Ultrasound Characteristics of Carpal Tunnel Syndrome in the Spanish Population

ABSTRACT

Objectives

The aim of this study was to determine the cross‐sectional area (CSA) (cm²) of the median nerve and its relationship with patient age and sex, together with the frequency of other qualitative ultrasound patterns in patients with carpal tunnel syndrome (CTS) in the Spanish population.

Methodology

An observational, cross‐sectional, prospective, descriptive study was conducted among 79 patients diagnosed with CTS. The CSA of the median nerve was measured at the carpal tunnel inlet (CTI) and outlet (CTO) by ultrasound examination. In addition, qualitative variables such as hyperechoic enhancement, thickening, and loss of ultrasound pattern of the median nerve were measured. The images were analyzed using Image J software.

Results

The sample comprised 73.4% women and 26.6% men. In 68.4% of the cases, the dominant hand was involved. Professional cleaners and domestic staff were the most affected (22.8%). The CSA of the median nerve in patients with CTS was 0.16 cm² (95% CI 0.15–0.18) at the CTI and 0.17 cm² (95% CI 0.16–0.18) at CTO. At the CTI and CTO, the CSA showed significant differences in men under 50 years. As for the qualitative variables at the CTI and CTO, the most notable finding was the alteration in the ultrasound pattern, which was present in 87.3% and 91.1% of subjects, respectively.

Conclusion

The median nerve CSA has a homogeneous value at the CTI and CTO in the Spanish population with CTS. These values show differences in terms of patient age and sex. The alteration in the ultrasound pattern of the nerve and the presence of hyperechoic reinforcement are the most frequent qualitative findings.

The median nerve CSA has a homogeneous value at the CTI and CTO in the Spanish population with CTS, approximately 0.16 cm² and 0.17 cm² respectively. These values show differences in terms of patient age and sex.

1. Introduction

Carpal tunnel syndrome (CTS) is a common condition that causes pain, numbness, and tingling in the hand and fingers [1, 2]. It occurs due to compression of the median nerve, when it crosses the carpal tunnel in the wrist [1, 2]. The incidence of CTS varies, although it is one of the most common compression neuropathies [3, 4, 5]. Women tend to have a higher incidence than men, which may be due to anatomical differences in the wrist or hormonal factors [3, 4, 5, 6]. Additionally, certain medical conditions such as diabetes, rheumatoid arthritis, and obesity [7, 8] are recognized risk factors for the development of CTS.

This condition affects diverse populations and may be influenced by genetic, occupational, and socioeconomic factors [9, 10, 11]. Regarding nationality and geographic setting, variations in CTS incidence may be related to differences in predominant types of work and environmental factors. For example, higher rates of CTS may be observed in countries with industries that require repetitive manual work (repetitive wrist motions, prolonged wrist positions, and exposure to vibrations), such as manufacturing or assembly [12, 13, 14].

Some studies suggest that ethnicity may play a role in the prevalence and severity of CTS [15, 16, 17]. These differences could be due to genetic factors affecting the anatomical structure of the wrist or the median nerve itself, as well as differences in working conditions and general health between different racial groups [18, 19, 20, 21, 22, 23]. For example, certain specific anatomical or morphological characteristics may increase the risk of CTS [24, 25].

Further studies are necessary for the accurate identification of risk groups and to improve our understanding of the differences in the incidence of CTS according to nationality or ethnicity.

In recent years, high‐resolution musculoskeletal ultrasound has emerged as a revolutionary tool in the field of healthcare, from a clinical and scientific standpoint, marking a before and after in the diagnosis and treatment of many pathologies [26, 27, 28]. In the context of CTS, ultrasonography offers a rapid, dynamic, noninvasive, and cost‐effective method compared to other diagnostic techniques such as magnetic resonance imaging (MRI) and electromyography/nerve conduction studies (EMG/NCS) [29, 30, 31, 32]. In this regard, ultrasonography offers invaluable insight by enabling the measurement of quantitative and qualitative parameters [33, 34, 35, 36, 37] of the median nerve within the carpal tunnel. This can be instrumental in the assessment of the severity of nerve compression, based on hallmarks of CTS, such as the cross‐sectional area (CSA) (cm²), which has been demonstrated as an accurate and reproducible test for diagnosis of CTS [29, 31]. Other qualitative variables, such as loss of the ultrasound pattern of the median nerve, can be evaluated to help clinicians assess severity and customize treatment approaches. However, it should be noted that these ultrasound measurements may vary between populations due to anthropometric differences [24, 25]. This highlights the importance of establishing population‐specific reference values to enhance diagnostic accuracy.

