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Schwannomas

Description of Orbital Schwannomas

• Schwannomas are slow-growing, benign tumours that develop within the outer covering of peripheral and sympathetic nerves formed by Schwann cells, called the nerve sheath. They are sometimes referred to as neurilemmomas and often involve sensory and motor nerves supplying the orbital region. Schwannomas are rare tumours and constitute about 1% of all orbital tumours and 35% of peripheral nerve tumours within the orbit.
• It primarily affects individuals between 20 and 60 years old, with 10-15% cases accompanied by neurofibromatosis.

Pathophysiology of Orbital Schwannomas

• Orbital schwannomas are seen as encapsulated growths within a peripheral or sympathetic nerve, which distinguishes them from a neurofibroma that affect nerve fibres themselves. They are benign tumours and are rarely associated with malignant transformation. They usually occur in the superior temporal region or muscle cone of the orbit pushing the eyeball forward and downward.
• The orbital schwannomas has the following histologic components that help in a pathologic diagnosis:
  • Antoni A type areas: Constitute solid areas of tumour cells forming the bulk of the tumour.
  • Antoni B type areas: Contains loose cystic spaces with no axons.
  • Verocay body: Represents palisading of the tumour nuclei in acellular zones of the tumour.
• Verocay bodies, when present, are a useful marker for orbital schwannoma. Histochemical staining with S-100 could be positive.

Symptoms and Signs of Orbital Schwannomas

• A patient with orbital schwannoma usually presents with a slowly progressive painless bulging of the eyeball, proptosis and takes years or even decades to produce symptoms.
• Other symptoms and signs commonly seen are:
  • Oedema of the eyelids
  • Dystopia of the eyeball
  • Exophthalmos
  • Impaired ocular motility
  • Disturbances in vision including visual loss
  • Changes in the optic disc such as choroidal striae with hyperopia

Diagnosis of Orbital Schwannomas

• Clinical history of a slow-growing tumour within the orbit producing symptoms hinting to a diagnosis of orbital schwannoma. This suspicion is further supported by the presence of an encapsulated mass.
• Diagnosis is confirmed by Computed Tomography and Magnetic Resonance Imaging of the orbital region. Histologic diagnosis is obtained after surgical excision of the tumour and shows Antoni A or Antoni B areas, Verocay bodies and positive for S-100 immunohistological stain.

Treatment of Orbital Schwannomas

• Complete surgical excision is the treatment of choice for orbital schwannoma. Being well-encapsulated, it is easily removed. There is however a small risk of recurrence. The risk of malignant transformation is minimal.