Median nerve deformation in differential finger motions: Ultrasonographic comparison of carpal tunnel syndrome patients and healthy controls

Carpal tunnel syndrome is a compression neuropathy of the median nerve in the wrist. Patients with carpal tunnel syndrome (CTS) experience pain and weakness in the hand, and numbness and paresthesias in the first three digits. These symptoms were first described by Sir James Paget in 1854, although widespread recognition of the condition only happened in the 1950s because of the work of Phalen.1 However, the etiology of CTS remains idiopathic in most cases. Various anatomic, systemic, and occupational factors such as repetitive use of the wrist and digits have all been described as potential causative factors.2, 3 In other studies, the focus has been on biomechanical factors that might influence the development of CTS.4-6 The carpal tunnel contains nine different tendons and the median nerve, bound by the carpal bones on the dorsal side and the transverse carpal ligament on the volar side. Recent studies have demonstrated that even in healthy people, the median nerve gets compressed between the flexor retinaculum and the tendons during active finger motion.3, 7 In addition, several studies have shown that there is reduced longitudinal gliding of the median nerve in CTS patients.8, 9 This suggests that monitoring the motion and deformation of the median nerve by ultrasound may offer new insights into the mechanics within the carpal tunnel, and potentially serve as a new means by which CTS can be better understood, or perhaps even diagnosed. It is therefore important to characterize the deformation of the median nerve during finger motion in both CTS patients and normal controls. We hypothesized that there are detectable differences in deformation and motion of the median nerve in individuals with CTS, compared to healthy controls. If our hypothesis is supported, then these parameters would potentially be useful to use ultrasound as a noninvasive tool to study the genesis of CTS, and to monitor at risk individuals.

METHODS

Image Acquisition

This study was approved by our Institutional Review Board, and all participants gave written informed consent. We recruited 29 healthy volunteers (15 women, 14 men, age range 22–67 with a mean age of 35.5 years) without any history of CTS, and 29 patients with idiopathic CTS (18 women, 11 men, mean age 51.1 years with a range of 26–70 years) which was diagnosed by electrophysiological studies. All but two volunteers had bilateral CTS. CTS patients with a history of systemic disease associated with a higher incidence of CTS, such as thyroid disease, obesity, or rheumatoid arthritis, as well as all patients with any upper extremity surgery in their medical history, were excluded. We evaluated both left and right wrists in the healthy volunteers; in CTS patients we evaluated the affected side(s). Cross-sectional images of the carpal tunnel were obtained by placing the 15L8 linear array transducer of a Siemens Sequoia C512 ultrasound machine (Siemens Medical Solutions, Malvern, PA) set to a 15 MHz acquisition frequency, transversely at the wrist crease and perpendicular to the long axes of the forearm, just proximal to the carpal tunnel. The participants were positioned with the supinated hand fixed in a custom made device, with the wrist in neutral position. They were asked to flex and extend all four fingers (index, middle, ring, and little) together as well as to move three digits (either middle finger, index finger, or thumb) independently, from 0° (i.e., full) finger extension to the maximum flexion, that is, until the finger tip touched the palm. In the case of single digit motion, the participant was asked to keep the other fingers as much extended as possible. Five cycles of motion were recorded for each of the four movements. Using Analyze 8.1 software (Mayo Clinic, Rochester, MN) the recorded clip was reviewed and the initial and final frames of the motion cycle were selected. Based on these images, the outer hypoechogenic rim of the median nerve was outlined for both the full extension and the full flexion positions (Fig. 1). We then calculated the median nerve cross-sectional area and perimeter. We also calculated the circularity, defined as:

(1)

Also, the aspect ratio of the median nerve minimum enclosing rectangle (MER), was an indication of the flattening of the median nerve. To calculate the aspect ratio of the MER, the software calculated the smallest possible rectangle that would fit around the cross-section of median nerve and divided the short axis by the long axis. We then created a deformation index (DI), calculated as:

(2)

where DI is an indication for the amount of deformation of the median nerve throughout an extension to flexion motion. Since we recorded five cycles of each finger motion within each participant, we were able to calculate the intra-rater reliability from these measurements.

RESULTS

The results of the absolute parameter measurements of the median nerve for all finger motions are shown in Table 1 and the deformation indices in Table 2.

DISCUSSION

In this study we have shown that in flexion and extension, regardless of the specific finger motion, the median nerve cross-sectional area and deformation of the median nerve are greater in CTS patients than in controls. We believe that these are important observations for several reasons.