This study hypothesizes that there are specific ultrasound characteristics in the CSA of the median nerve within the Spanish population that may differ based on age, sex, or work‐related factors. Establishing these standards for the Spanish population will enhance the clinical reliability of ultrasound diagnostics and ensure that criteria reflect the nuances of this demographic thereby reducing the likelihood of false positives or negatives.

Therefore, the objective of this study was to determine the CSA of the median nerve and its relationship with patient age and sex, and the frequency of other qualitative ultrasound patterns in patients with CTS in the Spanish population.

2. Materials and Methods

2.1. Participants

An observational, cross‐sectional, prospective, descriptive study was carried out on patients diagnosed with CTS. The patients were consecutively admitted to the Hospital General Universitario de Ciudad Real (Spain) and subsequently referred to the “Puerta de Atocha” Health Center (Ciudad Real) for ultrasound examination, over a 12‐month period. Demographic data such as age and sex, hand dominance, and occupational activity were recorded.

The study protocol received approval from the Institution's Ethics Committee (Minute No. 01/2014) and was registered in https://www.clinicaltrials.gov/ (registration number NCT04216147). All patients provide written informed consent.

The inclusion criteria required subjects to be over 18 years old and diagnosed with idiopathic CTS. The diagnosis of CTS was established when the patient presented the electromyographic abnormalities described as the gold standard for CTS diagnosis [38, 39].

To achieve the highest level of diagnostic certainty, the CTS diagnosis was defined according to the criteria proposed by Harrington et al. [40], which combines the findings from electrodiagnostic studies with symptomatic characteristics. To be included in the study, all patients had to meet the inclusion criteria listed in Table 1.

TABLE 1.

Inclusion and exclusion criteria.

INCLUSION CRITERIA

People over 18 years of age

Positive electromyography examination [38, 39]

Pain, or paresthesia, or sensory loss in the median nerve distribution and one of: Tinel's test positive, Phalen's test positive, nocturnal exacerbation of symptoms, motor loss with wasting of abductor pollicis brevis [40]

EXCLUSION CRITERIA

Expected inability to complete the entire study follow‐up

Received surgery on the hand for any reason

Received injections in the last 3 months

Presents diagnostic imaging indicative of previous disease or trauma

Diabetes Mellitus type 2

Polyneuropathies: cervical‐brachialgias, neuropathic diseases

Rheumatism (Rheumatoid arthritis, Lupus erythematosus, Gout…)

Pregnancy

Hyperthyroidism

Patients who could not provide accurate or quality information due to illiteracy, blindness, or language barriers were excluded from the study. Other exclusion criteria included cognitive impairment, individuals who might introduce information bias, those with a history of CTS treatment involving wrist surgery or corticosteroid injections within the previous 3 months, patients with a history of wrist trauma or surgery, and individuals with a potential concurrent cause of idiopathic CTS (such as diabetes, thyroid disease, chronic rheumatoid arthritis, renal failure with hemodialysis, pregnancy, or breastfeeding). Additionally, patients with diffuse peripheral neuropathy or cervical radiculopathy were excluded.

2.2. Variables

The study variables were as follows: CSA (cm²) at the inlet and outlet of the carpal tunnel (Figure 1), obtained through ultrasound image analysis. Additionally, three qualitative sonographic features were assessed: hyperechoic enhancement, thickening, and loss of ultrasound pattern (Figure 1), evaluated using a dichotomous scale (Yes or No).

FIGURE 1.

Quantitative and qualitative sonographic examination of the median nerve at CTI and CTO. Quantitative variable (cross‐sectional area [CSA]) and triad of qualitative variables analyzed (thickening, hyperechoic enhancement, and loss of the ultrasound pattern). Thickness is defined as the distance from the superficial to the deep layer of the median nerve. Abnormal increase in the thickness of the median nerve is consider positive. Hyperechoic enhancement is defined as an increase in rim thickness. Abnormal hyperechoic enhancement is defined all around the MN or at some parts (up or down). Loss of ultrasound pattern is defined as an abnormal echo pattern without honeycomb appearance.

MN: Median nerve. P: Pisiform. S: Scaphoid. CTI: Carpal Tunnel Inlet. H: Hamate. T: Trapezium. CTO: Carpal Tunnel Outlet.

2.3. Ultrasound Examination

All ultrasound examinations were performed by an examiner with over 10 years of experience in musculoskeletal ultrasound who was blinded to the results of the electromyography so as not to influence the ultrasound examination. The patient was seated facing the examiner with the forearm in supination, resting on a flat surface, and the fingers slightly flexed. The probe was placed perpendicular to the longitudinal axis of the forearm, and three images of the transverse section of the median nerve were taken just proximal to the level of the scaphoid‐pisiform at the carpal tunnel inlet (CTI), and three more images were taken at the carpal tunnel outlet (CTO) at the hamate‐trapezium level (Figure 1). A portable General Electric Logic‐E ultrasound device was used with a 12 L‐RS linear probe. The examiner was blinded to the electrodiagnostic findings.

Once the ultrasound images were recorded, they were analyzed by an expert evaluator using the ImageJ^®️ tool (Figure 2). An intraobserver and interobserver reliability study was carried out. To evaluate intraobserver reliability, the mean of three recordings of each image was taken, and an external evaluator performed the evaluation of 20 of the images at random for interobserver validity. The analysis was performed using the “polygon” tool to obtain the CSA, after previously calibrating the tool in centimeters.

FIGURE 2.

Step‐by‐step use of Image J® tool for measurement of CSA.

First, we calibrate the scale to measure (cm). To do this, we can use the same image that we are going to analyze later:

1. We select the image in the menu by clicking on “file”.

2. We click on the straight line icon, and we use the scale of the ultrasound image to make a straight line that occupies one centimeter on the ultrasound scale.

3. After this, in the menu bar, we select “analyze” and in the drop‐down menu, “scale adjustment”.

4. In the pop‐up window, the pixels occupied by the selected distance are marked. In the “known distance” option, we will mark 1, and in “unit of measurement” we will indicate “centimeters.” We will mark the “global” box so that the scale is applied to all the images to be evaluated.

Once the measurement unit has been calibrated, we proceed to analyze the different images. To do this:

1. “File”, “new”, select the image to be measured.

2. Select the “polygon” icon.

3. Mark the CSA of the nerve.

4. “analyze” and “measure”.

5. A screen appears with the measurement performed.

2.4. Data Analysis

The data analysis was performed using SPSS version 25.0. Descriptive and frequency analyses of the main variables were conducted. The Mann–Whitney U test was used to determine differences between two independent groups. A p value of < 0.05 was considered statistically significant.

For the reliability of quantitative US measures (CSA), we calculated the intraclass correlation coefficient (ICC) of the intra‐ and interobserver reliability of US measures. For qualitative US measures, we calculated the Kappa index. The values of Kappa statistics range from 0 to 1: 0.01–0.20 indicates slight agreement, 0.21–0.40 fair agreement, 0.41–0.60 moderate agreement, 0.61–0.80 substantial agreement, and 0.81–0.99 almost perfect agreement [41, 42].

3. Results

In total, 79 patients diagnosed with CTS were examined, consisting of 58 women (73.4%) and 21 men, with a mean age of 49 years (SD: 11.9). In 68.4% of the cases, the condition coincided with the dominant hand, and most frequently affects people working as professional cleaners (16.5%) and domestic cleaning staff (22.8%).

The mean CSA value for all subjects at CTI was 0.16 cm² (95% CI 0.15–0.18) and 0.17 cm² at CTO (95% CI 0.16–0.18).

At the CTI, no significant differences were identified between men and women across all subjects (p = 0.515). However, when stratified by age, CSA showed significant differences in men under 50 years (p = 0.05) and in women over 50 years (p = 0.03) (Table 2).

TABLE 2.

CSA at CTI and CTO. Distribution by patient age and sex.

				Age
	All subjects			< 50 years			≥ 50 years
	n	Mean	CI 95%	n	Mean	CI 95%	n	Mean	CI 95%	P value
CSA at CTI
Women	58	0.163	0.149–0.177	37	0.153	0.136–0.169	21	0.182	0.159–0.204	0.038
Men	21	0.172	0.146–0.197	15	0.184	0.150–0.217	6	0.143	0.110–0.175	0.131
P‐value	0.515			0.054			0.031			—
	79	0.165	0.154–0.177	52	0.161	0.146–0.177	27	0.173	0.154–0.192	0.193
CSA at CTO
Women	58	0.163	0.152–0.173	37	0.160	0.148–0.172	21	0.167	0.147–0.187	0.540
Men	21	0.185	0.164–0.205	15	0.188	0.160–0.216	6	0.176	0.138–0.214	0.424
P‐value	0.039			0.029			0.408			—
	79	0.168	0.159–0.178	52	0.168	0.157–0.180	27	0.169	0.152–0.186	0.971

At the CTO, significant differences were observed overall in the group of men with a mean CSA value of 0.18 cm² (p = 0.03), and when stratified by age, in men under 50 years (p = 0.02) (Table 2).

As for the qualitative variables at the CTI, the most notable finding was the alteration in the ultrasound pattern, which was present in 87.3% of subjects, compared to less significant features such as hyperechoic reinforcement around the nerve, found in 49.4% of subjects, and nerve thickening, present in 44.3%. In 68.4% of cases, at least two of these features were present in the studied sample.

