Archives of Physical Medicine and Rehabilitation
Volume 90, Issue 9 , Pages 1489-1494, September 2009

Ultrasonographic Median Nerve Changes After a Wheelchair Sporting Event

Presented to the State of the Science Symposium: Ultrasound at Walter Reed Army Medical Center, September 25–26, 2008, Washington, DC.

  • Bradley G. Impink, BS

      Affiliations

    • Human Engineering Research Laboratories, Veterans Affairs Pittsburgh Healthcare System, Pittsburgh, PA
    • Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA
    • ,
  • Michael L. Boninger, MD

      Affiliations

    • Human Engineering Research Laboratories, Veterans Affairs Pittsburgh Healthcare System, Pittsburgh, PA
    • Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA
    • Department of Physical Medicine and Rehabilitation, University of Pittsburgh, Pittsburgh, PA
    • Corresponding Author InformationReprint requests to Michael L. Boninger, MD, Human Engineering Research Laboratories, VA Pittsburgh Healthcare System, 7180 Highland Dr, 151R1–H, Building 4, 2nd Fl, East Wing, Pittsburgh, PA 15206
    • ,
  • Heather Walker, MD

      Affiliations

    • Department of Physical Medicine and Rehabilitation, University of Pittsburgh, Pittsburgh, PA
    • ,
  • Jennifer L. Collinger, PhD

      Affiliations

    • Human Engineering Research Laboratories, Veterans Affairs Pittsburgh Healthcare System, Pittsburgh, PA
    • Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA
    • Department of Physical Medicine and Rehabilitation, University of Pittsburgh, Pittsburgh, PA
    • ,
  • Christian Niyonkuru, MS

      Affiliations

    • Department of Physical Medicine and Rehabilitation, University of Pittsburgh, Pittsburgh, PA

Article Outline

Abstract 

Impink BG, Boninger ML, Walker H, Collinger JL, Niyonkuru C. Ultrasonographic median nerve changes after a wheelchair sporting event.

Objectives

To investigate the acute median nerve response to intense wheelchair propulsion by using ultrasonography and to examine the relationship between carpal tunnel syndrome (CTS) signs and symptoms and the acute median nerve response.

Design

Case series.

Setting

Research room at the National Veterans Wheelchair Games.

Participants

Manual wheelchair users (N=28) competing in wheelchair basketball.

Intervention

Ultrasound images collected before and after a wheelchair basketball game.

Main Outcome Measures

Median nerve cross-sectional area, flattening ratio, and swelling ratio and changes in these after activity. Comparison of median nerve characteristics and patient characteristics between participants with and without positive physical examination findings and with and without symptoms of CTS.

Results

Significant changes in median nerve ultrasound characteristics were noted after activity. The group as a whole showed a significant decrease in cross-sectional area at the radius of 4.05% (P=.023). Participants with positive physical examinations showed significantly different (P=.029) and opposite changes in swelling ratio compared with the normal group. Subjects with CTS symptoms had a significantly (P=.022) greater duration of wheelchair use (17.1y) compared with the asymptomatic participants (9y).

Conclusions

Manual wheelchair propulsion induces acute changes in median nerve characteristics that can be visualized by using ultrasound. Studying the acute median nerve response may be useful for optimizing various interventions, such as wheelchair set up or propulsion training, to decrease both acute and chronic median nerve damage and the likelihood of developing CTS.

Key Words: Carpal tunnel syndrome, Median nerve, Ultrasonography, Wheelchairs

List of Abbreviations: BMI, body mass index, CSA, cross-sectional area, CTS, carpal tunnel syndrome, FSS, functional status scale, SSS, symptom severity scale

 

CARPAL TUNNEL SYNDROME is the most common entrapment neuropathy and a major public health problem in the general population1 with a prevalence between 1.5% and 2.7%.2, 3, 4, 5 Our previous research6, 7, 8 has shown a direct link between manual wheelchair propulsion and CTS. Manual wheelchair propulsion requires high-force, high-repetition actions of the wrist, which are believed to contribute to median nerve damage.9, 10 Therefore, it is not surprising that in manual wheelchairs users the prevalence of CTS is much greater than in the general population, with a range from 49% to 73%, and that prevalence of CTS increases with the duration of disability.11, 12, 13, 14, 15, 16 The symptoms associated with CTS can be disabling for manual wheelchair users because they rely heavily on their arms to complete activities of daily living. In this study, we sought to investigate the median nerve response to manual wheelchair propulsion via ultrasound imaging before and after an intense sporting event.

