Ulnar Nerve Enlargement at the Medial Epicondyle Negatively Correlates With Nerve Conduction Velocity in Cubital Tunnel Syndrome

Abstract

Background: In cubital tunnel syndrome (CuTS), chronic compression often occurs at the origin of the flexor carpi ulnaris at the medial epicondyle. Motor nerve conduction velocity (NCV) across the elbow is assessed preoperatively to corroborate the clinical impression of CuTS. The purpose of this study was to correlate preoperative NCV to the direct measurements of ulnar nerve size about the elbow at the time of surgery in patients with clinical and/or electrodiagnostic evidence of CuTS. Methods: Data from 51 consecutive patients who underwent cubital tunnel release over a 2-year period were reviewed. Intraoperative measurements of the decompressed nerve were taken at 3 locations: at 4 cm proximal to the medial epicondyle, at the medial epicondyle, and at the distal aspect of Osborne fascia at the flexor aponeurotic origin. Correlation analysis was performed comparing nerve size measurements to slowing of ulnar motor nerve conduction velocities (NCV) below the normal threshold of 49 m/s across the elbow. Results: Enlargement of the ulnar nerve at the medial epicondyle and nerve compression at the flexor aponeurotic origin was a consistent finding. The mean calculated cross-sectional area of the ulnar nerve was 0.21 cm² above the medial epicondyle, 0.30 cm² at the medial epicondyle, and 0.20 cm² at the flexor aponeurotic origin (P < .001). There was an inverse correlation between change in nerve diameter and NCV slowing (r = −0.529, P < .001). Conclusions: For patients with significantly reduced preoperative NCV and clinical findings of advanced ulnar neuropathy, surgeons can expect nerve enlargement, all of which may affect their surgical decision-making.

Introduction

Entrapment of the ulnar nerve at the elbow is the second most common compressive neuropathy in the upper extremity after carpal tunnel syndrome.¹³ Multiple sites of ulnar nerve compression about the elbow have been described, with the cubital tunnel being the most common site.¹⁴ The cubital tunnel is located posterior to the medial epicondyle with a roof formed by Osborne ligament/fascia. Proximal compression above the medial epicondyle occurs at the Arcade of Struthers between the medial triceps and the intermuscular septum. Chronic compression is often found distally at the flexor pronator aponeurotic origin.⁵

Patients with chronic ulnar nerve compression often present with sensory deficits to the ring and little fingers, some motor weakness in the hand, and finger clawing with advanced disease.¹⁴ The diagnosis of cubital tunnel syndrome (CuTS) is multifaceted, involving thorough history-taking, clinical examination, and electrodiagnostic studies. Electrodiagnostic testing is usually performed preoperatively to corroborate the clinical impression of CuTS. The most useful parameter is motor nerve conduction velocity (NCV) across the elbow, with an absolute conduction velocity of less than 49 m/s consistent with the diagnosis of CuTS.⁵ High-resolution ultrasound of the ulnar nerve has also demonstrated a correlation between motor NCV and nerve enlargement at the elbow.¹⁷ Nerve enlargement is much more common than focal narrowing in entrapped ulnar nerves, presumably because nerves slide distally and proximally creating a zone of compression and irritation.¹⁷ It has been postulated that nerve enlargement in CuTS encompasses a cascade of inflammatory events including endoneurial edema, demyelination, remyelination, and perineurial thickening secondary to chronic compression.¹²

The purpose of this study was to evaluate direct measurements of the size of the ulnar nerve about the elbow at the time of surgery in patients with clinical and/or electrodiagnostic evidence of CuTS and to correlate the nerve size with preoperative NCV. We hypothesized that preoperative slowing of ulnar motor NCVs across the elbow correlates with ulnar nerve enlargement at the cubital tunnel.

Materials and Methods

We prospectively collected data from consecutive patients who underwent surgical decompression for CuTS between 2014 and 2015. The study was approved by the institutional review board prior to data collection. All surgeries were performed by the senior author (MJW) on patients who demonstrated clinical symptoms and electrodiagnostic findings consistent with CuTS. The clinical criteria could include numbness of the ring and little fingers, pain in the elbow ulnar forearm and hand, positive Tinel at the cubital tunnel, and weakness of muscles innervated by the ulnar nerve. Patients with history of neuropathy, elbow trauma, brachial plexus injury, and osteoarthritis of the elbow were excluded. The decision to perform anterior transposition of the ulnar nerve was dependent on intraoperative assessment of the nerve based on subluxation/dislocation, the size of the nerve, and the site of compression. Nerve subluxation was defined as the ability of the nerve to approximate the edge of the epicondyle during elbow flexion, and dislocation was defined as complete anterior displacement out of the ulnar groove.⁷

Electrodiagnostic Testing

All patients underwent preoperative NCV testing. The temperature of the affected arm was maintained at a minimum of 32°C. The testing was performed in the standard fashion with cathode placement at about 5 cm above the elbow, 4 cm below the medial epicondyle, and at the proximal wrist crease. Diagnostic criteria for ulnar neuropathy include absolute NCV below the normal threshold of 49 m/s across the elbow or a decrease ⩾10 m/s in NCV across a measured interval.¹¹ The difference in NCV (ΔNCV) from the normal value of 49 m/s across the elbow was calculated, where a negative value indicates NCV below this threshold.

