الفهرس 
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Abstract
Keratoconus is a condition in which the cornea assumes a conical shape, as a result of non inflammatory thinning of the corneal stroma. The corneal thinning induces irregular astigmatism, myopia, and protrusion, leading to mild to marked impairment in the quality of vision. It has Incidence of approximately 1 in 2000 in the general population. Keratoconus, classically, has its onset at puberty and is progressive until the third to fourth decade of life, when it usually arrests.
Keratoconus has a disproportionate impact on the quality of life, and causes significant loss of productivity as it is a disease of young.
Despite extensive investigations, the etiology and underlying mechanism of stromal thinning in keratoconus is not understood.
Etiology is most likely multifactorial. A positive family history has been reported in 6-8% of the cases and its prevalence in first-degree relatives is 15-67-times higher than the general population.
It is most commonly an isolated condition, despite positive association between keratoconus and many conditions has been suggested, including atopy, eye rubbing, contact lens wear, handedness, cardiovascular disease (especially mitral valve prolapse), ocular trauma, collagen vascular disorders, pigmentary retinopathy, Marfan’s syndrome, and Down’s syndrome.
The treatment of keratoconus depends on the severity of the disease.
Rigid contact lenses are the principal optical means for managing visual impairment produced by keratoconus in early stage of disease.
In advanced cases with severe corneal irregular astigmatism and stromal opacities penetrating keratoplasty (PKP) may be required to restore visual function.
Several devices are available for detecting keratoconus by measuring anterior corneal topography. These range from simple inexpensive devices, to expensive sophisticated devices. Computer-assisted videokeratoscope, which generate color-coded maps and topographic indices, are an excellent tool for confirming the diagnosis of keratoconus even when signs of the disease are not obviously apparent at the slitlamp.
Methods of measuring corneal topography fall into two broad categories. Those which use the reflection principal to measure the slope of the corneal surface and calculate the curvature and elevation, they are represented by the placido disc systems. The other category includes elevation based systems that measure the true corneal shape directly in terms of elevation from which curvature can be calculated, they are represented by rasterphotogrammeter, slit scanning (Orbscan) and the Scheimpflug camera, which had become the most important and accurate topographic device. These new diagnostic systems now allow to image the back surface of the cornea enabling point-to-point pachymetry they also enable modeling of the cornea, yielding information about curvature, power, and shape .
The Pentacam is the first instrument to use a rotating Scheimpflug camera to take multiple images of the anterior segment and use these to generate three-dimensional images and calculate measurements of the eye. It provides:
• Detailed Scheimpflug images of the anterior segment from the posterior surface of the crystalline lens to the anterior corneal surface.
• Three-dimensional visualization and animation of the anterior segment.
• Measurements of anterior chamber angle, chamber volume, chamber depth, pupil diameter and corneal characteristics such as eccentricity, central radius and astigmatism.
• Pachymetry (limbus to limbus, accurate to ±5 μm). We can locate any point on the cornea manually, onscreen, and see the thickness at that point. The map notes the thinnest region.
• Densitometry data for the cornea and the crystalline lens, including the subcapsular layer, shown both graphically and quantitatively. The quantitative data make it easy to identify a developing cataract, categorize existing cataracts, and document their development.
• Topographic maps of the anterior and posterior corneal surfaces, based on measurement of 25,000 elevation points (when using the two-second scan), including tangential and sagittal (axial) curvature and limbus-to-limbus elevation, with freely selectable reference bodies.
• A “true net power map” that shows the refractive power of the cornea at any given point. (The instrument calculates the refractive power of both surfaces and adds them together.)
Because the Pentacam only measures to the posterior capsule of the crystalline lens, it can’t provide a measure of axial length. Also, because it’s optically based, it can’t measure from sulcus to sulcus.
The key advantages of the rotating imaging process are the precise measurement of the central cornea, the correction of eye movements, the easy fixation for the patients and the extremely short examination time.
The high reliability of the rotating Scheimpflug system is a result of the measuring principle in which highly precise measurements are obtained through many repeated, mostly central, corneal measurements.