الفهرس 
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Abstract
The choroid is a vascular sheet that forms the most posterior part of the middle coat of the eye, extending from optic nerve to the ciliary body. Its function is to provide the blood supply of the retinal pigmented epithelium (RPE) and the outer half of the sensory retina. Moreover, it absorbs excess light energy that penetrates the retina through its pigmented cells preventing light reflection. The choroidal thickness (CT) is 250 µm at the posterior pole, thickest at the macula with 300 µm and thinner anteriorly 100 µm.
There are many factors that may affect the CT, such as myopia, intraocular surgery, diabetic retinopathy, age-related macular degeneration, central serous chorioretinopathy, glaucoma, laser therapy and some medication.
Myopia has many classifications; one of them is by etiology into axial myopia and refractive myopia. Axial myopia is caused by the increase in the antero-posterior diameter of the eye and is subdivided into simple, degenerative and congenital. Refractive myopia is caused by the increase of the power of the eye either by increased curvature of the refractive media of the eye as cornea and lens or index myopia caused by increase in the index of the refractive media as in nuclear sclerosis of the lens.
Another classification is by degree into: low from -0.25 to -3.00 Diopters (D), medium from -3.00 to -6.00 D and high more than -6.00 D.
Likewise, hypermetropia is classified by anatomical features into: axial hypermetropia, curvature hypermetropia and index hypermetropia. Another classification is by degree into: low from +0.25 to +3.00 D, medium from +3.00 to+5.00D, and high more than +5.00D.
Choroidal thickness can be measured by the optical coherence tomography (OCT) where a light in the near-infrared spectrum 810 nm from a super luminescent diode is used to create high-resolution cross-sectional images of the retina. A partially reflective mirror is used to split the coherent light beam into a measuring beam and a reference beam. The measuring beam is directed into the eye, where successive optical interfaces (e.g., retinal layers, RPE, choriocapillaris) reflect the beam to a variable extent.
Early OCT models were based on time-domain (TD) OCT technology. More recent models take advantage of spectral domain (SD), or Fourier domain, technology which circumvents use of a reference beam, enabling faster acquisition of a larger amount of data, ultimately providing higher resolution images that can yield three-dimensional reconstructed views.
Time domain OCT light source wave length is 820 nm but in SD OCT it is 840 nm and with broader band width. The resolution in TD OCT is 10 μm axially and 20 μm in transverse direction. In contrast, in SD OCT they are 7 μm and 10 μm respectively which gives better visualization. The scanning speed in TD-OCT range from 400 to 500 A-scans per second in comparison with SD-OCT the speed reaches up to 52 000 A-scans per second.
This study was conducted on 60 eyes of 55 Egyptian healthy subjects, 24 males and 31 female were selected by convenient sample with the mean age of 36 years old, they equally divided into two groups, myopia group and hypermetropia group. All procedures were done at Demerdash hospital. This study it was used the RS-3000 SD-OCT machine.
In this study it was found that in hypermetropic group SFCT was 98.58±31.31μm and in myopic group, SFCT was 96.13±35.24μm.
In myopes group, there was negative correlation between refraction and SFCT. Similarly, there were mild negative (reverse) correlation between refraction and upper, lower, nasal and temporal parafoveal and upper and lower perifoveal CT. There was negative correlation between refraction and upper, lower and temporal mean parafoveal and perifoveal CT. There was negative correlation between refraction and mean total parafoveal and mean total perifoveal CT.
In hypermetropic group the lower parafoveal CT is the thickest and the nasal parafoveal CT was the thinnest these differences were statistically significant (P =0.001). However, in myopic group the temporal parafoveal CT is the thickest and the nasal parafoveal CT is the most thin also these were statistically significant (P =0.001).
In hypermetropic group the temporal perifoveal CT is the most thick and the nasal perifoveal CT is the thinnest and these differences were statistically highly significant (P = <0.001). in myopic group the temporal perifoveal CT is the thickest and the nasal perifoveal CT is the thinnest and these differences were statistically highly significant (P =<0.001).
The mean total parfoveal CT in hypermetropes is (105.5±31.6μm) and in myopes is (99.7±34.6μm) and the mean total perifoveal CT in hypermetropes is (116.4±30μm) and in myopes is (112.2±25.3μm). The difference between SFCT, mean total parafoveal and perifoveal in hypermetropic group is significant (P- value <0.001) and in myopic group is significant (P-value <0.001) also this comparison revealed that SFCT is the thinnest and perifoveal is the thickest in both groups.
There was no statistically significant difference between the choroidal thickness in the myopic group and the hypermetropic group.