Ancient schwannoma involving the median nerve: a case report and review of the literature

Schwannomas can be asyptomatic or can produce pain, a positive Tinel’s sign or a Tinel’s like sensation and sensory alterations [5, 7]. The slow growth pattern of benign nerve tumors, allows for adaptation of the nerve function to the pressure effects. Thus, although schwannomas can rarely induce impaired motor function, neural tumors producing motor deficits should always raise a high suspicion of malignancy [7].

On pathologic analysis, a schwannoma has a true capsule composed of epineurium. The hallmark of schwannomas is the alternating pattern of Antoni A and B areas [1]. Antoni A are cellular areas with spindle cells and nuclear palisading forming Verocay bodies whereas Antoni B are hypocellular areas in a myxoid background. Ancient schwannomas display pronounced degenerative changes attributed to the vascular insufficiency during growth of the tumor and the tumor size has been correlated with progressive degenerative features [8]. They are characterized by cystic necrosis, stromal edema, xanthomatous change, fibrosis, perivascular hyalinization, calcification and degenerative nuclei with pleomorphism and hyperchromasia [1, 6, 9]. Antoni A and Antoni B areas are also observed as in conventional schwannomas. There is, however, a relative loss of cellular regions, which tend to be fibrosed or sclerotic. These areas may degenerate into hematomas and cysts [1, 9]. Nuclear atypia, and hyperchromasia, however, with low mitotic rate, can mislead to the diagnosis of malignancy.

On MRI the morphology and signal intensities reflect the histologic components of schwannomas. A rim of low intensity surrounds the lesion corresponding to the epineural capsule [10]. Antoni A areas are cellular areas containing collagen, thus exhibiting low signal in T1 and T2 images. In Antoni B, the water containing myxoid matrix produces a low in T1 and high signal intensity in T2 images [10]. In conventional schwannomas, central location of the Antoni A areas produces the so-called target sign on T2 images [7]. Cystic and necrotic areas have low signal in T1 sequences and are not enhanced with gadolinium [10]. These heterogeneous signal areas combined with cystic formations are easily interpreted as aggressive features seen in malignant peripheral nerve sheath tumors or other high grade sarcomas.

In our case, the cystic areas involved a higher percentage compared to the viable tumor on an axial view. On MRI the viable tumor areas were enhanced after gadolinium administration and, because of their peripheral location, a bright ring surrounded the nonenhancing irregular central cystic areas.

As a rule, biopsy of a soft tissue mass should be performed before surgical treatment is scheduled. Closed biopsy techniques are highly diagnostic with a low complication rate [11]. However, biopsy of a nerve tumor can cause neuropathic pain [5]. Excision biopsies should be performed only when imaging studies reveals a mass that can be completely excised and the surgeon considers that it is going to be the final surgical procedure. It is well known that unplanned excision biopsies are associated with a high rate of residual tumor in the surgical bed [12]. In this case, clinical examination in association with MRI features was suggestive of a long standing peripheral nerve tumor with possible diagnosis of a malignant peripheral nerve sheath tumor vs degenerated schwannoma. In our case, a closed biopsy could be either nondiagnostic or false positive for malignancy due to the extensive cystic areas and atypical histological features commonly seen in degenerated, old neural sheath tumors. Based on the fact that the lesion was well defined on MRI and frozen sections were negative for malignancy, we decided to perform a marginal tumor resection to the negative histological margins. So, even in the worst scenario, if the final pathology diagnosed a malignant tumor, the negative intra-operative margins protected the patient from contamination of the surgical bed from tumor remnants.

In contrast to neurofibromas and primary malignant nerve sheath tumors, schwannomas are eccentrically placed in relation to the axis of a large peripheral nerve, well encapsulated and can be dissected from the fascicles of the parent nerve without jeopardizing permanent or severe nerve dysfunction [5, 13]. Minor nerve dysfunction after schwannoma excision can be the result of three possible reasons [14]. The first is neuropraxia from nerve handling during tumor removal. In these cases nerve function recovers within weeks. The second reason is that small nerve fibers running through the capsule of the tumor are divided during capsule opening. In these cases a minor nerve function loss is noted. The third reason is that schwannomas arises from neural sheaths and the fascicles surrounded by this part of the sheath may have to be rejected. The resultant deficit depends on the specific function of this fascicle. Additionally, there are few cases were schwannomas demonstrate an infiltrative pattern and tumor excision necessitates some fascicle resection. Sawada et al. [14] demonstrated that schwannomas that could not be enucleated were those located proximal to the middle upper extremity and pre-operatively had sensory disturbances. When motor function loss is expected by intra-operative nerve fascicle stimulation, nerve grafting is recommended [15]. Tumor recurrence is rare, even in incomplete resections [15].

Here we present a rare example of a longstanding, symptomatic ancient schwannoma of the median nerve. Histology findings from small tissue sampling and MRI features may suggest malignancy leading to unnecessary wide tumor excision with loss of nerve function. Careful evaluation of the patient’s history with the relatively long time for existence of the nodule and the subtle clinical findings, in association with the specific frozen section histology findings and imaging features, are essential for intra-operative decision making to avoid unnecessary sacrifice of the parent nerve.