الفهرس 
Diagnosis of astigmatismOnly 14 pages are availabe for public view
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Abstract
Astigmatism is the condition of refraction of the eye wherein a point of focus of light cannot be formed upon the retina, but is refracted into two focal lines separated from each other by focal interval.
Clinically detectable refractive astigmatism reportedly is present in as many as 9 out of 10 eyes. However, refractive astigmatism in most of these eyes would not be clinically significant.
Astigmatism is due to corneal, lenticular causes or due to obliquity of eye elements as in high myopia with posterior staphyloma. It may be regular or irregular, regular type is classified into simple, compound and mixed astigmatism.
Irregular astigmatism can occur as a result of trauma, keratoconus, corneal disease, corneal infection and others. Sometimes it is the result of surgical procedures, such as pterigyum removal, penetrating keratoplasty (PK), and cataract extraction.
Clinically, astigmatism is presented by diminution of vision, blurring, discomfort and headache. Diagnosis of astigmatism can be done by ophthalamoscopy, retinoscopy, keratometry or refractometry either automated or conventional.
Recent techniques in diagnosis also include corneal topography which can map the surface curvature of the cornea, the procedure is carried out in seconds and is completely painless.
Astigmatism is corrected either by optical or surgical methods. Optical methods include eye glasses with cylindrical or spherocylindrical lenses and contact lenses either soft toric, hard or rigid gas permeable contact lenses.
Surgical techniques to correct astigmatism can be divided into three major categories: Incisional, lamellar and intraocular. Incisional methods for correction of astigmatism include astigmatic keratotomy (AK), wedge resection, limbal relaxing incisions (LRIs), lamellar methods include photoastigmatic refractive keratectomy (PARK), laser-assisted in situ keratomileusis (LASIK) and intraocular methods include toric intraocular lens implantation either in phakic or aphakic eyes.
AK is a simple surgical procedure aims to flatten the steep cylinder axis which accompanied by steeping of the flat axis which known as coupling, it is done either by straight transverse or arcuate incisions.
Limbal relaxing incisions (LRIs) are a modification of AK, They are placed on limbus to reduce astigmatism, and it can be performed at the time of cataract surgery or as an independent procedure.
More recently, excimer laser photoastigmatic refractive keratectomy (PARK), conventional LASIK, IntraLase, LASEK, Epi-LASIK techniques and wavefront-guided LASIK have been used to reduce astigmatism.
In PARK the outermost layer of the cornea (epithelium) is manually scraped away and excimer laser reshapes tissues located on the underlying (stromal) surface of the cornea by non radially symmetrical ablation with greater ablation in the steep axis and minimal or no ablation in the flat axis of astigmatism.
LASIK uses a microkeratome metal blade to cut a corneal flap and excimer laser is applied to the underlying stroma. After the procedure, the flap is returned to its position.
Wavefront guided ablation uses the same hardware as conventional LASIK but the excimer laser is guided by advanced software called Wavefront. In this procedure vision is measured by mapping the optical system using technology that is more accurate than traditional methods. The map is so exact likened to a fingerprint. Error induced from cyclotorsional movements will be significant. So Iris image registration permits compensation for cyclorotational errors and allows better alignment of astigmatic correction.
The correction of astigmatism includes also toric intraocular lens implantation either in phakic or aphakic eyes.
The phakic toric IOLs may be placed in anterior chamber or in posterior chamber, anterior chamber phakic toric IOL may be angle supported or iris supported type.
Irregular astigmatism can be surgically treated by topographically guided corneal ablation. In cases of high but stable corneal irregular astigmatism, treatment by superficial anterior lamellar keratoplasty (SALK) with or without additional topographically guided ablation can be done. In treating high and unstable corneal irregular astigmatism, ICRS, penetrating keratoplasty (PK), or deep anterior lamellar keratoplasty (DALK) with or without the use of the laser (LA-DALK) are the best procedures.
The Intacs based on the idea that implantation of rings in the corneal stroma can flatten the cornea and makes its anterior surface more regular, thus correcting both myopia and irregular astigmatism, allows the patient to use spectacles and delays or eliminates the need for keratoplasty.
SALK for the correction of an induced high irregular corneal astigmatism after single or multiple corneal refractive procedures had an extremely low anatomical complication rate, SALK alone may compensate only for very low irregularities and, when they are moderate or high, topographically guided ablation must be performed sometime after the lamellar surgery.
DALK is done by removing all stromal tissue overlying leaving just the recipient Descemet’s membrane and endothelium and creating the smoothest possible recipient bed.
FALK using IntraLase femtosecond laser, designed for use during corneal transplantation allows the surgeon to create complex, shaped incisions that allow the recipient and transplant tissue to fit together much like an intricate puzzle. The result is a potentially stronger graft that may require fewer sutures with patients reporting faster healing and visual recovery.
By applying the excimer laser in the PTK mode and using masking substances, such as viscoelastics, several groups, have been able to regularize some irregular corneas, thereby leaving a fairly smooth surface.