Diagnostic tests and procedures
Ocular ecography is a diagnostic procedure that uses ultrasounds to visualize the structure of the eye and its orbit. Ocular ecography is at its most useful when a direct exploration of the interior of the eye is not possible, due for instance to corneal opacity, advanced cataracts, or hemovitreous. The clinical conditions that have been most studied through this imaging technique include: retinal detachment or lacerations, choroidal detachment, vitreal haemorrhages or endopthalmitis, advanced proliferating diabetic retinopathy in the presence of tractive membranes, solid endobulbar tumours (choroidal melanomas, haemangioma, metastases). It is also extremely important in the post-surgical follow-up of many of the conditions mentioned above. Orbital ecography instead makes it possible to study the tear gland, oculomotor muscles, the optic nerve, and the other structures in the rear of the bulb; in certain cases and with experienced practitioners, it can be a valid alternative to CT and NMR.
Fluoroangiography is an exam that makes it possible to view and photograph in rapid succession the aspects of retinal ocular hemodynamics and related tissue alterations. This exam is performed through the intravenous subministration of a dye, sodium fluorescin (20%), which is 80% bound to plasma proteins, while the remaining 20% circulates freely in the serum. This exam makes it possible to specifically point the alterations in retinal and choroidal circulation. It is therefore particularly useful in all vasculopathies (high blood pressure, thrombosis, vascular occlusions, diabetes), inflammations (retinitis, choroiretinits, papillitis), macular degenerations (hereditary, metabolic, senile, myopic), in pathologies of the optic disk, and in tumors. In spite of the advent of new diagnostic techniques for the study of retinal and choroidal pathologies, fluoroangiography continues to be one of the most valid methods for the study of the physiopathology of chorioretinal disorders and for the short-term and long-term assessment of their treatment.
IIndocyanine green angiography
Similar to fluoroangiography, indocyanine green angiography is an exam that uses the intravenous injection of a dye (indocyanine green) to study the choroidal cycle through a photographic sequence, thanks to the use of infrared light. At this wavelength, unlike fluorescein, diffusion is more effective through less transparent dioptric means, ocular pigments, exudations, and some hemorrhages. This dye binds almost completely (98%) and rapidly to plasma proteins; it does not diffuse like flourescein, but remains in choroidal circulation, thus highlighting it. This exam is particularly well suited for functional and anatomic alterations of the choroid which can be observed in vasculopathies, inflammations, neovascular maculopathies, pathologies of the optic nerve, and tumours.
A retinography is a photographic image of the fundus ocularis. This image can be obtained with a fundus camera or with laser scan tools, and the procedure is usually carried out after administering mydriatic eye drops, although there currently are instruments that make it possible to obtain such an image even without dilating the pupil. Traditional retinography produces a colour image of the ocular fundus, which is generally used in the screening of certain ocular pathologies, such as diabetic retinopathy, or for monitoring, as in the case of choroidal nevi. There are also other types of retinography, each of which is suited for studying certain aspects in more detail: red-free light retinography (vessels, hemorrhages, drusen, exudates), blue-green light retinography (nervous fibre layer, inner limiting membrane, folds, retinal cysts, epiretinal membrane) and red-light retinography (pigmented lesions, ruptured choroid, choroidal vessels).
Retinography using autofluorescence makes it possible to highlight the spontaneous fluorescence of the retina emitted at various wavelengths, especially lipofuscin under blue light and melanin under infrared light. In order to obtain such an image, it is currently possible to use two different types of angiographs, those using a scanning laser as a light source (the most widespread is the Heidelberg Retina Angiograph – HRA) and those equipped with a fundus camera (Topcon, Canon, Zeiss). In clinical practice, this method is useful in the study of various pathologies, including atrophic and exudative senile macular degeneration, central serous chorioretinopathy, cystoid macular edema, Stargardt’s disease, Best’s disease, macular fora, and choroidal nevi.
