2.5. Technical characteristics of the indirect diode laser

Monochromatic laser light allows the specific selection of a wavelength that is going to be

Laser light is much more intense, which makes it possible to use low energies to obtain a

Therapeutic energy can be pre-selected by the ophthalmologist according to the effect on the retina: lower energies are used initially which are progressively increased, up to the moment

Clinical effects of laser photocoagulation are obtained by thermal reaction. When the temperature in the tissue reaches a critical level, proteins are denatured and coagulation occurs. The factors that influence the results of coagulation for a certain power are: the energy of a specified wavelength, the spot size and the exposure time. According to the degree of coagulation, ocular reaction manifests by cell proliferation, cell migration, and scar formation [5].

Tissue effects of lasers depend upon the interaction between the laser wavelength and ocular

Melanin is found in the RPE as melanosomes and in the choroid as granules. The maximum absorption of melanin is for wavelengths of 400–600 nm, followed by blue, green, red and infrared. Subsequently, shorter wavelengths are better absorbed by melanin as compared to longer ones. When it comes to the penetration through tissues of the laser radiation, longer wavelengths will manifest their effect deep in the choroid. Because the quantity of melanin varies between individuals and from one region to another of the fundus, the coagulation effect of longer wavelengths is unequal. Longer wavelengths require higher energies to obtain

Hemoglobin is more selective in terms of absorption. Its maximum absorption is for blue,

Xanthophyll pigment is located exclusively in the fovea, and it absorbs the blue and blue-green laser radiations. Because of the damaging effects on the central vision of these radiations, they

In ischemic retinopathies, such as ROP, the indirect effect is used to induce the regression of

Several mechanisms of action have been described. It is postulated that tissue destruction by laser photocoagulation decreases the need for oxygen of the tissues, and in the same time, it

Lasers that create thermal reactions have direct and indirect effects on the ocular tissues.

green and yellow wavelengths, whereas for red and infrared, there is no absorption.

Also there is no chromatic aberration allowing the obtainment of a small retinal impact.

absorbed by a specific pigment in the eye according to the ocular condition.

Parallelism allows the obtainment of small retinal lesions in the periphery.

clinical effect with a significantly lower risk for complications.

where the desired effect is obtained [5].

pigments: melanin, hemoglobin and xanthophyll.

2.4. Laser retinal photocoagulation

110 Laser Technology and its Applications

similar effects with the shorter ones.

are no longer used in the clinical practice [5].

new vessels that appear due to retinal ischemia.

lowers the stimulus for the production of angiogenic factor.

Diode laser has an infrared emission (810 nm), and it cools in the environment and has a life span of 30,000 h. The transmission optic system is very simple: it is constituted by mirrors leading the laser beam from the slit lamp to the patient's retina. When attached to the indirect ophthalmoscope, such as in the ROP treatment, this role is taken by a system of fiber optics.

The laser fiber is connected from the laser to the indirect ophthalmoscope, and the laser impacts are delivered by acting on a pedal, which is also connected to the laser. The console allows the operator to choose the adequate parameters of the laser impact: dimension, exposure time and power. The operator wears the helmet throughout the treatment and moves his head in order to deliver the light from the indirect ophthalmoscope and the laser beam into the infant's eye (Figure 1).

Figure 1. Indirect diode laser.

However, at low exposure times, high powers are required in order to obtain a clinical effect. This is associated with the risk of explosive effects. Usually, exposure times between 200 and

Indirect Diode Laser in the Treatment of Retinopathy of Prematurity

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When laser energy is delivered through the indirect ophthalmoscope, the dimension of the laser spot effectively arriving on the retina is the result of the distance between the helmet (the

The number of laser spots is correlated with the area of nonvascular retina. Practically, all the surface of non-vascular retina should be destroyed by laser. By consequence, more the disease is posterior, more laser spots are needed. ROP represents an instance in which all photocoagulation must be completed in one session due to its rapid progression. The number of laser

At the moment of application, the laser marks should be whitish. During the next days, they become less net because of the inflammatory reaction, which is proportional to the extension of treatment. Within 1–2 weeks, the impacts develop pigment in the central area surrounded by atrophy. The extension of scars at the fovea in posterior ROP may induce significant loss of

The premature retina is incompletely developed and when exposed to high levels of oxygen which is necessary for life support, a relative hyperoxic environment is created, leading to the decrease of VEGF and subsequent delay or even cessation in the development of retinal vessels. By consequence, the retinal tissue does not get enough oxygen (relative hypoxia) and synthesizes angiogenic factor with the development of new vessels at the limit between the

The rationale of laser use in ROP is to destroy the nonvascular retina, which is the source of

Cryotherapy was used before laser to destroy the nonvascular retina, and its results were first published in 1988 by the Cryo-ROP Study proving its efficacy in preventing ROP-related

A few years later, in 1994, the Laser Study Group demonstrated the laser was as effective as cryotherapy in preventing blindness produced by ROP and with less severe side effects. Besides the fact that it causes less trauma and manipulation on the globe as compared to

new vessels, thus interrupting the pathogenic chain ultimately leading to vision loss.

cryotherapy, laser was more effective in zone 1 and zone 2 disease [2, 7].

500 ms are preferred [5].

2.6.3. Dimension of the laser spot

2.6.4. Number of the laser spots

2.6.5. Evolution of laser impacts

vascular and nonvascular retina.

vision [5].

blindness [6].

site where the laser fiber is fixed) and the retina [5].

spots may vary between 1000 and 5000/eye [5].

2.7. Rationale for laser use in the treatment of ROP

Figure 2. Delivery of the laser treatment.

The delivery mode is chosen: repeat at various intervals/single impact and the number of impacts is counted. The repeat mode is preferred because when pressing the pedal once, a series of impacts is delivered on the retina, at time intervals that are selected by the operator. However, this more rapid modality of treatment is chosen by the experienced surgeons. The retina is visualized with a +20 or +28 diopter lens and the peripheral retina is reached by the help of a scleral indentor (Figure 2).

### 2.6. Parameters used during the laser retinal photocoagulation

The clinical effect of laser photocoagulation on the retina depends on two main parameters: power and exposure time. If low power is used, higher exposure time is required. On the contrary, at high power, a low exposure time is sufficient to obtain a significant retinal reaction [5].
