Systematic review: hand activity and ultrasound of the median nerve

Abstract

Background

Ultrasound is an established method of viewing the median nerve in the carpal tunnel syndrome (CTS). There is some evidence to suggest that immediate changes may occur in the median nerve before and after hand activity. The evidence for the validity and reliability of ultrasound for testing acute changes in the median nerve has not been systematically reviewed to date.

Aims

To evaluate the evidence for visible change in ultrasound appearance of the median nerve after hand activity.

Methods

A literature search was designed, and three reviewers independently selected published research for inclusion. Two reviewers independently appraised papers using the Evidence Based Library and Information Practice (EBLIP) appraisal checklist, while the third reviewer resolved discrepancies between appraisals.

Results

Ten studies were appraised and the results showed an increase in median nerve cross-sectional area following activity, with a return to normal size within 1 h following activity. Both healthy individuals and those diagnosed with CTS participated, all were small convenience samples. Ultrasonographic measurements of the median nerve were reliable in the four studies reporting this, and the studies demonstrated high quality.

Conclusions

Good-quality evidence as identified by the EBLIP appraisal checklist suggests that following hand activity, the median nerve changes in size in the carpal tunnel. The results may not be generalizable to all people and activities due to the use of small convenience sampling and narrow range of activities studied, in all of the studies appraised.

Introduction

Carpal tunnel syndrome (CTS) has prevalence in the general population ranging from 1 to 5% [1]. Symptoms of CTS include motor weakness in muscles that abduct and oppose the thumb, as well as pain, tingling or numbness in the thumb, index finger, or middle finger [2]. The carpal tunnel is bounded by the carpal bones of the wrist and enclosed by fibro-osseous structures [3]. The median nerve is positioned deep to the flexor retinaculum and superficial to the flexor digitorum superficialis tendons [3]. It is here between the flexor tendons and flexor retinaculum, where compression of the median nerve is likely to occur [4,5].

CTS is considered to be an occupational condition with an increased prevalence among occupations which involve repetitive wrist movements [2,5,6]. There are a number of risk factors associated with occupational CTS, including local compression over the carpal tunnel, exposure of the hand to vibration, repetitive hand or wrist movements and maintained awkward postures with wrist deviation, flexion or extension [2,6,7]. However, the mechanisms and process of median nerve compression or ischaemia, in response to specific activities, are not measurable and remain largely unreported in both healthy people and patients with CTS.

One study about keyboarding [8] reported the change in median nerve dimensions as visualized with ultrasound following 60 min of continuous keyboarding in 12 females and 9 males. An increase in median nerve cross-sectional area (CSA) was found following the 60-min keyboard activity (P < 0.05), allowing the authors to conclude that there may be a causal relationship between keyboarding and CTS.

Ultrasound is a suitable method of imaging the median nerve within the carpal tunnel as it shows the carpal tunnel anatomy with great accuracy [3,9]. The median nerve is distinguished from the flexor tendons by its appearance, position in the carpal tunnel and distinct motion when compared with that of the flexor tendons [9].

Ultrasound imaging of the median nerve in CTS diagnosis is becoming increasingly common due to its accessibility, low cost, non-invasiveness and shorter examination times [3,6,10,11]. Ultrasound does not provide information relating to median nerve function; however, it is helpful in displaying a variation in nerve size, shape, echogenicity and vascularity [10]. Echogenicity is the brightness of structures visualized on diagnostic ultrasound images, which can help distinguish pathological nerves from normal nerves [12]. An increase in intraneural oedema results in reduced echogenicity, therefore pathological nerves appear hypoechogenic, or darker, in comparison to normal nerves [13].

The use of ultrasound in identifying CTS is reliable [3]. A past protocol [3] gave reproducible measurements in order to detect changes in the median nerve accurately. This protocol included strict patient positioning with the forearm supinated and the fingers relaxed [3].

Ultrasound has been used to measure the change in median nerve appearance following upper limb exercise. Past studies [5,9] reported an increase in CSA of the median nerve in asymptomatic subjects, following a provocative hand exercise. The authors propose an explanation for this increase in median nerve CSA, attributing it to the anatomy and vascularity of the nerve. The vasa nervorum are the blood vessels which supply and drain peripheral nerves. They are coiled which allows their expansion within the nerve, and they contain reserve vessels which may dilate in the case of exertion [9].

