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This is an effort from Neoretina Eyecare Institute to
provide the right information that makes your more knowledgable about your eyes.
 

Diagnostic tests

 

Fundus Fluorescein Angiography: Innermost structures of the eye including the retina (light sensitive screen), optic disc (the head of the nerve to the eye) and the macula (the small spot in the retina where vision is keenest) are included in the term Fundus.

Fundus fluorescein angiography (FFA) is a diagnostic procedure in which a dye is injected into a vein in the arm or hand and multiple digital photographs are taken to display the blood vessels on the Fundus. The test shows any abnormal blood vessels and leaking spots. This information is essential for the eye consultant to find the problem and to plan for the treatment. The test is often carried out by the technician.


Preparation for the test:
•    The person undergoing FFA should be on empty stomach for two to three hours prior to the test
•    Questions regarding allergies, medications, blood pressure and other systemic conditions are asked
•    Contact lenses should be removed before the test
•    Pupils should be dilated to the maximum extent (which takes around 30 -45minutes) to ensure the best photographs possible
Procedure for the test:
After dilatation the person is seated in front of the camera with the chin and the forehead resting firmly on the frame. A series of colour photographs of the Fundus are taken. A small needle is inserted into the vein of arm or hand and the dye is injected. The pictures of the vessel filling are taken during the following 10 minutes. Photographs of both the eyes should be taken even if the problem is with one. Lights from the camera flash are quite bright but it is very important to keep the eyes open. Multiple pictures are taken during the first minute of the test and another set of pictures are taken a few minutes later. The pictures are copied into the C.D while the person waits in the waiting room.
Fluoresceine dye- the risks involved
The dye is deep orange in colour. It has been in use for many years and causes few side effects. It has been proved safe to use in young as well as in older patients. The common side effects are nausea and vomiting. Skin rashes and itching are seen rarely. Extremely rare side effects include spasm of the airway, anaphylactic shock and collapse.
After injection the skin shows temporary yellow discoloration and exposure to direct sunlight should be avoided. If the dye leaks outside the vein it causes temporary pain that can be relieved by cold compressions.
The dye is metabolized in the liver and excreted through urine. The urine remains orange green in colour for 24-48 hours. The dye gives false high reading of the urine and blood sugar tests, hence the diabetic personnel should not adjust the insulin or any diabetic treatment based on these results during the first 2 days following FFA.


Optical coherence tomography (OCT):
It is a non-invasive, non contact painless technology used for imaging the retina (the sensory tissue lining the back of the eye). It allows doctors to see the cross sectional images of the layers of the retina so that eye conditions such as macular holes, pre-retinal membranes, macular swelling and optic nerve damage etc can be diagnosed early. 

The test does not require any patient preparation. She/he is placed in front of the machine and made to fix at a cross. The picture is acquired within seconds.   
Few of the indications for retinal OCT are
•    To examine the retina and its sub layers for atrophy, edema, traction, sub retinal fluid, irregularities of retinal pigment epithelium which is the outermost layer of retina.
•    To monitor progression of a disease
•    To aid in treatment planning
•    To monitor response to therapy


Anterior segment OCT:
It is a non contact diagnostic test for high resolution, cross-sectional, Three dimensional imaging of the cornea (outer black portion of the eye) and other anterior segment structures. Abnormalities of the cornea, anterior chamber angle, iris and lens can be evaluated.

•    Keratoconus (developmental abnormality where the cornea is cone shaped) and other structural changes of the cornea can be visualized by mapping the corneal thickness
•    Complications of Lasik including any growth under the Lasik flap; post Lasik Keratoconus etc can be visualized
•    The corneal implants and lamellar procedures done for therapeutic purposes can be visualized
•    The anterior chamber structures can be seen through thin corneal opacities
•    Anterior chamber angle can be visualized and measured to diagnose the narrow angle glaucoma, to see any angle abnormalities like congenital defects, deposits, synechial  closure etc.

•    -Dimensions of the anterior chamber and the fit of IOL implants can be measured and visualized

•    Lens abnormalities can be visualized

B-Scan ultrasound:
It is an important   test used for assessing various eye and orbital diseases when direct visualization of the intraocular structures is difficult or impossible.
 
 The conditions that prevent visualization of intraocular structures include lid problems like severe oedema, partial or total torsorrhaphy (suturing of the lids), opacities of the cornea due to edema and scars, hyphema (blood in the anterior chamber) hypopyon (pus in the anterior chamber), pupillary membranes, dense cataracts or vitreous opacities (blood or inflammatory debris).

It is also performed in clear corneas for differentiating iris or ciliary body lesions, for differentiating intraocular tumors, for differentiating serous versus hemorrhagic choroidal detachments, for differentiating rhegmetogenous (detachment due to hole) versus exudative retinal detachment (due to sub retinal fluid).
B-Scan ultrasonography gives valuable information on the status of the lens, vitreous, retina, choroid and sclera.
Procedure:
The person is seated in front of the technician. The technician then performs the test by placing a probe on the closed lids after placing a small quantity of gel over the lid. By asking the person to look at different directions the technician evaluates the status of various intraocular structures.

The test requires no patient preparation and the report can be given within minutes.
A-scan biometry:
This is a contact technique used widely for measuring the power of the intraocular lens that needs to be implanted after the cataract surgery.

Procedure
The test is performed by placing a probe on the cornea after anaesthetizing it with paracaine eye drops and asking the person to look straight ahead or into the fixing red light. Axial length so measured along with the keratometry readings (that measure curvature of the cornea) are used to calculate the power of the intraocular lens. The test is completed with infew minutes and the reports can be given immediately.

