A thorough assessment and recording of the patient optic nerve heads seems to be a skill that is often overlooked, especially due to the advent of modern technologies such as Optical Coherence Tomography (OCT) and highly analytical visual field analysers. Additionally, many optometrists focus solely on the cup-to-disc ratio alone as opposed to the general appearance of the disc, which the ratio alone means very little in direct care for the patient1,2.
JUMP TO:
The Best View
Locate the Edges of the Disc
Measure the Size
Colour
Rim Assessment
The Cup-to-Disc Ratio
Vessel Changes
Additional Features
Summary
Skill Activity
Further Reading
This over reliance on technology and disregard for features beyond the cup-to-disc ratio is no substitute for evaluating all aspects of the optic nerve. This guide looks at how optometrists and ophthalmologists of all levels can improve their disc analysis to better manage their patients. Whilst this guide will look at the basics of optic nerve head assessment, it is important to note that a basic level of understanding of the optic nerve, how it works and its physiological function is expected.
An image of a left optic nerve head, labelled with some of the key parts of the nerve head anatomy.
Please note, this is educational advice geared at eye care professionals and NOT a source for personal medical management. For registered practitioners, please keep in mind that this article does not replace your professional judgement when examining patients and no responsibility will be assumed by the author or website for misdiagnosis of patients that you see.
The Best View
Direct Ophthalmoscopy or Slit Lamp Biomicroscopy?
There is a heavy emphasis on using binocular indirect ophthalmoscopy via a slit lamp biomicroscope in modern day optometry training, with a wider field of view than usually achieved with a direct ophthalmoscope that is stereoscopic in nature.
Whilst an assessment with a direct ophthalmoscope is possible, subtle topological features may be missed without having that stereo view. It is of my own experience of following on from multiple optometrists through my locum role that those who are known to use direct ophthalmoscopy tend to underestimate features such as the cup-to-disc ratio compared to my colleagues that use a slit lamp biomicroscopy with a Volk lens, however the research around this fact reveals that there is no statistically significant differences between the methods3,4.
In an ideal world, a dilated view of the optic nerve head will provide the best chance of obtaining a clear stereoscopic image. Personally I like using a Super 66 Volk Lens when assessing the disc, as this provides excellent 1:1 magnification2 and suitable high resolution views of both the disc and macula.
That said, many like to use the 78D, 90D or Superfield lenses during routine assessments and, with correct slit lamp set up coupled with knowing the magnification factors of each lens, all will provide a good starting point for optic nerve head assessment.
It is vital that you get the patient to fixate accurately in order to first obtain, but also to hold on to a view of the optic nerve. From the slit-lamp, I ask my patient to look towards my ear (the one directly opposite the eye not being examined)1. This provides a relatively central view on the posterior pole, with a very minor adjustment then being needed to find the optic nerve head.
Additionally, I would strongly remind you that the process of assessing the optic nerve head is highly dynamic. You will need to move your beam of light across the optic nerve head multiple times in order to see how light interacts with both the topology of the nerve and also to assess the composition of the nerve tissue.1 Keeping the light static/still will only provide so much information – keep the light moving to fully assess the nerve.
Locate the Edges of the Disc
A common mistake many optometry students (and in some cases, qualified optometrists) make is rushing in to assess the cup-to-disc ratio. It is for this reason that cup-to-disc ratio measurement is further down this article in the hope that by the time it is reached that the other factors of the optic nerve are considered!
Take time to look at where the disc begins and where it ends, as often the scleral insertion1 or changes around the edges (such as peripapillary atrophy) can mimic the rim and cause you to underestimate the size of the cup. You need to make sure you do this else you will not be able to accurately measure the cup-to-disc ratio!
Furthermore, the edges of the disc (or “disc margins”) are vital to record as some conditions; such as papilloedema or optic nerve head drusen, can cause these margins to become indistinct or blurry. If you can note the conditions of the margins, it will improve your record keeping, helping both continuation of patient care and providing protection should a complaint about a missed sign be made.
