Perineural Lipoma of the Ulnar Nerve Within the Cubital Tunnel: A Brief Review of the Literature

Abstract

This study presents a case of a patient with a 6-month history of progressively worsening paresthesia, numbness, weakness, and shooting pain in the ulnar nerve distribution of the left hand. Ultrasound measurements showed an enlarged cross-sectional area of the ulnar nerve, confirming the diagnosis, and an isoechogenic and homogenous mass alongside the ulnar nerve. An open cubital tunnel release was performed for ulnar nerve decompression. Intraoperatively, a perineural lipoma was identified within the cubital tunnel and subsequently confirmed by histopathological analysis. Soft tissue lipomas rarely cause symptomatic compressive neuropathy. Although other space-occupying lesions within the cubital tunnel are well-documented in the literature, a perineural lipoma causing extrinsic ulnar nerve compression has not been previously reported. After surgery, shooting pain resolved, sensation normalized in digits four and five, and hand strength gradually improved. Therefore, we report the first case of a perineural lipoma extrinsically causing ulnar nerve compression in the cubital tunnel.

Cubital tunnel syndrome (CuTS) is the second most common compressive neuropathy of the upper extremity, following carpal tunnel syndrome. Its annual incidence ranges from 25.2 to 36 per 100,000 in men and from 17.2 to 26 per 100,000 in women.¹ The ulnar nerve's unique anatomical relationship with the elbow increases its susceptibility for entrapment. Throughout the range of motion of the elbow, the ulnar nerve undergoes compression, traction, and frictional forces, as canal volume decreases by 55% as the arcuate ligament elongates during elbow flexion.²

There are several anatomical sites of compression of the ulnar nerve; however, the cubital tunnel is the most common site of entrapment. In addition to these known anatomic sites of compression, trauma, anomalous anatomy, and tissue overgrowth can also contribute to symptoms consistent with entrapment neuropathy. Potential compressive pathologies include malunions, nonunions, hypertrophic callus, osteophytes, synovitis in rheumatoid arthritis or gout, fibrous bands, tumors, and hematoma formation. In most cases of CuTS, no space-occupying lesions are identified.³ Nevertheless, certain benign and malignant tumors have been reported to cause peripheral nerve compression, such as ganglions, hemangiomas, desmoids, ganglioneuromas, lymphangiomas, myoblastomas, and lipomas.⁴

Case Report

A 54-year-old, right-handed Caucasian man was examined at the Plastic and Reconstructive Surgery Department of the Anna Hospital in Geldrop. The patient presented with a 6-month history of progressively worsening paresthesia, numbness, muscle weakness and shooting pain in the fifth digit and the ulnar side of the fourth digit of the left hand, with the right hand being unaffected. Physical examination showed no evident atrophy of the intrinsic hand muscles, but it did show grade 4 muscle weakness. Froment’s test was negative. Decreased sensation to light touch was evident in the fifth digit and the ulnar side of the fourth digit in the left hand. Tinel sign was positive at the left ulnar sulcus, and Wartenberg sign was inconclusive. There was no solitary mass palpable at the level of the left elbow that could indicate the presence of a lipoma.

The ultrasound measurements revealed an enlarged cross-sectional area of the ulnar nerve, measuring 13 mm² at the ulnar sulcus. At the level of the flexor carpi ulnaris muscle and Guyon canal, the ulnar nerve had decreased to its normal size. Aside from the enlarged cross-sectional area, the clinical neurophysiology technologist found an isoechogenic and homogenous mass alongside the ulnar nerve. The mass, approximately 6 cm in size, was compressing the ulnar nerve in the ulnar sulcus. Given the severity of the symptoms and lack of response to conservative measures, a decompression of the nerve by open cubital tunnel release was attempted (Fig. 1).

Figure 1.

Ultrasound image showing an enlarged cross-sectional area (13 mm²) of the ulnar nerve at the cubital tunnel. A well-defined, homogenous, isoechogenic mass consistent with a lipoma is seen enveloping the nerve. Both heads of the flexor carpi ulnaris and the olecranon are visible at the level of the cubital tunnel.

