▃Cryotherapy

Cryotherapy or cryopexy involves the application of intense cold by a cryoprobe in certain eye diseases. It is used:

•As a pain treatment that produces tissue injury by localized freezing temperature but it can leave numbness or tingling as a side effect of cryotherapy treatment.

•To treat localized areas in some cancers such as prostate cancer.

•To treat abnormal skin cells but can cause redness and irritation of the skin, as side effects, which are generally temporary.

■Cryounit

It is made of the following:

•A cryoprobe is made of silver, a highly conducting metal. Cryoprobes are available in different sizes for different indications (Table 25.1).

•A cryomachine to which the cryoprobe is attached.

•Coolant: Liquid nitrogen, nitrous oxide, or carbon dioxide, is used as a cooling agent.

Temperature produced (−20 to −80) depends upon the size of the cryoprobe tip, duration of freezing process, and gas used.

■Mode of Action

Cryotherapy produces different actions on tissues in different disease conditions:

•Tissue necrosis as in cyclocryopexy in glaucoma.

•Tissue adhesion as in retinal detachment.

•Vascular occlusion as in Coat’s disease.

•Ice crystal formation as in cataract extraction.

■Indications

Cryotherapy is used in the following eye problems:

•Lids: In lids, it is used for trichiasis, Molluscum contagiosum, hemangioma, and basal cell carcinoma.

•Conjunctiva: It is used for giant papillae of vernal keratoconjunctivitis.

•Lens: It is used for intracapsular cataract extraction but, nowadays, its use in cataract surgery is limited because of extracapsular cataract extraction.

•Glaucoma: In absolute and neovascular glaucomas, the cyclodestruction is done with cryo application to the ciliary body, which destroys the ciliary epithelium and reduces the aqueous formation and intraocular pressure (IOP). For cyclocryotherapy, the tip of cryoprobe is placed 1.5 mm from the limbus on bulbar conjunctiva. The tip of probe at −80°C overlies the pars plicata and is applied for 60 seconds. 3 to 4 cryo applications per quadrant are given. If IOP is not controlled even after first cyclocryopexy, it can be repeated after 1 month.

•Retina: Cryotherapy is used in more anteriorly placed lesions in retina or in opaque media. Posteriorly placed retinal lesions are treated with laser photocoagulation. In retina, cryopexy is used for the following problems:

◊For prophylactic treatment of retinal degeneration and retinal breaks.

◊Retinal detachment.

◊Retinopathy of prematurity (ROP) to prevent neovascularization.

◊Cryodestruction of hemangioma and small-sized retinoblastoma.

▃Lasers in Opthalmology

LASER is an acronym for “Light Amplification by Stimulated Emission of Radiation.”

■Properties of Laser Light

The properties of laser light that make it useful to ophthalmologist are as follows:

•Monochromaticity means that laser light is only a single-color light and emits only one wavelength. To prove the monochromaticity of laser light, we can use prism. When white light is passed through prism, it breaks into component colors (VIBGYOR) but if we a pass beam of laser light through the prism, then the beam is only changed in direction and not separated into different colors.

•Coherence means that the light waves are in phase which improve focusing. Laser light is much more coherent than ordinary light. The incoherent waves have no relationship to each other and do not have the same wavelength.

•Collimation is the process by which a beam of radiant electromagnetic energy is lined up to minimize divergence or convergence. A collimated beam is a bundle of parallel rays. Laser light does not spread out or diverge and stays together in a beam. Divergence is more in ordinary light in comparison with laser light. Usually a laser generates a beam of less than 0.001 rad which means that a beam from the laser will spread to less than 1 foot diameter circle at a distance of 1000 feet from the laser.

•Ability to be concentrated in a short-time interval.

•Ability to produce nonlinear effects.

The combination of these properties makes laser light focus 100 times better than ordinary light (Table 25.2).

■Production of Laser Beam

The production of laser beam involves three basic components:

•Laser medium: Solid, liquid, or gas.

•Exciting methods (for exciting atoms or molecules in the medium): Light and electricity.

•Optical cavity (laser tube) around the medium which acts as a resonator.

■Delivery System

The laser can be delivered to the eye by:

•Slit-lamp biomicroscope: It is most common and the delivery is transpupillary.

•Laser indirect ophthalmoscope: The delivery is transpupillary in this method also. It provides wider field, better visualization, and laser application in hazy medium.

•Transscleral probes.

•Endolaser probes: It involves fiber-optic probes used within the eye during pars plana surgery.

■Classification of Lasers

•Gas lasers:

◊Argon.

