Cubital tunnel syndrome (CuTS) ranks second after carpal tunnel syndrome, as a common cause of entrapment neuropathy in the upper extremity with an incidence of 24.7 per 100,000 people [1,2]. Ulnar nerve compression of the cubital tunnel during repeated elbow flexion can cause frequent friction and increased pressure on the ulnar nerve, which may lead to ulnar neuropathy presenting as paresthesia. This may involve the hypothenar region, pain at the elbow, sensory symptoms with prolonged flexion of the elbow, and in severe cases motor deficit of the ulnar nerve innervated hand muscles [3].

The ulnar neuropathy at the elbow may be treated by surgical intervention or conservative therapy including corticosteroid injection [2]. Despite the high incidence and disease severity associated with CuTS, there are few reports regarding steroid injection therapy specifically as a non-surgical treatment, in contrast to a number of studies related to carpal tunnel syndrome [4,5]. However, a few studies have examined ultrasound (US)-guided injection as a treatment option for CuTS, but failed to show consensus in methodological guidance [6,7]. Recently, a cadaveric study demonstrated real-time visualized US-guided injection by in-plane approach for cubital tunnel that can accurately provide the injection. This approach can also identify dynamic morphologic changes and size differences in the cubital tunnel after injection [8]. The cadaveric study findings suggested that this approach can be potentially translated to clinical applications and improve the outcome in live patients with CuTS.

Therefore, in this study, we applied this new in-plane technique of the real-time visualized US guidance of steroid injection therapy for patients with CuTS. This study aimed to verify the clinically relevant therapeutic effects on symptomatic, morphologic, and electrophysiological responses of the ulnar nerve.

After injecting 10 patients who were diagnosed with CuTS, the patients were evaluated at the time of pre-injection, and at 1st week and 4th week after injection. The mean age of patients was 63.1 years and the mean duration of symptoms was 9.5 months at the baseline of pre-injection. A positive Tinel sign was found in 9 patients, however, positive Wartenberg and Froment signs were found in only 1 patient (Table 1). There were no procedural side effects immediately after the injection and during the follow-up period.

Among the symptom scores, the visual analog scale was significantly decreased at 1st week and 4th week after injection, as compared with the time of pre-injection, with mean 5.4 points at the time of pre-injection, 4.05 points at 1st week and 3.55 at 4th week, respectively (p<0.05). McGowan score and SQUNE tended to be decreased after injection, although it did not reach statistical significance (Fig. 2).

The cross-sectional area of the ulnar nerve was measured by US at 2 cm proximal to the epicondyle, the level of the epicondyle, and 2 cm distal to the epicondyle. The cross-sectional area measured at 2 cm proximal to the epicondyle showed significant decreases at the 1st week and 4th week, as compared to the time of pre-injection, with mean 11.50, 8.80, and 8.10 mm², respectively (p<0.05). The mean cross-sectional areas at the level of the epicondyle were 7.0, 6.5, and 6.3 mm² at the time of pre-injection, 1st week, and 4th week, respectively. There was a significant size difference in the cross-sectional areas at the level of the epicondyle between the time of pre-injection and 4th week (p<0.05), while significant morphologic change was not observed between the 1st and 4th week. The cross-sectional area measured at 2 cm distal to the epicondyle did not show significant morphologic changes according to the different evaluation times.

On electrophysiologic analysis of the ulnar nerve, we observed that the electrophysiological severity scale was significantly decreased at the 4th week, as compared to pre-injection and 1st week (p=0.011 and p=0.026, respectively). Among the quantitative parameters of electrophysiological severity scale, motor conduction velocity in the forearm segment from the elbow to the wrist did not show significant changes throughout the study duration. However, motor conduction velocity across the elbow was significantly increased and the difference of motor conduction velocity between the segment of forearm and across the elbow were significantly decreased correspondingly at 4th week, as compared to pre-injection and 1st week, respectively (p=0.005, p=0.025 and p=0.028, p=0.017). The amplitude of CMAP at each point of above and below elbow, showed no significant electrophysiological changes. However, drop rate of the amplitude of CMAP was consistently decreased not only at the 1st but also at the 4th week, (p=0.015 and p=0.013, respectively) (Table 2).

