انت هنا الان : شبكة جامعة بابل > موقع الكلية > نظام التعليم الالكتروني > مشاهدة المحاضرة
الكلية كلية الطب
القسم الجراحة
المرحلة 5
أستاذ المادة اسعد جعفر عبد السادة الطائي
4/19/2011 8:46:59 PM
Uses of Laser & Cryotherapy in Ophthalmology
***********************************************
Laser is Light Amplification by Stimulated Emission of Radiation.
Properties of laser light:
The unique properties of laser light make it particularly suitable for many medical applications. These properties are: monochromaticity, directionality, coherence, polarization and intensity. The first 4 properties enhance the fifth, which is the most important (intensity which means brightness or radiance). Monochromaticity allows target selection making laser a unique tool in ophthalmology, since the eye has got several pigments and it is designed to allow light transmission to most of its structures.
Elements of laser:
All ophthalmic lasers currently in use require 3 basic elements:
(1) Active medium: that emits coherent radiation.
(2) A means of energy input known as pumping.
(3) The opportunity of oscillation and amplification by stimulated emission of radiation. The opportunity is created by the optically resonant cavity formed by the mirrors. One mirror is 100% reflectant and the other is less than 100% reflectant. The distance between the mirrors must equal a multiple of the wavelength of light emitted by the active medium. Lasers are usually named after their active media.
Types of media:
1- Gas: e.g: argon, krypton, carbon dioxide or helium with neon.
2- Liquid e.g: a dye.
3- Solid: an active element supported by a crystal e.g: neodymium supported by yttrium-aluminum-garnet (Nd:YAG) and erbium supported by yttrium-lanthanum-fluoride (Er:YLF) or a semiconductor (diode).
Types of pumps:
1- Electrical discharge: between electrodes in the gas for gas lasers.
2- Other lasers: to pump dye lasers.
3- Incoherent light: xenon arc flash lamp for solid crystals.
Delivery systems:
l- S/L biomicroscopy: using special contact lenses if needed.
2- Indirect ophthalmoscopy: for lesions far in the peripheral fundus.
3- Endolasers: along with fiberoptic endoscopic devices as in closed intraocular microsurgery (ClOM) or endolaser DCR.
Mechanisms of laser effects:
A- Photo-coagulation: depends on the absorption of light energy by ocular pigments (melanin, haemoglobin & xanthophyll) and its conversion into heat. The objective of treatment is to produce a therapeutic burn to a preselected area of the eye causing minimal damage to surrounding tissues.
B- Photo-disruption: disruption of tissue due to the blast effect of the very high energy content producing mini punctures and tears in membranes, capsules and thin sheets of tissues.
C- Photo-evaporation: long wave-length infrared laser that is absorbed by water & therefore will not enter the interior of the eye. These lasers are used for evaporating away surface lesions such as lid tumours, bloodless incisions in skin or sclera, controlled superficial skin burns that can tighten the eyelid skin for cosmetic improvement. This class of laser includes carbon dioxide, erbium, & holmium lasers.
D- Photo-decomposition: by the production of excited dimmer (excimer) UV laser light which causes disintegration of organic molecules by breaking molecular bonds, thus allowing precise etching of structures such as the cornea or the lens.
Types of useful effects possible:
a- Destruction of an unwanted lesion. b- Creation of adhesions.
c- Change of shape of a membranous structure. d- Closure of harmful blood vessels.
e- Stopping leakage of diseased vessels or membranes.
f- Puncturing or cutting membranes or vitreous or stitches. g- Eating away, or melting away tissues by photodecomposition (ablation).
Uses of laser in ophthalmology:
l- Therapeutic:
a- On the retina (retinal photocoagulation):
i- Retina and disc neovascularization due to diabetes or central retinal vein occlusion.
ii- Exudative maculopathy & retinopathy to treat leaking areas depicted on fluorescine angiography.
iii- Sealing retinal breaks in retinal detachment surgery.
iv- Treating lesions predisposing to retinal detachment.
v- Treating subretinal neovascular membranes (SRNVM).
vi- Small retinal/choroidal tumours posterior to the equator.
vii- Retinopathy of prematurity.
b- Uses in glaucoma:
i- Laser peripheral iridectomy (LI).
ii- Argon laser trabeculoplasty (ALT).
iii- Treating an iris bombe.
iv- Making an opening in the anterior vitreous face in malignant glaucoma.
v- Laser gonioplasty (to deepen the angle of AC).
vi- Trans-scleral cyclophotocoagulation.
c- Uses in cataract:
i- Treating posterior capsule opacification.
ii- Laser anterior capsulotomy.
iii- Lens photoablation (phakolasers).
iv- Laser coreoplasty.
v- Laser synechialysis.
vi- Laser vitreolysis.
vii- Treatment of epithelial downgrowth.
d- Other uses:
i- Laser punctoplasty. ii- Trichiasis treatment. iii- Thermal keratoplasty. iv-Treatment of eyelid tumours. v- Cosmetic laser eyelid surgery.
e- Uses of excimer lasers:
i- Photorefractive keratectomy (PRK).
ii- Superficial laser keratectomy e.g: to treat band keratopathy.
iii- Laser in situ keratomeleusis (LASIK).
f- Photodynamic therapy: (PDT) consists of two essential components, the photosensitizer (dye) that accumulates in the target tissue and a specific laser light corresponding to the absorption peak of the dye. Verteporfin and a diode laser are used in PDT to treat subretinal neovascular membrane (SRNVM) in diseases like age-related macular degeneration (AMD) and presumed ocular histoplasmosis syndrome (POHS). The photosensitizer molecule is excited following the light absorption from the laser. The energy from the excited molecule is transferred to release free radicals and production of singlet oxygen. Both mechanisms destroy SRNVM.
II- Diagnostic:
a- Laser interferometry: to assess macular function in the presence of cataract so we can have an idea about the outcome of surgery. By this method we utilize the interference patterns of 2 laser beams.
b- Diagnosis of the extent of epithelial downgrowth as a complication of cataract surgery.
c- Laser aiming beam test for the diagnosis of macular hole.
d- Laser diagnostic imaging: optical coherence tomography, confocal scanning laser ophthalmoscopy, panoramic-200 are new confocal imaging high resolution 3-dimensional cross sectional techniques used to study the central retina, optic disc & retinal nerve fiber layer as well as possibly the cornea for diagnosis & follow-up of glaucoma, macular diseases, and many other diseases.
Cryotherapy in ophthalmology:
Freezing can be made use of clinically in one of 3 ways:
(1) In cataract surgery: the relatively old method of intracapsular cataract extraction (ICCE), the lens is removed as a whole using a cryoprobe ( the frozen tip sticks to the lens capsule).
(2) Cryoablation: Freezing is used to destroy unwanted living tissues by subjecting them to a double cycle of freeze-thaw procedure. Examples:
a- Malignant eyelid tumours: e.g: BCC that is relatively small or situated adj acent to lacrimal canaliculi because their epithelium is resistant to freezing.
b- Cyclodestruction: treating intractable glaucoma in eyes with no useful vision to relieve pain.
c- Trichiasis:
d- Retinoblastoma: small tumours anterior to the equator where photocoagulation is difficult to apply.
e- Retinopathy of prematurity: successful in 75% of cases.
f- Epithelial downgrowth.
(3) Cryotherapy used to seal adjacent membranes: a- Lesions predisposing to retinal detachment. b- In retinal surgery to seal retinal breaks.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
الرجوع الى لوحة التحكم
|