A-Scan is used for IOL power calculation by using ultrasound. In A-Scan biometry ultrasound waves of approximately 10 MHZ are used. The probe emits the sound waves which on contact with interface bounce back to the Probe. An interface is the junction between any two media of different densities and Velocities. In the eye the interfaces include tears corneal surface, aqueous/anterior lens surface, posterior lens capsule/anterior vitreous, posterior vitreous/retinal surface, choroid/anterior sclera surface. Echoes received by the probe from these interfaces are converted into spikes arising from baseline. Greater the difference in density/velocity in two media, higher the spike. I0L power is calculated form axial length acquired and 'K' readings obtained from keratometer using required formulae.
B-Scan ultrasonography is an important instrument used for the clinical assessment of a variety of ocular and orbital diseases. It is most useful when direct Visualization of intraocular structures is difficult as in corneal opacities including scars, severe oedema, hyphema, hypopyon, papillary membranes, dense cataracts or vitreous opacities (e.g. haemorrhage, inflammatory debris). In these cases diagnostic ultrasonography gives valuable information on the status of the lens, vitreous, retina, choroid and sclera. Ophthalmic ultrasonography uses high frequency sound waves. These waves when transmitted into the eye from a probe are reflected back to the probe and converted into an electric signal. The signal is subsequently reconstructed on the monitor as an image. In B-Scan ultrasonography, an oscillating sound beam is emitted which passes through the eye and images a slice of tissue, the echoes are represented as multiple dots that together form an image on the screen. Stronger the echo, brighter the dot.
During cataract surgery intra ocular lens is placed over the posterior capsule following removal of the lens. The living cells at the equator on the posterior capsule may produce cloudy cells which may cause posterior capsular haze. This can be treated by using yag laser. Yag laser capsulotomy is a simple out patient procedure to create an opening in the centre of the hazy capsule.
OCT (ANT SEGMENT)
Anterior segment OCT is a non contact high resolution topographic device for imaging anterior segment structures. In this imaging technique low coherence near infrared light is split into a probe and a reference beam. The probe beam is directed at the tissues while the reference beam is sent to a moving reference mirror. The probe beam is reflected from tissues according to their distance, thickness and refractive index which is then combined with the beam reflected from the moving reference mirror. When the two light beams undergo constructive interference, depth and reflectivity of the tissue can be measured that is analogous to an ultrasound A-Scan at a single point. Computer than corrects for axial eye movement artefacts and constructs a two dimensional image. Anterior segment is useful in mapping of corneal thickness and keratoconus evaluation, measurement of Lasik flap and stromal bed thickness, visualization and measurement of anterior chamber angle and diagnosis of narrow angle glaucoma. Other uses include measuring the dimensions of the anterior chamber and assessing the fit of intraocular lens implants, visualizing and measuring the results of corneal implants and lamellar procedures. Anterior segment OCT can also be used for imaging through corneal opacity to see internal structures of the eye.
OCT (POST SEGMENT)
Optical coherence tomography is a non invasive and non contact imaging technology. The technique of imaging is analogous to ultrasound B mode except that it uses light instead of sound. A beam of near infrared light is directed at the retina and the echo delay and magnitude of back reflected and back scattered light is measured giving axial information similar to ultrasound A-Scan. The light beam is then scanned in the transverse direction to generate a cross sectional image similar to an ultra bound B-Scan. The images can be displayed in grey scale or false colour and represent a cross section through the tissue. OCT is a powerful diagnostic technology as it enables visualization of the cross sectional structure of the retina. It enables earlier and more sensitive detection of disease as well as disease progression and response to therapy. OCT is capable of scanning the peripapilapillary retina, optic nerve head (ONH), and macular region. The final image provided by the OCT appears in a colour- coded map. Regions of high reflectivity are depicted in bright colours and regions of minimal optical reflectivity are depicted in dark colours.
GDx or scanning laser polarimetry is based on the retardation of polarized light. Parallel arrangement of retinal nerve fibres cause net change in the retardation of passing light, therefore the amount of retardation is proportional to the amount of axonal tissue. The back scattered light is captured and analyzed. The amount of retardation is calculated per pixel and displayed in a retardation map of the scanned area. Areas of high retardation represented more axons hence thicker retinal nerve fibre layer. As cornea and lens also show birefringence, their retardation needs to be compensated. GDxVCC is provided with corneal compensation (VCC- Variable corneal compensation). The two major applications of the instrument are early glaucoma detection as were as monitoring progression.
IOL master provides an accurate axial length measurement and accurate IOL power calculation by non contact technique. IOL master uses partial coherence interferometry of the optical coherence light to measure the length of the eye. Optical coherence light passes through the visual axis and reflects back from the RPE. Distance from cornea to retina is then measured by interferometry. It has accuracy of +/- 0.02mm or better in the measurement of axial length as against +/- 0.1 to 0.12mm by standard A-scan ultrasound. Additionally this device calculates anterior chamber depth, Radii of curvature of cornea and white to white measurement. Surgeon can have his/her personalised 'A' constant calculated by IOL master by feeding the post operative refractive status of 10 of his/her patients.