Intraoperative Measurement

After surgical release of Osborne ligament/fascia, the diameter of the ulnar nerve was measured by the senior author with the elbow at 70° of flexion, by applying a graded caliper (Figure 1) to the decompressed nerve at 3 locations: (1) at 4 cm proximal to the medial epicondyle; (2) at the medial epicondyle; and (3) at the distal aspect of Osborne fascia at the flexor aponeurotic origin. Cross-sectional area (CSA) of the nerve was calculated based on the diameter. The change in nerve CSA (ΔCSA) from location 1 to 2 was calculated.

Figure 1.

A graded caliper was used to measure the ulnar nerve diameter in a standardized fashion, with minimal tension on the nerve and minimal force placed on the caliper.

Statistical Analysis

Descriptive statistics were presented as mean ± SD. Student t test was performed to compare the measured parameters between patients who underwent in situ decompression or anterior transposition. One-way analysis of variance (ANOVA) was performed to compare the ulnar nerve CSA at the 3 measured locations. Post hoc analysis with Bonferroni correction was used to further assess differences between the 3 locations. Pearson correlation coefficient was analyzed to correlate nerve size parameters with NCV, with statistical significance set at 0.05.

Results

Fifty-one patients (31 females, 20 males) met inclusion criteria, with mean age of 54.6 ± 14.3 years (range: 17 to 79 years). Swelling of the ulnar nerve at the medial epicondyle with relative narrowing of the nerve at the distal aspect of Osborne fascia/flexor origin was a consistent intraoperative finding in all patients. The mean CSA of the ulnar nerve were 0.21 ± 0.08 cm² above the medial epicondyle, 0.30 ± 0.14 cm² at the medial epicondyle, and 0.20 ± 0.07 cm² at the flexor aponeurotic origin (P < .001). Post hoc analysis showed that CSA at the medial epicondyle was significantly greater compared to above the medial epicondyle (P = .001) and compared to the flexor aponeurotic origin (P < .001).

Of the 51 patients, 39 underwent in situ decompression of the ulnar nerve, and 12 patients underwent anterior transposition. Patients who underwent anterior transposition demonstrated greater ΔCSA, indicating more substantial nerve swelling, compared to patients who underwent in situ decompression (0.17 cm² vs 0.06 cm², P = .005).

The mean NCV and ΔNCV were 40.5 ± 10.5 m/s and −8.7 ± 10.5 m/s, respectively. The intraoperative nerve measurements were correlated with electrodiagnostic measurements, demonstrating a statistically significant inverse association between change in nerve diameter and ΔNCV (r = −0.529, P < .001) (Figure 2).

Figure 2.

The scatterplot demonstrates the negative association between change in nerve size from the proximal cubital tunnel to the medial epicondyle and change in nerve conduction velocity from a threshold of 49 m/s.

Note. NCV = nerve conduction velocity.

Discussion

In the present study, we quantified ulnar nerve size intraoperatively at 3 potential sites of nerve compression. We demonstrated that the nerve swelling was greatest at the medial epicondyle, which inversely correlated with preoperative NCV, corresponding to a moderately strong but statistically significant association based on the Pearson correlation coefficient.

The treatment of CuTS remains controversial with no consensus on the optimal surgical intervention when conservative measures fail. Surgical treatment options include in situ decompression, anterior transposition of the ulnar nerve (via subcutaneous, intramuscular, or submuscular techniques), and medial epicondylectomy, which is less commonly performed.^6,20 Ultrasonography of the ulnar nerve has greatly added to our understanding and diagnosis of CuTS and ulnar nerve pathophysiology. Various studies have demonstrated that the CSA of the ulnar nerve on ultrasound in patients with CuTS was significantly greater compared to normal volunteers.^4,17,18 Comparing the CSA at the location of maximal swelling to unaffected sites provided improved diagnostic accuracy. Our study sought to delineate the relationship between the observed nerve swelling to changes in preoperative NCV. For the neurologist or physiatrist performing the electrodiagnostic testing, our data may provide useful clinical correlation to localize areas of maximal nerve compression. Preoperative NCV values may also allow the surgeon to anticipate the location and severity of nerve swelling and compression intraoperatively, which may factor into the surgeon’s decision for ulnar nerve transposition versus in situ decompression.

