Ulnar Neuropathy Caused by a Giant Epidermal Cyst at the Elbow: Case Report

Abstract

Here, we report an unusual case of ulnar neuropathy at the elbow caused by a giant epidermal cyst. A 76-year-old man was assessed on an outpatient basis for ulnar numbness of the left hand that had persisted for 6 months. A soft, elastic subcutaneous mass 6 cm in size was noted on his left elbow. He felt numbness on the ulnar aspect of the left fourth and fifth fingers, corresponding to the area innervated by the ulnar nerve, which worsened upon elbow flexion. An electrophysiological study revealed ulnar neuropathy at the elbow. To remove the subcutaneous mass at the left elbow and open up the ulnar tunnel, surgery was performed. There were no signs of nerve impingement or a neuroma on the ulnar nerve. The histological diagnosis was an epidermal cyst. On the day after surgery, numbness on the ulnar aspect of the left hand upon elbow flexion was markedly abated.

Introduction

Ulnar neuropathy at the elbow is the second most frequent type of entrapment peripheral neuropathy after carpal tunnel syndrome.¹⁾ Most cases are caused by osteoarthritis, an elbow deformity such as cubitus valgus or cubitus varus, and overuse of the elbow.^1-4) Nonetheless, ulnar neuropathy cases due to neoplastic lesions such as ganglia, neurofibromas, and schwannomas have also been reported occasionally.^5-10) We encountered an extremely rare case of ulnar neuropathy at the elbow caused by a giant epidermal cyst at the elbow, as reported here.

Case Report

A 76-year-old man was assessed on an outpatient basis for numbness on the ulnar aspect of the left hand that had persisted for approximately 6 months. He had a cerebral infarction 4 years before but had no particular sequelae. A soft, elastic subcutaneous mass with a maximum diameter of 6 cm was noted on the left elbow (Fig. 1A). On the basis of an examination, a dermatologist suspected lipoma. Nerve findings included numbness in the left fourth and fifth fingers that intensified upon left elbow flexion. The left fingers had no obvious motor impairment, and dorsal interosseous atrophy of the hand was not noted. An electrophysiological study failed to detect the sensory nerve action potential of the left ulnar nerve (Fig. 1B). The amplitude of the compound muscle action potential of the left ulnar nerve was 7.7 mV at below elbow-wrist and 5.2 mV at above elbow-below elbow, and the conduction velocity was 42.2 m/s at below elbow-wrist and 25.1 m/s at above elbow-below elbow (Fig. 1C). The electrophysiological diagnosis was ulnar neuropathy at the elbow. A Computed Tomography scan of the elbow revealed no deformity of the elbow or development of osteophytes at the medial epicondyle. MRI revealed that the mass was isointense on T1WI and hyperintense on T2WI, and there were no signs of fat suppression (Fig. 1D, E). On the basis of the diagnostic imaging, lipomas were ruled out and an epidermal cyst was suspected. The brightness of the ulnar nerve in the cubital tunnel had no changes. Surgical removal of the mass was performed while taking care to preserve the tumor capsule (Fig. 2A). However, the capsule was tense; thus, the capsule was punctured during the surgery to decompress the capsule. The slack capsule was detached from the surrounding soft tissue. There were no adhesions between the tumor capsule and the ulnar nerve. Following tumor removal, the ulnar tunnel was opened up from Osborne's band and the cubital tunnel to the arcade of Struthers, but there were no signs of nerve impingement or a neuroma (Fig. 2B). A histological examination revealed a cystic lesion lined by stratified squamous epithelium and containing layers of keratotic debris (Fig. 2C). The patient was diagnosed with an epidermal cyst. On the day after surgery, numbness of the ulnar aspect of the left hand upon elbow flexion was markedly abated.

Fig. 1.

A. Photographs of the left elbow showing the large mass at the posterior side of the olecranon.

B. An electrophysiological sensory nerve action potential study of the left ulnar nerve (left ulnar–digit V antidromic nerve).

