Understanding the structure function relationship in glaucoma

Underpinning much current glaucoma research is the drive to understand how the condition develops in order that eye care professionals can improve early diagnosis rates and monitor the effect of treatments.

One area of particular interest at the moment is the link between structural and functional changes in the eye that lead to chronic open angle glaucoma (COAG), the second most common cause of global blindness. This was explored in a recent article by Professor David Henson in Eye News magazine.

Currently, glaucoma is diagnosed using three clinical signs which can be categorised into functional or structural changes. There are two functional changes – raised intraocular pressure (IOP) and the development of visual field (VF) defects. Structural changes happen in the optic nerve head (ONH) and the nerve fibre layer (NFL).

The key over-arching questions in the structure-function relationship were raised by Professor David Henson in his Eye News article earlier this year:

“Clinical interest in the structure function relationship is primarily focused on two questions. First, do detectable changes occur first in functional or structural measures? Second, for the monitoring of diagnosed cases is it better to use functional or structural measures?”

Professor David Henson, Eye News Vol 23, No. 6.

An awareness of the type of changes that happen, focusses the clinician on the most appropriate method to use, enabling effective early detection and thus expediting treatment and helping to preserve sight.

Structure or function?

Traditionally eye health professionals have been told that structural damage will come before visual field loss, but new research cited by Professor Henson shows that functional measures now play a greater role.

The new research suggests that in some cases visual field analysis is the more effective measure because it can detect glaucoma earlier than associated structural measures. This necessitates taking a more sophisticated approach to glaucoma diagnosis.

But it does not mean that there should be a wholesale switch to functional (visual field) testing – both must work together. In some patients, glaucoma is detected earlier using structural measures but, in others, you will receive a better outcome from a functional measure. There is no catch all scenario – some patients may have structural damage first but others may have visual field loss first.

Taking this into account, we can’t definitively say that one approach is more effective than the other, but that both must be used. Eye care professionals should acknowledge that each measure supports the other, leading to a more comprehensive diagnostic protocol.

Currently, many glaucoma cases are detected exclusively via OCT (optical coherence tomography) which measures the structure of the eye – commonly the retinal nerve fibre layer thickness and the condition of the retinal ganglion cells.

In the paper, Professor Henson notes that structural measures are more likely to show the progress in early-stage glaucoma, compared with visual field testing. However, visual field testing will still detect a ‘significant’ number of cases that remain undetected with a structural measurement approach.

Improving glaucoma diagnostics with structural and functional measures

Professor Henson concludes that while more research is needed we still need to be mindful of the complex relationship between structural and functional measures:

“The findings from recent studies show that during the early stages of glaucoma SD-OCT measures are more likely to detect progression than the widely used VF measures but that VF measures will still detect a significant number of cases undetected by structural measures.”

Professor David Henson, Eye News Vol 23, No. 6.

He goes on to say that protocols need to include both structural and functional measures which will result in high diagnostic performance and sensitivity to the progression of the condition.

Glaucoma is a common disease but one that we still need to learn a lot about. Only recently have we begun to understand its relationship to the brain and how this affects treatment, for example.

As people live longer and are therefore more at risk of the condition, we need to develop protocols that include specific and sensitive diagnostics to ‘cover all bases.’ This will enable eye health professionals to detect glaucoma as efficiently as possible and monitor it on an ongoing basis.

To learn more about the detection and management of glaucoma, including visual field analysis, read our Henson 9000 technical profile.