Other authors [8] suggest how these findings in healthy participants may impact on patients with CTS. It may be possible to grade the risk associated with particular activities in association with the likelihood of developing CTS, by determining how repetitive activity affects the median nerve [6]. Therefore, the aim of this review was to determine the evidence for visible change in ultrasound appearance of the median nerve after upper limb activity.

Methods

The population, intervention and outcome were identified to plan the search strategy. Using this framework, studies looking at how the median nerve responds to activity as visualized with ultrasound were identified. An academic librarian assisted in the search design. Table 1 displays the search terms used for Medline, EMBASE, CINAHL and Scopus databases. The systematic search was performed on 17 December 2015. Figure 1 shows the number of studies the search returned from each database. An additional seven studies were identified through the process of ‘pearling’, which searches the reference lists of published papers. Database searches offering ‘papers citing this reference’ were also searched. Duplicates were removed before the abstracts were screened using the eligibility criteria. The abstracts were screened by one reviewer, and the resulting 12 abstracts were reviewed by the other two reviewers. Abstracts without full texts available were excluded from the search.

Inclusion criteria for the studies were that healthy adults screened for upper limb and systemic disorders were included and/or patients with a diagnosis of CTS, median nerve dimensions in the carpal tunnel region, as visualized with ultrasound were acquired before and after activity of the upper limb. Studies were excluded if they only investigated nerve mobility, or ultrasound measurements were only acquired during different postures of the upper limb.

Ten studies were appraised using the Evidence Based Library and Information Practice (EBLIP) critical appraisal checklist. The questions are grouped into four sections: A, population; B, data collection; C, study design; D, results [14]. The studies were appraised independently by two reviewers.

Three questions in Section A were not applicable to the studies being assessed: ‘If a comparative study: Were participants randomized into groups? Were the groups comparable at baseline? If groups were not compar able at baseline, was incomparability addressed by the authors in the analysis?’ The question, ‘Is the sample size large enough for sufficiently precise estimates?’ was interpreted as being positive if power analysis for sample size was performed and described in the methods and results.

One question in Section B, ‘If a face-to-face survey, were inter-observer and intra-observer bias reduced?’ was not applicable to the studies being assessed. This is because surveys were not used for data collection [14], therefore the remaining items are displayed in Results section of Table S1 (available as Supplementary data at Occupational Medicine Online).

Each question in the appraisal tool was answered with either yes (Y), no (N), unclear (U) [14]. If a question was answered with ‘unclear’, this was considered a negative response [14]. The validity of each section for each study was determined by calculating the percentage of questions answered yes within that particular section [14]. Questions answered with ‘not applicable’ were excluded from these calculations [14]. The total number of responses (T) was given by the following formula: Y + N + U = T [14]. Hence, the percentage of questions answered yes was given by Y/T [14]. If this percentage was <75%, it was concluded that there were noticeable omissions within the section and that the validity of the study was questionable [14].

Results

Four hundred and sixty-one studies were identified through the search, and processed as seen in Figure 1. Twelve studies remained to be screened by full text. Two conference abstracts were excluded, as the authors did not respond to requests for further details of their studies. Ten studies satisfied the selection criteria of the search and passed screening by three reviewers (Figure 1).

Ten studies met the eligibility criteria, while nine studies showed a statistically significant change in median nerve CSA following repetitive activity; seven studies reporting increased CSA and three reporting a decrease in median nerve CSA, as seen in Table S1. One study reported no significant change in median nerve dimensions.

Eight of the ten studies achieved an overall validity score of ≤75% when the EBLIP criteria were applied, and there was little discrepancy (1–2 appraisal criteria) between the two independent appraisers, all of which could be resolved by discussion. All studies used convenience sampling, and none provided a power calculation to justify their sample size. The study population of four studies was considered unlikely to represent the general adult population [6,8,9,15], as they only recruited young adults [9,15], or did not describe the age or sex of the study population, simply referring to subjects as volunteers [6,8]. One study only described in detail their population of hemiparetic patients [16].

All studies achieved >75% validity for their data collection methods; ultrasound methods were considered valid, described to allow reproduction, and there was no missing data in any study. In four of the studies, the measurements collected were not deemed free from subjectivity due to a lack of blinding among investigators [1,5,15,17]. Reliability of measurement was reported in three studies only; however, all studies employed experienced sonographers in data collection.

Study design achieved >75% validity in nine studies by both reviewers. Ethical approval was obtained in nine studies, with one study [5] not to confirm ethic approval. All studies showed appropriate study type/methodology, face validity, research methodology in sufficient detail to allow replication and outcomes were clearly stated.