Ultrasonography has become a popular imaging tool because of its noninvasiveness, shorter examination time, and lower cost.17 High-frequency transducers allow for the depiction of peripheral nerves and tendons, such as those seen in the carpal tunnel. Ultrasound has been shown to be a very precise method of viewing musculoskeletal anatomy. In a study by Kamolz et al,18 the cross-sectional areas of the median nerves of 20 cadavers were measured both anatomically and by using ultrasonography. There were no significant differences found between these measurements, and they concluded that ultrasound can precisely display the anatomy of the carpal tunnel and median nerve. When viewing structures via ultrasound, those containing more fluid appear hypoechoic or darker (eg, nerves, blood vessels), whereas those containing less fluid appear more hyperechoic or lighter (eg, bones, tendons). Ultrasound images of carpal tunnel anatomy are presented in figure 1.

  • View full-size image.
  • Fig 1. 

    Unlabeled (left) and labeled (right) cross-sectional images of the wrist at the level of (A) the distal radius and (B) the pisiform. Abbreviations: DR, distal radius; MN, median nerve; PIS, pisiform; T, tendon.

Many studies have shown ultrasound to be useful in the diagnosis of CTS. Buchberger et al19 found that the median nerve of participants with CTS had significantly larger CSA at the pisiform level and flattening ratio (nerve width divided by nerve height) at the hamate level. Other investigators19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 have confirmed these findings, with the most consistent finding being an increased CSA of the median nerve at the pisiform level. Other common findings include an increased swelling ratio (CSA at pisiform level/CSA at distal radius) and increased palmar bowing of the flexor retinaculum. In summary, larger flatter nerves tend to be more associated with CTS than smaller, rounder nerves. Ultrasound has also been used to study the acute nerve response to activity. Two studies30, 31 investigating acute changes in median nerve CSA found increases after activity and that those increases were larger in participants with CTS. Altinok et al30 investigated the effects of bidirectional squeezing and twisting with the hands over 10 minutes and found increases in CSA at the level of the pisiform. Massy-Westropp et al31 investigated the changes in the median nerve after a 5-minute cutting task and found increases in CSA at both the pisiform and radius levels.

In this study, we used ultrasound to investigate median nerve characteristics commonly associated with CTS during a baseline examination. We also examined changes in these variables by comparing ultrasound images before and after an intense wheelchair sporting event. We investigated how these baseline and change values differed between groups with and without signs and symptoms of CTS. We hypothesized that subject characteristics (age, years of wheelchair use, weight, and BMI) would be associated with a larger baseline median nerve CSA, flattening ratio, and swelling ratio. We expected the intense activity associated with a wheelchair basketball game to induce detectable changes in median nerve characteristics and that subject characteristics would be associated with greater changes in those variables. For example, the longer one's been propelling a manual wheelchair, the larger and flatter their nerves would be at baseline and their nerves would be more likely to change in response to activity. We further hypothesized that participants with signs and symptoms of CTS would show greater changes than the asymptomatic participants.

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Methods 

Subjects 

Subjects were recruited at the 2004 and 2005 National Veterans Wheelchair Games. A convenience sample of wheelchair athletes who expressed interest was asked to participate in the research study. To be eligible to participate in the study, participants were required to use a manual wheelchair as their primary means of mobility and participate in wheelchair basketball. Subjects were excluded if they reported a history of traumatic upper-extremity injury from which they had not fully recovered or which affected their ability to propel a manual wheelchair. They were excluded because trauma could affect wrist anatomy, and we wanted to avoid introducing these spurious elements in this study. All participants provided institutional review board–approved written informed consent before testing.

Data Collection 

Subject demographics were collected via a questionnaire. Participants completed brief questionnaires focused on symptoms of CTS. We used the SSS and FSS questionnaires developed by Levine.32 These scales have been found to have high reproducibility (Pearson correlation of .90), internal consistency (Cronbach α=.89), validity, and sensitivity to clinical change. Additionally, 1 of 2 physicians blinded to the questionnaire and subject history completed a physical examination of the wrists and hands focused on common findings associated with CTS. The 2 physicians were physiatrists, 1 with more than 15 years postresidency experience who thoroughly trained the other physician, a resident physiatrist, on the physical examination protocol. This physical examination included a series of physical examination techniques commonly used in assessing CTS. The physical examination consisted of the following tests: thenar muscle atrophy, thumb weakness with abduction, fifth digit weakness to abduction, 2-point discrimination, impaired sensation to pinprick, Tinel sign, carpal compression test, and Phalen test.33 Only the nondominant scores of the SSS, FSS, and physical examination were used because the ultrasound images were only collected on the nondominant wrist.

