Ulnar nerve entrapment (UNE) at the elbow is one of the most common entrapment neuropathies. The diagnosis basically relies on the combination of both clinical and electrodiagnostic techniques [1]; however, in many cases, they might provide insufficient or inaccurate data that may be misleading and cause delay in patient’s diagnosis.
Electrophysiological false negative results may result from the fact that nerve conduction studies may fail to localize and diagnose ulnar nerve lesions with selective fascicular or pure sensory fiber involvement. Besides, ulnar nerve displacement at the elbow can also cause errors in distance measurements with overestimated nerve conduction velocities, and in those instances, imaging is often used to confirm the diagnosis [2].
Being a non-costly, fast, available, and non-invasive technique for the assessment of musculoskeletal disorders including evaluation of the elbow joint abnormalities related to or causing UNE syndrome, high-resolution ultrasound (HRUS) has gained increasing popularity as a diagnostic tool for musculoskeletal assessment [3]. According to previous studies, the cutoff point of the cross-sectional area of the ulnar nerve which is 10 mm2 or higher proves its sensitivity and specificity of more than 88% for cubital tunnel syndrome diagnosis [4]. The cutoff value for the cross-sectional area of the asymptomatic ulnar nerve is suggested by Thoirs and his colleagues [5] to be 9 mm2, derived as the upper limit of the 95% confidence interval. Also, Patel and his colleagues [6] recommend the use of the cross-sectional area (CSA) rather than the flattening ratio as it does not change with change of angle at the elbow.
Factors such as technical developments and improvements, increased experience, and detailed knowledge of elbow anatomy and its pathologic conditions together with awareness of different sonographic pitfalls, limitations, and artifacts have significantly improved sonographic elbow assessment results.
Study design and population
Our case-control study was performed between January and July 2015; patients were referred to the Radiology Department, or Neurosonology Unit in the Neurology Department, Faculty of Medicine, Cairo University, for HRUS of the elbow from neurology specialists.
The study was approved by the Neurology Department Faculty of Medicine, Cairo University Ethical Committee, and an informed consent to participate in the study was obtained from all subjects.
A total of 62 elbows were examined, and 39 were assigned as the healthy control group without any clinical symptoms or signs, but surprisingly out of the 39 control elbows, 15 limbs which had no previous signs or symptoms were found to have sonographic features suggestive of UNE and therefore classified as a separate subgroup named asymptomatic UNE group while 24 limbs had no symptoms or sonographic features of UNE at the elbow and were classified as a non-entrapment control group; the remaining 23 elbows were limbs with both symptoms and sonographic features of UNE and were assigned to the symptomatic UNE group.
Inclusion and exclusion criteria
Limbs were enrolled in the symptomatic UNE group if the participant had typical signs and symptoms of UNE syndrome, with no prior history predisposing to UNE at a site other than the elbow (e.g., shoulder or wrist pain), and had an electrophysiologically confirmed diagnosis of UNE for that limb, and were excluded in case of acute trauma to the elbow or if any surgical procedures for UNE was previously performed.
Methods
Static and dynamic grayscale ultrasound and Doppler examination of the ulnar nerve using the 6–7.5-MHz linear array transducer for the musculoskeletal system, Logiq P5, GE Medical Systems, USA ultrasound device, were performed for all participants.
Examination technique
For patient position, patients were instructed to either lie supine or sit with their arms abducted and externally rotated. Images were obtained while the elbow is fully extended then in different degrees of flexion.
Greyscale sonography was done using static and dynamic techniques for examination of the ulnar nerve at the elbow.
The cross-sectional area (CSA) of the ulnar nerve was measured circumferentially by direct manual tracing just inside the hyperechoic rim of the nerve with the transducer perpendicular to the nerve to ensure that the smallest and most accurate CSA was being obtained. Ultrasound was used to scan the entire ulnar nerve segment across the elbow and other common sites of nerve entrapment along the length of the nerve (Figs. 1, 2, and 3): (1) 2 cm proximal to the tip of the medial epicondyle, (2) tip of the medial epicondyle, (3) just at the entrance of the cubital tunnel (between the two heads of flexor carpi ulnaris (FCU) muscle, and (4) 2 cm distal to the tip of the medial epicondyle (in the FCU muscle).
The CSAs of ulnar nerves with two or more fascicles were calculated as the sum of the individual measurements of the CSA of each fascicle [4].
The flattening ratio of the ulnar nerve at the elbow was obtained by measuring the widest transverse and anteroposterior cross-sectional nerve diameters (i.e., long and short axes) on axial images obtained by a direct tracing method during elbow extension and flexion. To obtain the proper axial images, the probe is placed in a plane traversing the olecranon process (OLE) and the medial epicondyle (ME) in full extension then during elbow flexion. (Images clearly showed the osseous floor of the cubital tunnel, the medial epicondyle, the olecranon process, and the ulnar nerve in a cross section).
The nerve was also assessed in terms of its mobility in association with elbow flexion. The patient was instructed to abduct and internally rotate his arm while the elbow is extended; at first, the probe was placed transversely between two bony landmarks: the medial epicondyle of humerus and the olecranon process of the ulna on the posteromedial aspect of elbow, then the patient was asked to flex his elbow gradually till 90° and 120° and the movement of ulnar nerve within the cubital groove was assessed in relation to the medial epicondyle. The nerve was categorized as stable if it remained posterior to the medial epicondyle, subluxing if it moved to the level of the medial epicondyle, or dislocating if it moved anteriorly to the medial epicondyle.
The purpose of this study was to evaluate the role of dynamic high-resolution ultrasonography in the detection of different abnormalities including both the causes and sequelae of ulnar nerve entrapment at the elbow.
Statistical methods
The data were coded and entered using the statistical package SPSS version 15. The data were summarized using descriptive statistics, and statistical differences between groups were tested using the chi-square test for qualitative variables while the nonparametric Mann-Whitney test and Kruskal-Wallis test were used for quantitative variables which are not normally distributed.
For the ethical statement, the ethical committee of the Faculty of Medicine, Cairo University, has allowed this study.