Six studies achieved >75% for their reporting of results, all clearly outlined the results and their conclusions accurately reflected their results. Confounding variables were accounted for in seven studies, with three studies [5,15,16] failing to do so. The reviewers considered potential confounders to be the other activities that participants may have done to affect their median nerve, or the intensity of the activity they performed within the study, as this was very difficult to regulate.

Seven out of ten studies reported an increase in CSA following repetitive activity; three showed a decrease in CSA and one study found no significant change in median nerve dimensions following exercise. Six of the studies measured median nerve response in normal healthy subjects, while three compared the median nerve response to exercise between normal healthy adults and a study group of patients with CTS. The remaining study compared the effect of activity on the median nerve between normal healthy adults and hemiparetic stroke patients. Both genders were represented in six studies, although the proportions of males to females varied. The three studies about athletes included only males, while one [12] did not specify the gender of their subjects.

The type and duration of activity used to investigate median nerve response varied from a short simulated cutting activity [9] to longer typing and wheelchair propulsion activities [1]. Two studies differ to the remaining eight studies as the median nerve was not measured before and after a specified activity, rather at a cross-sectional time point.

Overall, the majority of the studies included in the systematic review provided valid results and conclusions, the major deficit in generalizability due to the use of small, convenience samples who tended to be younger adults. Two studies [8,15] were at most risk of bias as they did not achieve an overall validity of ≥75%.

Discussion

The majority of studies reported an increase in median nerve size following hand activity, and these were of good quality, but there were aspects of all of the studies that prevent comparison of median nerve responses between studies. The difficulty of standardizing effort, repetition and activity duration of the hand, then choosing activity relevant to different populations is appreciated.

There were also some aspects of the studies that could be standardized to facilitate replication and generalizability, namely sonographic measurement technique and participant selection. The technique used to determine the CSA of the nerve varied among studies, as seven studies used a continuous boundary trace of the median nerve to derive the CSA while others used electronic callipers to measure the anterior posterior diameter and the transverse width of the nerve before multiplying the two together to derive the CSA. Although both methods showed reliability, this factor decreases comparability of studies.

We propose a potential reason for the three studies that reported different median nerve responses to hand activity to the other studies reviewed. Two studies reported that asymptomatic wheelchair athletes showed a decrease in median nerve dimensions following wheelchair propulsion [12,18]. The type of motion required in wheelchair propulsion is unique and differs from the activities used in other studies included in the systematic review.

The third study showing a decrease in median nerve dimensions instructed subjects to perform 30 min of smartphone use [15]. Smartphone use also requires a unique motion of the wrist, possibly accounting for the findings. However, the other study that also investigated the effect of smartphone use on median nerve CSA, reported an increase in nerve dimensions [19]. This contrast in findings may be attributed to the duration of smartphone use. Perhaps these two studies cannot be directly compared as one used a cross-sectional [15] design while the other [19] employed a pre-post design. This outlines another gap in the current research, in that immediate effects of various activities have been measured, yet there are no longitudinal studies observing the nerve or any changes through a person’s occupational history, period of smartphone use or period of wheelchair sport participation, for example. It is possible that some activities lead to a gradual and irreversible increase in the size and fluid content of the nerve, as patients with CTS demonstrate when ultrasound imaging is applied [10–12].

One study reported no significant changes in median nerve CSA following a 30-min bilateral sanding activity [16]. This finding may also be attributed to the type of activity which involved large shoulder and elbow motions, without repetitive gripping, twisting or squeezing, therefore it may be appropriate to suggest that the median nerve may not respond in the same way.

This systematic review was unbiased as the authors and institutions had been blinded to the appraisers, and appraisals were performed separately by two authors. On the other hand, some of the exclusion criteria applied to the systematic search may have limited the results obtained. Only English papers were included in the results and the author’s attempts to request conference proceedings were not successful.

The data from the systematic review conclude that some hand activities are followed by significant change in the size of the median nerve as imaged using diagnostic ultrasound, and that the nerve returns to its pre-activity size following 10–30 min of rest. The small, convenience samples and the variation in type and duration of activity among studies pose difficulties in generalizing results. Despite these limiting factors, the data collection methods, study design and reporting of results in most studies were well reported and to be internally valid.

Conflicts of interest

None declared.

References

Author notes