- Slow-growing tumor, often managed with prolonged observation
- Tumor can grow to large sizes and extend into the orbital apex, often without decreased vision or motility deficits, thus supportive of a benign, encapsulated, very slow-growing mass
- Can be associated with von Recklinghausen's disease, but not typical for most orbital schwannomas
- Represent approximately 1% of orbital tumors
- Malignant transformation extremely rare
- Can extend through the superior orbital fissure (rarely the inferior orbital fissure)
- Enlargement of the foramen rotundum, foramen ovale, cavernous sinus, or pterygopalatine fossa also reported
- Cellular schwannoma variant is relatively uncommon, but important to recognize.
- High cellularity, fascicular growth pattern, increased mitotic activity, and occasional locally destructive behavior, including bone erosion, often lead to consideration of malignancy.
- Composed almost entirely of a compact, fascicular proliferation of well-differentiated, cytologically bland Schwann cells, lacking Verocay bodies (Casadei 1995), and showing no more than very focal Antoni-B pattern growth (<10% of tumor area)
- Important findings include the presence of foamy histiocyte aggregates, a well-formed capsule containing lymphoid aggregates, and diffuse strong S-100 protein and pericellular collagen IV expression.
- In contrast, diffuse S-100 protein expression is extremely uncommon in spindled malignant peripheral nerve sheath tumors.
- Cytokeratin immunoreactivity can be seen in some cellular schwannomas, but might represent cross reactivity with GFAP.
- Cellular schwannomas lack expression of smooth muscle actin, desmin, CD117, and DOG1, allowing exclusion of other tumors in the differential diagnosis such as leiomyosarcoma and gastrointestinal stromal tumors.
- Despite the very high cellularity and mitotic activity, these typically lack malignant potential for practical purposes and never metastasize.
- Local recurrence is variable (5%–40%), and might be higher than in conventional schwannomas, perhaps due in part to its propensity for deep anatomic regions that are not always amenable to gross total resection.
- However, even recurrent lesions have been shown to grow slowly and do not result in death (White 1990).
- Plexiform schwannoma variant usually occurs in superficial (cutaneous or subcutaneous) locations and is defined by a plexiform (intraneural-nodular) growth pattern.
- Although might be associated with NF2 and schwannomatosis, the association is weak, seen in only about 5% of cases (Berg 2008).
- Tumor might be less circumscribed than conventional schwannoma or even lack a capsule.
- Usually composed of Antoni-A type areas
- Melanotic schwannoma variant is a rare, distinctive, potentially malignant neoplasm characterized by epithelioid cells with variably sized nuclei and marked accumulation of melanin in neoplastic cells and melanophages
- Psammoma bodies can be seen.
- Differential diagnosis must include other melanin-producing neoplasms, i.e., melanoma.
- Can be used after excision to decrease recurrence
- More recently, multisession gamma knife radiosurgery (GKRS) has been reported to be an effective primary management strategy to treat solitary, benign, well-circumscribed orbital apex tumors
- Goh et al. (2013) reported a retrospective interventional case series of 5 patients with visual disturbances from a well-circumscribed orbital apex tumor (3 cases of cavernous malformation, 2 schwannoma).
- Each patient underwent treatment with GKRS in 4 sessions with a radiation dose of 10 Gy to the tumor, and 5 Gy to the tumor margin with a prescription isodose of 50%.
- This was a cumulated radiation dose of 40 Gy to the tumor and 20 Gy to the tumor margin.
- All patients demonstrated improvement in visual acuity, pupillary responses, color vision, and visual field.
- Tumor shrinkage was observed in all patients and remained stable until the last follow-up.
- No adverse events were noted during or after radiosurgery.
- No patients experienced any radiation-related ocular morbidity.
- Multiple sessions achieved the cumulative total radiation doses needed to cause tumor shrinkage while avoiding radiation to orbital lesions of more than 8–10 Gy in a single session, which can result in injury to the anterior visual pathway.
Surgical excision in symptomatic patients
Observation is typical primary management unless there is visual compromise to the optic nerve, corneal exposure from significant proptosis, or worsened discomfort.
Complete excision is optimal and often achievable due to its well-circumscribed, encapsulated nature.
As the tumor originates from the Schwann cells and is thus located along the periphery of the nerve, it can usually be carefully removed from the nerve with minimal trauma.
Approach governed by location
Most often a superolateral orbitotomy or upper lid crease anterior orbitotomy, with an extraperiosteal approach, is used due to the predilection of the tumor to the superior orbit.
Transfrontal craniotomy/orbitomy in conjunction with neurosurgery might be indicated for extensive superior orbit and apical lesions.
- Complications include CSF leak, intracranial injury, meningitis, postoperative ptosis, frontalis palsy.
Transcaruncular approach or transnasal endoscopic removal can be useful for medial lesions.
- Interactive image guidance systems can be useful to aid in locating the tumors intraoperatively.
Cryoprobe can be useful in extracting posterior lesions with a small dumbbell extension into the superior orbital fissure (Shields 1986).
Ideally avoid injury to vital orbital structures
If the tumor extends into the superior orbital fissure, incomplete excision might be necessary to preserve the structures traversing this space.
- Shields (1986) reported a case of a large encapsulated ovoid schwannoma involving the entire orbit and extending into the superior orbital fissure without causing motility deficits or supraorbital nerve sensory changes.
- CT demonstrated enlargement of the superior orbital fissure, and showed that the majority of the tumor was within the orbit except for a small dumbbell extension.
- Complete removal via a lateral orbitotomy with blunt dissection and cryoprobe was successful without postoperative visual or motility complications.
- Can be a therapeutic alternative for a small subgroup of tumors in the intraconal orbital apex — particularly inferior to the optic nerve — that would otherwise cause significant ocular morbidity with excision (Kloek 2006)
- Kloek et al. (2006) reported a retrospective case series of 5 patients with compressive optic neuropathy secondary to an orbital apex tumor who underwent surgical decompression.
- All patients had progressive visual decline measured by visual loss, afferent pupillary defect, decreased visual field, and in some cases, color vision deficits.
- No patients had a history of malignancy or rapid clinical decline.
- All tumors were clinically diagnosed as benign tumors (schwannoma or cavernous malformation) based on radiologic features such as well-defined margins, lack of infiltration, and homogeneous enhancement pattern.
- 3/5 patients underwent serial preoperative imaging that documented no change in tumor size.
- Surgical decompression of the orbit was via transnasal endoscopic decompression of the medial orbital wall, lateral orbitotomy, or transcaruncular decompression approaches.
- None had biopsy or resection of the tumor.
- All patients had improved visual function by 2 weeks to 6 months after surgery.
- Postoperative complications included
- Recurrent optic neuropathy
Common treatment responses, follow-up strategies
- Typically good response to surgery, even if tumor is only partially excised
- Rose (1991) reported 54 cases of peripheral nerve sheath orbital tumors and found that despite incomplete resection in 24% benign schwannoma cases, there were no recurrences requiring additional surgery in 23 years of follow-up.
- Low rate of recurrence
- Close follow-up after surgery