Read below about:
- Measuring CSA
- Advantages & disadvantages of reference values
- How to avoid problems
When we talk about reference values in nerve ultrasound, we are talking about the upper limit normal of the cross-sectional area (CSA) of a nerve.
The CSA is the by far most widely used parameter in nerve ultrasound and its enlargement the by far most frequent morphologic expression of a pathologic condition of a nerve.
NOTE: in this article we focus on adults. Studies were usually carried out including participants aged 18 and up with the mean age of the study population between 40 and 50.
How do I measure the CSA?
To correctly measure the CSA you have to take care of two things:
- Perpendicular probe orientation
Make sure your probe is perpendicular to the nerve, in other words: make sure that what you see is really the short axis view and not some diagonal cross-section (See Figure 1) that gives you the wrong CSA – which is, by the way, then also bigger than in reality and could wrongly be interpreted as enlarged. A beautiful short axis view can easily be achieved in nerve segments with a straight course, e.g. the median or ulnar on the forearm. It might be a little trickier in nerve segments with a distinctly winding course like the peroneal at the fibular head or the radial around the radial groove. It might be virtually impossible in small nerves with a course oblique to the skin through the whole visible segment like the branches contributing to the long thoracic nerve (which is why we suggested to rather evaluate the diameter than the area in this case).
- Measurement without the epineurium
The CSA is measured without the epineurium. So place your measurement tool just inside the hyperechogenic rim. Again, disregard could lead to overestimation of the CSA.
Factors influencing the CSA
Factors that were checked for correlation with CSA are typically age, sex, height, weight and BMI. The first problem is that many of those interfere with each other, like BMI is calculated from height and weight and height is influenced by gender.
Among the 18 studies [1-18] we included here, 17 reported about correlations of CSA with various factors.
BMI was investigated in 13 [1-3, 5-7, 11, 12, 14-18] and showed at least some correlation in 12 of them. The correlation with height was investigated in 14 [2-9, 11, 12, 14-18] studies and showed correlation in 12 studies. Weight – 16 studies [2-4, 6-9, 12, 14-18], correlation in 13. Age – 14 [1, 2, 4, 7-9, 12, 14-17], correlation in 8. Sex – eight studies [2-4, 6, 7, 10, 16, 17], at least some correlation in 5, but not at all sites. So we are confronted with conflicting results.
The important conclusion for you is: There may be some influence on the CSA by biological factors, but at the moment there is no recommendation or need to use different reference values for people of different height or weight, for example. On the contrary, concerning age it has been shown by Cartwright et al.  that CSA is positively correlated with age, but values do not become pathologically high and stay within usual reference values – with one exception of the ten sites checked.
If you are interested in the details – these can be found in Table 1.
How do I know if a CSA is pathologically enlarged?
There are basically two ways:
- Comparison within the patient in unilateral disorders – this is also addressed in our basic course
- Using reference values
How do I get reference values?
This is a big task, no question. But believe us: it also helps you a lot to get better with nerve ultrasound. Once you have examined 50 people, your eye will be well trained in normal anatomy and you will benefit from that a lot.
The second possibility is to use reference values from literature. There are several studies out there focusing on the topic. For the purpose of this article we focused on those with the declared aim of providing reference values.
Table 2 gives you an overview on the nerves for which reference values were published until May 2019. We tried to include as many studies as possible, but cannot guarantee completeness. Most of the studies would give a mean plus standard deviation. We then calculated the upper limit normal as mean + 2x standard deviation.
With the upper limit normal, we created the pocketcard for you. You can download here.
However, the pocketcard can only be a good start for you and of course you can use it to reference suspicion of pathological alteration in your reports, but: if you compare reference values from literature (as you find in the table), you will soon realize that they are not free of problems, which we would like to point out to you and also explain how we tried to take these problems into account.
First and foremost: For most nerves for which more than one study was published, there is of course a certain range of the mean or calculated upper limit normal, e.g. the CSA of C5 was reported with an upper limit normal between 8 and 11 mm2 [4, 10, 16]. In that case, we report the 11mm2 on the pocketcard.
But there are some nerves in which published reference values vary over 100%, maybe the most extreme example being the tibial nerve in the popliteal fossa, which comes with an upper limit normal of 56 mm2 in one study  and 14 mm2 in another . This may partly be explained by the exact site of measurement, as the study with the larger CSA states that measurement was done “right after splitting of the sciatic nerve”, where the tibial nerve exhibits a larger CSA than at the distal end of the popliteal fossa, just before diving between the heads of the gastrocnemius muscle, where it already gave off branches. But even this does not seem to fully explain the discrepancy.
On the pocketcard we provide you with the highest values that were reported and added a * which tells you about the discrepancy between studies. On the website you can check the details.
Use our pocketcard as a start, to get an idea, see how those values fit your own experiences and then set the next level by creating your own.