Ultrasound images were collected by using the Diasusa ultrasound machine using a 10- to 22-MHz linear array transducer. One set of images, the baseline images, was collected anytime before participation in the sporting event. Immediately after the event, an investigator escorted the participant to the ultrasound examination room where another set of ultrasound images, postactivity images, was collected to investigate acute changes. Images of the carpal tunnel, with primary emphasis on the median nerve, were collected at 2 levels of the wrist: (1) at the distal radius and (2) at the pisiform. The radius level was at the most distal ridge of the radius, which was found by scanning distally along the wrist until the radius disappeared from the image, at which time the probe was slid proximally until the bone reappeared. This corresponds to the point at which the distal ridge of the radius is closest to the skin. The level of the pisiform was found by scanning distally along the wrist until the pisiform first appeared in the image. Figure 1 shows sample images collected with the bony landmarks and median nerve labeled. These 2 regions are easily viewed by using ultrasound, and nerve characteristics at these locations have previously been linked to CTS both electrodiagnostically and symptomatically.19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 Hand and arm position was maintained in a neutral posture during all imaging to ensure reliability of the images collected. The amount of time played was self-reported by the subject, and we recorded the time between the end of the event and postevent ultrasound.

Data Analysis 

Median nerve CSA was determined by using custom image analysis software to perform a boundary trace. The software presented the images in a random fashion, thus blinding the investigator completing the trace to subject characteristics and whether the image was taken before or after activity. A biomedical engineer with 2 years of experience collecting and analyzing ultrasound images performed the boundary trace. The trace included the hypoechoic portion of the nerve, ignoring the hyperechoic boundary. This hyperechoic boundary represents the epineurium (the edges can be difficult to distinguish from its surroundings) and therefore was not included. The software also measured nerve width and height based on user input for the purpose of calculating the flattening ratio. Changes in these variables were calculated by subtracting the baseline value from the postactivity value. Research34 has shown good reproducibility of median nerve characteristics when measured in this fashion. An unpublished repeatability study was conducted as part of this series of studies by capturing and analyzing median nerve images of 15 participants at 2 different times separated by 30 minutes of rest. By using intraclass correlation coefficients, we found values of .85 and .90 for distal radius and pisiform level CSA, respectively.

Statistical Analysis 

There were 3 basic steps in our analysis. The first step was to investigate relationships between baseline median nerve ultrasound variables and subject characteristics. The second step was to investigate any relationships between change in median nerve variables and subject characteristics. Playing time and time between the end of the event and the postactivity ultrasound were analyzed as possible predictors of median nerve change. Pearson correlations were used to determine relationships between subject characteristics and median nerve variables. The third step in the analysis was to compare differences in subject characteristics and changes in median nerve characteristics between participants with signs and symptoms of CTS with those without signs and symptoms. For this analysis, the group was dichotomized in 2 ways: (1) participants with 1 or more positive physical examination finding (ie, sign[s] present) and those with none (ie, sign[s] absent) and (2) participants with a score of 1 or more on the SSS and FSS (ie, symptomatic) and those scoring 0 (ie, asymptomatic). After dichotomizing the groups, independent sample t tests were used to investigate any differences between the 2 groups. The Benjamini-Hochberg method was used to control for multiple testing. Statistical analyses were conducted by using SPSS version 15.b

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Results 

Subjects 

Twenty-eight men with a mean age of 40.3±8.86 years and mean years of wheelchair use of 13.4±9.46 years participated. The average playing time was 28.6±15.8 minutes, and the average time between the end of the event and the postactivity ultrasound was 9.7±5.5 minutes.

Baseline Median Nerve Ultrasound Variables 

There were no significant relationships (all P>.217) between baseline median nerve variables (CSA, flattening ratio, swelling ratio) and subject characteristics (age, years of wheelchair use, weight, BMI).

