Cataract Surgery Tests
Potential Acuity Meter testing (PAM)
Potential acuity meter testing enables one to have an idea about the possible visual recovery following cataract surgery. In this testing the doctor projects a chart of letters or numbers into the back of the eye through the gaps in the cataract that enables the patient to read the letters. Depending upon the number of lines that one could read, the potential for recovery of vision is estimated. One should realize that this is only an approximate estimate and very often the true recovery of vision is greater that the estimate.
Glare Testing
The glare testing permits one to assess the deterioration in vision that occurs with glare. Cataract can produce significant scattering of light. Hence people with early cataract may have good vision under ideal conditions of testing but the vision may deteriorate rapidly under conditions of glare. This testing enables one to decide on the need or otherwise for cataract surgery.
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Special Tests for corneal diseases
Schirmer's Test
Schirmer's test is a measure of the tear secreting capacity of the eye. Deficiency in tear secretion can lead to a chronic condition called dry eye. The test involves placement of a special filter paper strip across the lower eyelid margin and measuring the length of the strip that is wetted by the tears over a one-minute period.
Keratometry Testing
Keratometry involves the measurement of the corneal curvature in two meridians. The cornea is the front portion of the eye that is clear like a watch glass. The curvature of the cornea helps it to focus the light partly. Measurement of the corneal curvature is needed for fitting proper contact lens. It is also needed for the calculation of the IOL power before cataract surgery.
Corneal Topography
Corneal topography is the detailed mapping of the surface of the cornea. Advanced computer analysis of several spots on the surface of the cornea using the study of the reflected image is done. Color coded graphs of the surface map enable the doctor to diagnose certain conditions such as keratoconus. Before undergoing excimer laser treatment for getting rid of glasses, one needs to perform this test to understand the surface of the cornea better and plan the treatment accordingly.
Pachymetry Testing
Pachymetry is the study of the thickness of the cornea. The accurate measurement of the thickness is made possible by using ultrasound or optical means. Measurement of the thickness is important in the diagnosis and management of certain corneal conditions such as keratoconus, corneal endothelial dystrophy etc.
Specular Microscopy
Specular microscopy is a test that enables the evaluation of the back most layer of the cornea called the endothelium. The health of this layer is important in maintaining the clarity of the cornea. With age, injury, surgery and in some diseases this layer may have reduced number of cells and become abnormal. Study of this layer is done by counting the number of cells per square millimeter as well as study the type of the cells. This study is important in planning certain surgeries.
Special Tests for Glaucoma
Field Charting
Field of vision describes the side vision when one is looking straight ahead. The testing of the extent of the side vision is important in the diagnosis and follow-up of several disease including glaucoma, and diseases relating the eye with the brain (neuro ophthalmology). The test is usually done on computerized machines (Humphrey field analyzer). The machine is programmed to test several points, sometimes repeatedly with varying illumination. The test may take time depending upon the defect in a given patient. The computer has inbuilt software that enables comparison of the field charting on repeat testing of the same patient.
Optic disc photography Testing
Optic disc is the only part of the optic nerve visible to the eye doctor in the back of the eye. The appearance of the disc gives valuable information to diagnose and treat conditions such as glaucoma. It is important to be able to compare the appearance of the disc over a period time in cases of chronic glaucoma. This is made possible by several techniques- one of which is the photography of the disc using the Fundus camera.
GDx nerve fibre analyzer
This recent innovation allows measurement of the thickness of the nerve fiber layer, which is the part of the retina that is first affected in the disease of glaucoma. The nerve fibril layer defect is detected long before any defect is noted in the function of the eye including the field examination. This test may help in the early detection of significant damage caused by glaucoma and help in the follow up of these patients. The test involves the use of scanning laser that passes through the nerve fiber layer and in the process undergoes a process called retardation. By measuring the extent of retardation the machine calculates the thickness of the nerve fiber layer.
Ultrasound biomicroscopy (UBM)
This is an advanced technology in ultrasonography, which permits high-resolution pictures of the front of the eye. The technology enables the measurement of the angle of the eye, which is otherwise not accessible for measurement. The angle of the eye is the path through which the fluid in the eye finds access outside. The angle can become closed in certain individuals. This propensity to closure of the angle can be more adequately predicted using this advanced testing. Following injury to the eye, sometimes abnormal communications develop leading to excess drainage of fluid and resulting soft eyes. These abnormal sites can be best identified by UBM.
