Refractive Surgery in Keratoconus
Peter Farouk Saba;
Abstract
Keratoconus is a disease characterized by ectasia of the cornea in the absence of inflammation.
The etiology of keratoconus is still largely unknown. Although many biochemical and pathological changes at the structural and cellular level of the cornea abnormalities have been suggested.
Histopathological abnormalities have been documented in every layer of the keratoconic cornea.
Patients with keratoconus often complain of decrease in visual acuity which can be mild or severe depending on the amount of corneal tissue affected. Some develop photophobia, eye strain from squinting in order to read and monocular polyopia.
The clinical manifestations of keratoconus include steepening of the cornea, especially inferiorly, thinning of the corneal apex, scarring at the level of Bowman's layer, Vogt’s striae and Fleischer's ring. In advanced keratoconus, two external findings are associated with keratoconus diagnosis; Munson's sign and corneal hydrops.
Measurements of corneal thickness are required in keratoconus as the cornea tends to be thinner than normal; ultrasound pachymeter is the most widely used method.
Corneal tomography is another new diagnostic systems now allow us to image the back surface of the cornea and directly evaluate elevational changes of both anterior and posterior corneal surfaces; enabling point-to-point pachymetry. It is the creation of three-dimensional models created from two-dimensional cross sections, such as combination tomography and topography (Orbscan), Scheimpflug imaging based (Pentacam), ultrasound based and optical coherent principle based have greatly expanded our capability and precision in measuring the structure of the cornea.
Clinical studies on the effectiveness of intrastromal rings on keratoconus have so far been generally encouraging, a small incision is made in the periphery of the cornea and two thin arcs of PMMA are slid between the layers of the stroma. The segments push out against the curvature of the cornea, flattening the peak of the cone and returning it to a more natural shape. The procedure, offers the benefit of being reversible and exchangeable as it involves no removal of eye tissue.
Collagen crosslinking by the photosensitizer riboflavin and ultraviolet A-light is an effective means for stabilizing the cornea in keratoconus. Collagen crosslinking might become the standard therapy for progressive keratoconus in the future diminishing significantly the need for corneal transplantation and with more long-term experience, prophylactic treatment of keratoconus at an early stage might become possible.
Penetrating keratoplasty is the most treatment option used if the contract lens is no longer provides acceptable vision. In a transplant the weakened central portion of the patient's cornea is removed and the donor cornea is stitched into place. PK is associated with many complications as graft rejection, Suture-related problems and in a small number of cases, keratoconus has been found to occur in the newly transplanted cornea.
Although keratoconus has been described as a contraindication to LASIK but several studies have documented successful LASIK following PKP in patients with keratoconus also Surface ablations like PRK can be used for correction of postkeratoplasty astigmatism. Furthermore topography-guided surface ablation is a promising option to rehabilitate vision in contact lens-intolerant patients with forme fruste keratoconus.
An alternative is deep anterior lamellar keratoplasty (DALK), a
The etiology of keratoconus is still largely unknown. Although many biochemical and pathological changes at the structural and cellular level of the cornea abnormalities have been suggested.
Histopathological abnormalities have been documented in every layer of the keratoconic cornea.
Patients with keratoconus often complain of decrease in visual acuity which can be mild or severe depending on the amount of corneal tissue affected. Some develop photophobia, eye strain from squinting in order to read and monocular polyopia.
The clinical manifestations of keratoconus include steepening of the cornea, especially inferiorly, thinning of the corneal apex, scarring at the level of Bowman's layer, Vogt’s striae and Fleischer's ring. In advanced keratoconus, two external findings are associated with keratoconus diagnosis; Munson's sign and corneal hydrops.
Measurements of corneal thickness are required in keratoconus as the cornea tends to be thinner than normal; ultrasound pachymeter is the most widely used method.
Corneal tomography is another new diagnostic systems now allow us to image the back surface of the cornea and directly evaluate elevational changes of both anterior and posterior corneal surfaces; enabling point-to-point pachymetry. It is the creation of three-dimensional models created from two-dimensional cross sections, such as combination tomography and topography (Orbscan), Scheimpflug imaging based (Pentacam), ultrasound based and optical coherent principle based have greatly expanded our capability and precision in measuring the structure of the cornea.
Clinical studies on the effectiveness of intrastromal rings on keratoconus have so far been generally encouraging, a small incision is made in the periphery of the cornea and two thin arcs of PMMA are slid between the layers of the stroma. The segments push out against the curvature of the cornea, flattening the peak of the cone and returning it to a more natural shape. The procedure, offers the benefit of being reversible and exchangeable as it involves no removal of eye tissue.
Collagen crosslinking by the photosensitizer riboflavin and ultraviolet A-light is an effective means for stabilizing the cornea in keratoconus. Collagen crosslinking might become the standard therapy for progressive keratoconus in the future diminishing significantly the need for corneal transplantation and with more long-term experience, prophylactic treatment of keratoconus at an early stage might become possible.
Penetrating keratoplasty is the most treatment option used if the contract lens is no longer provides acceptable vision. In a transplant the weakened central portion of the patient's cornea is removed and the donor cornea is stitched into place. PK is associated with many complications as graft rejection, Suture-related problems and in a small number of cases, keratoconus has been found to occur in the newly transplanted cornea.
Although keratoconus has been described as a contraindication to LASIK but several studies have documented successful LASIK following PKP in patients with keratoconus also Surface ablations like PRK can be used for correction of postkeratoplasty astigmatism. Furthermore topography-guided surface ablation is a promising option to rehabilitate vision in contact lens-intolerant patients with forme fruste keratoconus.
An alternative is deep anterior lamellar keratoplasty (DALK), a
Other data
| Title | Refractive Surgery in Keratoconus | Other Titles | تأثير جراحات الانكسار الضوئي على القرنية المخروطية | Authors | Peter Farouk Saba | Issue Date | 2014 |
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