Median Nerve Changes After Activity Compared With Baseline 

Before investigating changes between baseline and postactivity measurements, we investigated the effects of playing time and the time between the end of the event and the postactivity ultrasound. Neither of these time variables was related to the postactivity or change in median nerve measurements (all P>.349).

On investigating changes in median nerve variables, there was a significant (P=.023) decrease in CSA at the level of the distal radius. The mean baseline CSA was 11.86mm2 and 11.38mm2 after activity representing a 0.48mm2 or 4.05% decrease in CSA. Although the CSA at the level of the pisiform also decreased, it was not a statistically significant change. There was no significant change in flattening ratio at either level, nor was there a significant change in swelling ratio. There was a positive trend between age and change in flattening ratio at the distal radius (P=.081). No other median nerve change variables were significantly correlated with subject characteristics (all P>.125).

Comparison Between Groups 

Grouped based on physical examination findings 

The change in the swelling ratio was significantly (P=.029) different between the positive (n=19) and negative (n=9) physical examination groups (fig 2). No other significant differences were noted (all P>.184).

Grouped based on symptoms 

On comparing means between symptomatic (n=15) and asymptomatic (n=13) groups, we found that the symptomatic group had a significantly (P=.022) larger duration of wheelchair use with a mean of 17.1 years compared with 9.0 years in the asymptomatic group (fig 3). No other significant differences were noted (all P>.145).

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Discussion 

Previous research19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 has shown that ultrasound is a useful tool for diagnosing CTS by measuring the shape and size of the median nerve at various levels of the wrist. It is our belief that it may also be useful at predicting the likelihood of developing CTS rather than simply diagnosing it after the fact. It may also be useful for assessing how detrimental a certain task is by examining the median nerve before and after a provocative activity, providing insight into the repetitive trauma that the nerve undergoes during various activities.

In our study, the baseline characteristics of the median nerve were not sufficient for determining any relationships with characteristics commonly linked to CTS such as age, weight, BMI, or years of wheelchair use.11, 12, 13, 14, 15, 16, 35, 36, 37, 38 This was contrary to our hypotheses but could be explained by the fact that only 2 participants reported they had been previously diagnosed with CTS. We also had a relatively small number of participants (n=28) and a unique population with a broad range of ages and activity levels, which could contribute to the lack of baseline relationships. This leads to the need for another measure such as assessing the median nerve acute response.

Based on our literature search, we found 2 previous studies investigating the acute median nerve response to activity.30, 31 Altinok et al30 investigated the effects of repeated forceful bidirectional squeezing and twisting with the hands over 10 minutes and found significant increases in CSA at the level of the pisiform. Their findings at the level of the distal radius were not significant, but there was a CSA increase of 5.4% in control participants and 4.2% in CTS participants. A study by Massy-Westropp et al31 investigated the changes in the median nerve after performing a cutting task repeatedly for 5 minutes and showed increases in CSA at both the level of the pisiform (4.1% in men) and the level of the radius (2.7% in men).31 Based on these 2 studies, we developed our hypothesis that median nerve CSA would increase after wheelchair propulsion.

We found changes in CSA at the distal radius of similar magnitude to these studies, but our changes were in the opposite direction, contrary to our hypothesis. We found a significant decrease in the CSA of the median nerve at the level of the radius. At the pisiform, we also saw a decrease in CSA, although it was not statistically significant. We believe the difference between our study and the 2 mentioned here is the nature of the repetitive activity. The 2 previous studies involved low-impact repetitive gripping and finger movement, whereas wheelchair propulsion during wheelchair basketball is a high-impact activity with the hand striking the pushrim with each propulsive stroke. More research is needed to investigate how different hand and wrist activities may affect the median nerve at various levels of the wrist.

