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Ophthalmology 5th year, 6th lecture (Dr. Tara)


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The lecture has been given on Apr. 18th, 2011 by Dr. Tara.

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Ophthalmology 5th year, 6th lecture (Dr. Tara)

  1. 1. <ul><li>laser </li></ul><ul><li>in ophthalmology & refractive surgery </li></ul>
  2. 2. Laser <ul><li>Is an acronym for the instrument's mode of action: ( L ight A mplification by the S timulated E mission of R adiation ) . </li></ul><ul><li>Laser light is coherent: all photons, have same wavelength and are in phase. </li></ul><ul><li>A laser beam is collimated, i.e. the waves of light are parallel. </li></ul>
  3. 3. Production of laser energy
  4. 5. Effects of the Laser energy on ocular tissues <ul><li>Radiation wavelengths from 400 to 1400 nm can enter the eye and reach the retina. </li></ul><ul><li>Effects of laser energy on ocular tissues depend on:- </li></ul><ul><ul><li>the wavelength and pulse duration of the laser light </li></ul></ul><ul><ul><li>the absorption characteristics of the tissue (largely determined by the pigments contained within it). </li></ul></ul><ul><ul><li>the duration of exposure </li></ul></ul><ul><li>When the laser energy exceeds the threshold , causes tissue damage. </li></ul><ul><li>The effects can be ionizing, thermal or photochemical. </li></ul>
  5. 6. 1. Argon blue-green gas laser <ul><li>It is a mixture of 70% blue (488 nm) and 30% green (514 nm) light. </li></ul><ul><li>They are most commonly employed for retinal photocoagulation & for trabeculoplasty </li></ul><ul><li>Photocoagulation aims to treat the outer retina and spare the inner retina to avoid damaging the nerve fiber layer. </li></ul><ul><li>Argon green (blue screened out) photocoagulation of the macula does not cause direct retinal damage. It is well absorbed by melanin and haemoglobin , but Xanthophyll ( in the inner layer of the macula) absorbs blue light (but not green) and thus the use of blue light at the macula is contraindicated in order to avoid, direct damage to the retina in this region. </li></ul>
  6. 7. 2. Diode lasers <ul><li>It emit an infrared (wavelength of 810 nm) in continuous wave mode. It is absorbed only by melanin i.e why most commonly used for retinal photocoagulation ; </li></ul><ul><li>low scattering of this wavelength ensures good penetration of the ocular media and of oedematous retina. It also penetrates the sclera. Thus if retina is obscured from view through the pupil, coagulation may still be performed by placing the probe on the sclera. </li></ul>
  7. 8. 3. Nd-YAG laser <ul><li>The neodynium-yttrium-aluminium-garnet: laser emits infrared (1064 nm) radiation, it is Continuous wave (C'W) </li></ul><ul><li>It is commonly used to:- </li></ul><ul><ul><li>the posterior capsule of the lens following cataract surgery </li></ul></ul><ul><ul><li>the iris (Peripheral iridotomy) in narrow angle glaucoma. </li></ul></ul>
  8. 9. 4-The excimer laser <ul><li>Its name derived from ‘excited dimer' two atoms forming a molecule in the excited state but which dissociate in the ground state. </li></ul><ul><li>In clinical use employ an argon-fluorine (Ar-F) dimer laser medium to emit 193 nm ultraviolet (UV) radiation. </li></ul><ul><li>High absorption of UV by the cornea limits its penetration. Each photon has 6.4 eV, sufficient to break intramolecular bonds. </li></ul><ul><li>The delivery of a relatively high level of energy to a small volume of tissue causes tissue removal (i.e. ablation ) </li></ul><ul><li>The ablation depth may be precisely determined. </li></ul>