The increased nerve diameter occurred proximal to the flexor aponeurotic origin, the site where the ulnar nerve was most consistently compressed. The swollen surgical appearance of the ulnar nerve in CuTS has been previously described.² Our findings are consistent with the ultrasonographic location of nerve swelling demonstrated by Yoon et al.¹⁸ There are multiple potential sites of compression of the ulnar nerve along its course about the elbow. The cubital tunnel itself starts at the proximal edge of the medial epicondyle and extends to the distal edge of the medial epicondyle at the flexor aponeurotic origin.⁵ The pathophysiology behind ulnar nerve swelling in CuTS involves endoneurial edema, demyelination, and compensatory remyelination from repeated compression.⁵

In a previous study, nerve CSA on ultrasound in symptomatic patients after prior ulnar nerve transposition was significantly greater compared to symptomatic patients who had not yet undergone surgical intervention; however, the authors were not able to delineate the reason for this disease progression.¹⁶ Similarly, a drawback of our study was the inability to explain the variability in nerve size in the cohort, which was likely inherent to the patients, as all caliper measurements were performed in a standardized fashion by the same surgeon. Nevertheless, our findings suggest that for patients with significantly reduced preoperative NCV, surgeons can expect specific locations of nerve enlargement and maximal compression at the flexor aponeurotic origin. An enlarged ulnar nerve at the cubital tunnel is potentially more prone to subluxation, especially if there is a shallow groove in which it rests. This may factor into the decision of the surgeon to perform an anterior transposition.

Advocates of performing simple decompression argue that it allows for symptomatic relief while preserving the blood supply, maintaining the native anatomy, and allowing for early physical therapy, an important aspect of any elbow surgery. A recent cadaveric study demonstrated that simple decompression of the ulnar nerve from the flexor aponeurosis to the intermuscular septum does not lead to a significant increase in nerve subluxation or translation.³ The laboratory conditions for this cadaveric study, however, may not reflect clinical presentation of CuTS, specifically with respect to nerve swelling and baseline subluxation.

Proponents of nerve transposition argue that although the procedure requires a more extensive dissection and a longer operative time,¹³ it is the only procedure that addresses symptoms caused by dynamic compression of the nerve during elbow flexion.²⁰ Some surgeons suggest waiting to make a final decision until the ulnar nerve can be visualized intraoperatively to examine the environment in which it is found.⁹ Nathan et al¹⁰ suggested that anterior transposition of the ulnar nerve should be preferred in cases where there is narrowing of the cubital tunnel by the presence of bony spurs, synovial swelling, or cases of recurrent nerve subluxation. The results of the present study provide additional preoperative information to the surgeon to anticipate the need for anterior transposition.

Often, the decision between simple decompression and anterior transposition remains solely based on surgeon preference and experience. A systematic review found simple ulnar nerve decompression demonstrated better clinical outcomes compared to anterior transposition, although the authors pointed out the low level of evidence and inherent selection bias of the included studies, in which patients with less severe disease often underwent simple decompression while patients with more severe disease underwent anterior transposition.¹ Subsequent meta-analyses of randomized controlled trials and observational studies comparing simple decompression to anterior ulnar nerve transposition did not find a significant difference between the 2 procedures with respect to outcome scores and postoperative NCV.^8,20 More recent studies looking at complications rates between the 2 procedures found low complication rates for both procedures but that the rate of secondary surgeries (11.1% vs 2.5%)¹⁹ and postoperative narcotic consumption¹⁵ was greater in the transposition group compared to the decompression group.

Given this evolving body of evidence, the rate of transposition has seen a downward trend from 49% of cubital tunnel procedures in the 1990s to 38% in 2006.¹³ Stability of the nerve at the groove may still prompt certain surgeons to opt for transposition.¹⁴ In our series, the patients who underwent transposition had significantly greater nerve CSA compared to those who underwent in situ decompression. This is in line with the preference of the senior author (MJW) to perform nerve transposition based on the degree of nerve swelling, with the underlying assumption that nerve enlargement potentially gives rise to instability at the nerve groove.

The present study was limited by the small sample size of patients and the lack of outcome data to assess the effectiveness of treatment based on the magnitude of nerve swelling. This mitigated the prospective nature of the study. Another limitation was the precision of measurements of nerve enlargement using a caliper; however, a standardized approach by a single evaluator was used, with minimal tension on the nerve and minimal force on the caliper to maintain consistency of the measurements.

In conclusion, this is the first study to our knowledge to show relationship between intraoperative measurement of nerve enlargement and nerve conduction slowing in CuTS. For patients with significantly reduced preoperative NCV and clinical findings of advanced ulnar neuropathy, surgeons can expect nerve enlargement, all of which may affect their surgical decision-making. Further investigation is needed to determine how parameters derived by ultrasound, nerve conduction studies, or direct intraoperative measurements can be best used to predict the optimal surgical approach to a patient with CuTS.