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12 January 2007

Diabetes: Proliferative retinopathy assessment

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The hallmark of proliferative diabetic retinopathy (PDR) is neovascularisation. Retinal ischaemia is thought to play a major role in this process. New vessels originate as endothelial proliferations, which pass through defects in the internal limiting membrane of the retina and become attached to the posterior vitreous cortex.

(The absence of the internal limiting membrane at the optic disc may explain the frequency of neovascularisation at this site.) The vitreous then serves as a scaffold for new vessel and fibrous tissue growth. PDR is associated with several serious complications:

The new vessels are structurally abnormal. They leak fluid and are prone to haemorrhage – either into the vitreous or into the potential space between the internal limiting membrane and the rest of the retina (often termed pre-retinal or sub-hyaloid haemorrhage)

Contracting fibrovascular membranes exert traction on the underlying retina, causing tractional and/or rhegmatogenous retinal detachment

Widespread retinal ischaemia can result in new vessels at the iris, leading to neovascular glaucoma (of which diabetes is one of several causes).

Symptoms

Complications of PDR can cause sudden and severe loss of vision. Less catastrophic haemorrhage or retinal detachment can produce floaters, blots or flashing lights in the visual field. Patients may experience ocular pain, photophobia and decreased vision with neovascular glaucoma.

Signs

Significantly, visual acuity may be normal in the presence of vision-threatening PDR.

When complications occur, acuity may be reduced to hand movements or worse. The iris is examined for new vessels in the iris (NVI) and neovascular glaucoma prior to dilation. (The pupils of diabetic patients often have a slow or poor response to mydriatics.) In diabetic patients, PDR is defined as one or more of:

Any definite neovascularisation, or

Pre-retinal or vitreous haemorrhage.

In addition, neovascularisation is described as new vessels on the disc (NVD, on or within one disc diameter of the optic nerve head), new vessels elsewhere (NVE), or NVI (rubeosis iridis). NVE are less likely to bleed than NVD. Other critical signs relate to the presence of complications of PDR and the indications for surgery:

Pre-retinal haemorrhage is often oval- or crescent-shaped. Vitreous haemorrhage diffuses through the vitreous cavity and may obscure the retina. In eyes without haemorrhage, a detached posterior vitreous suggests that vitreous haemorrhage is less likely to occur

Significant fibrosis increases the risk of retinal detachment, and may obscure the macular. Tractional retinal detachment has a shiny surface, which is concave towards the pupil. The retinal surface with rhegmatogenous retinal detachment is dull, and undulates with shifting sub-retinal fluid

Early NVI may be easily missed without magnification. The vessels usually grow radially from the pupillary margin towards the corneal angle.

Prevalence

As with DR in general, the incidence of PDR correlates best with the duration of diabetes. Over a period of 10 years, approximately 10 per cent of patients with Type 2 diabetes may develop PDR.

Significance

Prompt treatment of PDR with high-risk characteristics improves the prognosis for vision.

See also Diabetes – Proliferative retinopathy management.

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