  9. 10. Laser refractive surgery <ul><li>Although refractive errors are most commonly corrected by spectacles or contact lenses, laser surgical correction is gaining popularity. </li></ul><ul><li>The excimer laser precisely removes part of the superficial stromal tissue from the cornea to modify its shape. Myopia is corrected by flattening the cornea and hypermetropia by steepening it. </li></ul><ul><li>In photorefractive keratectomy (PRK),the laser is applied to the corneal surface. In laser assisted in situ keratomileusis (LASIK), a hinged partial thickness corneal stromal flap is first created with a rapidly moving automated blade, the flap is lifted and the laser applied onto the stromal bed. </li></ul>
  10. 11. Photorefractive Keratectomy <ul><li>Schematic representation of the pattern of corneal ablation necessary to correct hyperopia with the excimer laser. The greatest amount of tissue is removed from the periphery of the ablation zone with a gradual transition toward the anterior corneal surface. </li></ul>
  11. 12. Photorefractive Keratectomy <ul><li>Schematic representation of the excimer laser flattening the central cornea for the treatment of myopia . </li></ul>
  12. 13. Keratomileusis <ul><li>Laser in situ keratomileusis: </li></ul><ul><li>A. Hinged flap made with microkeratome. </li></ul><ul><li>B. Laser ablation (shaded area destroyed) of bed to alter curvature. </li></ul><ul><li>C. Flattened cornea after replacement of flap. </li></ul>
  13. 14. Laser In Situ Keratomileusis <ul><li>Schematic representation of laser in situ keratomileusis with formation of a corneal flap followed by stromal ablation. The flap is then easily replaced into its original position. </li></ul>
  14. 15. Laser In Situ Keratomileusis <ul><li>The flap is replaced and the interface is irrigated under oblique illumination to assure that all debris is removed. </li></ul>
  15. 16. Photorefractive keratectomy ( PRK ) Indications <ul><li>Stable myopia up to 6D with astigmatism no more than 3D </li></ul><ul><li>Hypermetropia up to 2.5D </li></ul>Main complication Subepithelial haze which usually resolves after 1-6 months Reshaping of cornea by excimer laser ablation of Bowman layer and anterior stroma Technique
  16. 17. Laser in-situ keratomileusis (LASIK) Indications - similar to PRK but corrects higher degrees of myopia <ul><li>Thin flap of cornea fashioned </li></ul><ul><li>Bed treated with excimer laser </li></ul><ul><li>Flap repositioned </li></ul>Complications <ul><li>Wrinkles in flap </li></ul><ul><li>Cellular interface proliferation </li></ul>Technique
  17. 18. FACTORS INFLUENCING RETINAL PHOTOCOAGULATION <ul><li>Factors in retinal photocoagulation </li></ul>
  18. 19. Retinal photocoagulation <ul><li>The goals in retinal photocoagulation vary with the clinical entity being treated. For example, in pan photocoagulation for diabetic retinopathy the aim is the elimination of abnormal retinal blood vessels through direct treatment to the blood vessels or destruction of the ischemic areas of the retina . </li></ul>
  19. 20. LASER IRIDECTOMY <ul><li>Laser iridectomy is now the standard surgical treatment of angle-closure glaucoma. </li></ul>
  20. 21. LASER TRABECULOPLASTY <ul><li>Laser trabeculoplasty is probably the most widely employed laser technique for the treatment of glaucoma. </li></ul><ul><li>It also is useful in some secondary open-angle glaucomas such as exfoliation syndrome glaucoma and pigmentary glaucoma; </li></ul><ul><li>In eyes that have had previous filtering surgery; </li></ul><ul><li>and in eyes that have had surgical or laser iridectomies because of acute angle-closure glaucoma attacks. </li></ul>
  21. 22. A. NVD and a small vitreous hemorrhage. Panretinal photocoagulation was given. B. Two months later, the NVD has completely regressed.
  23. 35. References. <ul><li>Clinical optics,2 ND edition 1997. </li></ul><ul><li>Wright interactive ophthalmology. </li></ul><ul><li>By K.Wright ,1997 on CD. </li></ul><ul><li>Duane's ophthalmology ,basic science, on CD,2003. </li></ul>