Special Tests for Neurophtalmology
Hess and Diplopia charting
These two tests enable the measurement of misalignment between the two eyes. This type of problem leads to a condition of double vision in a patient. The extent of the double vision and the direction in which it is maximal can be charted by using these two tests. The tests are done using red and green goggles where in one colored glass is placed in front of one eye and the other in front of the other.
Contrast sensitivity Testing
Certain disease of the retina and optic nerve leave behind subtle defects of sensitivity. A patient is very symptomatic of these deficiencies but the commonly performed tests like the vision testing do not reveal the true extent of the defect. Measurement of contrast sensitivity enables one to understand these subtle defects in the visual function. This test involves identification of patterns of gray on gray background.
Color Vision Testing
Color vision is an important component of human vision. Defects in this can be by birth or due to any acquired diseases. The testing is done using one of the two methods. Ishihara's charts- In this, many charts are presented and the patient is asked to identify the letters or numbers in the chart. Farnsworth- Munsell 100 hue test- In this the patient is asked to arrange several caps of different hues in their order. The test is done in good illumination.
Visually evoked potential (VEP)
In this test bright light or patterns of dark and light bands are projected on to the eye. The electrical potentials that are generated in the brain as a result of the light or pattern are recorded. This gives valuable information regarding the functional intactness of the optic nerves and the optic pathways that normally conduct these impulses to the brain.
CT Scanning
CT scanning is a computerized system where in x- rays are used to construct images of thin slices of tissues allowing detailed evaluation of the tissues under consideration. By manipulating the soft ware, the image quality and detail can be enhanced. Injecting some drugs called contrast agents can get additional information. CT scanning is very useful in the evaluation of diseases of the orbit (bony cage in which the eye is located) as well as some diseases of the eye itself. Injury related problems - especially presence of foreign bodies is easily picked up and located on CT scanning.
MRI Scanning
MRI scanning is a different technology and looks at the tissues in a different perspective. Sometimes both CT and MRI scanning may be needed to understand some diseases. MRI is especially useful in diseases of brain that may affect the eye. By using some specialized soft ware, one can even image the blood vessels of the brain without injecting any drug (MR Angiography).
Special Tests for Retinal diseases and Uveitis
Fundus photography
Fundus photography permits documentation of the structures of the eye. This documentation may be important to compare with other investigations such as Flourescein angiography as well as for follow up. Fundus photography of the optic disc is important in the management of glaucoma.
Fundus Flourescein angiography (FFA)
This is an important test to evaluate a variety of retinal disease such as diabetic retinopathy. This is one of the commonest tests performed for retinal diseases. The test involves injecting a dye called sodium Flourescein into the blood stream and taking photographs of the retina using special filters. The test is important to stage the disease as well as to guide treatment with laser photocoagulation. Present generation digital cameras permit manipulation of the pictures and for instant viewing without need for development of the film etc.
Indocyanine angiography (ICG)
Indocyanine angiography is similar to the Flourescein angiography but involves injection of a different dye called Indocyanine green. The test utilizes a special infrared sensitive camera to capture the images digitally. Very often Indocyanine and Flourescein angiography are combined in a given patient to give maximum information. Indocyanine angiography gives more information regarding the choroidal vessels compared to Flourescein angiography that gives more information regarding the retinal blood vessels.
Electroretinography & Electrooculography
These two tests are done to evaluate the function of the retina. Light is projected onto the retina and electrical potentials that occur normally in the eye are recorded using special electrodes placed near and on the eye. Certain retinal degenerative diseases are diagnosed only on testing with Electroretinography. Specialized computer soft ware is needed to analyze the data.
Low Vision aid Testing
There are certain diseases that may lead to permanent partial loss of vision. These patients can be sometimes helped to some extent by using special aids called low vision aids. There are a variety of these available and most of them are fine tuned for a specific function. Most of them have been made to enable reading fine print. It is important that the patient should be motivated to use them. They are used at a closer range than normal working distance and hence one needs to get used to the same. Computers and closed circuit television are also useful as low vision aids. One has to test different varieties before choosing what is appropriate for them.
MRI Scanning
MRI scanning is a different technology and looks at the tissues in a different perspective. Sometimes both CT and MRI scanning may be needed to understand some diseases. MRI is especially useful in diseases of brain that may affect the eye. By using some specialized soft ware, one can even image the blood vessels of the brain without injecting any drug (MR Angiography).