Understanding structure-function relationships is crucially important in controlling glaucoma. Fundamentally, glaucoma is a progressive optic neuropathy, and structural damage to the optic nerve is directly responsible for loss of vision.
Although there are several forms of glaucoma intervention, the main aim of treatment is to control IOP as the only modifiable risk factor. IOP reduction is intended to mitigate the structural damage caused by glaucoma progression that ultimately yields functional deficits.
The importance of structure and function in glaucoma is also relevant for diagnostic and prognostic purposes. In addition to clinical examination and pressure monitoring, several diagnostic modalities may be used to gather information on the relative health of the eye.
For instance, fundus photographs are used to observe and monitor the optic nerve (for analysis of structure), while visual field testing provides insight into the functional losses a patient may be experiencing due to his or her glaucoma. Both tests provide important pieces of information, despite their output and interpretation being highly subjective.
Optical coherence tomography
In recent years, optical coherence tomography (OCT) has become an important tool for assessing glaucoma. Studies indicate that loss of ganglion cells at the optic nerve may be observable on OCT before functional vision loss is demonstrated on visual fields. It is thought that some of the earliest signs of glaucoma may be evident on such imaging, which presents an opportunity for earlier diagnosis and an ability to intervene before additional irreversible damage occurs.
The primary benefit of OCT is that it can be used to objectively measure the optic nerve and retinal structures. OCT testing in glaucoma, though, has some important caveats.
Firstly, while it may be useful in mild to moderate glaucoma, it becomes less useful after there is severe loss of the nerve fibre layer. Secondly, OCT is most useful when it is used over a period of time to detect disease progression.
Thus, due to the critical need to intervene before glaucoma damages the structures of the eye, OCT is unreliable for making appropriate and timely treatment decisions in early glaucoma, specifically those patients without obvious abnormalities or conflicting test results.
1. Resende A, et al. Repeatability of Steady-State Pattern Electroretinogram and Full-Field Electroretinogram using a novel office-based testing platform in normal subjects. Invest. Ophthalmol. Vis. Sci. 2016;57:3949.
2. Shengelia A, Derr PH, Tello C. Evaluation of pattern ERG responses using various electrodes. Invest. Ophthalmol. Vis. Sci. 2016;57:3943.
3. Ventura LM, et al. Pattern Electroretinogram progression in glaucoma suspects. J Glaucoma. 2013;22:219-225.
4. Osborne NN, et al. Ganglion cell death in glaucoma: what do we really know? Br J Ophthalmol. 1999;83:980-986. Review.
5. Carr MD RE. The Pattern Electroretinogram (PERG. Paper presented at: NANOS, 1988).
6. Dodt E. The electrical response of the human eye to pattern stimuli: clinical observations. Doc Ophthalmol. 1987;65:271-286.
7. Porciatti V. Electrophysiological assessment of retinal ganglion cell function. Exp Eye Res. 2015;141:164-170.
8. Banitt MR, et al. Progressive loss of retinal ganglion cell function precedes structural loss by several years in glaucoma suspects. Invest Ophthalmol Vis Sci. 2013;54:2346-2352.
9. Ventura LM, Feuer WJ, Porciatti V. Progressive loss of retinal ganglion cell function is hindered with IOP-lowering treatment in early glaucoma. Invest Ophthalmol Vis Sci. 2012;53:659-663.