The Eye in Diabetes Mellitus

Diabetes is associated with an increased risk of:

• Retinopathy (and associated maculopathy)
• Glaucoma
• Cataracts

Diabetic retinopathy is thought to be the commonest cause of blindness among people of working age in the developed world. The increased risk of glaucoma in diabetics is largely due to the effects of retinopathy. Cataracts are thought to occur due to the effect of hyperglycaemia on the structure of the lens and are the commonest cause of visual impairment in the elderly diabetic population. Hyperglycaemia damages retinal vessels via glycosylation of key vascular proteins, and hyperactivation of protein kinase C.

Diabetic retinopathy and its sequelae are amenable to early detection through a screening programme, and subsequent beneficial intervention. This can prevent or slow disease progression and reduce the burden of visual impairment. There has been a marked change in emphasis over recent years in the management of diabetic retinopathy. Formerly it was seen as an inevitable and inescapable consequence of diabetes, possibly noted on ophthalmoscopy at annual diabetic clinic review, with intervention often delayed until frank or severe disease existed, when it was least likely to be effective.

Nowadays, it should be considered as a preventable/treatable condition that must be carefully monitored for and managed through improved diabetic control and ophthalmological intervention. The widespread use of digital retinal photography should help to improve the detection and treatment of diabetic retinopathy and hopefully improve visual outcomes in the diabetic population at large.

Digital retinal photography with mydriasis is now seen as the gold standard screening test to detect diabetic retinopathy. Indirect slit-lamp ophthalmoscopy also meets sensitivity and specificity criteria, but lacks a hard record to allow comparisons for quality assurance.¹ The use of digital retinal photography with mydriasis is specified in the National Service Framework for diabetes, with a target of a minimum of 80% of people with diabetes being offered screening for the early detection (and treatment if needed) of diabetic retinopathy. This is as part of a systematic programme that meets national standards, rising to 100% coverage of those at risk of retinopathy by end of 2007.¹

Screening should be carried out on an at-least annual basis and include assessment of visual acuity, examination of the iris and pupil, checking of the pupillary reflexes and fundoscopy of dilated eyes. Screening for glaucoma using tonometry may be carried out at the same time. The screening programme may vary in its execution from region to region, depending on available resources and patterns of provision. It may involve GPs, optometrists, hospital physicians and other healthcare professionals. See the National Screening Committee's details for the programme in each region, via the further reading reference in the internet section below.

Blindness is one of the most feared complications of diabetes with an incidence of 50–65 per 100,000 diabetic population per year in Europe.² In Type I diabetes, microaneurysms start to appear after 5 years, affect half of cases at 10 years and nearly all patients after 20 years. Proliferative retinopathy, as defined by a formation of new vessels, appears after 10 years, and affects about 40% after 20 years. Maculopathy follows similar pattern finally affecting 10–20% of cases. In Type II diabetes, these changes may be found at diagnosis because sub-clinical hyperglycaemia may have been present for a prolonged preceding period. ³

Risk factors

• Progression of retinopathy is associated with the severity and length of time that hyperglycaemia exists.
• Smoking, hypertension and other vascular risk factors such as dyslipidaemia can influence the onset and progression of retinopathy.
• There is marked individual variation in susceptibility to retinopathy for a given vascular risk profile.

Diabetic retinopathy

In all cases of diabetic retinopathy, improved glycaemic control (usually aiming to bring HbA1_c levels to <7%) is associated with improved long-term outcomes and delayed progression of retinopathy.⁴ However, in individual cases, particularly with pre-proliferative and proliferative retinopathy, improved glycaemic control can initially bring on a decompensation and worsening of symptoms and signs. It is important to address other vascular risk factors, particularly smoking, hypertension and correction of dyslipidaemia.⁵

• Background retinopathy: Clinically significant macular oedema is treated by laser photocoagulation of specific vessels, or using a grid pattern of laser burns spaced 1 spot-width apart over the affected area where specific leakage points cannot be identified, sparing the macular area.
• Pre-proliferative and proliferative retinopathy:Pan-retinal photocoagulation is used. This involves placing laser burns over the entire retina, sparing the central macular area. Moderate intensity burns are placed 1 spot-width apart , except where there is neovascularisation where the whole arterial tree is treated. 1200–1600 burns may be placed on the retina over 2–3 sessions. If macular oedema is present it is often treated first and separately before treatment with pan-retinal photocoagulation. It is thought that the procedure works by reducing the release of vasoproliferative mediators by hypoxic retinal vessels, and allowing easier direct diffusion of oxygen from the choroid blood supply.⁶ Vitrectomy may be needed in case of long-standing vitreous haemorrhage. Patients who have suffered vitreous opacification due to haemorrhage may be treated with retinal cryotherapy rather than photocoagulation.

Cataracts and glaucoma

Cataracts are usually of the typical 'senile' nuclear type and treatment is by conventional surgical means. See separate article. Glaucoma can be prevented/ameliorated by appropriate treatment of retinopathy. Established glaucoma, of both varieties, is treated by standard medical and surgical therapies. See separate article.

• Of focal/grid photocoagulation:⁴
  • Impaired central vision
  • Paracentral scotoma
  • Choroidal neovascularisation
  • Epiretinal membrane formation
  • Worsening of macular oedema in a minority

• Of pan-retinal photocoagulation:⁶
  • Constriction of visual field
  • Nocturnal diminution of vision/blindness
  • Burns affecting the fovea centralis
  • Worsening macular oedema
  • Serous and/or choroidal detachment
  • Ocular pain
  • Anterior chamber adverse effects, eg burns affecting cornea or lens

Background retinopathy will eventually progress to the more severe forms in the majority of individuals, if left untreated. Patients who undergo treatment have their risk of moderate visual loss reduced from 30% to 15% over the subsequent 3 years.⁴ Patients with proliferative retinopathy, if left untreated, have a 90% chance of having no useful vision after 10 years. Those who have pan-retinal photocoagulation have their risk of severe visual loss reduced by 50%, compared to untreated individuals with a similar severity of disease.⁶

• Improved glycaemic control in the diabetic population, achieved through education, better therapeutics and evolving healthcare support systems.
• Better screening for sight-threatening diabetic retinopathy through the national screening programme.
• Adequate metabolic and ophthalmological intervention in those identified as suffering from retinopathy.