The purpose of this study was to determine the clinical applicability of the US-guided injection by the new in-plane approach for the potential treatment of CuTS. The results presented here showed clinically relevant symptomatic, anatomic, and electrophysiological improvements in CuTS patients using US-guided injection by in-plane approach.

There have been few reports on the treatment of steroid injection for CuTS until the mid-2000s [14,15]. Because US was not actively used, the early studies performed a blind injection and failed to demonstrate a significant injection effect, as compared to placebo effect or conventional splinting. In response, this generated a controversy related to steroid injection. Recently however, Rampen et al. [6] briefly reported that US-guided injection using an out-of-plane technique improved symptoms in patients of CuTS, however, statistical verification was not conducted. On the other hand, Alblas et al. [7] described that there was no significant improvement of morphologic changes through their study on the feasibility of US-guided injection in dispersing steroid near the ulnar nerve. In these studies, the treatment outcomes were different and the injection techniques differed, thus raising concerns about the US-guided injection procedure in CuTS. Kim et al. [8] used cadaveric specimens to introduce a new in-plane approach and demonstrated that it was a feasible and more accurate cubital tunnel injection technique. They demonstrated the importance of the hydrodissection effect to separate the ulnar nerve from adjacent connective tissues, and suggested that it may disrupt adhesion and alleviate inflammation. Using the real-time US in-plane approach, needle advancement can be visualized throughout the process and the nerve can be accurately targeted through the shortest path. This maximizes the injection accuracy and limits incidental complications [16].

In the present study, we found that symptom scores and cross-sectional areas measured at 2 cm proximal to the epicondyle tended to decrease with time throughout the study. Interestingly, the recovery time related to morphological improvements did not correspond to the time of electrophysiological improvement. The symptomatic recoveries and morphologic changes appeared before the electrophysiological improvement. The morphological changes began to significantly decrease at the 1st week after injection. On the other hand, electrophysiological improvement was not identified until the 4th week after injection, because the difference of motor conduction velocity between the segment of forearm and across the elbow was decreased at the 4th week, as compared to pre-injection and 1st week. Consistent with our finding, the earlier studies also described the rapid improvement in inflammation and edema in entrapment neuropathy, when steroid injection was applied to the swollen ulnar nerve at the proximal portion of cubital tunnel [5,13]. It is estimated that neural regeneration occurs subsequent to the early anti-inflammatory effect of steroid. In fact, an in vivo study reported that regenerative myelination is observed at 3 weeks after nerve damage on glucocorticoid treatment [17], which may explain why electrophysiological improvement was found at the 4th week after the injection in our study.

The amplitude of CMAP at each point of above and below the elbow, showed no significant electrophysiological changes. However, drop rates of the amplitude of CMAP measured at both 1st week and 4th week were decreased. The improvement of motor conduction velocity and the drop rate of CMAP across the elbow appeared to contribute to the enhancement of overall electrophysiological severity. Because the drop rate of the CMAP suggests the presence of a conduction block, the steroid injection is thought to resolve focal demyelination [17].

This pilot study was limited by the small number of participants, lack of a control group, and the short-term study period. Nevertheless, some important conclusions were derived from these results. Accurate US-guided injection using the new in-plane approach provided clinical improvements and therapeutic effects, based on improvements in the symptom scale scores, anatomic morphologic changes, and electrophysiological recoveries in patients with CuTS. Our results indicated that the new approach is a safe and accurate non-surgical treatment for patients with CuTS. In future, a large scale and long-term follow-up study, compared with other treatment modalities, will be necessary to determine the effectiveness and efficiency of the new approach to injection.

In conclusion, our study demonstrated that the new maneuver of US-guided injection using in-plane technique might be a safe and effective treatment tool for CuTS in terms of symptomatic, morphologic, and electrophysiological outcomes. We found significant symptomatic, morphological, and electrophysiological improvements in patients with CuTS following the steroid injection. However, the recovery time, as measured by morphological improvements did not correspond with the time of electrophysiological improvement, suggesting that the symptomatic and morphologic improvements may have occurred before electrophysiological improvements.