To further elucidate how the median nerve response to activity may be related to pathology, we split our group into 2 based on the presence of physical examination findings. This resulted in findings supporting our hypotheses, namely the positive physical examination group showed significantly different changes in swelling ratio in response to activity compared with the negative examination group. The median nerves of the positive examination group showed an increase in swelling ratio likely caused by increased trauma and strain of the nerve during activity compared with the negative examination group. Nerve entrapment often results in increased swelling of the nerve just proximal to the point of entrapment.39, 40 In the case of CTS, the level of the pisiform is at the carpal tunnel inlet, likely the location just proximal to entrapment within the carpal tunnel. This supports the finding of an increase in swelling ratio in the positive examination group. The positive examination group had an increase in CSA at the pisiform level paired with a decrease at the radius level, resulting in an increased swelling ratio and suggesting that the nerve is entrapped causing swelling just proximal to the point of entrapment. In the negative examination group, there may not be entrapment, and, therefore, we did not see an increase in CSA at the pisiform level but rather a decrease. In fact, the decrease at the level of the pisiform was greater than the decrease at the radius level, resulting in the decreased swelling ratio in this group. An increased swelling ratio has been noted in participants with CTS,19, 23 and it may be that this swelling ratio is exacerbated by activity. Altinok et al30 also observed a significant increase in swelling ratio after activity and to a greater extent in those participants with CTS.

In addition to dichotomizing the group based on physical examination, we split the participants based on the presence of symptoms, expecting that those participants with symptoms would show greater median nerve changes in response to activity. Contrary to our expectations, there were no significant differences in median nerve changes between groups. However, as expected, the symptomatic group had a longer duration of wheelchair use compared with the asymptomatic participants. This is in line with previous research12, 14 suggesting that a longer duration of disability, and thus wheelchair use, is a risk factor for the development of CTS.

One of the common median nerve characteristics associated with CTS is flattening of the nerve.19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 In our study, we found neither baseline relationships nor significant relationships with changes in flattening ratios during our analyses. We did see a positive trend between age and change in flattening ratio, suggesting that the median nerves of older participants may be more likely to flatten in response to activity. The lack of significant findings is most likely caused by the fact that the flattening is typically noted at the level of the hook of the hamate and we did not collect images at this level because of limitations of our ultrasound equipment and time constraints associated with this study design. We measured the flattening ratio at the distal radius and pisiform levels as exploratory measures.

Study Limitations 

There are several limitations associated with conducting the study at a sporting event. One limitation is different amounts of activity among participants. Future studies could use a more uniform task in which each subject performs the same amount of activity. A second limitation of our study is that not all participants were imaged at a uniform time after activity. Because the study was conducted at a sporting event, we could not tightly control the time between the event and the postactivity ultrasound. However, a mean time of 9.68 minutes indicates a relatively quick postactivity examination and we found no statistically significant relationships between this variable and our median nerve ultrasound changes. Another limitation is that we could not control or account for activity before the baseline ultrasound examination. Subjects were idle (not propelling) for at least 30 minutes during the informed consent process before the initial ultrasound, but activity that occurred earlier in the day or on a previous day may have affected our results. However no participants participated in activities with the same level of intensity as wheelchair basketball in the 12 hours preceding their ultrasound. Having studied what occurs in the median nerve acutely, our future studies will focus on what changes occur postacutely (after several minutes or hours) and in the long-term (over months or years) and how these changes relate to the biomechanics of the activity. Future studies will likely include more measures of CTS, such as nerve conduction studies.

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Conclusions 

Baseline median nerve ultrasound characteristics were not different between participants with and without signs and symptoms of CTS. However, after intense activity (ie, wheelchair propulsion), the median nerve responded differently in those participants with signs and symptoms compared with those without. We have shown that ultrasound is a valuable tool for investigating changes in median nerve characteristics in manual wheelchair users and may be useful in future studies. Obtaining a better understanding of the median nerve response and how it relates to pathology will help us to study the impact of various interventions to prevent acute compression of the nerve. Such interventions may include different wheelchair setups, redesigned pushrims, or altering a patient's propulsive stroke to minimize the impact experienced by the wrist. Using ultrasound in this manner has research implications outside of the wheelchair user population. We may be able to study repetitive strain injuries in other high-risk populations such as those performing frequent typing and/or mousing.

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  • a Dynamic Imaging Limited, 9 Cochrane Sq, Brucefield Industrial Park, Livingston, EH54 9DR, Scotland.
  • b SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606-6307.

 Supported by the Human Engineering Research Laboratories, Veterans Affairs Pittsburgh Healthcare System; the U.S. Department of Veterans Affairs (grant no. B3142C); the National Institute on Disability and Rehabilitation Research (grant no. H133N000019); and the National Science Foundation (grant no. DGE0333420).

 No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated.

PII: S0003-9993(09)00317-7

doi:10.1016/j.apmr.2009.02.019

Archives of Physical Medicine and Rehabilitation
Volume 90, Issue 9 , Pages 1489-1494, September 2009

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