Do we get additional relevant findings by imaging CTS?
1) Before treatment
Researchers who have studied the utility of ultrasound imaging in CTS frequently claim that the imaging study often reveals structural anomalies, or even outright abnormalities which either help to explain why that patient has developed CTS or have some bearing on treatment. These findings include:
High branching of the median nerve
Invasion of the carpal tunnel by normal muscles on movement - either the lumbrical muscles from the palm on finger flexion, or the long flexor muscles of the forearm on finger and wrist extension
Anomalous muscles within the carpal tunnel - some individuals have 'extra' muscles which may extend into or pass through the carpal tunnel.
Evidence of inflammation of the flexor tendons
Space occupying lesions within the carpal tunnel - ganglion cysts, tumours, granulomatous deposits, gouty tophi.
Anomalous vessels - persistent median artery, either patent or thrombosed.
The frequency with which such findings are present varies in different studies. We will add some figures from Canterbury here in due course.
It is often simply assumed that any anatomical peculiarity on the imaging constitutes important information but in practice some of these findings may make no difference at all to either the choice of treatment or the risk of complications while in other cases the significance of the imaging may depend on what course of treatment is proposed. A median nerve which branches proximal to the carpal tunnel may make endoscopic surgery more risky but for the traditional open operation it probably makes little difference. Those who argue that the ability of imaging to show these structural changes which NCS miss makes ultrasound a superior diagnostic modality to nerve conduction studies for CTS are generally neglecting the fact that the neurophysiology can show a variety of physiological changes which will be missed by ultrasound. At present the counsel of perfection appears to be to carry out both investigations where possible.
2) After surgery
There are as yet only a few studies of what the median nerves look like on imaging after surgery. There is reasonably good evidence that the nerve swelling reduces with successful treatment (Abicalaf 2007, Smidt 2008) and another study was able to show that the patients who did better after surgery had a larger decrease in size of their median nerves when comparing the pre and post-operative ultrasound images (Vogelin 2010), but it is not clear what proportion of nerves, if any, return to the normal range nor how much improvement we should expect. We have seen asymptomatic patients many years after successful surgery who still have significant enlargement of their median nerves.
Another issue entirely is the assessment of failed carpal tunnel decompression. Naranjo et al, in their cohort of surgical patients, found that the decrease in CSA was similar overall in both the groups of patients with good results and the group with bad ones (Naranjo 2010) but there are anecdotal reports of ultrasound imaging being able to demonstrate incomplete section of the transverse carpal ligament when this is the reason for failure (Tan 2011).
This 83 year old lady had bilateral carpal tunnel surgery in 2003 for a grade 5 right and grade 3 left CTS. The operations were entirely successful in relieving CTS symptoms until 2012 when she began to experience symptoms in the left hand which she recognised as similar to those she had had before surgery. The right hand remained asymptomatic, though she had sustained a Colles fracture on that side in the interim from which she had also made a good recovery, albeit with some deformity. Repeat nerve conduction studies showed a grade 1 right and grade 6 left CTS. The residual nerve conduction abnormality on the right side is not unusual after surgery for a severe CTS and the ultrasound imaging in the right hand showed a nerve cross sectional area of 14 mmsq - again probably a residual abnormality of little significance. The newly symptomatic left side however showed the following:
The left median nerve can be seen to be sharply indented by a dense fibrous stucture superficial to the nerve. The site of this band is just proximal to the wrist crease 2-3 cm proximal to the usual site of maximal compression in untreated carpal tunnel syndrome. The nerve shows marked enlargement just proximal to this indentation (27 mmsq) and is also enlarged distally being 11mmsq at the level of the hook of the hamate. Although it is impossible, in the absence of studies performed during the 8 years when she was free of symptoms, to tell for certain whether this constriction of the nerve is something which has developed anew, or is a residual fragment of transverse carpal ligament from the original operation, the clinical history strongly suggests that it is a new lesion and in any case it is clear that a surgical exploration proximal to the carpal tunnel will be required to release the nerve now.
As with other topics of interest we will continue to collect more data as patients pass through the clinic.
Revision date - 3rd July 2012