To identify the boundaries of the optic disc, search for the slender white ring surrounding the disc. This represents the inner edge of the peripapillary scleral ring, which is more noticeable on the temporal side and when observing the disc in 3D5.
Measure the Size
Why do we measure the size?
Taking note of how big the optic nerve head is can really aid your assessment of it, as there can be a large variation on disc sizes that may influence your interpretation of the cup-to-disc ratio1,5.
In general, smaller discs tend to have smaller cups and larger discs can have larger cups. Therefore a measurement of a cup-to-disc ratio of 0.6 could be concerning in a small optic nerve head, but of low clinical concern in a large optic nerve head1.
The way I like to think of it is that the optic nerve is a tube for straws – we all get approximately the same number of straws and if the tube is narrower, the straws will be closer together with less room for a divot at the top but if the tube is wider, those same straws will be able to spread out and allow for a bigger divot. This is demonstrated in the image below.
An example of how disc size can make a difference to the cup-to-disc ratio. The circle on the left is smaller and as such the 95 circles are much more bunched together leaving less space at the centre. The circle on the right is much larger and as such the same 95 circles are spread out and there is a lot of space at the centre.
Interestingly, a good estimate for the number of axons (or “straws”) each optic nerve has is 1.2 million6,7 – but in general, larger optic nerves tend to have more axons than smaller optic nerves. Even with these additional axons, the cup will be bigger given the additional amount of space.
Additionally, smaller, crowded discs with small cup-to-disc ratios can be at risk of Non-Arteritic Anterior Ischemic Optic Neuropathy, so being aware of disc size can help with patient advice and management.
What is considered a normal disc size?
Optic nerve heads typically range from 1.20 mm to 2.50 mm in size4,8,9, with an average vertical height of 1.88 mm and horizontal height of 1.77 mm4,8,9. Discs that are smaller than 1.50 mm are considered small discs and discs larger that 2.20 mm are considered large discs.
However, these measurements can vary due to patient ethnicity and the above is given for Caucasians. Generally Asian and African-American ethnicities have larger optic nerve heads9.
How do you measure the size?
To measure the size, you can adjust the beam height of the slit lamp to match the size of the optic nerve head and take the reading from the beam height scale on the slit lamp8. Remember to adjust for the magnification factor for the volk lens that you are using and calculate the size from there.
| BIO/Volk lens used | Magnification Factor |
| Superfield | 1.5 |
| 90 D | 1.4 |
| 78 D | 1.1 |
| 66 D | 1.0 |
| 60 D | 0.9 |
To estimate the height of the optic disc, multiply the measurement obtained from the slit-lamp by the corresponding magnification factor listed in the right-hand column8.
Colour
Noting the colour of the optic nerve head is vitally important as a normal optic nerve head should be a pinkish colour. Any disc that is not this pinkish hue should be investigated. If it is pale or whitish in colour, wither fully or in a sector, this could indicate optic atrophy.
Pallor can be relatively subjective and, in the cases of glaucoma, usually appear later in its progression rather than an early sign8.
Rim Assessment
This will be part of assessing the colour of the disc, but looking at the shape and viability of the rim will be a key component to your evaluation as to whether or not the optic nerve is healthy or not.
Look at the shape of the rim, is it shaped like an oval (general shape of a healthy cup) or more like a circle (which could indicate concentric cupping1 and be a sign of rim loss).
Does it follow the ISNT rule (generally in normal optic nerve heads the rim is thickest inferiorly, then superiorly, then nasally with the temporal rim being thinnest1,5,8).
Are there any notches? A notch is a localised loss of nerve fibre1,5 and this is definitely an abnormal finding5. If you see one, document it and look for other signs on the disc, retina and from the visual fields.
