Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_2589_Библиотеки_им_академика_М_И_Перельмана

get disturbed by several metabolic, inammatory, post-surgical, vascular occlusions, and pharma­cological or degenerative conditions that lead to a collection of the intraretinal and the subretinal uid. The collection of uid in the extracellular space and some swelling of the cellular elements in the neurosensory retina is called macular oedema (ME). It causes signicant visual distur­bances [2]. The OPL in the watershed zone is the most favoured site for collecting uid, the fol­lowing common site being the INL.

ocular indirect ophthalmoscope. The direct oph­thalmoscope offers a magnication of 15× but only a 5° eld of view. In the hands of an experi­enced physician, it can detect the retinal MAS and tiny dot haemorrhages besides detecting ne new vessels on the optic disc. If any abnormality is detected on direct ophthalmoscopy, it calls for a referral to a retina specialist for a detailed reti­nal examination.

developed OCT technology to examine the cross­sectional details of biological tissues non­invasively. The basic principle of the use of OCT was similar to the use of ultrasonography. Like the sound waves, the reected light signals are detected as a low-coherence light beam passes through the retina, which either gets reected or transmitted. The initial time-domain technology used a time-of-ight delay between the reected light from a standard mirror and the time the light took to reect from the various structural inter­faces in the retina. It gave an axial resolution of ~10μ. Rapid technological developments have taken place in this eld. The spectral domain technology replaced the time-domain technol­ogy. It used a broad-band near-infrared superlu­minescent diode as a light source and a spectrometer to detect the Fourier transformation of the reected light from the tissue interfaces. The axial resolution improved to 1–3μ. At the same time, there was a tremendous change in the acquisition time to obtain information from each point of the retinal structures enabling a 3D con­struct of the retina. Apart from the cross-sectional image of the various microstructures, a layer-by­retinal layer information is obtained as en face imaging. On the other hand, the swept-source OCT (SS-OCT) uses a narrow band of a tunable laser source. The higher acquisition speed with SS-OCT to 100,000 scans/second compared to 50,000 scans/s with the SD-OCT has allowed a 12×12mm wider scan line compared to a 6 × 6 mm line scan with SD-OCT.Using a higher wavelength of 1050 nm than the SD-OCT (840 nm), SS-OCT gives higher resolution images (1μ) of the deeper retinal structures and choroid and can delineate even the choroido­scleral interface [13, 14].

relatively consistent and reproducible measure­ments, it has become standard practice to use OCT to study thickness and microstructure in all retinal studies. The ETDRS grid is used to dene the various sectors of the macula. The area within the innermost central 1-mm-diameter circle is the central subeld (CSF), the inner subeld is the area between the innermost circle, and a circle of 3-mm-diameter centred on the fovea and the outer subeld is the area between the 3-mm circle and the outer circle of 6-mm diameter. Each of these circles is further subdivided into sectors. The measurements of macular thickness with OCT come with an important caveat that differ­ent OCT machines come with different resolu­tions and different algorithms to identify reective interfaces; hence, the results obtained from one machine cannot be transposed to the other machine. Likewise, racial, gender, and age factors may vary the CST measurements [15]. The TD-OCT (Stratus; Carl Zeiss Meditec, Inc.) machine used an algorithm that measured the dis­tance between the ILM and the IS/OS junction. This junction is now named the ellipsoid zone. This zone of the outer segments has the maxi­mum density of mitochondria. This OCT gave a CST of 176.4μ in a North China population [16]. The SD-OCT machine (Cirrus; Carl Zeiss Meditec, Inc., Dublin, CA) measures the retinal thickness between the ILM and the mean reec­tance of the outer segment/RPE junction. It thus gives a higher value of CST by ~60μ than the Stratus [17].