Procedures of the Eyes
Fluorescein angiography, or fluorescent angiography, is a technique for examining the circulation of the retina. It involves injection of sodium fluorescein into the systemic circulation, and then an angiogram is obtained by photographing the fluorescence emitted after illumination of the retina with blue light at a wavelength of 490 nanometers.
Pathologic changes are recognized by the detection of either hyperfluorescence or hypofluroescence.
Causes of hyperfluorescence:
Leaking (i.e. Capillary leakage, aneurysm, neovascularization)
Pooling (serous retinal detachment)
Staining
Transmission defects
Abnormal vessels
Causes of hypofluorescence:
Optical barrier (i.e. Blood)
Filling defect (capillary blockage)
Fluorescein dye
In order to detect injuries, ulceration, or foreign bodies, a fluorescein dye is applied to the cornea and conjunctivae. Then, the surface of the area is examined with the cobalt blue light.
Electroretinography
This helps in the determination of the electrical potential of the retina in response to the light stimuli. It also identifies visual abnormalities due to retinal disease. It is used to measure the electrical responses of various cell types in the retina, including the light-sensitive cells (rods and cones) and the ganglion cells. Electrodes are placed on the cornea and the skin near the eye. During a recording, the patient is watching a standardized stimulus and the resulting signal is interpreted in terms of its amplitude (voltage) and time course. Stimuli include flashes (flash ERG) and reversing checkerboard patterns (pattern ERG).
Electroretinogram (ERG) is used for the diagnosis of various retinal diseases:
- Retinitis pigmentosa and related hereditary degenerations
- Retinitis pigmentosa sine pigmento
- Retinitis punctata albescens
- Leber's congenital amaurosis
- Choroideremia
Funduscopic Examination
Fundus refers to the rear interior of the eye, which contains the retina along with its blood vessels. Funduscopic examination is performed with an ophthalmoscope, which has a light source and some changeable lenses to Focus on the fundus. An ophthalmoscope is able to detect vascular disorders of the retina, swelling or cupping of the disk, and retinal detachment.
In order to identify intraocular or orbital foreign body, an orbital imaging is performed with X-rays or MRI of the skull emphasizing the orbits.
Perimetry is used to assess the peripheral vision by testing the patient's ability to notice the moving objects or flashing lights at the extreme periphery of the visual field.
Refraction
This procedure gives the accurate determination of near and distant vision. The instrument allows the use of greater number of lenses in order to determine the correct lens for each eye and is more effective than the vision charts. This also helps in the detection and measurement of astigmatism:
Retinal arteriography is the process of recording the images of the retinal arteries with the fluorescein injected into the arm vein.
Slit lamp has a microscope and a built-in illumination, which is projected through a narrow slit. With the help of this slit lamp, a physician can view the cross sections of the anterior structures of the eye. Flares and cells can be detected through the slit lamp examination.
To test the color blindness, printed figures of different sized dots in different colors are used, which a normal person can perceive against a backdrop of differently colored dots. A person with color blindness perceives only the scattering of the dots.
Tonometry refers to the procedure eye care professionals perform use to determine the intraocular pressure (IOP), the pressure found inside the eye. It is an important test in the evaluation of ocular conditions such as glaucoma as well conditions such as phthisis bulbi, and iritis.
- Methods of Tonometry
- Applanation tonometry (Goldmann tonometry)
- Noncontact tonometry (or air-puff)employs a puff of air to flatten the cornea. This type of tonometry is the least accurate way to measure IOP. It is often used as a simple way to screen for high IOP and is the easiest way to test children.
The Tono-Pen is an electronic, digital pen-like instrument that determines IOP by making contact with the cornea, after use of topical eye anesthetic eye drops. This is especially useful for very young children and special population patients.
Indentation (Schiötz) tonometry historically was used to determine IOP. This type of tonometry makes use of a plunger to indent the cornea. The IOP is determined with small metal weights.
Vision Testing is considered the gold standard in the determination of intraocular pressure. Applanation tonometry uses a special probe to flatten part of the cornea to measure eye pressure with a slit lamp. Because the probe makes contact with the cornea, a topical eye anesthetic, such as oxybuprocaine, tetracaine, Alcaine, proxymetacaine or proparacaine is introduced onto the surface of the eye in the form of one or a few eye drops. Yellow Fluorescein is also used to aid the examiner in determining the IOP.
The standard way to test the vision is to place the Snellen chart 20 feet away from the subject, and visual acuity is recorded on the basis of the smallest line of type, which a person can accurately read more than half the letters of. Normal vision is designated as 20/20. A score of 20/80 indicates that the person can see an object clearly from a distance of 20 feet what a normal person can see from a distance of 80 feet. To test the near vision, cards with different-sized letters are used. For i1literates and children, these cards have pictures and symbols on them.
Visual field testing
A black felt sheet or screen mounted on a wall is used to assess the areas of impaired or absent vision. Doctors make use of the Amsler grid, which consists of a network of lines-usually white on black-around a central point at which the subject is instructed to gaze while the examiner moves a small object through the various parts of the visual field.
Amsler Grid Testing
The Amsler grid, used since 1945, is a grid of horizontal and vertical lines used to monitor a person's central visual field. It is a diagnostic tool that aids in the detection of visual disturbances caused by changes in the retina, particularly the macula (e.g. macular degeneration), as well as the optic nerve and the visual pathway to the brain.
Amsler grids can be obtained from an ophthalmologist or optometrist and may be used to test one's vision at home.
The original Amsler grid was black and white. A color version with a blue and yellow grid is more sensitive and can be used to test for a wide variety of visual pathway abnormalities, including those associated with the retina, the optic nerve, and the pituitary gland.
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