I'm Britt Moore and this is the ultrasound consult on carpal tunnel syndrome, the most common peripheral nerve entrapment in the body. Historically, electrodiagnostic testing is a gold standard to diagnose carpal tunnel syndrome. Ultrasound is a very reliable way to diagnose carpal tunnel syndrome as well and it's becoming a lot more popular. This is a normal ultrasound of the carpal tunnel. It's important to know the borders of the carpal tunnel. At the carpal tunnel inlet, which is the proximal aspect of the carpal tunnel, you've got the scaphoid bone radially with the FCR tendon overlying that. The pisiform bone ulnarly with the FCU tendon overlying that. At the carpal tunnel outlet or the distal extent of the carpal tunnel, you've got the traqueatrum radially, hook of the hamate ulnarly. Floor of the carpal tunnel are the other mid-carpal bones. The roof of the carpal tunnel is the transverse carpal ligament, abbreviated TCL here. Transverse carpal ligament is the same thing as the flexor retinaculum. Whatever verbage you want to use, totally fine. At the ulnar aspect, just superficial TCLs where Ghion's canal lives, through the carpal tunnel is the one median nerve, and then the nine hand flexor tendons. This is the ultrasound example of carpal tunnel syndrome. In carpal tunnel syndrome, the median nerve will become swollen. A swollen nerve will appear larger. A swollen nerve will appear darker or hypoechoic, and will lose its typical fascicular honeycomb appearance. If you turn Doppler on, you might see associated flexor tenosynovitis or hyperemia around the median nerve and the transverse carpal ligament might appear bowed superficially because of all the pressure in the carpal tunnel. If you flip into a sagittal plane so that you're long axis to the median nerve, you might see a notch sign. What the notch sign refers to, as seen in this picture, is the median nerve will be thick and swollen approximately, and then as the transverse carpal ligament comes into view, it will be squished down and you'll see a notch in the nerve. There are formal diagnostic criteria for carpal tunnel syndrome on ultrasound. This is not a comprehensive list, but a good general guide. You can use cross-sectional area. This is more reliable if you use it at the carpal tunnel inlet compared to the carpal tunnel outlet. You can choose your size based off of how sensitive or specific you want to be. If you use a cutoff value of 10 millimeters squared or greater, being abnormal or consistent with carpal tunnel syndrome, you get sensitivity in the mid 80s and specificity to diagnose carpal tunnel syndrome in the low 90s. I specifically use the range of 10-12 millimeters squared, suggesting that if it's just 10 millimeters squared and you're not seeing any other associated features of carpal tunnel syndrome, it's low-level ultrasound criteria. If it's 12 millimeters squared or greater, that certainly means carpal tunnel syndrome criteria, and you're probably seeing other associated findings as well. This is an example of the cross-sectional area criteria. The top picture is pretty standard. The nerve at the carpal tunnel inlet measures 20 millimeters squared. That certainly meets the greater than 12 cutoff. The bottom picture is a bifid nerve example. With a bifid nerve, you want to circle both sections of the nerve and use the summative value. On this example, the patient's summative value is essentially 17 millimeters squared. That meets the criteria, the cutoff of greater than 12. The other reason I chose this picture was, this isn't the classic picture at the carpal tunnel inlet where you have the bony contours, the pisiform, and the scaphoid. It's just a hair proximal, and that's because that's where I got the clearest neural borders. If you're using cross-sectional area, we want to be as accurate as possible. Make sure you're as short axis, axial to the nerve as possible, so you're not creating a falsely larger size of the nerve, and that you have the clearest border so you can as accurately circle around that nerve. You can also use a comparison of cross-sectional areas. Most commonly, we'll compare the carpal tunnel inlet to the pronator quadratus level. You can either use the difference between those two called the delta or a ratio between the two. I will use the difference or the delta because it's faster for me just to do in my head as I'm dynamically scanning. If the median nerve at the carpal tunnel inlet is greater equal to or greater than two millimeters squared larger than it was at the pronator quadratus level, that is abnormal consistent with carpal tunnel syndrome. If the patient has a bifid nerve, you have to use four millimeters squared or greater as the cutoff to say they have carpal tunnel syndrome. This is an ultrasound example of that. The top picture is measuring the median nerve at the pronator quadratus level. You see the median nerve between the muscle bellies of the FDS superficially, the FTP deep, and then the pronator quadratus is just coming into view between the radius and ulna deep to the FTP at that level. In this patient, it measures 7.6 millimeters squared. When I scan distally, the median nerve enlarges to 20 millimeters squared. That's a difference of 12.4 millimeters. That certainly meets the greater than two cutoff for carpal tunnel syndrome. There are also published numbers in the literature you can use to categorize the severity of carpal tunnel syndrome. This paper from 2019 is a very good quality study if you want to use these numbers. That's more than fine. It's a very good quality study. I think it's a little misleading to use numbers alone. What I do is I use the cross-sectional area cutoff numbers that I just went over. Then also too, I'll comment on, is there thenar atrophy or denervation changes? If they meet the cross-sectional area cutoffs and there are changes of thenar denervation or atrophy, then I'll say it's consistent with electrodiagnostically moderate to severe carpal tunnel syndrome. If I'm just seeing the enlarged nerve without signs of thenar denervation changes, then I'll say it's more consistent with mild, maybe moderate carpal tunnel syndrome. When you're looking for thenar muscle atrophy or denervation changes, I take a comparison ultrasound like seen in this picture of the thenar muscle at the mid first metacarpal level compared to the hypothenar muscle at the mid fifth metacarpal level. This ultrasound picture is a normal picture. What you're looking for with thenar denervation changes is a smaller muscle bulk or a brighter, more hyper-echoic muscle. In summary, you can quickly and effectively diagnose carpal tunnel syndrome and ultrasound using median nerve cross-sectional area of 10-12 millimeters squared or greater at the carpal tunnel inlet. A difference in cross-sectional area with the nerve being two millimeters squared or larger at the carpal tunnel inlet compared to the perineal quadratus level four millimeters squared if it's bifid. If you feel comfortable, comment and thenar atrophy because that gives you more of a clue about if there is motor involvement, which is consistent with electro-diagnostically moderate or severe carpal tunnel syndrome. Also, if electro-diagnostic testing is normal, if ultrasound is fairly normal, but history and exam is still compelling for carpal tunnel syndrome, use an ultrasound-guided steroid injection around the median nerve of the carpal tunnel inlet as a third test to see do they or do they not get better. If they get better, that's a sign that this still could be carpal tunnel syndrome. It's just subtle enough that they don't meet our frank criteria. Happy scanning.

3rd Edition, CASE 58

3rd Edition

Wrist

ultrasound

carpal tunnel syndrome

median nerve

diagnosis

steroid injections