UNEXPECTED ULNAR NERVE SCHWANNOMA. THE REASONABLE RISK OF MISDIAGNOSIS

INTRODUCTION

Nerve tumors are classified as benign and malignant, and further sub-classified as neural or non-neural in origin.

Neurogenic tumors comprise less than 5% of all tumors of the hand and upper extremity (1).

Tumors of the nerve sheaths are the most prevalent of the peripheral nerve tumors and are divided in two categories, schwannomas and neurofibromas.

Malignant nerve tumors are rare; the most common of these is the neurogenic sarcoma.

In upper extremities schwannoma may arise from digital nerve, ulnar nerve, median nerve, radial nerve and from brachial plexus; its incidence is 0.8-2.1% of all upper extremity tumors (1, 2).

Schwannomas are benign, slow-growing neoplasms of neuroectodermal origin; they are the most frequent of all nerve sheath tumors: these lesion arise from Schwann’s cells that enveloped axons of the peripheral, cranial and autonomic nervous system.

The tumor originates from the nerve sheath in a focal manner as a solitary and well-encapsulated mass; it tends to arise from a single fascicle spreading and displacing the remaining fascicles within the nerve sheath, occasionally it tends to protrude from the nerve.

Schwannomas occur as a solitary lesion, but they can occur as multiple lesions and can affect one or several nerves.

Multiple schwannomas occur in two inherited tumor syndromes. Bilateral vestibular schwannomas are pathognomonic of neurofibromatosis 2 (NF2), while in the absence of other NF2 features, they are characteristic of a newly described syndrome, schwannomatosis (3).

The tumor can also occur in association with neuro-fibromatosis.

Malignant trasformation may occur in patients with Von Recklingausen’s disease and with large benign tumors, but the pathogenesis of this transformation is not yet fully understood.

The isolated ulnar schwannoma usually presents as a painless mass with or without positive Tinel’s sign, and the mass may remain asymptomatic for a long period of time (the irritative symptoms are mass size related).

Schwannoma must be suspected in patients with a palpable mass of the upper extremity, mobile, with or without neurological symptoms or signs.

A careful history and physical examination is necessary for diagnosis and management of ulnar nerve tumors; the history should include the growth rate of the lesion, the presence of neurological deficit or pain, a family history of neurofibromatosis or its stigmata, the presence of other masses or systemic disease.

The physical examination should focus on the size and location of the lesion (olecranic region), the presence of tenderness and mobility, Tinel’s sign, and any neurological deficits including sensory loss and paresthesia.

Usually ulnar nerve tumors are superficially located and can be moved laterally rather than proximally-distally.

Schwannoma of the ulnar nerve may remain asymptomatic for a long period of time because of the natural resiliency of the ulnar nerve and due to its ability to tolerate the physiological stresses of normal daily activities.

The paucity of symptoms in a large nerve such as the ulnar nerve is a result of the slow growth and non infiltrative nature of schwannomas, to minimize nerve dysfunction, and to explain the normal nerve conduction studies.

Ulnar nerve schwannomas arise from a single nerve funiculus; other funiculi of the nerve remain unaffected while being pushed slowly to the periphery of the neoplasm and may be subjected to only slight compression, maintaining their integrity and function.

These considerations explain the absence of clinical neurological symptoms, pain, and the absence of significant alteration of the electromyographic examination in many patients.

Neurological symptoms can be vague, tend to present late with an average interval of up to 5 years before the diagnosis is established.

Local pain or paresthesia are reported in less than half of affected people. Diagnostic tests such as ultrasound, CT, MRI can aid in diagnosis.

All these ulnar nerve tumors are usually on relatively superficial locations and therefore are suitable for US assessment (4).

MRI can further localize the extent of solid mass but it is expensive and difficult to obtain or to propose when there are not neurological signs of nerve injury. Nerve conduction studies may be normal.

Diagnostic tests are not specific so there is a potential for high rate of misdiagnosis with inadvertent nerve resection!

The absence of specific clinical diagnostic tests and the rarity and variable clinical presentation of Schwannoma result in the diagnosis being determined intraoperatively.

An appropriate index of suspicion is necessary for inclusion of peripheral nerve tumors as a differential diagnosis of an upper extremity mass (5, 6).

CLINICAL CASE

A 64-year-old male with 5-year history of a mass in the olecranic region, referred to the Department of Plastic and Reconstructive Surgery complaining of a growing, mobile and fusiform mass of his left elbow, causing discomfort (not pain) and interfering with his common daily activities.

The patient denied any trauma or any pathology.

The pathological anamnesis was not useful in obtaining further information about the nature of the mass.

The mass was associated with mild and sporadic discomfort due to its size; clinical neurological examination was not helpful (Tinel’s sign was negative).

Palpation of the olecranic region revealed a mobile mass, measuring 6x4 cm, that displaced side to side, perpendicular to the axis of the ulnar nerve, but not mobile in a longitudinal direction.

The sensory and motor examination did not show any deficits.

Nerve conduction velocities were normal, as were motor and sensory latencies.

Sonography showed a circumscribed hypoechogenic homogeneous mass with distal acoustic enhancement. Diagnostic tests were not specific.

Surgery confirmed the nervous origin of the mass.

A surgical exposure of the ulnar nerve from the level of the epicondyle and through the cubital tunnel was performed, splitting the flexor carpi ulnari heads and delivering the ulnar nerve and its associated tumor mass. The mass had a white-pink color, an elastic consistency and a wrinkled surface (Fig. 1).

A careful microscopic dissection of the ulnar nerve mass was performed, separating the ulnar nerve involved fascicles from the mass, which was completely excised with the fascicle involved using an electrostimulator.

The mass was sent to the pathologist, and the histology confirmed this large mass as a benign schwannoma with B Antoni cells.

Recovery was quite uneventful and satisfactory. The patient had a normal ulnar sensory and motor function postoperatively (ulnar nerve function remains intact).

Sonography follow-up showed no evidence of residual or recurrent tumor.

CONCLUSIONS

Ulnar nerve schwannoma is benign encapsulated neoplasm that arises from the Schwann cells of a single nerve funiculus.

These tumors are difficult to diagnose clinically and have been confused with neurofibromas or other benign neoplasms.

They present as a painless mass, occasionally with paresthesia and sensory deficit. They are mobile in the plane transverse to the course of the nerve and painful in rare cases. Nerve conduction studies may be normal.

The clinical importance of diagnosis when suspicion is present is related to the surgical technique of schwannoma excision; in order to preserve the nerve function, the mass requires careful enucleation from the affected nerve. With an operating microscope and an electrostimulator it is possible to show the tumor-nerve interface.

The most significant morbidity of misdiagnosis of schwannomas is the risk of nerve resection during the surgery.

Diagnostic tests such as US, CT, MRI can aid in diagnosis; the MRI can further localize the extent of a solid mass but is very expensive and difficult to propose when there are no neurological signs or symptoms of nerve involvement.

Awareness of this clinical possibility is important to prevent, during the surgery, the unfortunate resection of the nerve.

Frequently the diagnosis is made intraoperatively, so an appropriate index of suspicion is necessary to include peripheral nerve tumors in the differential diagnosis of an upper extremity mass.

Address for correspondence:

Sara Di Lorenzo, M.D.

Via del Vespro 129

90127 Palermo, Italy

E-mail: dilsister@libero.it