◊Krypton.

◊He-Neon.

•Solid-state lasers:

◊Nd:YAG.

◊Ruby.

◊Krypton chloride.

•Excimer laser:

◊Argon fluoride.

◊Krypton fluoride.

•Diode laser:

◊Gallium–Aluminum–Arsenide (GaAlAs).

Choice of optimal wavelength depends on the absorption spectrum of the target tissue. There are three types of ocular pigments: hemoglobin, xanthophyll, and melanin. Melanin is present in retinal pigment epithelium (RPE) and choroid, hemoglobin in blood vessels, and xanthophyll in inner and outer plexiform layers of macula.

Argon laser emits coherent blue–green light and is absorbed by all three ocular pigments. Krypton laser emits red light and is well absorbed by melanin but poorly or not at all by hemoglobin and xanthophyll. Therefore, the main effect of krypton is on choroid, RPE, and outer retinal layers. Argon blue is not recommended for treating macular lesions which are absorbed by xanthophyll. The main advantage of krypton over argon is that it is not absorbed by xanthophyll pigment, so the lesions inside the foveal avascular zone (FAZ) may be treated with krypton laser.

■Wavelength and Tissue Interactions of Lasers

Three basic light tissue interactions are:

•Photocoagulation.

•Photodisruption.

•Photoablation.

Photocoagulation

Photocoagulation refers to denaturation (coagulation) of proteins following laser light exposure on target tissue. When exposed laser light is absorbed by the tissue pigments, heat is generated which denatures proteins. Rise in temperature of approximately 10 to 20°C will cause coagulation of tissue. Lasers used for photocoagulation are argon, krypton, Nd:YAG, and diode.

Photodisruption

The laser ionizes the electrons of the target tissue producing plasma and shock waves which disrupt and exert a cutting effect upon the ocular tissues. Nd:YAG laser is used for photodisruption.

Photoablation

Laser breaks the chemical bonds between cells that hold tissue together. It converts them to small molecules that essentially diffuse away. Excimer laser (argon fluoride 193 nm) is used for the purpose of remodeling the cornea in refractive surgery.

Lasers operate in ultraviolet (UV), visible, and infrared regions (Table 25.3). Usually:

•Visible wavelength results in photocoagulation.

•Ultraviolet yields photoablation.

•Infrared results in photodisruption and photocoagulation.

■Mode of Operation

Lasers can be used in distinct modes of operation, the most important of which are:

•Continuous wave lasers: This laser is continuously pumped which emits light continuously.

•Pulsed lasers: These are lasers which emit light not in a continuous mode, but rather in the form of optical pulses. Depending on the pulse duration, pulse energy, pulse repetition rate and wavelength required, very different methods of pulse generation and very different types of pulse lasers are used.

■Uses

Lasers are used owing to their diagnostic as well as therapeutic uses.

•Diagnostic uses:

◊Scanning laser ophthalmoscopy.

◊Laser interferometry.

◊Wavefront analysis.

◊Optical coherence tomography (OCT).

•Therapeutic uses:

◊Lids.

◊Lacrimal surgery.

◊Anterior segment.

◊Posterior segment.

Diagnostic Uses

Lasers are widely used as tools in imaging and diagnosis, for example, in early detection of cancer and other diseases in patients. The diagnostic applications of lasers include:

•Scanning laser ophthalmoscopy (SLO).

•Laser interferometry.

•Wavefront analysis.

Scanning Laser Ophthalmoscopy

In the scanning laser ophthalmoscope (SLO), a narrow laser beam illuminates the retina one spot at a time, and the amount of reflected light at each point is measured. The amount of light reflected back to the observer depends on the physical properties of the tissue which, in turn, define its reflective, refractive, and absorptive properties. Media opacities, such as retinal hemorrhage, vitreous hemorrhage, and cataract, also affect the amount of light transmitted back to the observer.

Scanning laser acuity potential (SLAP) test is performed on SLO and determines a subject’s potential visual acuity.

Laser Interferometry

Interferometry is a measurement method using a phenomenon of interference of waves. Laser interferometer uses a laser as a light source. As the light enters into the eye, the two beams interfere with each other and form light and dark fringe patterns on the retina. A rough estimate of visual acuity can be made by changing the distance between two beams, resulting in the alteration of fringe pattern.

Wavefront Analysis

Lasers are used in the measurement of complex optical aberrations of the eye using wavefront analysis.

Therapeutic Uses

Lasers can be used for various structures of the eye, for example (Fig. 25.1):

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