Introduction:

The most frequent referral to our Neurophysiology lab is for Carpal Tunnel Syndrome (CTS). The syndrome is well recognised as arising from compression of the median nerve in the carpal tunnel. Typically, this induces intermittent hand paraesthesia, which is often worse at night or with activities requiring sustained hand grip such as driving. Despite Nerve Conduction Studies (NCS) being the gold standard diagnostic test for some decades, the sensitivity has been reported to fall between 56% to 85% with a specificity of 94% [1]. Some authors have reported false negative findings between 10% to 20% [2]. Due to the relatively low sensitivity of NCS, and also because these tests are uncomfortable, ultrasound has slowly been introduced over the past 15 years as a diagnostic test for CTS. The advancements in ultrasonography, with high frequency (high-resolution) probes providing clear images of the median nerve at the wrist, enables one to determine if there is swelling of the median nerve just proximal to the carpal tunnel. It is commonly believed that compression of the nerve causes increased endo-neural fluid pressure, which in turn to produces this nerve swelling.

PubMed search shows that many papers have been published on the use of ultrasound in the diagnosis of carpal tunnel syndrome over the past 20 years. Many different parameters have been suggested by different authors. However, the most common measurement is determining the cross-sectional area of the median nerve at the wrist, just at its entry into the carpal tunnel. Most studies use a cut-off value between 9 and 11 mm2. In a recent study Maire and colleagues [3] reported the highest sensitivity (87%) and specificity (91%) for a cross- sectional area of 11.5 mm2 when a range of different cut-off values, between 8.5–12.5 mm2, were studied. Another common parameter reported is the wrist-to-forearm cross-sectional area ratio [4]. Here the cross-sectional area of the median nerve at the wrist is divided by the cross-sectional area of the median nerve in the forearm (approximately 12 cm above the wrist). In this study a ratio of 1.4 or greater was reported to show 100 % sensitivity.

I have worked in Neurophysiology labs for over 25 years and have the same experience of false negative results in CTS. Patients with typical symptoms of intermittent pain, paraesthesia, and numbness which is worse at night, can have normal studies. To further complicate this there has been times when the symptomatic hand shows a normal NCS while the asymptomatic hand shows median nerve slowing across the wrist ☹. We decided to purchase an ultrasound machine. My neurologist (my boss) and I visited a known lab that was performing ultrasound for CTS to obtain our initial training. Once I was confident in my technique, we started including this in our reports. As I am not a trained sonographer, we did not bill any patients directly or through Medicare for the service. It was simply meant to improve the sensitivity of our diagnoses. Our results are not published, this material is simply from my experience and point of view and for my fellow scientific officers’ awareness of this technique.

We used 10mm2 as the cut off value for the median nerve cross-sectional area at the wrist, and wrist-to-forearm cross-sectional area ratio of 1.6. We needed to have both of these criteria met in the absence of an electrophysiological abnormality to diagnose carpal tunnel.

Our findings:

1) In a majority of patients with abnormal electrophysiology, the ultrasound findings were also abnormal.

2) At least 3/10 people with normal electrophysiological findings, but with symptoms typical of a carpal tunnel syndrome, had an abnormal ultrasound.

3) Some patients with a mild electrophysiology abnormality had normal ultrasound studies.

4) Our biggest surprise was that patients with long-term CTS who had marked electrophysiological abnormalities had no significant swelling of the median nerve in the carpal tunnel. This has been reported previously but I was not aware of this observation at the time.

5) There was some correlation between the severity of the electrophysiological findings and the swelling of the median nerve, but it was not a strong relationship.

Discussion from our findings.

1. Ultrasound was most useful when the electrophysiology was normal with only a short history of carpal tunnel syndrome. The likely explanation for this finding is that the nerve conduction studies predominantly assess large fiber conduction. In early CTS the compression may not have caused significant slowing (or demyelination) to be detected. However, the nerve swelling caused by endo-neural edema is picked up by the ultrasound.