Stands for electro-oculogram. It measures the resting potential of the eye which is the electrical difference that exists between the cornea and the retina. The test is performed after dilating the pupil. Two electrodes are attached to the left and right of each eye and one to the forehead. Subjects look into a hollow sphere, following small red lights that turn on and off. The back ground light is also turned on or off during the test. Fast and slow oscillations of the EOG will be studied. Peak to peak amplitude, peak to trough ratio and phase will be measured in the analysis of fast oscillations slow EOG oscillations will include measurements of the ratio of light peak to dark trough (Arden ratio), implicit time (latency) of the light peak and amplitude of the dark through, congenital retinal pathologies, Chloroquine retinopathy, siderosis retina, early diabetic retinopathy are few conditions where EOG is abnormal.
Stands for electroretinogram and is the electrical response of the retina to brief flashes of light. It is used in the diagnosis of retinal diseases. Retina is a collection of rods cones and neutral cells which are the source of electrical signals. By measuring the changes in these signals, it is possible to determine how the different cells in the retina are working.
ERG is recorded by keeping a small contact lens on the cornea or front surface of the eye. Electrodes of different sizes are used for infants and adults. In an ERG result a +Ve 'a' wave phase from photoreceptors and a +Ve 'b' wave phase from bipolar cells are seen. Retinal diseases like retinitis pigmentosa, retinal degenerations and other retinal conditions alter the ERG pattern.
COLLAGEN CROSS LINKING
It is a new non surgical treatment done for early keratoconus using Riboflovin eye drops and ultra violet A light. The riboflavin activated by UV-A light, augments the collagen cross-links within the stroma recovering some of the corneas mechanical strength.
Slitlamp photos allow the doctor to compare conditions in the eye or the eyelid over a period of time, for research purposes and for presentations. Photographs are taken using slitlamp and a photographic image aquisition system.
Fundus photography of the retina including disc and macula in diabetic patients in a sensitive and cost effective means for detecting diabetic retinopathy. Retinal photography can be added to the screening for complications of diabetes to detect retinopathy changes, early.
FUNDUS FLUORESCEIN ANGIOGRAPHY (FFA)
To ensure the best photographs possible pupil dilation is mandatory. A series of colour photographs of the retina are taken initially. Dye is injected into a vein of the arm or hand. A series of pictures will be taken from 1st minute of early to 8th minute of late phase. Any circulation problems, abnormal blood vessels and leaking blood vessels can be detected by this method. Possible treatment options can be suggested by the doctor on seeing the photographs.
Focal laser, Macular grid and PRPC (Pan retinal photocoagulation) are the different modes of laser delivery given based on the FFA analysis. Laser is given to the focal areas of leaking micro aneurysms in focal Laser. In grid photocoagulation laser is given to the areas of diffuse leakage or non perfusion. Laser in PRPC is focussed on all parts of the retina except the macula. This treatment causes abnormal new vessels to shrink and often prevents them from growing in the future. Tiny spots of laser are placed on the retina in PRPC. PRPC is recommended for proliferative retinopathy. Laser is used to destroy the dead area of retina where blood vessels have been closed. When these areas are treated with the laser, the retina stops manufacturing new blood vessels and those that are already present, tend to decrease or disappear. The goal of pan retinal photocoagulation is to prevent the development of new vessels over the retina and elsewhere, not to regain lost vision. This procedure sacrifices peripheral vision in order to save as much of the central vision and further complications like vitreous haemorrhage and tractional retinal detachments.
VEP stands for Visual Evoked Potential. VEP is the electrical response of the brain to a simple patterned stimulus. VEP measures amplitude or size of the response and the time it takes for nerves to respond to stimulation. Electrodes are applied to the scalp at the back of the head. Flashing lights or patterned stimuli like checker board are presented to the person. Response is recorded by the electrodes. Amplitude and latency are the two factors which are considered in VEP. Diseases affecting optic nerve like optic neuritis, multiple sclerosis, degenerative retinopathy, ischemic optic neuropathy, tumours compressing the optic serve cause delays in VEP. Amplitude is reduced in macular diseases like central serous retinopathy, traumatic macular oedema etc.
Perimetry is the simple and cost effective method of diagnosing neurological diseases. A neurological disease often causes specific pattern of visual loss, depending on the location of the insult. Lesions of the optic nerve like optic neuritis, toxic reactions to the drugs and mechanical compression of the optic nerve due to injuries or tumours cause central scotomas. Optic chiasma lesions like pituitary adenomas, craniopharyngiomas, suprasellar meningiomas and aneurysms of the vessels of circle of Willis cause bitemporal hemianopias.Involvement of post-chiasmal optic nerve pathway causes homonymous hemianopias. Congruity of defects may be used to help localize the lesion. Lesions situated close to the visual cortex are more congruous.