The tendons and median nerve move in a three-dimensional plane during finger motion.3, 7 Because the carpal tunnel is a closed space, the median nerve cannot move away from the tendons and thus gets compressed causing a change in shape and area. As noted by others, these parameters may be useful as a tool for diagnosing CTS with ultrasound.8-12 Our results show that the best parameters seem to be the cross-sectional area and the area deformation index (Tables 1 and 2). While circularity measurements were also different in all finger motions, this deformation index was only significantly different in four finger and middle finger motion, so this measure would probably not be useful for clinical purposes. As shown in Tables 1 and 2, the results of perimeter and MER measurements are also too inconsistent for clinical use. Other investigators have studied median nerve cross-sectional area in CTS, with reported values ranging from an average cross-sectional area of the median nerve at the distal wrist crease of 7–9 mm² in asymptomatic volunteers to 13.7–16.8 mm² in CTS patients.10, 12-14 Klauser et al. compared the cross-sectional area of the median nerve (CSA) at two different levels. They found an average CSA of 16.8 mm² in patients and 9.0 mm² at the carpal tunnel level, and 9.5 mm² in patients and 8.7 mm² in controls at a more proximal level.12 They also calculated the difference between those two measurements and found a 99% sensitivity and 100% specificity for these measurements. However, absolute value measurements in the carpal tunnel may also be dependent on confounders such as gender and wrist size. We believe that the amount of compression is therefore best shown by a deformation index, as calculated in this study, since it is unaffected by absolute size. Since the measurements for the median nerve area were different both in normal measurements as well as in deformation index, we believe this would be the best potential parameter to distinguish between patients and healthy individuals in supporting a clinical diagnosis of CTS. Sernik et al.14 showed the median nerve cross-sectional area was increased compared to their control group and they suggest a cut off point of 10 mm², but the reliability, specificity, and sensitivity of such measures were not rigorously compared to other diagnostic measures, such as electrodiagnosis, clinical findings, or differences between flexion and extension values. Other studies suggest cut-off values of 9–11 mm², with high sensitivity and specificity levels.10, 13 Based on our results we would suggest using a cut-off value of 10 mm², measuring the area with middle finger motion since we noticed during image acquisition that the third superficial flexor digitorum tendon is the easiest tendon to recognize compared to the other tendons.

Finally, and most importantly, we believe the measures described here may be useful in noninvasive study of mechanical behavior related to the median nerve in health and disease. A combination of longitudinal and cross-sectional data could generate three-dimensional images of the carpal tunnel contents; the dynamic aspect imparts a fourth dimension, that of movement over time. Such imaging could be used to investigate, and even monitor, the mechanical behavior of the nerve and tendons within the carpal tunnel, not only for simple motions, as described here, but also for more task-related activities, such as pinching, gripping, or keyboarding. Such investigations may shed further light on activities likely to deform the median nerve.

Our study has some short comings. As shown in Tables 1 and 2, differences in healthy volunteers are very small between flexion and extension, and one finding was different from the general trend in results: the perimeter in middle finger motion was higher at flexion than in extension in controls.

Ultrasound is known to have measurement errors due to operator-differences like experience, but also technical differences like the angle of the transducer to the wrist. Even though fixed the transducer in a special holder, motion of the patient may have influenced the results. The angle in which the transducer is placed on the patients' wrist is also important: in case of a smaller angle (than 90°, like ours) the cross-section of the median nerve might become bigger, for example. Another cause could be the level of the carpal tunnel at which the measurement is taken: our measurements were taken at the wrist crease level, just proximal to the tunnel. If images were taken more distally, within the tunnel, the median nerve might be more compressed. Also, during finger motion, the tendons move towards the median nerve, thereby compressing it. This happens in both healthy controls and in CTS patients, as shown with this study. It is logical to think that the median nerve might flatten with finger flexion, as the tendons press against the nerve from below. For a given cross-section, a circular shape will have a smaller perimeter than an elliptical shape. This means that even in healthy subjects, due to the change in shape, the perimeter might change as well, but this effect might differ between different locations along the course of the nerve.

Secondly, our results are not categorized by severity of CTS. We do believe it would be useful to study the relation between duration of symptoms, severity of the electrophysiological changes, and deformation of the median nerve. This would be a useful next step towards using ultrasound parameters for diagnosing CTS in an early stage. There was a statistically significant age difference between our patient and control groups which may have caused bias in our results, since theoretically the difference in shape may have been caused by a normal aging process rather than disease.

The intra-rater reliability was calculated from five repetitions within each finger motion measurement, all done by one not-blinded investigator within the same session. Ideally, a more true representation of the intra-rater reliability would have been to compare multiple repetitions within more than one session. Also, even though it is known that ultrasound is a highly operator-dependent tool, we did not calculate inter-observer reliability. This remains for future studies.

We conclude that it is possible to investigate the deformation of the median nerve in CTS by ultrasonography and that there is more deformation of the median nerve in CTS patients during active finger motion. These parameters might be useful in the future as an additional tool for diagnosing or assessing the biomechanics of CTS.