2.6.5. Evolution of laser impacts

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

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,

Power is the parameter that influences the most the photocoagulation effect. Selecting a certain power is relative, as it depends on individual factors: retinal pigmentation, edema, pre-retinal fibrosis, the presence of vitreous hemorrhage. Therefore, the key element in choosing a certain laser power is clinical (retinal reaction), not technical (mW). Usually, treatment is initiated with low powers (150–200 mW) in order to test retinal reaction, never on pigmented retina. If there is no visible mark, power is increased with 20–50 mW, until the obtainment of the desired effect: whitish lesion. When high powers are used, the lesion appears gray with well-defined margins and sometimes in its center a vapor stream is emitted. This effect can be identified when passing from a thicker to a thinner retina, for example from the posterior pole toward the periphery, without reducing the power accordingly. During retinal photocoagulation, power

Exposure time can vary between 50 ms and 1 s. At low exposure times, the effect in the tissue is cylindrical: the area of the lesion is the same at the superficial (RPE) and deep (choroid) levels.

at high power, a low exposure time is sufficient to obtain a significant retinal reaction [5].

help of a scleral indentor (Figure 2).

Figure 2. Delivery of the laser treatment.

112 Laser Technology and its Applications

2.6.1. Power

2.6.2. Exposure time

2.6. Parameters used during the laser retinal photocoagulation

must be varied in order to obtain the appropriate tissue reaction [5].

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 vision [5].

### 2.7. Rationale for laser use in the treatment of ROP

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 vascular and nonvascular retina.

The rationale of laser use in ROP is to destroy the nonvascular retina, which is the source of new vessels, thus interrupting the pathogenic chain ultimately leading to vision loss.

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 blindness [6].

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