2. The ultrasound findings were extremely helpful following carpal tunnel surgery. Following carpal tunnel surgery, the symptoms often disappear quickly but it takes several months for the electrophysiological abnormalities to resolve. We found that the ultrasound findings could be used to determine persistent nerve entrapment in those cases where symptoms persisted after carpal tunnel decompression, in contrast to the confounding finding of residual electrophysiological abnormalities even following a successful release. Ultrasound is also helpful in recurrent carpal tunnel syndrome, especially when it has occurred shortly after release and when one is unsure of the relevance of residual slowing.

3. In chronic nerve entrapment the nerve swelling may disappear due to nerve degeneration, in which case the cross-sectional area returns to normal. This suggests that ultrasound in chronic carpal tunnel syndrome is of little diagnostic use. Therefore, one must be aware of this potentially confounding finding if a patient walks in with a normal ultrasound examination.

4. I cannot confidently say that the degree of ultrasound abnormality can be used to categorise the severity of median entrapment, although some authors have reported otherwise. For example, Partea and colleagues [5] have reported a normal cross-sectional area to be between 7.0-10.0 mm2, mild median nerve compression as 10-13 mm2, moderate compression as 13-15 mm2 and severe compression when the area is >15 mm2.

Conclusion:

While NCS remains the gold standard for the diagnosis of CTS, in my experience the use of ultrasound together with NCS can improve both the sensitivity and specificity in the diagnosis of CTS. Although we as neurophysiology scientific officers do not have the qualification to perform ultrasound, median nerve ultrasound is easy to learn, not time consuming and painless. However, the cost of equipment and the time leaning may not be justified in providing the service for free. Ultrasound should be considered when the NCS is normal in the presence of typical CTS symptoms. It is very helpful in distinguishing between persisting nerve entrapment versus post-operative residual slowing. Lastly, ultrasound is widely available these days and should be considered whenever one is unsure of the source of a patient’s symptoms, as can be encountered in the “double-crush” syndrome.

Some pictures from our Lab

A) Normal: CSA 6.0mm2 wrist; 4.3mm2 forearm; Ratio 1.39

(Right panel shows CSA at the wrist, left panel shows CSA in the forearm)

B) Abnormal: CSA *16.4mm2 wrist; 6.0mm2 forearm; Ratio *2.74

(Right panel shows CSA at the wrist, left panel shows CSA in the forearm)

C) Abnormal: CSA *13.5mm2 wrist; 6.0mm2 forearm; Ratio *2.26

(Right panel shows CSA at the wrist, left panel shows CSA in the forearm)

References

[1] Ahcan U, Arnez ZM, Bajrović FF, et al. Nerve fibre composition of the palmar cutaneous branch of the median nerve and clinical implications. Br J Plast Surg 2003;56:791–6. [PubMed] [Google Scholar]

[2] Duncan I, Sullivan P, Lomas F : Sonography in the diagnosis of carpal tunnel syndrome. AJR Am J Roentgenol 173 : 681-684, 1999

[3] Maire Ratasvuori, Markus Sormaala , Antti Kinnunen and Nina Lindfors 1Journal of Hand Surgery (European Volume) 2022, Vol. 47(4) 369–374

[4] Hobson-Webb L.D., Massey J.M., Juel V.C., Sanders D.B. The ultrasonographic wrist-to-forearm median nerve area ratio in carpal tunnel syndrome. Clin. Neurophysiol. 2008;119:1353–1357.

[5] Pert ̧ea M, Ursu S, Veliceasa B, Grosu OM, Velenciuc N,Lunca ̆ S. Value of ultrasonography in the diagnosis of carpal tunnel syndrome–a new ultrasonographic index in carpal tunnel syndrome diagnosis: a clinical study.
Medicine 2020;99:29(e20903).

Use of Ultrasound in Diagnosing Carpal Tunnel Syndrome (A personal experience)