Benign intracranial tumours cause harm by the pressure they exert on adjacent structures. The commonest benign tumours arise from cells lining the brain or from the cranial nerves emerging from the brain and they can pose siginificant treatment challenges because of the delicate tissues they involve.
Meningiomas arise from meningothelial cells in th earachnoid layer covering the brain. This discrete layer is closely applied inside the dural covering. They account for about 20% of all primary brain tumours. The vast majority are benign (WHO grade 1) but atypical types which are more prone to recurrence (WHO grade II) and even frankly malignant tumours (WHO grade III) are encountered. Meningiomas may occasionally involve the skull bone and cause it to thicken. Very rarely the scalp itself may become involved by malignant tumours.
Small meningiomas which are not growing over time and not causng symptoms may be safely left alone. It is however advisable to monitor them for a period of time. The duration of such surveilence will be affected by the size and location of tumour as well as the patient's time in life. Surgical removal offers the prospect of cure for most meningioma although it is usual to monitor the patients with CT or MR scans for some years after surgery. It is our experience that if WHO grade I lesions reccur they will do so within two years. The best prospect for cure is to remove not only the tumour but also the section of dura to which it is attached. This is not always possible especially with tumours arising along the skull base or enclosing important venous sinuses. Treating the dura that remains with electrocautery can reduce the chances the tumour will come back. If recurrent but benign tumour is an ongoing problem radiosurgery or radiotherapy to the origin can be effective at controlling growth. For some small, symptomatic meningiomas of the skull base radiosurgery may be considered as primary treatment.
Schwannomas (Neurilemmoma) arise from from cells forming a sheath around nerves which emerge from the brain and brainstem. The most common arise from the vestibular nerve, one of the nerves serving the balance organ of the ear. The vestibular nerve is closely applied to the cochlear which transmit hearing and for practical purposes they consitute a single structure the vestibulo-cochlear nerve. Vestibular Schwannomas may therefore present with hearing loss, tinnitus and vertigo. They were formerly known as Acoustic Neuroma when thought wrongly that the tumour arose form the hearing portion of the nerve. These tumours may grow sufficently large to press on the adjacent brainstem causing high-pressure headaches and hydrocephalus. Schwannomas on other nerves are very rare with the next most common being on the trigeminal nerve- Trigeminal Schwannoma. The early symptoms depend on the function of the nerve affected.
Neurofibromatosis type 1 is a condition which results from a mutation in a gene on chromosome 17. It affects approxomately 1 in 3000 people. The gene is called NF1 and codes for a protein called neurofibromin. It is a tumour supressor gene meaning that when it is not functioning the following can develop:
Patients benefit from genetic counselling and surveillence in specialist clinics designed to identify when adverse features develop in tumours meriting treatment. The nervous system tumours may behave quite indolently for long periods. The role of the specialist is as much to prevent unecessary treatment of tumours as to carry out such intervention. Specialist teams are made up of neurologists, neurosurgeons, geneticists, psychiatrists, orthopaedic and plastic surgeons.
Neurofibromatosis type 2 is a genetically and clinically distinct disease to the above. It is less common affecting aproximately 1:35000 individuals. All develop vestibular schwannomas and in most these are bilateral. Other tumours assosciated with this condition include schwannomas of other cranial nerves, meningioma and spinal cord ependymoma. The gene mutation in NF2 is on chromosome 22 and its gene prodict is the Merlin protein. Juvenile cataracts may be the earliest detectable evidence of the disease.
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