The image to the left has a clear inferior notch and a subtle superior notch
Does it appear moth-eaten appearance? In some cases, the rim may have bits missing within it due to loss of axons caused by glaucomatous damage. Sometimes passing the light over the rim can show this appearance; sometimes the rim is clear and with the light elsewhere, it can appear missing1. This is why it is vital to be dynamic when assessing the disc and not just leaving the light static. Furthermore, noting this appearance is important as it may explain why different examiners arrive at wildly different cup-to-disc ratios1 – especially if one is looking at the static light of a direct ophthalmoscope and the other using the slit lamp dynamically.
The Cup-to-Disc Ratio
What is it?
The cup-to-disc ratio (also frequently referred to as the C:D ratio or CDR) is a measurement of how much the cup makes up of the overall disc expressed as a ratio. This measurement should be taken vertically, for the reasons described above regarding the ISNT rule.
Why do we measure it?
You may wonder why I am discussing the cup-to-disc ratio when I have stated that it isn’t something that should be rushed to obtain. On its own, it means very little, but when used in combination with the other factors described above and compared both between eyes and over time, it is a useful measurement.
From a basic point of view, the neuroretinal rim (or the “disc” part of the CDR) is essentially the nerve fibres from the retina exiting the eye through the nerve and the cup is an absence of these nerve fibres. In a physiologically normal eye, there is often some space that forms the cup due to the amount of space in the nerve exceeding the amount of space required to house all these healthy axons. However, in diseases such as glaucoma, retinal cells die and the axons are lost. The loss of axons causes there to be more space in the optic nerve and thus the rim gets thinner and the CDR increases.
We therefore measure the cup-to-disc ratio as it can be hard to determine if the space (the cup) is physiologically normal or if the space is there because of axon loss from retinal disease. A CDR measurement should be taken as a baseline measurement and we can then monitor if this space (the cup) gets bigger over time, which could indicate pathology is present.
This measurement on its own is relatively useless as it doesn’t account for disc size. As described when discussing disc size measurement, a larger disc is likely to have more space for all the axons and therefore will have a larger cup and a smaller disc will have less room and is likely to have a smaller cup (see the diagram regarding disc size measurement above). That said, as a rule of thumb, optic nerves with a CDR of 0.6 and above generally are more concerning and would require further investigation into glaucoma and often a CDR difference of more than 0.2 between the eyes is unusual5.
How is it calculated?
As stated, the CDR is a measurement of how much the cup makes up of the overall disc expressed as a ratio. For example, a cup that is approximately 40% of the size of the optic disc overall will have a CDR of 0.40 and a cup that is approximately 80% of the size of the disc would have a CDR of 0.80 (and so forth).
Another method of calculating the cup-to-disc ratio includes working out how many cups would fit into the overall disc and working that out as a ratio (for example a cup that would fit in to the overall disc three times would have a C:D ratio of between 0.3 and 0.35 or a cup that would fit in two-and-a-half times would have a C:D ratio of 0.4. Whilst this method is often taught at universities, I personally find the percentage method easier. That said, find a way that works for you that provides repeatable and comparable results as the C:D ratio is one of many recordings that you will compare for the duration of the management of your patient.
Please note, that when we are measuring the cup, we are measuring the depression in the centre of the nerve, and not the colour at the centre of the nerve. Be sure to use your stereoscopic view where possible and use topographical clues from the emerging vessels to help guide your assessment.
Difficulties in measuring the cup-to-disc ratio
There can be some cases where measuring the cup-to-disc ratio is more difficult:
Tilted Discs: Tilted optic nerves are optic discs that have an anomalous angle of insertion, which gives it the appearance of being tilted on its axes5. This can often make assessment of where the cup begins and ends more difficult to ascertain and thus cause difficulties in measuring the CDR.
Myopic Discs: Highly myopic discs can be hard to assess due to the structural changes caused by the high myopia. Often the disc is surrounded by myopic atrophy, making it difficult to see where the disc begins and ends, with many high myopes having shallow cups, further adding to the difficulty in assessing a CDR.
Additionally, these discs may not demonstrate the ISNT rule5, so do not be overly alarmed by this. Generally these discs are best assessed with dilation8 using a Volk lens (ideally 66D or 78D).
