J HOW DO WE TREAT SIGHT THREATENING RETINOPATHY?
The definition of sight threatening retinopathy is controversial. The Scottish Diabetic Retinopathy Grading system avoided the phrase “sight threatening retinopathy” in preference to the term referable for levels of retinopathy or maculopathy that require referral to an ophthalmologist for further assessment. This is because many of the patients with referable retinopathy, when assessed by ophthalmologists, are not found to be at risk of losing vision. In particular many patients with maculopathy (retinopathy at the macula) do not have, nor do they progress to, macular oedema. From an ophthalmic point of view there are three forms of retinopathy that are truly sight-threatening retinopathy:
- new vessel formation
- the development of “clinically significant” macular oedema
- the presence of capillary “dropout” (occlusion) at the macula (ischaemic maculopathy), with or without macular oedema
New vessel formation potentially threatens total sight from an eye. Macular oedema and ischaemic maculopathy threaten central sight.
New vessel formation is amenable to laser treatment. “Clinically significant” macular oedema may respond to laser treatment, particularly if it is from a discrete source of leakage. Ischaemic maculopathy does not respond to laser. “Tight” blood pressure control and “tight” metabolic control may ameliorate progression of ischaemic maculopathy.
1 Macular Oedema
Macular oedema has been shown to be amenable to laser therapy when there is “clinically significant macular oedema” in the absence of extensive retinal ischaemia.
“Clinically significant macular oedema” has been defined as:
- Retinal thickening at or within 500µm* of the centre of the macula
- Hard exudates at or within 500µm* of the centre of the macula, if associated
with thickening of the adjacent retina - Retinal thickening of one disc area or larger any part of which is within
one disc diameter of the centre of the macula
Most of the evidence for laser treatment in macular oedema comes from the Early
Treatment of Diabetic Retinopathy Study and work by Olk.
EARLY TREATMENT OF DIABETIC RETINOPATHY STUDY
Although early studies showed little benefit from laser therapy, the Early Treatment of Diabetic Retinopathy Study showed that photocoagulation of clinically significant oedema, reduced visual loss by one-half. Clinically macular oedema is defined as thickening of the retina that is twice the diameter of a retinal ein at the optic disc margin. This trial primarily addressed patients with focal leakage (circinate macular oedema) and recommended direct photocoagulation of leaking retinal microaneurysms. More recent studies have shown that treatment of the retinal pigment epithelium underneath the leaking microaneurysm is just as effective. For areas of diffuse leakage or capillary non-perfusion grid laser treatment was recommended. This entailed burns of 50-200 µm placed one-burn width apart. Follow up treatment was recommended if there was still clinically significant macular oedema 4 months after previous treatment.
OLK’S MODIFIED-GRID PHOTOCOAGULATION
Olk first recommended modified-grid photocoagulation treatment in 1986 for diffuse diabetic oedema. This had not really been addressed by the Early Treatment of Diabetic Retinopathy Study, which looked mainly at focal macular oedema. Grid laser therapy was applied to areas of diffuse leakage with occasional focal treatment to focal leakage. Diffuse macular oedema was defined as retinal thickening of two or more disc areas and involving some portion of the foveal avascular zone. Visual results at two and three years were comparable with the ETDRS. At five years 85% of patients so treated had not lost or gained sight compared to baseline.
2 Proliferative Retinopathy
Proliferative diabetic retinopathy is characterised by the presence of new vessels growing primarily from retinal veins. Untreated they will eventually bleed causing sudden loss of vision secondary to vitreous haemorrhage. They may cause scarring of the retina leading to traction on the retina and subsequent retinal detachment.
THE DIABETIC RETINOPATHY STUDY (DRS)
The Diabetic Retinopathy Study identified 4 risk factors for severe vision loss:
- Presence of new vessels.
- Location of new vessels on or within one disc diameter of the optic disc (NVD).
- Severity of new vessels-
- NVD equal or greater than one-fourth to one-third disc area
- NVE equal or greater than one-half disc area
- Vitreous or pre-retinal haemorrhage
High-risk proliferative retinopathy was defined as having three out of the four risk factors.
The DRS, however, showed no clear benefit from laser treatment for less severe proliferative retinopathy or severe non-proliferative retinopathy.
EARLY TREATMENT OF DIABETIC RETINOPATHY STUDY
The Early Treatment of Diabetic Retinopathy Study (ETDRS) looked specifically at the question of when to initiate panretinal or scatter laser treatment. Early treatment of patients with non-proliferative or early proliferative retinopathy, compared to deferral of treatment, was associated with a small reduction in the incidence of severe visual loss, but 5-year rates were low in both the early treatment and deferral groups (2.6% and 3.7% respectively). They concluded that scatter photocoagulation should not be recommended for mild or moderate non-proliferative retinopathy providing careful follow up could be maintained. When retinopathy is more severe then scatter photocoagulation should be considered and usually should not be delayed if the eye has reached the high-risk proliferative stage. There is a risk, however, of exacerbating any pre-existing macular oedema and, if appropriate, consideration should be given to treating this first. It is the usual practice of ophthalmologists to initiate scatter laser treatment at the early proliferative retinopathy stage in concordance with the recommendations of the ETDRS. However, the reduction was small and the risk was low in the deferral group