(1)
St. Johns, FL, USA
(2)
Helen Keller Foundation for Research and Education, International Society of Ocular Trauma, Birmingham, AL, USA
(3)
Consultant and Vitreoretinal Surgeon, Milos Eye Hospital, Belgrade, Serbia
(4)
Consultant and Vitreoretinal Surgeon, Zagórskiego Eye Hospital, Cracow, Poland
Laser, as photocoagulation in general, uses light to cause tissue coagulation as the energy from the light source is absorbed by the tissue and converted into thermal energy.1 Laser’s significant therapeutic benefit is thus delivered via a force that is destructive in nature.
Laser to the retina is typically applied at the slit lamp or via the IBO. Unlike endolaser, with these techniques the laser is delivered from outside the eye. Both involve at least some inconvenience to the patient,2 and even if retrobulbar anesthesia is used, treatment close to the fovea poses some risk of misdelivery.
All other parameters (see Table 30.1) being equal, three factors determine the effectiveness of the laser therapy: the depth of penetration of the laser beam3 and the type and amount of tissue pigment4 that absorbs the laser energy. Table 30.2 provides a list of selected conditions in which (endo)laser5 therapy is applied.
Table 30.1
The parameters influencing the efficacy of the endolaser
Variable | Variable | Increasing effect | Decreasing effect |
|---|---|---|---|
Laser parameter | Duration | Longer | Shorter |
Power | Higher | Lower | |
Size | Smaller | Larger | |
Distance of probe from retina | Closer | Further away | |
Angle of probe relative to the retinaa | Perpendicular | Maximum | N/A |
Parallel | N/A | N/A | |
In-between perpendicular/parallel | Closer to 90° | Closer to 0° | |
Pigment content of the retina | More | Less | |
Fluid content of the retina (edema) | Less fluid | More fluid | |
Table 30.2
Selected indications for endolaser therapy
Indicationa | Comment |
|---|---|
Central serous chorioretinopathy | To close the vascular leakage as seen on fluorescein angiography |
CNV | To destroy a (parafoveal) membraneb |
Diabetic retinopathyc | Panretinal or focal treatment, depending on the severity of the disease; the goal is to destroy ischemic areas that produce vasoproliferative agents and to prevent bleeding and exudation |
High IOPd | Endocyclophotocoagulation to destroy (some of) the ciliary processes |
IRMAe | To close the vascular leakage |
Macular edema in diabetes, vein occlusion etc. | To dry the macula – the mechanism how laser may work is not known |
Prophylacticf | To prevent RD (endolaser cerclage) |
PVR | To prevent redetachment (endolaser cerclage) |
RD | To seal the break and to prevent redetachment (endolaser cerclage) |
Retinal tear | To seal the break to prevent RD development |
Retinal telangiectasiasg | To reduce exudation and prevent progression |
Retinal vascular tumorsh | Destruction of the feeder vessel or the tumor itself |
Retinoschisis | To prevent progression or mark the central border of the pathology |
Retinotomy/retinectomy | To seal its edge to prevent redetachment |
ROP | Retinal ablation to halt disease progress |
Vessel, abnormal | A feeding vessel of a tumor or other pathology containing one or vessels in a proliferative membrane may be closed with high-intensity laser application. Argon green (512–534 nm) is the best option for this purpose because it is absorbed by blood. Nonetheless, diathermy or preoperative bevacizumab is typically preferred to laser in these cases |
Endolasers of different types6 are available; the use of Argon green (514 nm) is described here.
30.1 The Consequences of Laser Treatment
The retina becomes edematous after laser treatment. The surgeon notices that within a few seconds, the spot, which was white with sharp borders upon delivery, turns fuzzy-bordered, and its color turns a bit grayish.
With time,7 the spot turns into a scar. The RPE, the photoreceptors, and the retinal layers external to the inner nuclear layer are involved in the scar; there is some hyperplasia and hypertrophy of the remaining RPE. Glial cells also contribute to the development of the scar.
Q&A
Q
What may be the undesirable consequences of an “overpowering” (endo)laser spot?
A
Pain, even if the rest of the operation was painless; corresponding defect in the visual field; significant retinal bleeding; retinal break formation; break in Bruch’s membrane with development of a neovascular scar; if a spot is close to the fovea, RPE migration into it with loss of the central vision; and the vitreous “burned” into the scar, making PVD creation impossible.
After treatment, the oxygen consumption of the retina is reduced.
Oxygen is able to diffuse through the laser scars without being consumed by the photoreceptors, relieving the inner retinal hypoxia and raising its oxygen tension.
The retinal arteries constrict and the blood flow decreases. The hypoxia relief reduces VEGF production, and the neovascularization process stops or regresses.
30.2 The Setup
The initial step is to ensure that the laser filter is properly situated on the microscope. It should be mounted so that it provides protection for the surgeon, his assistant, the nurse, and the video camera. The first three are a matter of safety9, the fourth of convenience. It is advisable for all in the OR to wear safety goggles when the endolaser is in use.
The laser pedal should be in the middle (see Fig. 16.3a) and operated by the nondominant foot.
30.3 The Technique of Endolaser Treatment
Unlike cryopexy, laser is ineffective if applied over detached retina.10 This is the reason why, for instance, in RD surgery laser is delivered after drainage of the subretinal fluid and under air.
Pearl
As a general principle, the parameters should be adjusted so that the effect is a discreet but visible whitening of the retina with no apparent major tissue disruption (strong whitening, bubble formation, significant bleeding, audible “pop”).
30.3.1 General Considerations
To deliver a spot that causes mild retinal whitening, all three main parameters must be properly set.11
My default parameters are the following:
Duration, 100 ms; spot size, 100 μ; energy, 150 mW.
Since the effect also depends on the pigmentation of the fundus,12 major changes in the settings may become necessary. It is best to test the parameters at a peripheral location (see below).
An additional parameter to set is the “repeat” mode (see below).
Ideally the laser probe is curved so that the surgeon can always keep it perpendicular to the surface and be able to reach any retinal area without difficulty or risk in the phakic eye.
The straight probe has significant limitations regarding reach and safety.13
The curved probe (may be retractable if made of memory material) can be held truly perpendicular to the retina at all locations – but the surgeon must remember to pull the laser probe into the shaft before exiting the cannula with it (see Sect. 21.7).
In a phakic eye it is possible with the curved probe to avoid damaging the lens, but extra caution is still needed: the surgeon tends to focus on delivering the laser spots, not on the actual position of the laser probe’s shaft.
The spots should not be confluent; as a general rule, the distance between spots should be roughly the same as the spot diameter.
When panretinal treatment is employed, this rule is difficult to keep (see below).
The working distance14 is typically ~1–2 mm – with some caveats.
When lasering under air (see Sect. 31.2), it may be difficult to clearly see the tip’s distance from the retina, especially in the periphery. The laser probe must be advanced close to the retina,15 and the surgeon should carefully observe the distance between the shaft’s tip and the shadow it casts.
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