C. An electrophysiological compound muscle action potential study of the left ulnar nerve (left ulnar–ADM 3).

D. Axial T1WI of the left elbow showing a well-defined mass (white arrow, mass; yellow arrow, ulnar nerve).

E. Axial T2WI of the left elbow showing a well-defined mass.

Fig. 2.

A. Intraoperative photograph showing a subcutaneous tumor mass.

B. Photomicrograph of hematoxylin–eosin stain showing a cystic lesion lined by stratified squamous epithelium and containing layers of keratotic debris.

C. A histological examination of the cystic lesion.

Discussion

Entrapment sites in ulnar neuropathy at the elbow are classified into (1) the arcade of Struthers, (2) the intermuscular septum, (3) the medial epicondyle of the humerus, (4) the cubital tunnel, and (5) Osborne's band.^11-13) Cubital tunnel syndrome is a condition causing entrapment ulnar neuropathy at the elbow, but ulnar neuropathy at the elbow is not always limited to the cubital tunnel. Therefore, “ulnar neuropathy at the elbow” is appropriate phrasing.¹⁾ In this case, the mass was located directly above Osborne's band; therefore, the ulnar nerve was compressed presumably through Osborne's band during elbow flexion. Anatomically, Osborne's band is a band of fibrous tissue that attaches to the olecranon from the medial epicondyle.^1,11-13) Osborne's band is stretched by elbow flexion, and simultaneously, the medial collateral ligament expands medially, which tends to increase the pressure in the ulnar tunnel.^1,11-13) In this case, an epidermal cyst above Osborne's band contributed to the increase in internal pressure during elbow flexion. During surgery, the mass was removed and then the cubital tunnel and Osborne's band were opened up, but there were no signs of entrapment such as impingement of or a neuroma along the ulnar nerve, and elbow deformities and osteophyte development were not noted. Thus, ulnar neuropathy at the elbow was presumably caused by the epidermal cyst alone.

An epidermal cyst causing ulnar neuropathy at the elbow is extremely rare, and only one case has been reported by Yamazaki et al.¹⁴⁾ However, in this report, ulnar neuropathy was caused by olecranon bursitis associated with an epidermal cyst. This case is the first report of ulnar neuropathy at the elbow caused by an epidermal cyst alone. An epidermal cyst is a skin mass, but, pathologically, the cyst wall structure is the same as that of the epidermis or infundibulum with a granular layer and keratin debris in the cyst cavity.¹⁵⁾ The cyst tends to develop on the face, neck, and trunk, which are hair-bearing areas; therefore, its development on the peripheral portion of the extremities is relatively rare.¹⁶⁾

In this case, an examination by a dermatologist indicated that the subcutaneous mass was a lipoma. A lipoma must first be differentiated from epidermal cysts. A later MRI revealed that the mass had low and high signal intensities on T1WI and T2WI, respectively, which ruled out a lipoma. Typically, MRI findings for an epidermal cyst are well-defined boundaries, a slightly heterogeneous isointensity on T1WI, and hyperintensity on T2WI with no contrast enhancement.¹⁷⁾ The MRI findings in this case were consistent with those in an epidermal cyst.

Deciding on the surgical treatment in this case, i.e., whether to remove the mass, release Osborne's band, and open up the cubital tunnel, was difficult. The patient's symptoms could have been alleviated by the removal of the mass alone. However, associated idiopathic ulnar neuropathy at the elbow could not be ruled out; thus, the decision to open up the ulnar tunnel was also made. The basis for this decision was an electrophysiological diagnosis of ulnar neuropathy at the elbow. Imaging findings in cubital tunnel syndrome cases are not always accompanied by an elbow deformity or luminance change of the ulnar nerve on MRI.^18-20)

Conclusion

We encountered an extremely rare case of ulnar neuropathy at the elbow caused by an epidermal cyst alone. In such cases, not only tumor mass removal but also cubital tunnel release can guarantee symptomatic improvement.

Informed Consent

Informed consent was obtained from the patient involved in this study.

Conflicts of Interest Disclosure

The authors have no conflicts of interest to disclose.