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For your eyes only

Optical Coherence Tomography is a new tool in the management of diabetic retinopathy, write Dr. M. REMA and R. GUHA PRADEEPA.

OPTICAL COHERENCE TOMOGRAPHY (OCT) is a new non-contact, non-invasive imaging technique used to obtain the thickness and profile of different parts of the retina. OCT is identical to ultrasound B-scan imaging except that light rather than sound waves are used in order to obtain a much higher longitudinal resolution of approximately 10m in the retina.

It is being used increasingly to evaluate and manage a variety of retinal diseases including diabetic macular oedema (fluid collection in the critical central region of the retina, which is called as the "macula" — the "seeing" portion of the eye), macular holes, age-related macular degeneration (ARMD), epiretinal membranes and retinal inflammatory diseases.

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Increased eye pressure leading to damage of the optic nerve is called glaucoma. Early nerve damage and other diseases of the optic nerve can be measured using this technique. Glaucoma is three times more prevalent in diabetic individuals.

OCT III enables cross-sectional images of the macula and optic nerve and also facilitates evaluation and follow up of patients with diabetic macular oedema. The OCT-III has also a normodata base of Asian Indian subjects, which facilitates comparative studies.

The test takes approximately five minutes per eye. In clinical practice, OCT images add information to the results of other imaging technique or functional testing. This method co-relates well with Fundus Fluorescein Angiography (FFA) where retinal photography is done after an injection of a dye. FFA documents the changes on the outer surface of the retina, whereas OCT clearly shows the damage within the inner layers of the retina.

Non-invasive

It is especially useful when following up a patient with macula oedema after laser therapy. The actual fluid accumulation can be demonstrated and the decrease after laser can be documented in a non-invasive way.

In India with the epidemic increase in diabetes mellitus (affecting approximately 32 million) diabetic retinopathy is cause for great concern. Visual disability from diabetes represents a significant public health problem, as one in four diabetic subjects develop retinopathy. Early detection is critical in preventing visual loss, as there may not be any visual disability till the disease reaches a severe stage. To prevent diabetes related visual impairment, the treatment must be appropriately timed and rigorous. Diabetic eye disease is well suited to a systematic routine eye-screening programme and the national guidelines recommend a yearly retinal examination for diabetics. If sight-threatening diabetic eye disease is present more frequent examinations are necessary.

Ophthalmologists use a variety of imaging techniques to diagnose and assess diabetic retinopathy and to aid therapeutic decisions. Digital colour photography is the internationally recommended mode of screening for diabetic retinopathy. The other technique used in the diagnosis of diabetic retinopathy is the FFA where a dye injection is necessary.

FFA is useful in identifying subtle abnormal vessels in the retina, which are responsible for a bleed in the eye and documenting accumulation of fluid in the seeing area of the eye. FFA can also be used to track the progression of disease or efficacy of laser therapy. OCT is useful in patients where FFA may not be possible e.g. allergy to the dye etc., to actually measure the retinal thickening before laser therapy and to routinely screen diabetic patients for early retinal nerve fibre damage due to glaucoma.

Future directions

It is expected that OCT will be used increasingly to diagnose and manage retinal diseases and glaucoma. In the future, improvements in scanning hardware and software will facilitate its use. It also has the added advantage over the Retinal Thickness Analyzer (RTA) and other instruments in that there is no variation in images with corneal irregularities. OCT is a promising investigative tool in the management of diabetic retinopathy and other retinal abnormalities.

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