Ultrasound Imaging Appearances in CTS
The longest established and most extensively studied change in the nerve imaging in patients with CTS is local swelling of the nerve in the region of the carpal tunnel. This nerve, shown in a maximally zoomed in image for measurement...
... measures 20 mmsq in cross section and is severely impaired on nerve conduction studies (grade 5)
Many explanations have been suggested for why the nerve should become larger proximal to the site of entrapment including:
Endoneurial oedema - simple accumulation of fluid within the nerve
Disturbance of axonal transport - the 'damming up' hypothesis
Demyelination - degenerating myelin may swell?
Inflammation - invasion of the nerve by inflammatory cells
Distal axonal degeneration - might explain thinning of the nerve more distally
Fibrosis - increased fibrous tissue within the nerve itself
Growth of new axons - might contribute to swelling via formation of a 'neuroma'
Remyelination - newly formed myelin may be less compact and thus bulkier
Thickening of the perineurium
Many studies have been carried out trying to assess how useful ultrasound is as a diagnostic test for CTS. Their characteristics are summarised here:
|Author/year||Diagnostic standard||CTS hands||Control hands||Probe||us measure||u/l normal||sensitivity||specificity|
|Solbiati 1992||Response to surgeryrgery||90||none||???||Tenosynovitis etc||97%|
|Duncan 1999||NCS||102||68||7-10MHz linear||CSA pisiform||9||82%||97%|
|Keberle 2000||Clinical+NCS||15||19||LA and MultiD||Swelling ratio||1.3||100%||74%|
|Sarria 2000||Clinical+NCS||64||42||???||CSA 'proximal'||11||73%||57%|
|Swen 2001||Response to surgeryrgery||47||???||70%||63%|
|Kele 2002||Clinical||110||66||11MHz linear||Radiocarpal||48%||96%|
|Nakamichi02||Clinical||414||408||5-10MHz linearar||distal edge TCL||13||57%||97%|
|CT a/p diameter|
|El Miedany 2004||Clinical||96||156||12Mz linear||Mean CSA||10.03|
|Wong 2004||Clinical + NCS||193||none||???||CSA inlet||9||89%||69%|
|Yesildag 20044||Clinical + NCS||148||76||???||CSA pisiform||10.5||89%||95%|
|Altinok 2004||Clinical||40||40||???||CSA pisiform||9|
|Keles 2005||Clinical+NCS||35||40||???||CSA 'middle'||9.3||80%||77.50%|
|Ziswiler 2005||NCS||78||23||5-12MHz linearar||Largest CSA||9||86%||70%|
|Hammer 20066||Clinical+NCS||21||60||8-16MHz linearar||CSA pisiform||11 or 12||100%|
|Wiesler 2006||Clinical + NCS + Response to surgery||44||86||12/5MHz linearar||CSA wrist crease||11||91%||84%|
|Bayrak 2007||Clinical+NCS||41||40||10Mz linear||CSA r/u junction||10||not stated||not stated|
|CSA pisiform||9.9||not stated||not stated|
|CSA hamate||11||not stated||not stated|
|Peiteado Lopezez||NCS||48||22||12Mz linear||CSA radio-ulnar||9.5||80%||67%|
|Kwon 2008||Clinical||41||41||5-12Mhz linearar||CSA inlet||10.7||66%||63%|
|Padua 2008||Clinical||54||5-12MHz linearar||CSA 'max'||10||70%|
|Pinilla 2008||Clinical||40||30||7.5Hz linear||CSA pisiform||6.5||90%||93%|
|Visser 2008||Clinical||168||162||5-12MHz linearar||CSA 'inlet'||10||78%||91%|
|Moran 2009||NCS||52||20||5-12Mhz linearar||CSA 'inlet'||12.3||62%||95%|
|Ashraf 2009||NCS||70||CSA 'middle'||9.3||80%||77%|
|Yazdchi 2012||NCS||155||25||7.5-10 MHz linear||CSA 'outlet'||11.5||76%||57%|
|CSA 'proximal inletnlet'||11.5||76%||56%|
There is wide variation in what individual laboratories consider to be the upper limit of normal for a CSA measurement, ranging from 6.5 to 14 mmsq. This variation probably reflects the many different transducers and measuring methods and for the present at least every laboratory has to establish its own normal range for the method and equipment it is using. More studies are being published every year and several attempts have been made to synthesise the better quality studies into meta-analyses (Roll 2011, Fowler 2011, Descatha 2011).
|Sensitivity||95% CI||Specificity||95% CI|
|Roll||66% - 91%||47% - 87%|
It is clear from examining many patients in the Canterbury carpal tunnel clinic that patients with CTS show an enormously wide range of median nerve cross sectional areas, a few of which will be considered 'normal' by any standard.
These measurements were all made at about the level of the wrist crease/pisiform bone, choosing the point of maximal enlargement and measuring as described on this site using maximally enlarged images. The scanner is a Sonosite M-Turbo with an 8-14MHz small footprint linear transducer. The distribution is moderately skewed and applying my usual cut-off for abnormality of 9 mmsq would mean that 19% of these neurophysiologically abnormal hands would be classified as normal by this ultrasound measure. Thus, using a conventional methodology to compare ultrasound against nerve conduction studies as the 'gold standard' for the diagnosis, and normal asymptomatic control subjects to establish the specificity, we obtain a sensitivity of 81% and specificity of 95% for this method of imaging.
We have also compared both diagnostic techniques against a clinical diagnostic standard in unselected clinic referrals - a method which probably gives a much more realistic impression of the performance of both tests. The results of this were presented at the 2011 meeting of the American Association of Neuromuscular and Electrodiagnostic medicine and a version of the findings will appear here in due course.
Of the other imaging measurements which have been suggested none have been studied to anything like the extent of the simple CSA at the proximal carpal tunnel and it is not possible to draw any wholly reliable conclusions. The most obvious thing to do, given the one fifth of individuals whose nerves appear a fairly normal size at the wrist, is to compare the CSA at the wrist with that in the forearm in an attempt to demonstrate that a 9 mmsq nerve in that individual is actually enlarged compared to it's pre-morbid state (Hobson-Webb 2008). The original paper on this method found it useful with a wrist to forearm CSA ratio of >1.4 achieving 100% sensitivity and specificity in a small study of 18 normal control subjects and 44 CTS patients, but when checked in a subsequent study (Visser 2008) it performed less well achieving 69% sensitivity and 90% specificity (results published in a letter subsequent to the main study).
The next most promising assessment appears to be that of looking for evidence of increased blood flow in and around the median nerve using colour power doppler imaging but so far this method appears to be rather machine and operator dependent.
Revision date - 8th July 2012