Congenital Anomalies: Discs that are affected by congenital anomalies (such as a coloboma or morning glory disc anomaly) can pose additional difficulties in measuring the CDR5.
Take home message:
If there is anything that you can take from this article it is that CDR alone are not useful. They are prone to inter- and intra- observer errors (due to viewing technique, instrument used to assess, confidence in assessing and experience), mean nothing when taken into consideration alone and there is no definitive rule of what is a normal and what is a definitely abnormal CDR. Take all factors of the optic nerve head in consideration along with other ophthalmic testing and patient history to make your judgement as over-reliance on CDR alone will see you miss early glaucoma in small optic nerve heads and over-refer larger optic nerve heads5.
Vessel Changes
The vessels are often neglected in optic disc assessment and many only think to consider the retinal vessels in terms of their calibre, tortuosity and nipping. Quite often changes to retinal vessels are detected at the disc8,10 and as such may present a sign that conditions such as glaucoma are present8,10. Therefore, evaluating and recording any concerning vascular changes at the disc can help enhance your assessment and highlight any regions of interest.
Bayonetting/Undercutting Vessels: ‘bayonetting of vessels’ is a term used to describe areas of significant neuroretinal rim tissue loss that involves a particular blood vessel temporarily disappearing from view as it navigates along the excavated borders of the optic nerve, only to re-emerge at the edge of the rim from the excavated cup8.
Circumlinear Baring: As the supporting loss of neuroretinal rim occurs, the vessels resting upon them can start to follow a circumlinear path around the nerve head8. This “baring” is a sign that rim loss is occurring and should be documented if seen.
In some cases, the vessel may be left unsupported and become a “fly-over vessel”8 – again, this is an abnormal finding,
Nasalisation of Vessels: Over time, the loss of neuroretinal rim causes the blood vessels that are supported upon them to lean nasally8,10. The two images above also demonstrate some nasalisation of the central retinal trunk.
Additional Features
The optic nerve may have additional features to consider when evaluating if the disc is healthy or unhealthy. This section will briefly cover them:
Peripapillary Atrophy (PPA)
Peripapillary atrophy refers to the degeneration or thinning of the retina and retinal pigment epithelium layers surrounding the optic nerve at the rear of the eye. It’s typically symptomless, doesn’t cause vision loss, and currently has no available treatment. However, it can be associated with myopia and glaucoma1,5.
Peripapillary changes include alpha and beta atrophy. Alpha atrophy is irregular pigmentation of the retinal pigment epithelium, not associated with glaucoma. Beta atrophy results from atrophy of the RPE and choriocapillari5s, often seen as a first sign of glaucoma. It can develop in one region of the disc edge or extend around the entire disc circumference. While more common in eyes with glaucoma, it can be seen in other optic neuropathies.
An example of peripapillary atrophy can be seen in the image below.
Disc Haemorrhages
Optic disc haemorrhages (also known as Drance haemorrhages) are splinter- or flame-shaped haemorrhages that are orientated perpendicular to the optic disc margin1,5. Disc hemorrhages are a warning sign of glaucoma risk or indicate glaucomatous progression5. They appear as small vessels crossing the disc but do not continue like retinal vessels. They are also transient, often recur, and commonly develop at the inferior and superior rims, with notching occurring at the location of a prior hemorrhage1,5.
The image to the right shows a significant inferior disc haemorrhage emerging from a notch within the inferior rim of the disc. This patient had significant primary open angle glaucoma and a corresponding superior arcuate defect.
The image also demonstrates peripapillary atrophy (PPA) predominantly temporal to the disc.
It is absolutely vital to make an effort to look for these haemorrhages as some can be very subtle and easily be mistaken as small blood vessels. They mainly indicate active glaucomatous progression, but also indicate presence of a posterior vitreous detachment (albeit the shape of the haemorrhage is often different), diabetes5 and ischaemic optic neuropathies, so care must be taken to fully to investigate their presence fully.