IOL master:
It is a non contact optical device that is used for accurate and fast measurement of axial length
(anterior to posterior length in the visual axis), surface curvature and IOL power calculation using various formulae. It is especially useful in individuals with very long and very short eye balls and for those who had undergone refractive surgery. As it is a non contact technique no anesthesia is required and there is no risk of infection.

Procedure:
Prior preparation is not necessary. The person is placed in front of the machine and is advised to look at the fixation lights. The machine performs the selected tests automatically and calculates the IOL power using different formulae for the required lens. The test is completed within minutes and the reports are given immediately.

Glaucoma diagnosis test (GDx) :
It is one of the latest diagnostic tools that measure the thickness of the retinal nerve fiber layer around the optic nerve head (where the optic nerve leaves the eye). The loss of the retinal nerve fiber layer may be due to many causes including glaucoma, vein occlusion, optic neuritis, papilledema, toxicity, trauma, metabobolic diseases etc. Though GDx measures the nerve fiber loss in all the above mentioned diseases it plays a major role in glaucoma as early diagnosis of glaucoma helps in preventing visual loss by adapting to early treatment options.

Glaucoma represents a major public health problem in all parts of the world as it causes irreversible visual loss unlike that seen in cataract which can be reversed by surgery. The sooner glaucoma is detected the better the chance of preserving vision. Nerve fiber layer starts to become thinner when glaucoma is present and this thinning is measured by GDx. This specialized testing reveals nerve fiber loss before the loss is seen with a visual field defect.

Procedure:
The person undergoing the test is made to sit in front of the machine so that the chin and the forehead are placed on the machine and made to look into it. She/he has to look at the blinking lights presented on either side. In the presence of gross media opacities including cataract, corneal and vitreous opacities and glaucoma with advanced field defect nerve fiber analysis is difficult.

Clinical Specular microscopy:
It is a useful instrument to visualize and evaluate the corneal endothelium (deeper most layer of cornea which is the clear outer most layer of the eye). The young normal corneal endothelial cells as seen by specular microscope are regular array of hexagonal cells all having nearly the same size. This regularity is lost with aging, trauma and corneal diseases. Ophthalmologists’ especially corneal specialists rely on specular microscope to examine the corneal endothelium as it guides them in decision making when managing a corneal disorder. The test is also useful prior to cataract surgery in mature cataracts as extracting a large mature cataract may lead to endothelial damage and cell loss. Patients with relatively few endothelial cell counts undergoing cataract surgery may require extended periods to resolve their corneal edemas.


The test also provides us with corneal thickness and also determines the percentage of cells that show polymegathism.
Procedure:
It is a non contact technique and does not require any preparation. The person is made to sit in front of the machine and concentrate on a fixation spot. The machine takes 15 pictures of which the best is selected and given as a printout immediately.

Visual fields(HVF):
Field of vision refers to the total area in which objects can be seen in the central and side vision while we focus our eyes on a central point. It is a subjective examination requiring the person to understand the test.
Visual field test is frequently used for
        Screening for glaucoma
•    Testing patients with glaucoma for treatment response
•    Testing for macular diseases like macular degeneration
•    Testing for toxicity of certain drugs such as plaquerin used for rheumatoid arthritis
•    Testing for peripheral retinal diseases such as retinitis pigmentosa
•    Testing the function of the optic nerve
•    Testing the visual pathways (nerve bundles that carry information from the eye to the brain where it is processed) in the brain


Procedure:
The person undergoing the test is seated in front of the machine comfortably, made to place the chin in a bowl shaped chinrest. The test is done on one eye at a time with the opposite eye completely covered. The machine presents lights of various intensities at different locations of the bowl. When the person sees the light she/he pushes a button while maintaining straight head gaze. The test takes about 10 minutes for each eye and the reports are given immediately.


Electrophysiology tests: Several tests that measure the function of the various components of retina which is the light sensitive layer of the eye are included in electrophysiology. The tests included are Electroretinogram (ERG), Electro oculogram (EOG), and Visual evoked potential (VEP).
Electro retinogram (ERG): This test is to evaluate the light sensitive cells of Retina which include Rods and Cones and their connecting ganglion cells.
 Procedure: Person undergoing the test is seated comfortably in front of the machine. Usually the pupils are dilated with dilating drops. Anaesthetic drops are placed in the eyes and electrodes are placed in both the eyes. An additional electrode is placed on the skin to provide a ground for the very faint electrical signals produced by the retina. Electrodes measure the electrical activity of the retina in response to light. The person is presented with a light stimulus which includes flashes in flash ERG and reversing checkerboard in pattern ERG. The resulting signal is interpreted in terms of amplitude and time course. The test is repeated in the dark adapted state (person is seated in dark room for 20 minutes) and light adapted state (person sits in well lit room for 20 minutes) for eliciting the function of Rods and Cones respectively.


Hereditary and acquired rod and cone dysfunction including retinitis pigmentosa, rod and cone dystrophies, juvenile retinoschisis and drug toxicities produce abnormal ERG.

Electro oculography (EOG): It is useful to evaluate the function of retinal pigment epithelium which is the outer most layer of retina.

Procedure: The person is seated in front of the testing bowl. Two electrodes are placed at the two lateral canthi and one ground electrode is placed on the forehead. Fixation lights are presented to the person on either side alternately and the response is measured.  The test is done both in dark and light adapted state and the result obtained is calculated in terms of Ardens ratio.

Visual-evoked potential: Visual signals that reach the cortex are recorded by VEP. The test helps in identifying macular lesions, optic nerve lesions, and Occipital cortex abnormalities. It is also useful for visual acuity assessment in infants, pre-verbal children and in uncooperative persons.

Procedure: The person sits in front of the monitor which displays a checker board pattern. Electrodes placed over the occipital cortex record the response which is displayed on the screen as amplitude and time. Any deviation from the normal either in amplitude or latency is considered abnormal. 















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