FFB Visions 2013 Conference: ERGs vs OCTs

I attended the Foundation Fighting Blindness Visions 2013 conference this past week and had a chance to listen to and meet with a number of noted national experts on retinal degeneration, and some with specific expertise on BBS.

I’ll be posting various updates from conversations and sessions at the conference over the next week. The first one is a discussion of the differences and pros and cons of ERGs and OCTs, two different approaches to measuring retinal degeneration.

A brief primer on each.

An Electro-Retinograph or ERG tests the electrical response of various cells in the retina, including rods and cones. Various flashes of light and checkerboard patterns are projected and the electrical impulses are measured with electrodes attached to the cornea and/or skin near the eye.

Optical Coherence Tomography or OCT uses very high wavelength light to create a three-dimensional image. In Ophthalmology, the light is shone onto the retina; the scattering and reflection of the light is measured, and a micrometer resolution image of the retina is created. It is most similar to a sonogram which similarly creates three-dimensional images based on the scattering and reflection of soundwaves. OCTs however are much higher resolution than sonograms. A retinal OCT can show, in high resolution and a great deal of detail, the thinning of the retina that happens with BBS-related retinal degeneration. 

Historically, ERGs were used as a method to diagnose BBS. Before there were genetic tests for the BBS genes, a finding of retinal degeneration without other explanations (such as a family history of RP) with other features of BBS was key to diagnosis.

If there is a genetic finding of BBS, the use of ERGs or OCTs is mainly to understand the progress of retinal degeneration. For that purpose, there are several major differences between the two tests:

1.       An ERG measures the specific responses of rods and cones. While most people with BBS experience rod-cone dystrophy, meaning the rods deteriorate somewhat more rapidly than cones (beginning with night blindness and narrowing of field). But some people have cone-rod dystrophy which affects vision very differently.

2.       An OCT is a quick and painless test, requiring only that a person stare at a small blue light for a few seconds. An ERG can be very unpleasant, and in children usually requires sedation (which carries significant risks).

3.       An OCT can detect cysts in the vitreous fluid known as cystoid macular edema which some patients in BBS develop and which can further limit vision. The good news is that these cysts are treatable with drops—so there’s good reason to detect them and deal with them.

For either test, one reason to have the test done at regular intervals is to have an ability to track the progress of degeneration. This won’t tell you anything better than other tests of functional vision, but it may help future clinical trials to have an objective measurement of the pace and progress of degeneration to compare against.

Ultimately this is a decision for each family and their doctors to make. There is some usefulness to both tests and some downsides to each.