Neurofibromas of the skull base
Stanford Ear Institute
Overview
Neurofibromas are tumors that arise from the outer sheath of peripheral and cranial nerves. Although neurofibromas may arise sporadically, 90% of neurofibromas are asosociated with neurofibromatosis type I, an autosomal dominant neurocutaneous disorder associated with tumor development throughout the central and peripheral nervous system. Peak presentation is between 20 and 30 years of age, and both males and females can be equally affected. Most neurofibromas are benign tumors, however 5-10% of large neurofibromas may undergo malignant transformation into malignant peripheral nerve sheath tumors (MPNSTs). Cells harboring the NF-1 mutation undergo inactivation of the gene encoding neurofibromin, leading to unregulated cell growth via the RAS pathway. A subset of neurofibromas, known as plexiform neurofibromas, are more locally invasive and aggressive, and are associated with worse clinical outcomes.
Symptoms
Neurofibromas may present with headache, pain, neurologic deficits related to compression of important neurovascular structures by tumor growth, or unwanted cosmetic effects. Neurofibromas of the skull base may cause symptoms related to cranial neuropathies such as hearing loss, facial weakness, facial numbness, vision changes, or swallowing difficulties. In advanced stages their growth may cause hydrocephalus or brainstem compression leading to somnolence and depressed level of consciousness. Some skull base neurofibromas may be discovered incidentally as part of a workup for an unrelated condition.
Diagnosis
In cases where signs and symptoms suggest a lesion within the skull base, an MRI with and without contrast is used to better characterize the lesion. Neurofibromas of the skull base usually enhance with contrast and are associated with cranial nerves. They may expand the corridors of bone in which the nerves travel. In some cases, the tumors may contain cysts or areas of hemorrhage.
Treatment
Small neurofibromas that are asymptomatic may be followed with serial MRIs to monitor their growth and ensure that they do not cause symptoms. Small neurofibromas that have been shown to grow over time may be treated effectively with Cyberknife, a form of focused radiation developed at Stanford. Tumors larger than 3 centimeters are best treated with surgery, which involves performing a craniotomy to microsurgically remove the tumor and prevent further injury to the brain and nerves that would result from further cell growth. Malignant neurofibromas are treated with a combination of microsurgical resection, radiation, and chemotherapy.