place in the body in which blood vessels can be viewed directly and noninvasively. Various systemic diseases, such as diabetes, hypertension, and arteriosclerosis, can alter ocular vessels. To obtain a more complete view of the inside of the eye, topical drugs are administered to influence the iris muscles, causing the pupil to become enlarged, or mydriatic. The binocular indirect ophthalmoscope allows stereoscopic viewing of the fundus.
The outside of the globe and the eyelids can be assessed with a biomicroscope. This combination of an illumination system and a binocular microscope allows stereoscopic views of various parts of the eye.
Particularly beneficial is the view of the transparent structures, such as the cornea and lens. A number of auxiliary instruments can be used with the biomicroscope to measure intraocular pressure and to view the interior of the eye.
Technologic advances have produced instrumentation that can provide three-dimensional mapping of retinal and optic nerve head surfaces and measure the thickness of specific retinal layers.
Additional instrumentation can allow visualization of corneal layers, cells, and nerves and can aid in the differentiation of bacterial, viral, parasitic, and fungal infection in corneal tissue.
The visual field is the area that a person sees when looking straight ahead, including those areas seen “out of the corner of the eye.” A perimeter is used to test the extent, sensitivity, and completeness of this visual field.
Computerized perimeters provide extremely detailed maps of the visual field, as well as statistical information on the reliability of the test and the probabilities of any defects.
OPTOMETRY-SHARP VISION
Optometrist