Myofibroma mimicking peripheral nerve sheath tumour with ulnar nerve compression symptoms

Background

Tumours involving peripheral nerves are uncommon. Of these nerve tumours, benign peripheral nerve sheath tumours including schwannomas, neurofibromas and perineuromas are the most common, while malignant peripheral nerve sheath tumours are extremely rare.1 2 MRI is useful in preoperative diagnosis of a peripheral nerve sheath tumour, with characteristic features being a well-defined fusiform morphology, isointense on T1-weighted images and hyperintense on T2-weighted images.3 4

Myofibromas, in contrast, are rare spindle cell tumours, classically seen in children younger than 2 years of age, where the condition is termed infantile myofibromatosis.5 Solitary myofibromas in adults are extremely rare and not typically associated with peripheral nerves.6 The prognosis for adult non-visceral myofibromas is excellent, and these tumours can be cured by excision in most cases.7

Here, we report a case of myofibroma encasing the ulnar nerve in the upper arm with motor and sensory symptoms, initially diagnosed as a benign peripheral nerve sheath tumour based on preoperative imaging. The final diagnosis was only made after formal histopathological analysis.

Case presentation

An early adolescent man was referred for management of a presumptive peripheral nerve sheath tumour involving the ulnar nerve in the medial aspect of the left arm, 8 cm distal to the axilla. Four months prior, the left arm mass was incidentally noticed by a family member. A few months later, he developed intermittent numbness and tingling of his left fifth digit with shooting pains to the ulnar hand, progressive left weakness, and difficulty extending the fifth digit. He had an MRI and ultrasound of the mass done at another facility, which was read as a heterogeneously enhancing mass encasing the ulnar nerve, consistent with a benign peripheral nerve sheath tumour.

Investigations

On examination, he had left hand intrinsic weakness with Medical Research Council grade 4/5, notable clawing and a prominent Wartenberg’s and Froment’s sign (figure 1). A mass about 1.5 cm in diameter on his upper medial arm was palpable 8 cm distal to the axilla. After discussion with the family, decision was made not to proceed with a preoperative electromyogram (EMG) because there was an MRI and ultrasound read showing a clear diagnosis of a benign tumour as the aetiology for his muscle weakness. In addition, the patient as a child did not wish to have discomfort associated with having an EMG.

Figure 1

Preoperative clinical examination showing positive Wartenberg’s and Froment’s sign in the left hand.

Intraoperative exploration began through a longitudinal medial arm incision, and dissection was carried through the subcutaneous tissue and fascia. The ulnar nerve was identified proximally and distally. There was a lot of scarring around the mass, which appeared to circumferentially encapsulate a segment of the ulnar nerve (figure 2A). The median and medial antebrachial cutaneous nerves were noted to be anterior to the mass. The mass was carefully dissected off the ulnar nerve and off an abberant separate tubular structure deep to the ulnar nerve (figure 2B). There was an obvious hourglass constriction of the ulnar nerve (figure 2B) due to compression by the mass. The mass did not actually originate from the ulnar nerve proper, which raised suspicion intraoperatively that the mass might not be a peripheral nerve sheath tumour. Neurolysis and exploration of the left median nerve and medial antebrachial cutaneous nerve anterior to the ulnar throughout the surgical field was performed to ensure these were not involved by the mass.

Figure 2

Intraoperative images. (A) The mass was found to encase the ulnar nerve. (B) The mass was found to originate from a tubular structure deep to the ulnar nerve, causing a localised hourglass deformity of the ulnar nerve from circumferential compression.

Attention was then brought back to the tumour with careful dissection around the anomalous structure deep to the main ulnar nerve. Two fascicles of this structure, which appeared consistent with an anomalous nerve were dissected free from the mass, which was then excised and sent to pathology. The wound was closed in layers with subdermal 3–0 vicryl and running subcuticular 4–0 monocryl, followed by steri-strips and a dry sterile dressing.

Histopathological analysis of the specimen returned as a spindle cell neoplasm with features consistent with a myofibroma. Hypercellular areas (figure 3A) showed ovoid and short spindle cells, while nodular hypocellular areas (figure 3B) showed foci of stroma chondromyxoid-like changes as well as hyalinisation. Immunohistochemical stains were negative for S-100, Sox-10 and cytokeratin AE1/AE3, ruling out a schwannoma or epithelial lesion, while being diffusely positive for smooth muscle actin (figure 3C), a confirmatory immunohistochemical marker for myofibroma.8

Figure 3

Histopathological images. (A) Hypercellular areas show ovoid and short spindle cells (H&E stain, scale bar=50 mm). (B) Nodular hypocellular areas show foci of stroma chondromyxoid-like changes as well as hyalinisation (H&E stain, scale bar=100 mm). (C) Immunohistochemistry with diffuse positive staining for SMA (SMA stain, scale bar=100 mm). SMA, smooth muscle actin.

Outcome and follow-up

The patient underwent hand occupational therapy two times weekly postoperatively, with a focus on strengthening exercises. His postoperative recovery was uneventful, with resolution of pain and sensory symptoms shortly after surgery. There was resolution of clawing with improvement of strength in the left hand at the 3-month follow-up visit. Grip strength increased from 5 lb preoperatively to 12 lb at the 4-month postoperative visit.