During surgery, Osbourne's ligament was incised, decompressing the cubital tunnel. The soft tissue lipoma situated in the cubital tunnel, approximately 5 × 6 centimeters in size, immediately prolapsed when the ligament was opened. The lipoma was clearly encapsulated and situated perineurally in relation to the ulnar nerve which showed signs of compression being flattened and hyperemic. Histopathological examination demonstrated a well-circumscribed benign mesenchymal lesion composed of mature adipocytes, without evidence of atypia or malignant transformation, confirming the diagnosis of a benign lipoma. After surgery, the patient no longer experienced shooting pain, with normal sensation restored in digits four and five, and gradual recovery of strength in the left hand (Figs. 2 and 3).

Figure 2.

Soft tissue lipoma (5 × 6 cm) within the cubital tunnel prolapsed upon incision of Osborne’s ligament, indicating confinement and compressive pressure.

Figure 3.

Encapsulated perineural lipoma adjacent to the ulnar nerve. The nerve appeared flattened and hyperemic, consistent with compression caused by the mass.

Discussion

Four types of lipomatous masses are associated with peripheral nerve compression, including soft tissue lipoma, intraneural lipoma, macrodystrophia lipoma, and lipofibromatous hamartoma. Soft tissue lipomas, as seen in our case, are true benign neoplasms that can cause peripheral nerve compression extrinsically. Although soft tissue lipomas are the most common benign tumors in limbs and can occur in the vicinity of the peripheral nerve, they rarely cause symptomatic compressive neuropathy.^4,5

In the literature, few cases of soft tissue lipomas that cause compression extrinsically have been described. The first report of symptomatic compressive neuropathy caused by a lipoma was in 1953, by Richmond.⁶ He described a parosteal lipoma at the neck of the radius that caused paralysis in the muscles supplied by the posterior interosseous branch of the radial nerve.⁶ In 1962, Barber et al⁷ reported a series of benign extraneural soft tissue tumors causing compression of peripheral nerves. Of the 32 tumors in that series, 12 were lipomas. Here, we find the first two documented cases of ulnar nerve involvement. None of these were located in the cubital tunnel.⁷ In 1962, White and Hanna⁸ also reported a series containing four cases, with three involving the ulnar nerve and one involving the radial nerve. These lipomas were located at the wrist level or more distally. The often cited literature review by Phalen et al⁵ described 112 lipomas in the upper extremity. Only six of these cases involved some degree of nerve compression, none of which affected the ulnar nerve.⁵ Furthermore, none of the series or reviews included more than three cases of ulnar nerve compression caused by a lipoma, highlighting the rarity of this diagnosis.

In more recent years there have been some case reports that add to the scientific body of knowledge. Typically, symptomatic lipomas are not extrinsic but intraneural and located distally at the level of the wrist or hand. In the seven known cases involving the ulnar nerve, the location was consistently at the wrist, with four in Guyon canal, and three in the hypothenar region.9, 10, 11, 12, 13, 14, 15 In only four of these cases, the lipoma was perineural. In all of these cases, at the time of follow-up, all sensory and motor symptoms had improved or disappeared and no recurrence was found.

Additional space-occupying lesions occurring in the cubital tunnel are well-documented in the literature. The most prevalent space-occupying lesions are ganglion of the elbow, including intraneural and extraneural.¹⁶ The prevalence rate in patients with CuTS is estimated to range between 7.3% and 8.1%, as demonstrated in large series by Kato et al¹⁷ and Chang et al.¹⁶ These elbow cysts originate from nerve sheath, tendon sheath, ligament, bursa, joint capsule, or subchondral bone.¹⁸ Caused by ulnohumeral osteoarthritis, patients report experiencing pain at rest and a relative acute onset of symptoms. Radiographs of the joint show degenerative changes, which is why the diagnostic tool should be used to identify this risk factor.¹⁷ CuTS caused by these elbow cysts is generally treated successfully by surgery and recurrent cases are rare.¹⁹

Other, rarely reported, soft tissue masses that can also cause CuTS are peripheral nerve sheath tumors involving the ulnar nerve, giant cell tumor of the tendon sheaths, neurofibroma, synovial chondromatosis, and hemangioma.^4,17,18

From this brief review of the literature, it is apparent that peripheral nerve compression by lipomas is not common and it explains the limited epidemiological data concerning nerve-compressing lipomas. Ulnar nerve involvement appears to be even more rare. The authors report the first case of a lipoma extrinsically compressing the ulnar nerve inside of the cubital tunnel.

Conflicts of Interest

No benefits in any form have been received or will be received related directly to this article.