Swollen Nerve Heads / Indistinct Margins / Papilloedema
Whilst this article predominantly covers the assessment of the optic nerve with respects to commonly encountered disc features, I will mention briefly cover swollen optic nerve heads. This is outside of the intended scope of the article (and will feature in a future article) but is an important clinical finding.
A swollen optic nerve head is of great importance to note because is caused by pressure within or around the brain, causing the optic nerve to be pushed anteriorly into the posterior chamber.
This swollen appearance can indicate intracranial hypertension, malignant hypertension the presence of a brain tumour – and swift identification, referral and management is required to prevent permanent nerve damage, vision loss and even death
Signs of swollen nerve heads include indistinct and blurry margins, an elevated and hyperaemic optic nerve head, tortuosity of the retinal vessels and presence of haemorrhages on the optic nerve head. In order to think about the appearance of the disc, and to protect yourself, please note if the margins are distinct or indistinct on every disc assessment!
Summary
In summary, our comprehensive guide provides a holistic understanding of optic nerve head assessment, a crucial aspect of eye care. It underscores the importance of not solely relying on the cup-to-disc ratio and highlights the benefits of using modern examination methods like Slit Lamp Biomicroscopy with a Volk lens. The guide also emphasises the need to consider other key features of the optic nerve, such as the size, rim configuration, and changes in vessels. By understanding these elements, eye care professionals can make more accurate assessments and provide better care for their patients. This guide serves as a valuable resource for both novice and experienced practitioners in the field of ophthalmology.
Skill Activity
Be sure to check out our other skill guides in the Skill Centre!
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Further Reading
You may wish to read the following resources when learning and revising optic nerve head assessments:
- Piltz-Seymore J (2010). Improve your optic nerve evaluation. Ophthalmology Management. [Online]. Available at: https://ophthalmologymanagement.com/issues/2010/april/improve-your-optic-nerve-evaluation/ [Accessed: 2nd April 2024].
- Fingeret M, Medeiros F, Susanna R, and Weinreb RN (2005). Five rules to evaluate the optic disc and retinal nerve fiber layer for glaucoma. Journal of the American Optometric Association 76(11):661-8.
- Alfin RJ, Ramyil AV, and Mpyet CD (2023). Diagnostic accuracy of direct ophthalmoscopy in the rapid assessment of avoidable blindness protocol: A cross-sectional study. Journal of Clinical Ophthalmology and Research 11(1): 40-44.
- Robles R, Patel N, Neag E, Mittal A, Markatia Z, Ameli K, and Lin B (2023). A Systematic Review of Digital Ophthalmoscopes in Medicine. Clinical Ophthalmology 17: 2957-2965.
- Gandhi M and Dubey S (2013). Evaluation of the optic nerve head in glaucoma. Journal of Current Glaucoma Practice 7(3): 106-114.
- Browne J (2013). 4 quick and easy ways to assess optic nerve size. Optometry Students [Online]. Available at: https://www.optometrystudents.com/clinical-optometry/4-quick-and-easy-ways-to-assess-optic-nerve-head-size/#:~:text=The%20normal%20optic%20nerve%20head,large%20if%20%E2%89%A5%201.8%20mm. [Accessed: 4th April 2024].
- Johnson PT, Geller SF, Reese BE (1998). Distribution, size and number of axons in the optic pathway of ground squirrels. Exploratory Brain Research 118(1): 93-104.
- Harding A, and Harper R (2007). The glaucomatous optic disc: taking a closer look. Optometry in Practice 8: 151-162.
- Mark B (2023). A practical guide to measuring optic disc size and its clinical correlation in diagnosing glaucoma for the eye care professional. Austin Ophthalmology 7(1): id1042
- Shon K, Jo YH, Shin JW, Kwon J, Jeong D, and Kook MS (2020). Nasalisation of central retinal trunk predicts rapid progression of central visual field in open-angle glaucoma. Scientific Reports 10: 3789.
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