Until recently, I didn’t realize how important it is to assess the size of the optic nerve head along with the cup-to-disc ratio. Now I know that evaluating the C/D ratio without considering optic nerve head size is a lot like categorizing someone’s body mass index based only on weight without considering height! The overall size of the nerve head will influence the C/D, so both values must be considered together if you want to get a better idea of whether that optic nerve is outside the norm.
- A normal, large disc will have a relatively larger disc – and thus a larger C/D ratio – than a normal, smaller disc.
- A normal, small disc will have a relatively smaller disc – and thus a smaller C/D ratio – than a normal, larger disc.
For example: if all other glaucomatous risk factors were equal between 2 patients, and they both had a C/D ratio of 0.6, the person with the smaller optic nerve head would be of greater concern to us than the person with the larger nerve head.
Why does this size-and-C/D trend exist? It actually all relates back to the basic anatomy of the eye. There are roughly 1.2 million nerve fiber axons coursing through the neuroretinal rim of each optic nerve head. This means that, regardless of overall size, each normal optic nerve will need to allot the same area to accommodate 1.2 million axons – give or take a few thousand axons depending on the particular nerve in question!
Note: 1.2 million axons per nerve is a good estimate, but in general, larger nerves have more fibers than smaller ones. This number declines with age.
As an example, refer to the diagram below. The rim tissue is delineated in red. In both cases, the rim takes up the same area – in other words, each of the rims would house the same number of equally-spaced nerve fibers. If we imagine that each of the rims below holds 1.2 million axons, then both are normal. Yet we see that the smaller disc has a smaller C/D, by virtue of the geometry of a circle!
The reason why a large C/D ratio raises a red flag is because we’re concerned about the loss of the neuroretinal rim tissue. If you again refer to the above picture and imagine losing some of the red portion of the optic nerve head, you can appreciate that the C/D ratio would increase overall with the loss of rim tissue. In the end, it is important to understand that the fibers in the rim can be crowded together or more spaced out, which is why a disc of a particular size can be normal at a range of different C/D ratios.
Now that we know it is important to consider optic nerve head size, you’re probably wondering: what is normal? Below is a quick summary containing a few values you might want to keep in mind:
- The optic disc can be round, but is usually vertically oval in shape.
- Average horizontal size: 1.77 mm
- Average vertical size: 1.88 mm
- The normal optic nerve head diameter varies in size from 1.2 to 2.5mm.
- There is some inconsistency in the literature as to what cutoffs to use for a small or large disc, but in general a disc can be considered small if ≤ 1.2 mm and large if ≥ 1.8 mm.
- The key is to assess the C/D in conjunction with some knowledge of the disc size!
So now that we know it is important to assess optic nerve head size, let’s discuss a few ways to do that!
1. Measure Using High Plus
Probably the most accurate way to manually measure the optic disc is to do high-plus non-contact funduscopy using a 78- or 90-D lens and a slit lamp. To do this, simply bring the disc into view with your high plus lens using a narrow slit lamp beam. Adjust the beam height to match the diameter of the disc, and then read the beam height off of the indicator on the slit lamp.
Tip: You can rotate the slit lamp housing the set your beam at orientations, thus enabling you to estimate vertical diameter, horizontal diameter, or any orientation in between!
If you are using a particular slit lamp for the first time, it is a good idea to make sure that the beam height indicator is just right. In other words, you want to make sure that the 1 mm setting on the beam height indicator actually makes a 1 mm beam. If it doesn’t match up exactly, you’ll want to incorporate that into your final calculation of the optic nerve head size. Check the accuracy of the beam height indicator via the following steps:
1. Make a narrow slit beam and set the beam height to the 1 mm setting on your beam height indicator (H).
2. Place a millimeter rule at the plane of the patient’s face and shine the beam onto the face of the ruler. Read the beam height off of the ruler (X).
Once you’ve checked the accuracy of the beam height indicator and measured the diameter of the nerve head through your high plus lens, the only thing left to do is take into consideration the correction factor for your lens. This is crucial because the +78 D and +90 D lenses cause minification of the image. The correction factor (C) will depend on which power high plus lens you use:
- C = 1.0 for a +60D lens
- C = 1.11 for a +78D lens
- C = 1.33 for a +90D lens
All together, you can use the following equation to come up with an accurate optic disc measurement:
|Diameter (mm) = (X/H) x D x C|
- X = height of beam as measured by the millimeter rule (mm)
- H = height setting on the beam height indicator when checking with the millimeter rule (mm)
- This will be 1 mm if you checked the accuracy of the indicator as described above
- D = diameter of disc as measured by the beam height indicator (mm)
- C = correction factor based on high plus lens power
Want to get fancy? You can calculate the area of the optic nerve head using the diameter you just found!
|Area (mm2) = (∏r/4) x horizontal diameter (mm) x vertical diameter (mm)|
2. Using the Central Retinal Vein to Estimate Disc Size
The central retinal vein is a popular landmark often used for categorizing the size of drusen and the stage of AMD, but it can also be used to help estimate optic disc size! The average central retinal artery is roughly ~125 micrometers (i.e., 0.125 mm) in diameter as it crosses the inferior rim of the optic nerve. To use this to your advantage, estimate how many veins would fit side-by-side within the margins of the optic nerve head. By multiplying 0.125 mm by the number of times the vein would fit into the disc, you can estimate the disc diameter.
Tip: Cut time by using this as a ballpark method. If the disc is about 12-14 vein diameters wide, the nerve head is normal in size. Use this estimate in conjunction with the disc-to-macula estimation method (described below) to get a quick idea of disc size.
A larger nerve will hold a higher number of vein diameters, whereas a smaller nerve will hold fewer vein diameters.
3. Number of Disc Diameters to the Fovea
In normal eyes, the distance between the disc and center of the fovea is approximately 2 to 3 disc diameters, as measured from the temporal edge of the disc to the center of the fovea. Taken together with our knowledge about average optic disc size (i.e., 1.77 mm horizontally and 1.88 mm vertically), we can make the following statements:
- The smaller the distance between the disc and center of the fovea, the larger the disc.
- The larger the distance between the disc and center of the fovea, the smaller the disc.
You can use this estimation method while doing any type of fundoscopy, although I usually do this while using my 90-D lens. The foveola (i.e., the center of the fovea) is often conveniently marked by the foveal light reflex, making it easier to pinpoint the center of the fovea.
Tip: it can be difficult to visualize what 2.5 disc diameters looks like, so it may help (when doing high plus) to widen the beam of the slit lamp to match the diameter of the disc, and then see where you end up when you travel 2.5 beam widths from the temporal edge of the disc.
4. Use Your Direct Ophthalmoscope
If you use the correct aperture size while performing direct ophthalmoscopy, you can directly compare the size of the illuminated spot to the size of the optic nerve head. How much simpler can it get?! Most published data refers to the Welch Allyn 5-degree aperture (i.e. the middle setting) when using this technique, but I compared my Keeler ophthalmoscope to a Welch Allyn and the middle aperture sizes are pretty close.
To get an idea about the size of the optic nerve head, get a view with the medium aperture on your ophthalmoscope and bring the nerve into view. Place the spot of light directly over the nerve head and compare the size of the spot and the disc:
- If the optic nerve head is smaller than the spot of light from your ophthalmoscope, the disc is on the smaller side.
- If the nerve is more than 1.5 times the size of the spot of light from your ophthalmoscope, the disc is large.
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