Panretinal photocoagulation

Prompt laser treatment vital in high-risk PDR

Dermot McGrath

Posted: Saturday, June 1, 2019

Standard panretinal photocoagulation (PRP) is an established, effective therapy that greatly reduces the risk of severe vision loss in proliferative diabetic retinopathy (PDR) and its neovascular complications, according to Moin Mohamed FRCOphth.

“We know from studies that PRP more than halves the risk of severe vision loss in PDR. In the Early Treatment Diabetic Retinopathy Study (ETDRS), the five-year rate of severe vision loss was 6.5% with immediate PRP, which is very good. In the Protocol S study patients had a mean visual acuity of 20/25 after five years, which is also very respectable,” he told delegates attending the 9th EURETINA Winter Meeting in Prague, Czech Republic.

Poor outcomes with PRP were typically related to poor baseline vision, more severe PDR and diabetic macular oedema (DME), while patients with better baseline vision who were given a full early treatment dose usually had better outcomes, he added.

In PDR, new vessels grow in response to chronic, widespread, progressive retinal ischaemia from chronic hyperglycaemia, noted Dr Mohamed, Consultant Ophthalmic Surgeon at St Thomas’ Hospital, London, United Kingdom.

While the exact mechanism for how PRP achieves its therapeutic effect is not clearly defined, plausible theories suggest that destruction of ischaemic retina improves relative oxygenation by reducing the metabolic demand, decreases the production of VEGF and other angiogenic factors and allows an ingress of oxygen from the choroid towards the inner retina, via the laser burns that act as a portal of entry.

For a standard PRP, typical laser settings would be 500μm spot size, a 0.1-second exposure and lesions placed one burn width apart. A total of 1,200-1,600 burns are placed in one or more sittings, carefully avoiding the macular area and any areas of tractional elevation of the retina, aiming for a grade 2/grade 3 burn, using a suitable power setting. This ultimately equates to an area, akin to a dose of retinal ablation. When reducing the burn diameter, a commensurate increase in the number of burns must be taken into consideration (remembering that if the diameter is halved, the number must be quadrupled).

While pattern-scanning laser is quicker to perform and easier to tolerate for patients, there is now a significant body of evidence showing that it is less effective than standard PRP in inducing lasting regression of retinal neovascularisation in the setting of previously untreated high-risk PDR, and at least a 50% increase in the number of burns is advocated, added Dr Mohamed.

He said that treatment should be initiated on a case-by-case basis in patients with early PDR, but that it is critical to intervene rapidly in high-risk cases.

“It is vital to treat promptly if high-risk characteristics are present as there is a four-to-six times increased risk of severe vision loss for these patients if not treated,” he said.

High-risk PDR defined by the well-established DRS (Diabetic retinopathy Study) as any three of the following four factors, said Dr Mohamed: (1) presence of any active neovascularisation; (2) presence of vitreous haemorrhage or pre-retinal haemorrhage; (3) location of neovascularisation on or within one disc diameter of the optic disc; and (4) size of new vessels > ¼ disc area at the disc or > ½ disc area elsewhere.

Moin Mohamed: