Introduction

The dorsal ulnar cutaneous nerve (DUCN), a terminal branch of the ulnar nerve, plays a critical role in sensory functions of the ulnar aspect of the hand1,2. This branch supplies sensation to the dorso-ulnar aspect of the hand, dorsum of the little finger, and dorso-ulnar aspect of the ring finger3,4. Neuropathy of the DUCN may result in numbness, dysesthesia, and pain in the innervated area of the DUCN5. The DUCN has a variable origin in the forearm and may arise from either the proximal, middle, or most commonly the distal third of the forearm1,6. Due to its superficial positioning, the DUCN is susceptible to neuropathy caused by lacerations and blunt trauma. Additionally, its anatomical variability can lead to iatrogenic injuries during surgical procedures in the ulnar area5,6,7.

The point yielding the maximum amplitude in NCS is crucial because it provides the most reliable measure of the nerve's functional capacity. Because signal amplitude reflects the total number of neurons that depolarize, NCS is vital for accurate diagnosis and assessment of neuropathy severity. The sensory nerve conduction study of the DUCN is important not only for diagnosing DUCN lesions but also for localizing ulnar neuropathy. The assessment of normal DUCN functioning is helpful to distinguish between cubital tunnel syndrome and Guyon canal syndrome8. Although sensory nerve conduction studies is important to identify DUCN lesions9, the technique for nerve conduction studies (NCS) of the DUCN lacks standardization, and they may not adequately account for the variability in the branching pattern of the DUCN10, presenting challenges in accurately diagnosing ulnar neuropathies11,12. Meanwhile, in previous research, ultrasonography has been reported as a valuable tool in assessing conditions related to nerves13, as it enables precise localization through confirmation of the nerve's anatomical structure14.

Given that DUCN neurographic findings exhibit high specificity but relatively low sensitivity, and abnormal sensory nerve action potential (SNAP) of DUCN is associated with axonal damage of the ulnar nerve15, we seek to improve diagnostic accuracy for conditions involving the DUCN. This study aims to bridge this gap by investigating the impact of the anatomical separation point of the DUCN on NCS results, using ultrasonography to enhance precision. Specifically, this study aims to suggest the stimulation points for maximum amplitude based on the anatomical separation point of the DUCN during NCS in healthy subjects. We hypothesized that SNAP amplitude varies depending on the location of the DUCN separation.

Methods

This study was approved by the Institutional Review Board of Yeouido St. Mary’s Hospital (SC24RISI0024) and was conducted in compliance with the principles of the Declaration of Helsinki and its contemporary amendments and Good Clinical Practice. Written informed consent was obtained from all participating subjects. Asymptomatic healthy volunteers were recruited in this study. We collected demographic information including age, sex, height, weight, and body mass index. The exclusion criteria included a history of diabetes, any neuromuscular diseases involving upper extremities such as peripheral neuropathy or radiculopathy, or a history of cervical spine or upper extremity surgery. Total 26 healthy subjects were studied who showed normal ulnar sensory, motor, and DUCN NCS findings. Of the 26 participants enrolled, one was later excluded upon retrospective review for not meeting the inclusion/exclusion criteria, resulting in the analysis of data from 25 participants. The non-dominant limb was chosen to prevent potential confounding due to overuse injuries.

Nerve ultrasound

Nerve ultrasonography (HD15 Ultrasound System; Philips, Bothell, Washington) using a 7–12 MHz linear array transducer performed each participant’s unilateral arm, and the dorsal cutaneous ulnar nerve was evaluated by experts with more than 2 years of experience in musculoskeletal ultrasound. The transducer was maintained at a right angle to keep the anisotropy at a minimal level. By tracing the ulnar nerve proximally from the wrist at the scaphoid–pisiform level, the branching of the dorsal ulnar cutaneous nerve could be observed (Fig. 1, Supplementary Video S1). After identifying the branching point, markings were made on the skin along the course of the nerve, indicating proximal 2 cm, distal 2 cm, and distal 4 cm (Fig. 2). During the examination, the elbow was flexed to 90°–100° with the shoulder flexed to 60° while the subject was seated in a position with the fully supinated forearm supported by an arm-rest. We measured the distance between the separation point of the DUCN and the ulnar styloid process while tracking the ulnar nerve from wrist to forearm.

Fig. 1
figure 1

Ultrasound scan of dorsal ulnar cutaneous nerve. ⓐ: 2 cm proximal to separation point (P2). Ulnar nerve (arrow) runs beside ulnar artery (A), ⓑ: Separation point (S) of dorsal ulnar cutaneous nerve (arrow head), ⓒ: 2 cm distal to separation point (D2), ⓓ: 4 cm distal to separation point (D4), dorsal ulnar cutaneous nerve pierces the antebrachial fascia (dotted line) volar to ulnar bone (U).

Full size image
Fig. 2
figure 2

Stimulation point based on the separation point during the dorsal ulnar cutaneous nerve conduction study. The stimulation points were marked at 2 cm distances along the course of the dorsal ulnar cutaneous nerve. S: Separation point of dorsal ulnar cutaneous nerve, P2: 2 cm proximal to separation point, D2: 2 cm distal to separation point, D4: 4 cm distal to separation point.

Full size image

Nerve conduction study

The studies were performed by a qualified electromyographer. To study the dorsal cutaneous ulnar nerve, the active electrode was placed on proximal web space between fourth and fifth metacarpal bones. Reference electrode placed on base of the fifth digit. Since the styloid process was measured at an average distance of approximately 3.92 cm from the branching point of the DUCN, nerve conduction studies were performed at 2 cm intervals. This approach was deemed suitable to assess the correlation of NCS results at various locations relative to the separation of the DUCN. The DUCN was stimulated with the intensity of 25 mA at 4 different points which were identified with nerve ultrasound; 2 cm proximal to separation point (P2), DUCN separation point (S), 2 cm distal to separation point (D2), 4 cm distal to separation point (D4) according to DUCN course identified with ultrasound (Fig. 2). Peak latencies and amplitudes of each stimulation points were measured. Difference between these NCS data were statistically analyzed. The equipment settings for SNAP recording were: sensitivity, 20 mV/division; sweep speed, 1 ms/division; and bandwidth, 20–5000 Hz.

Statistical analysis

Demographic factors, distances between the separation points and anatomic landmarks, cross-sectional areas, and all nerve conduction study results are presented as mean ± SD. We performed Shapiro–Wilk normality test to check the normality of the variables. The Pearson correlation test, and Point-Biserial Correlation Coefficient were performed to investigate the correlation between the location of the separation point and demographic factors.

The Mann–Whitney U test, and Bonferroni correction were used to analyze peak amplitude, and peak latency according to stimulation site. P-values less than 0.05 were considered statistically significant.

Results

The demographics of all subjects are shown in Table 1. The age, height, weight, and body mass index of the 25 participants (9 men and 16 women) were 51 ± 16.06 years, 163.43 ± 8.68 cm, 60.82 ± 11.90 kg, and 22.64 ± 2.94 kg/m2, respectively. The normality test results showed the age, height, body mass index and distance between the separation point of the dorsal ulnar cutaneous nerve and the ulnar styloid process satisfied normality. The p-values are as follows: age is 0.12, height is 0.37, BMI is 0.19, and distance is 0.58. In examining DUCN via ultrasonography (Fig. 2) , we noted the average distance from the ulnar styloid process to the separation point of the DUCN to be 3.79 ± 1.19 cm (range 1.5–6.5 cm). The results of the sensory nerve conduction study are shown in Table 2. The mean peak amplitude values ranged from 24.7 ± 11.3 μV at the P2 to 33.0 ± 14.7 μV at the D2. In terms of latency, the values ranged from 1.7 ± 0.3 ms at the D4 to 2.5 ± 0.4 ms at the P2. The mean SNAP amplitudes at these points varied, with the highest amplitude observed 2 cm distal to the separation point (Fig. 3) . However, as shown in Table 3, statistical analysis revealed significant differences in SNAP amplitudes at specific point, with a noteworthy difference observed when comparing the point 2 cm proximal to the separation point and 2 cm distal to it (p-value: 0.02), suggesting anatomical variations at these points influence SNAP outcomes.

Table 1 Demographic characteristics of subjects.
Full size table
Table 2 Mean SNAP amplitude of dorsal ulnar cutaneous nerve.
Full size table
Fig. 3
figure 3

Boxplot of amplitude value by stimulation site. Box plot illustrating the distribution of amplitudes of DUCN across four different groups: P2, S, D2, and D4. The bold line within each box represents the median. The boxes represent the interquartile range (IQR). Whiskers indicate the range of the data, excluding outliers. Outliers are depicted as points beyond the whiskers.

Full size image
Table 3 The Mann–Whitney U test results of peak amplitude and peak latency at each points.
Full size table

The Pearson correlation coefficients for the distance between the separation point of the DUCN and the ulnar styloid process with height and body mass index were 0.09 and 0.24, respectively, with p-values of 0.69 and 0.29. In addition, the point-biserial correlation coefficient for the sex and between the separation point of the DUCN and the ulnar styloid process was − 0.12, with p-value of 0.59. Therefore, the distance between the separation point of the DUCN and the ulnar styloid process was not significantly correlated to sex, height, and body mass index.

When testing the normality of peak amplitude values, the p-values for P2 to D4 were 0.19, 0.02, 0.0008, and 0.3, respectively, indicating that the amplitude values for P2, and D4 satisfied normality. The Mann–Whitney U test showed a potential difference (p-value = 0.02) between P2 and D2 in peak amplitude. However, when Bonferroni correction was applied to the results, none of the comparisons yielded a p-value less than 0.05 (Table 3).

When testing the normality of peak latency values, the p-values for P2 to D4 were 0.18, 0.14, 0.25, and 0.03, respectively, indicating that the latency value for P2, S, and D2 satisfied normality. Mann–Whitney U test showed statistically differences in all peak latency values across the different stimulation sites, except for D2 and D4 (p-value = 0.21). Even after Bonferroni correction for the results, the p-value between D2 and D4 was 1.0, showing no significant difference, and all others showed statistically significant differences (Table 3).

Discussion

Our study shows that separation point may have a significant effect on sensory nerve conduction of the DUCN. The distance between the separation points and the ulnar styloid process has been reported to vary between 4.8 and 10.0 cm1,2,16, which is longer than the average of 3.79 ± 1.19 cm in the present study. This discrepancy may be attributed to the fact that the current study enrolled only Korean subjects and had a gender distribution skewed towards females. Additionally, as can be observed from the results of previous studies, it is apparent that there is considerable variation contingent upon the population group studied.

Based on the previous study17,18, the conventional conduction study for the DUCN is performed by placing the active electrode between the fourth and fifth metacarpals and positioning the reference electrode at the base of the fifth digit, with stimulation applied 8 cm proximal to the active electrode. However, conventional nerve conduction studies may not fully reflect the anatomical variations of the DUCN, and in previous study, they exhibited decreased latency compared to sono-guided conduction studies11.

DUCN can be well visualized from its point of branching from the ulnar nerve at the distal third of the forearm to the point where it pierces the antebrachial fascia to become subcutaneous in sonography1,14. We identified the separation point of the DUCN using ultrasonography and stimulated the DUCN at four different stimulation sites based on the separation point and the statistical analysis revealed a potential difference when comparing P2 and D2 from the separation point (p-value: 0.02) before Bonferroni correction. However, there was no significant difference between P2 and D4. This result may be caused by the mixed effect of anatomic variations such as the depth of the nerve from skin, fascia penetration effect, physiologic temporal dispersion or stimulus artifact. In addition, when performing DUCN conduction, the ulnar nerve is often unintentionally stimulated and the posterior part of the SNAP wave of the DUCN is obscured by the CMAP of thenar muscles such as the abductor digiti minimi. Stimulating only DUCN with appropriate intensity helps obtain a clear SNAP waveform.

The latency values observed are within the expected range based on the known characteristics of nerve conduction, with shorter latencies at distal points and longer latencies at proximal points. The lack of a statistically significant difference in latency between D2 and D4 can be attributed to several factors. Prior studies have shown that peripheral nerve conduction velocities tend to remain consistent over short distances, leading to minimal latency differences19. Additionally, the use of ultrasound to precisely locate and stimulate the DUCN allowed for a more accurate nerve conduction study. As a result, significant differences in latency were observed between P2, S, and D2, as expected. However, there was no statistically significant difference in latency between D2 and D4. Several factors may contribute to this. First, unlike the P2, S, and D2 segments, the DUCN between the D2 and D4 segments follows a curved course as it travels to the dorsum of the hand and becomes subcutaneous on the medial aspect of the wrist after piercing through the deep fascia. This fascia penetration effect has been reported in previous studies20. Since the point of electrical stimulation on the skin and the actual point of the nerve being stimulated can be influenced by various anatomical factors, these aspects should be considered when conducting a nerve conduction study. Secondly, D4 is quite close to the recording electrode, being only 4–6 cm away, which makes it more susceptible to stimulation artifacts. The amplitude of DUCN stimulation at D2 showed a higher value compared to P2, although this difference was not statistically significant after applying the Bonferroni correction (adjusted p-value = 0.13). This suggests that the amplitude at D2 may have a potential difference compared to other stimulation points.

Our study had several limitations. First, this study enrolled only healthy subjects. The parameters in this study may not be directly applied in the patient with dorsal ulnar cutaneous neuropathy. Second, the question of whether nerve ultrasound should be performed together during nerve conduction study is still controversial and requires consideration of various factors. It will cause additional costs and facilities. However, if it is performed by experts, it has the advantage of reducing painful electrical stimulation to the patient without increasing the test time20. Additionally, it can increase the accuracy and reliability of the diagnosis, as supported by studies on other nerves21. Third, ideally, it would be necessary to incorporate measurements of DUCN SNAP both with and without the aid of ultrasonography. Fourth, the small sample size and the sex ratio being biased toward women are other limitations of this study. Because of the small sample size, the Mann–Whitney U test indicated a statistically significant difference between the peak amplitudes of P2 and D2. However, after applying the Bonferroni correction, no significant difference was found. Large-scale studies with similar sex ratios are required for reaching a generalized conclusion. Fifth, measurements were only taken at four specific points from the DUCN separation point, which is another limitation of this study. Future studies could conduct more detailed examinations by using a more segmented approach. Sixth, this study did not compare side-to-side discrepancies. According to previously research, the bilateral difference in an individual's DUCN SNAP can reach up to 21%22. Therefore, further study is thus necessary in patients including on side-to-side discrepancies. Last, as the whole study relies on accurate identification of DUCN location using ultrasonography, an evaluation of inter- and intra-operator reliability should have been conducted to assess the repeatability in identifying the location. Future studies should address this by including assessments of both inter- and intra-operator reliability to ensure the consistency and reliability of DUCN localization using ultrasonography.

In conclusion, the result of this study suggests to stimulate between separation point and 2 cm distal to separation point according to the DUCN course identified with nerve ultrasound, to obtain the maximal amplitude of DUCN.