At the CTO, the alteration in the ultrasound pattern of the nerve was similarly notable, present in 91.1% of subjects, while hyperechoic reinforcement was observed in 44.3% and nerve thickening in 35.1%. In 64.6% of cases, at least two of these characteristics were present in the sample studied.

The CSA measurements at the CTI and CTO showed good intraexaminer (ICC = 0.846 and 0.796, respectively) and interexaminer reliability (ICC = 0.957 and 0.725, respectively). The intraexaminer agreement for the variables of thickening, hyperechoic reinforcement, and alteration in the ultrasound pattern was 100% (Kappa = 1.0) both at the CTI and CTO. The interexaminer agreement at the CTI and CTO was also complete in the ultrasound pattern and slightly lower for the variables of thickening (Kappa = 0.42 and 0.80, respectively) and hyperechoic reinforcement (Kappa = 1.0 and 0.783).

4. Discussion

In our study, the values found in the Spanish population for the CSA of the median nerve, both at the CTI and CTO, were homogeneous and approximately 0.16 cm² and 0.17 cm², respectively. These values are higher than those described in Anglo‐Saxon populations by Duncan et al. [43], reporting 0.13 cm², by Ziswiler et al. [44] in Switzerland, who measured 0.12 cm², by Mallouhi et al. [45], reporting 0.11 cm², by El Miedany et al. [46] in American subjects, measuring 0.10 cm², or by Sabag‐Ruiz et al. [19], reporting 0.10 cm² in the Mexican population. These data and their comparison should be interpreted with caution since most studies do not specify the exact location where the CSA of the median nerve within the carpal tunnel was measured. In this study, differences in the CTI and CTO were also identified. Future studies should consider this factor.

This study is relevant because, based on the values obtained, a subject with a CSA of the median nerve of 0.10 cm² would be considered an abnormal value in the Mexican population but would be defined as a normal value in the Spanish population since the mean is 0.16 cm² and 0.17 cm² at the CTI and CTO, respectively. Anthropometric differences may be decisive in a compressive clinical condition such as CTS. This circumstance should prompt future studies to consider anthropometric characteristics regardless of nationality.

Regarding sex, our study identified differences between men and women, in line with findings by Sabag‐Ruiz et al. [19]. In our case, this difference was found at CTI and CTO, with men under 50 years having a larger CSA of the median nerve.

One of the factors described in the scientific literature is fully identified in our sample: manual labor, with consequent overuse, appears to be a key determinant in the onset of CTS, as well as the importance of the dominant hand (in almost 70% of cases). In the study by El‐Najjar et al. [47], 65% of subjects with CTS performed repetitive cleaning activities in domestic settings.

This study has several limitations. First, our study was conducted on patients presenting to a single medical center. It would be advisable to confirm the results in a multicenter study with a larger sample size.

Second, measuring CSA with the ImageJ^®️ tool is a reproducible, inexpensive, and simple method; however, it requires some training, and it is time‐consuming to obtain the images and analyze them, which probably makes it difficult to use in daily clinical practice. From our point of view, it is a tool that has greater utility in CTS research. Conversely, from a clinical perspective, the triad of qualitative variables analyzed (thickening, hyperechoic enhancement, and loss of the ultrasound pattern) constitutes a quick and reliable way to describe median nerve compression in the carpal tunnel and may facilitate decision‐making for the healthcare professional, considering that in nearly 70% of cases, changes were identified in two of the three described variables. All variables measured showed good or excellent intraobserver and interobserver reliability (ICC > 0.75 and Kappa Index at least with moderate agreement [0.41–0.60]).

Third, ultrasound measurements were performed by a single examiner with extensive experience (> 10 years) and obtained moderate or high reliability. It is often assumed that measurements performed by less experienced examiners may lack accuracy. However, Crasto et al. [48] found that when measuring the CSA of the median nerve in the STC, even inexperienced examiners, with minimal training, were able to obtain measurements with a high level of agreement compared to experienced examiners.

Finally, this study has not evaluated Doppler sonography or elastography of the median nerve in the carpal tunnel. In our experience, it is difficult to generate a reproducible Doppler image for several reasons: choice of Doppler parameters (mainly the size of the Doppler window and the correct selection of the gain) and application of the transducer on the patient's wrist (even slight pressure through the transducer may cause compression of the small vascular structures and leave them occult). As for elastography, there is a great variability in the methodology and data obtained in previous studies [49, 50] therefore, its use was ruled out.

5. Conclusion

The median nerve CSA has a homogeneous value at the CTI and CTO in the Spanish population with CTS. These values show differences in terms of patient age and sex. The most frequent qualitative findings of the ultrasound exam include alterations in the nerve pattern and the presence of hyperechoic reinforcement.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.