A 47-year-old woman presented to our outpatient clinic with a soft elastic mass in the medial aspect of the right arm that had been progressively enlarging during the last 4 years. The patient referred no trauma history and clinical examination revealed 4th and 5th finger hypoesthesia.
An MRI examination was performed using a 1.5T scanner. The study consisted of T1-weighted fast spin-echo (FSE), T2-weighted fast spin-echo (FSE) with fat saturation and gradient echo T2 (FFE) sequences performed in the axial, coronal and sagittal planes, followed by post Gd dynamic and FSE T1 WI study.
The MRI study showed a fusiform-shaped mass in the medial aspect of the right arm close to the basilic vein in contact with the ulnar nerve. On T1-WI the mass was homogeneous and intermediate signal intensity and hyperintense and slightly heterogeneous on T2-WI fat saturation. After gadolinium injection the images showed avid and diffuse enhancement except non-enhancement central area.
Surgical resection followed with pathological examination confirmed the diagnosis of schwannoma.
Schwannomas are rare tumours but still the most common primary nerve sheath tumours of the upper limb, accounting for about 5% of all benign soft-tissue neoplasms. The most commonly affected peripheral nerves are the peroneal and the ulnar nerves. [1, 3]
Schwannomas are well-encapsulated benign tumours. Microscopically, schwannoma combines highly ordered cellular component and hypocellular loose myxoid component, Antoni A and Antoni B areas respectively. Tumour cells are highly inmunopositive for S-100 protein.
MRI is the best imaging modality for diagnosing nerve sheath tumours, however, sometimes the differential diagnosis from other soft tissue tumours is difficult if they do not have specific signs. [1, 2]
The most reliable sign for schwannoma is its localization in continuity (eccentrically) with a normal nerve. MR characteristic features show iso to slightly increased signal intensity relative to muscle on T1WI, a thin peripheral rim of fat (split-fat sign) is very characteristic and may have subtle muscle atrophy distal to lesion. On fluid-sensitive sequences schwannomas are hyperintense to muscle and may show a typical central low signal region (target sign) and multiple small ring-like structures (fascicular sign). Diffuse intense enhancement is typical. [1, 2]
Differentiating on imaging between schwannomas and neurofibromas may be impossible. However, schwannomas are more likely to contain cysts, haemorrhage, fibrosis or calcification. Neurofibromas might have target sign and central enhancement on MRI. The differential diagnosis with malignant peripheral nerve tumours (MPNT) is difficult. MPNT is usually a larger (>5 cm) ill-defined mass located more frequently at the sciatic nerve, brachial and sacral plexus. Heterogeneous on T1-WI, T2-WI and intense and especially peripheral enhancement. MPNT are more likely to produce pain and sensory deficit. [2]
Since the malignancy rate and recurrence are very low, resection of the tumour is recommended.
Ulnar nerve schwannoma
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Based on the provided MRI images and clinical information, the lesion is located within the medial soft tissue of the right upper arm, appearing relatively round or nearly round in shape with well-defined margins and a clear capsule. On T1-weighted images, the signal intensity is approximately equal to or slightly higher than that of muscle tissue. On T2-weighted and fat-suppression sequences, the lesion shows high signal intensity, and some areas may present “ring-like” or “septated” structures (possibly indicating internal small cystic changes or fibrous septa). In certain images, a thin fat rim (“split-fat sign”) can be observed around the tumor. After contrast administration, homogeneous or relatively homogeneous enhancement is evident. The tumor has a continuous, eccentric relationship with the surrounding nerve, and the overall shape is regular without signs of significant invasiveness. In addition, the patient’s sensory abnormalities in the 4th and 5th fingers suggest that the tumor originates from or compresses the related nerve (suspected ulnar nerve).
Rationale: Schwannomas commonly occur in peripheral nerves of the extremities; the ulnar nerve or common peroneal nerve are relatively common sites. On imaging, schwannomas often present with well-defined margins, an intact capsule, eccentric growth with respect to the nerve, and homogeneous enhancement. On T2-weighted images, they typically show high signal intensity and may display characteristic “split-fat sign,” “target sign,” or “fascicular sign.” The imaging features in this case are consistent with a schwannoma, and the tumor appears to have a slow growth rate, with the patient presenting only mild sensory abnormalities.
Rationale: Neurofibromas are also benign tumors arising from peripheral nerve sheaths, and some of their imaging characteristics overlap with schwannomas, such as high T2 signal intensity and continuity with the nerve. “Target sign” and central enhancement can also appear. Without pathological and clinical correlation, differentiating between schwannoma and neurofibroma based on imaging alone can be difficult. However, neurofibromas tend to show more central growth (symmetrically involving the nerve trunk) and are often seen in patients with neurofibromatosis.
Rationale: These tumors are typically larger (＞5 cm), with irregular outlines and poorly defined boundaries. On T1- and T2-weighted images, the signal is often heterogeneous, possibly showing necrosis or hemorrhage, and post-contrast enhancement is usually marked and uneven. They are also often accompanied by severe pain and deficits in sensory or motor functions. Given this patient’s long tumor history, well-defined margins, and lack of significant malignant features, malignancy is less likely. Nonetheless, pathological examination is necessary to rule it out.
Considering the patient’s age, the lesion’s location (the medial side of the right upper arm, seemingly associated with the ulnar nerve), clinical symptoms (numbness in the 4th and 5th fingers), imaging characteristics (well-defined capsule, signal features, eccentric relationship with the nerve), benign appearance, and slow growth, the most likely diagnosis is Schwannoma.
After surgical removal or during conservative observation, an individualized rehabilitation plan should be developed based on the patient’s nerve function and local pain levels. The following are general recommendations:
If the patient has comorbidities like osteoporosis or compromised cardiopulmonary function, exercise intensity and frequency should be reduced. Training should be conducted under the guidance of a professional rehabilitation therapist and physician.
This report is intended solely for medical knowledge and treatment reference and does not replace in-person clinical evaluation or direct professional medical advice. Specific diagnosis and treatment should be based on the patient’s actual condition, determined by a specialist physician. If you have any questions or experience changes in symptoms, please seek medical attention promptly.
Ulnar nerve schwannoma
