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scarring and visual field loss19 . As discussed above, the sub threshold diode micropulse (SDM, 810nm) laser targets the melanin within the RPE for photothermal effects, with minimisation of functional and structural damage to the outer retina since there is no absorption by photoreceptors and haemoglobin20 (Level 2). A major issue for clinicians is that ophthalmologists have found laser titration difficult in the absence of visible laser uptake, with risks of overlapping re-treatment burns. Additionally, the SDM laser burns are not visualised using fundus autofluorescence (FAF) or OCT techniques 21,22 There are few multisport laser delivery systems available with FDA clearance, such as the Pascal Photocoagulator Topcon23, 24 the Suprascan 532nm (Quantel Medical), and recently, the MC-500 Vixi (Nidek) with 532nm green, 577nm yellow, or 647nm red. Such systems allow a pulse duration of 10-30ms compared with 100-200ms with conventional laser. Additionally the procedure can be semi-automated by delivering multiple laser burns to the retina with a single depression of the foot pedal. Multispot laser treatment for PDR has been shown to be safe and effective, preserving central visual acuity as well as peripheral visual field 25(Level 1). Shorter duration of laser pulse has been demonstrated to be more favourable for pain 26, 27. It is recognised that if laser treatment is applied using shorted duration of pulse (e.g. 20ms) a larger number of burns are needed to achieve control of PDR, either in a single session or multiple sessions28,29,30 (Level 2) . 9.2.7 Healing Responses The in vivo effects of 20ms burns have been demonstrated in animal studies31 . A potential explanation for laser burn healing responses is related to fluence, which is calculated as (power x time)/area. The fluence required to produce a threshold ETDRS type PRP burn on the retina is significantly lower for a pulse duration of 20ms compared with conventional 100ms pulse duration. A lower fluence laser dose results in fewer structural alterations in the outer retina31 . At shorter and longer pulse durations, the RPE absorbs the laser light and is destroyed, and the adjascent RPE proliferates to fill the area destroyed. However, at shorter pulse duration, there is photoreceptor in-filling to sites of laser injury with healing responses produced over time. The MAPASS study showed that 20ms burns allow the tissues to undergo a healing response that may not occur after standard-duration (100- 200ms) photocoagulation19 .This healing response is associated with a significant reduction in burn size across time for 20ms pulse duration, with no significant disruption to either the inner retina or the basal RPE. Higher-fluence 100ms burns developed larger defects due to thermal blooming and collateral damage, with no alteration in burn size across time or any healing laser-tissue interactions. Furthermore, at 6-months, the 20ms laser burns reduced in size without any overlapping laser scars, as the laser burns show healing responses over time32 (Level 2). Hence, at different pulse durations, fluence should be titrated to achieve threshold burns in the outer retina, allowing for healing of laser burns and minimisation of photoreceptor injury. 76 9.2.8 Retinal Laser Ablation Area Calculation of the total retinal area has produced estimates between 1100 and 1368mm2 34 . Barr reported that a maximal number of 5500 laser burns could be applied to the retinal surface using a 500μm laser spot size35 . In 1995, Reddy and coworkers quantified the ablation areas using 500μm conventional argon PRP laser and reported that 2600–6500 laser burns, with a retinal coverage of 510–1280mm2 , is required to treat PDR with PRP dosage proportional to the number of retinopathy risk factors36 (Level 2). The ETDRS recommended multi-session 500μm PRP laser extending into farperipheral zones in high-risk eyes37 (Level 1) .The DRS study recommended a minimum laser ablation area of 236mm2 (range 157–314mm2 ) for standard PRP, and the ETDRS suggested a minimum area of 236 mm2 for PRP treatment (Level 1). In the UK, a snap-shot of single-session PRP reported a median treatment area of 98.2mm2 (range 6.7–682.5mm2 ) 38 (Level 2). At the time of the UK study in 1995, there appeared to be a trend to initially undertreat eyes compared with the DRS and ETDRS recommendations; however, subsequent PRP was often needed in clinical practice39 . The use of 1500, 20ms PRP burns in a single session was shown to be a safe regimen in the MAPASS trial. However, for long-term PDR regression, 72% of eyes required top-up PRP treatment27, and the laser burn treatment density and final treatment areas varied according to the risk profile of the PDR40 . Using 20ms PRP treatment, the retinal ablation areas needed to produce complete disease regression ranged from 292 to 657mm2 27 . Following primary PRP treatment of 1500 treatment burns, an additional 1000- to 2000-burn PRP was required in a single session to completely regress mild PDR (total 2500-3500 burns). The laser burn density and retinal ablation areas increase significantly for moderate PDR (approximately 4000 burns) and severe PDR (approximately 7000 burns). Overall regression rates for PDR showed between 67% and 75% for mild/moderate PDR and 43% in severe disease. Allowing for the laser healing responses that reduces the burn sizes of 20ms PRP burns over time, the retinal ablation area required to treat PDR using micropulse mode should be increased 33 (Level 2). 9.3 LASER APPLICATION: GENERAL PROTOCOLS Laser treatment can be carried out either as a single session or in multiple sessions. Both eyes can be treated in the same session for the macula as well as for the peripheral retina. Caution should be taken when treating in the macular area when there are associated exudates lying immediately adjacent to the fovea as sometimes when the oedema has been treated, the exudates increase and these can encroach into the fovea and permanently affect foveal function. Under these circumstances, the treatment should be fractionated.