الفهرس 
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Abstract
Keratoconus is a non inflammatory disorder characterized by ectasia of the central or inferior portion of the cornea. it is a cone-shaped cornea, with the apex of the cone being forward . keratoconus causes distorted vision. Also known as conical cornea. It typically presents in adolescence and progresses in a variable manner.
The term keratoconus suspect(KCs) may correspond to a subclinical kc (true positive)or not(false positive) these denominations can be confusing and have been employed interchangeably, all of these terms are clearly synonyms of increase risk of post lasik ectasia(true positive for keratoconus suspect and forme fruste keratoconus) .
Subclinical keratoconus (kcs &ffkc) is related to the stage in the development of the disease before the symptoms are observed & thus is not detectable by routine clinical tests such as maximal contrast visual acuity measurement or slit lamp examination.
With the current diagnostic tools, the classification of the cornea as normal may not indicate the absence of subclinical kc .Thus the sensitivity of a computer assisted placido-based videokeratotopography is not sufficient.
Defining early subclinical kc suspicion by the presence of abnormal thinning & posterior elevation is certainly more reflective of the physiopathology of the kc disease.
The detection of keratoconus is a major concern in the screening of refractive surgical patients, since it is known that its presence weakens the corneal stroma and can lead to iatrogenic ectasia. While clinical keratoconus is reliably detected with corneal topography and slit-lamp examination, means to detect the possibility of keratoconus in its earliest stages prior to the presence of slit-lamp findings have been extensively explored, and it is generally agreed that the first detectable sign of keratoconus is a localised steepening seen with Placido corneal topography.
High order aberrations (HOAS) are more complex vision errors which have relatively unfamiliar names such as coma, spherical aberrations& trefoil. These types of aberrations can produce vision errors such as difficulty seeing at night, glare, halos, blurring, double vision .
The eyes of keratoconus (KC) patients have five to six times more high-order aberrations(HOAs) than in a healthy eye. Vertical coma was also the most widely mentioned aberration that had a significant difference between subclinical keratoconus and normal eyes. 103, 104
The Scheimpflug technology, is based on the Scheimpflug principle, which provides a 3D mapping of the cornea from 25 000 actual lifting points, including measurement of anterior and posterior surfaces of the cornea, corneal thickness and angle anterior chamber. The Scheimpflug technology can also provide corneal wavefront analysis to detect higher-order aberrations. 81
Sirius is a high precision instrument for the tomography of the anterior ocular segment and the 3D cornea analysis. By merging Scheimpflug technology, which allows the measurement of the internal ocular structures, with Placido topography, still unsurpassed for the measurement of elevation and curvatures.
Keratoconus suspect is a catchall term to indicate a patient with inferior or central steepening on topography that the clinician suspects may progress to keratoconus .The term is not synonymous with subclinical keratoconus, because the practitioner only knows it is subclinical once it has progressed to keratoconus. Many patients labeled as keratoconus suspects never develop clinical keratoconus.one needs to be cautious about the localized steepening of the cornea, which could be caused by various other factors than earliest keratoconus.
Reaching appropriate sensitivity levels for screening methods is crucial to identify patients with mild forms of subclinical keratoconus and to prevent iatrogenic keratectasia
This study was conducted with aim of to detect pattern of high order aberrations in any of suspect keratoconus eyes, the study was conducted on 100 eyes . Fifty subjects have been included as a normal group.
We have been used for data collection Schiempflug topography (Sirius CSO) to determine the chosen parameters as : Flat k , Steep k , mean k, Total RMS , RMS Coma , RMS Spherical aberration , Anterior elevation and Posterior elevation.
Exclusion criteria:
-Diagnosed keratoconus
Our result show that there were statistically significant differences in:
- Flat k , Steep k and Mean k.
Total RMS, RMS Coma.-
- Anterior and Posterior corneal elevation.
Between control group versus keratoconus suspect .
The RMS and elevation (anterior and posterior) parameters achieves larger than 90% in both sensitivity and specificity. The accuracy of
RMS of HOAS and Coma for discrimination between the KCs group and the control group was (70.5% and 72%) respectively. And the accuracy of the anterior and posterior elevation was (75.5% and 76%) respectively.