MP is a symptom-based method that makes it possible to study the fixation (stability and location) and threshold of retinal sensibility (microperimetry) through the direct observation of the fundus oculis. Microperimetry (Fundus-Related Perimetry) is currently performed using a modern instrument called Micro Perimeter-1 (MP1) built by Nidek Technologies Inc. – Italy. This instrument makes it possible to have a personalized pattern, and can thus be adapted to any type of pathology and perform exams in automatic, semi-automatic, or manual mode. Current clinical applications for this exam include the diagnosis and long-term observation of numerous retinal pathologies, such as senile macular degeneration, diabetic retinopathy, macular holes, and disorder of the vitreoretinal interface. The MP1 makes it possible to acquire colour images of the central 45° of the fundus oculis. The procedure is non-invasive and repeatable, and is thus useful in the therapeutic follow-up of these pathologies. It is also useful for performing biofeedback during the visual rehabilitation of patients with macular damage.
Optical Coherence Tomography (OCT), until recently known as OCT Time Domain, is a non-invasive diagnostic technique that makes it possible to view high resolution retinal cross-sections (about 10 microns). In order to acquire these images, it uses a ray of light in a near-infrared wavelength (845 nm). The scans are acquired in one second and then show by the computer using a false-color scale, in which paler colors (from white to red) indicate areas of high optical reflectivity, and darker colors (from blue to black) indicate areas of low optical reflectivity. The different colors of the OCT image result from the different optical properties of the tissue under exam. The OCT signal that comes from a given layer of tissue is a combination of the reflectiveness of that layer and the absorption and dispersal properties of the layers above. Using a wavelength in the near-infrared and a non-contact methodology, the exam is easily tolerated by the patient, who will require a good level of mydriasis (dilatation of the pupil) and if possible a stable point of fixation.
OCT thus makes it possible to explore in detail the posterior segment of the eye using different scanning modes (linear or circular), while being able to set the size of the scan (length and diameter). Linear scans are used in assessing retinal and macular morphology, while circular scans, which are performed around the optic nerve, are used in assessing the thickness of nerve fibres. OCT exams are of fundamental importance in diagnosing disorders of the vitreoretinal interface. They allow for the early identification of any morphological variations in the retina, including the presence of macular puckers or anatomical anomalies such as partial or full-thickness macular holes. They also make it possible to assess and measure in real time the diameter of the retinal lesion in case of holes or pseudo-holes, and the retinal thickness in the presence of, for instance, an intraretinal edema of the perilesional margins.
By measuring the retinal thickness, OCT also allows for the early identification of retinal edemas caused by various pathologies, and makes it possible to monitor them over time. The presence of membranes or vitreous tractions can be highlighted in the form of highly reflective plaques connected to the internal surface of the retina. In the case of subretinal neovascular membranes, OCT makes it possible to point the lesion, study its relationship with the pigmented epithelium layer above and assess its invasiveness. In conclusion, OCT is a non-invasive method that is easily tolerated by the patient and which is used in diagnosing and assessing various macular pathologies, and in assessing the extent of damage caused by glaucoma by measuring the thickness of nerve fibers. In recent years, technological innovations in the field of optical tomography had brought about the production of increasingly sophisticated and highly-evolved instruments: high-definition OCTs (Spectral Domain OCT, HD OCT).
These instruments make it possible to acquire a huge number of scans per second (25,000-40,000 scans per second vs. 400 scan/ sec with Time-domain OCTs), to acquire high-resolution scans (4-7 micron, close to histological resolution) of the retinal neuroepithelium and the RPE-choriocapillaris complex, and to acquire both frontal and three-dimensional images of the structures being examined. It was recently acquired the SWEPT SOURCE OCT tool that searches , capable of a top detail in retinal layers deep , with its 100,000 A- scans / sec . and thanks to the used wavelength of 1050mm, the DRI OCT- 1 has the ability to penetrate deeper in the retina, viewing ocular tissues such as the choroid and sclera in a span of time incredibly restricted . Another method of new generation used increasingly for the purposes of scientific research is one that sees the use of Angio OCT , can exploit scans acquired with high resolution using new algorithms capable of displaying the retinal vessels in without contrast media. This method opens new scenarios in the study and understanding of some retinal diseases such as exudative macular degeneration , retinal vascular diseases (diabetes mellitus and venous thrombosis)