George Hogarth
Introduction
Glaucoma is a progressive optic neuropathy which can lead to permanent blindness. It is characterised by the death of retinal ganglion cells (RGCs) and their axons which leads to structural changes and corresponding visual loss. It is possible to manage the disease by medical and surgical means. Glaucoma is generally asymptomatic and regular eye examinations are therefore important to reduce the risk of vision loss. In fact, it is generally regarded as a bad sign if patients are aware of reduced peripheral vision as this means the disease has progressed to an advanced level. It is diagnosed by assessing the optic nerve along with IOP and visual field measurements. Characteristic optic nerve changes in glaucoma include an increase in cupping (increase in C/D ratio), neuroretinal rim thinning and disc haemorrhages. Although raised IOP (intraocular pressure) is a risk factor for glaucoma around 40% of people with glaucoma will have normal IOP (normal tension glaucoma)(1).
The Structure-Function Relationship
The term structure refers to the anatomical changes seen in the retinal nerve fibre layer in glaucoma (apoptosis of retinal ganglion cells). The term function refers to a disturbance of any test of visual function, in glaucoma this generally means visual field changes (2). Some studies have suggested that there can be as much as 30% loss of retinal nerve fibre before a visual field defect is detected. It is also possible for there to be visual field defects present without any apparent glaucomatous changes at the disc. At times this can lead to confusion for the practitioner, whether to use the structural changes, functional changes or both in assessing the severity of the disease. Due to the anatomy of the nerve fibre layer, visual field defects in glaucoma are specific to area of the axonal damage. Furthermore, the defects respect the horizontal midline. For example, an inferior area of retinal nerve fibre layer loss will correspond with a superior visual field defect.
The Role of OCT
OCT was first introduced in 1991, it is a non invasive imaging technology which uses low-coherence interferometry. It is effectively an optical version of ultrasound imaging. In 1996 it first became available in clinic practice (3). OCT allows the quantitative evaluation of the RNFL, the optic nerve head (ONH), and the macula. OCT has made a large improvement in the detection and management of glaucoma by taking it from a mainly subjective based assessment to a mainly objective based assessment. OCT is able to measure the thickness of different segments of the retina and then compare these values to a normative database to determine whether or not the thickness of the tissues is within normal limits. It is also possible to take baseline structural measurements and observe for change, this is useful to do in glaucoma suspects. Thinning of the nerve fibre layer is the earliest detectable sign in glaucoma and this tends to occur before visual field defects are apparent. This has become the most common approach in detecting glaucoma. It should be noted that OCT is not only useful in the detection and management of glaucoma but also many other posterior segment diseases such as macular degeneration, diabetic retinopathy and macula oedema. Additionally, Anterior segment OCT is an emerging modality for assessment of the angle in primary angle closure suspects.
As well as being useful in the diagnosis of glaucoma, OCT is useful in the progression of the disease. It is important to determine whether or not there is disease progression as this will affect the treatment of the disease. With multiple patient visits it is possible to track the trend of a patient’s glaucoma by looking at the changes in RNFL thickness over time. If the CD ratio is increasing and the RNFL thickness is decreasing then this is good evidence that the patient’s glaucoma is progressing. It is important to note that age-related changes occur in both normal patients and those with glaucoma. Thus the detection of progression should be done carefully to discriminate true progression from age-related changes.
Limitations of OCT
Although OCT is extremely useful in the detection and management of glaucoma there are limitations to its ability. One big issue is false positives, also known as ‘red disease’. This occurs when normal eyes are flagged as glaucomatous when, in fact, they are perfectly normal. The opposite also occurs – ‘green disease,’ false negatives, eyes which are labelled as normal when in fact they are glaucomatous. Why does this occur? One big factor is the normative databases which the machines use. They typically contain lots of eyes which are emmetropic and thus patients which are myopic are flagged up as red. Myopic retinas tend to be thinner due to increased axial length and thus this reduction in thinning is flagged up as red when compared to the normative database. Acquisition-dependent artifacts occur when the person acquiring the images incorrectly places the measurement of the nerve fibre layer. Eye movement during the scan, cataracts and dry eye can also all lead to ‘red disease’.
Segmentation errors can lead to both red and green disease. This type of error occurs when the software is unable to correctly distinguish the retinal layers. In doing so the RNFL being assessed is measured as thicker or thinner than it actually is (4).
It is also possible for other optic neuropathies to exist such as ischaemic optic neuropathy or optic neuritis, which can resemble glaucoma. It is important for practitioners to remember that patients may have multiple pathologies. A floor effect exists in advanced cases of glaucoma in which the RNFL becomes so thin that the machine can no longer detect loss of RNFL. This is a problem as it can lead practitioners to the conclusion that the disease is stable when in fact it is not. In situations such as these one needs to rely on visual field testing as a measure of disease.
Conclusion
OCT has proved to be a major step forward in the detection and management of glaucoma due to its ability to detect the disease earlier and monitor its progression accurately over time. It is safe, quick, non invasive and in the hands of experienced users leads to better overall patient care. One needs to be aware of its limitations such as segmentation errors, the need for well trained staff when acquiring scans and the largely emmetropic database which can lead to red and green disease. As useful as OCT is, the need to carry out a disc assessment using a stereoscopic viewing method along with visual fields and IOP measurement all remain important when examining patients and cannot be forgotten.
References
- Bourne RR, Khatib T. The optic nerve head in glaucoma. Community Eye Health. 2021;34(112):36-39. Epub 2022 Jan 31.
- Malik R, Swanson WH, Garway-Heath DF. ‘Structure-function relationship’ in glaucoma: past thinking and current concepts. Clin Exp Ophthalmol. 2012 May-Jun;40(4):369-80.
- Geevarghese A, Wollstein G, Ishikawa H, Schuman JS. Optical Coherence Tomography and Glaucoma. Annu Rev Vis Sci. 2021 Sep 15;7:693-726.
- Bayer A, Akman A. Artifacts and Anatomic Variations in Optical Coherence Tomography. Turk J Ophthalmol. 2020 Apr 29;50(2):99-106.