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Acoustic and Skull base tumors
ACOUSTIC TUMORS
What is an acoustic tumor?
Diagnosis
Treatment
Tumor size and removal options
SKULL BASE TUMORS
Skull base tumors
Meningiomas
Schwannomas
Chordomas
Glomus tumors
Chondrosarcomas
Symptoms of skull base tumors
Diagnosis
Treatment
What is an acoustic tumor?
Acoustic tumors (properly called Vestibular Schwannomas) originate from the balance nerve, which connects the balance portion of the inner ear and the brain. These tumors specifically arise from the nerve covering or sheath (Schwann) cells of the balance or vestibular nerve. The acoustic tumor is a benign growth and is not a brain tumor. The tumor exists in a space between the brain and the ear, called the cerebellopontine angle (CPA) and is directly next to vital brain centers. As they grow, they affect the adjacent facial and hearing nerves, and they can involve other cranial nerves with further growth. If they grow quickly and/or are allowed to grow over a long period of time, they may compress vital brain centers, eventually cause pressure on the brain, and ultimately can be fatal. In most cases, these tumors grow slowly (approximately 1-2 mm per year) however, in some people the growth is quite rapid and can actually double in size in a single year. Vestibular Schwannomas occur at a rate of approximately one per 100,000 individuals in the general population. They constitute approximately 10% of all brain tumors. Most (95%) acoustic tumors (AN) are unilateral; that is they occur on one side. About 5% are bilateral and are, by definition, associated with an inherited syndrome known as Neurofibromatosis type 2 (NF2).
Diagnosis
The diagnosis of a Vestibular Schwannoma is usually suggested by the presence of unilateral sensorineural hearing loss, ringing in one ear, and sometimes dizziness or unsteadiness. At times they cause subtle symptoms, such as only sound distortion, or ear fullness. Because of the possibilities of undiagnosed acoustic tumors, people with unilateral symptoms, consistent with acoustic tumors, should be thoroughly evaluated by a Neurotologist, or someone familiar with these tumors, their characteristics, and treatment options.
Generally when diagnosed, they will either be small, medium, or large. The small tumors remain in the canal (intracanicular tumors). Medium sized tumors are no longer only in the canal, but spread into the cerebellopontine angle. Large tumors extend further and will indent the brainstem and/or cerebellum.
Many diagnostic procedures are used to make an accurate diagnosis and to determine the extent of the tumor. The diagnosis begins with a complete history, physical examination, and conventional audiologic evaluation. Additional testing of the hearing and balance nerves will be recommended if there are any indications of a tumor. These indications include asymmetry in hearing, sudden or rapidly progressive hearing loss, unilateral tinnitus, and/or dizziness. Electrophysiologic tests or tests that measure nerve responses through electrodes include the Auditory Brainstem Response (ABR) test, Electro/Videonystagmography (ENG/VNG), Vestibular Evoked Myogenic Potentials (VEMP), and Electroneuronography (ENoG). Imaging tests (using contrast) include Magnetic Resonance Imaging (MRI) and Computerized Tomography; these tests further examine the brain for masses or growths.
The ABR test analyzes the electrical activity of the hearing nerves on both sides to determine if there is normal conduction of nerve signals from the inner ear to the brain. If the conduction is slowed down on one side, this may indicate a tumor.
ENG/VNG testing is also used to test for acoustic tumors. If the balance mechanism is damaged on one side, this may be indicative of a tumor on that side. If abnormal, it usually represents superior compartment involvement.
VEMP testing will help evaluate involvement of the inferior vestibular nerve. This may be important when considering removal of small intracanicular tumors.
Since these tumors are closely associated with the seventh cranial nerve, which gives movement to the face, an ENoG may also be performed to look for weakness on the affected side. It can provide prognostic value in preservation of facial nerve function post-operatively.
Magnetic resonance imaging (MRI) and CT scanning examine (by picture) the internal auditory canal and cerebellopontine angle region of the brain to determine if there is a tumor present. These studies generally both require contrast, although the MRI scan can be performed with a technique avoiding contrast material.
Treatment
The treatment of a vestibular schwannoma must be individualized to each patient by an experienced team of doctors. The treatment is totally dependent on the size of the tumor, the age and health of the patient, and the status of the hearing. If an acoustic tumor is identified, three main treatment options exist: observation, radiation, and surgery.
Observation – Since these tumors frequently grow slowly, sometimes the recommendation for observation is made, particularly when patients are older and have other health problems making surgery a riskier undertaking.
Radiation – Stereotactic irradiation, with Gamma knife or Cyber knife treatment, is also an option for helping to shrink the size of the tumor. As with conventional surgery, risks still exist to the surrounding nerves and brain structures with the radiation treatment. This type of therapy will not completely kill all the tumor cells. It may slow the growth of the tumor by decreasing the blood supply. Both may also complicate future "salvage" surgery should the tumor continue to grow, after having irradiation.
Surgery – The other treatment option for acoustic tumors is surgery, which may allow for complete tumor removal (i.e. cure). Microsurgical removal of these tumors remains the treatment choice for most patients. The main goal of surgery is the preservation of life. The secondary goal is removal of the entire tumor with a minimum of future physical disturbances. To accomplish these goals, a team consisting of a neurotologist, neurosurgeon, anesthesiologist, specially trained surgical nurses, audiologists and sometimes internists are involved in the pre and postoperative care. The neurotologist and the neurosurgeon function as co-surgeons during the surgery. The patient is usually admitted to the intensive care unit for 24-48 hours postoperatively for close observation. Patients usually are in the hospital for a total of 4-7 days.
When considering surgery, the technique or approach chosen is depended upon the age or health of the patient, tumor size, status of hearing in the opposite ear and the hearing in the ear involved with the tumor.
The techniques or approaches are numerous and include the middle fossa approach, as well as the translabyrinthine, retro-labyrinthine, partial translabyrinthine, suboccipital, and transcochlear approaches. Each approach has both advantages and disadvantages. All are done under general anesthesia utilizing an operating microscope. The approach which offers the best chance of preservation of the facial nerve, with complete removal of the tumor is generally the translabyrinthine technique. This approach is through the inner ear structures and provides the best ability to remove the tumor completely and preserve the facial nerve. Hearing cannot be preserved with this technique. Hearing also cannot be preserved with the more anterior trans-cochlear approach, which is usually selected for larger tumors. The suboccipital approach requires retraction of the cerebellum and can preserve hearing in selected tumors. The disadvantage of this approach is the blind dissection within the internal auditory canal, which may either leave residual tumor in the canal, or cause risk of facial paralysis. Occasionally a combination of approaches is utilized with some acoustic tumors. In most instances, the tumor is completely removed without major deficits. However in rare instances complications can occur resulting in other cranial nerve or brain function deficits.
Tumor size and removal options
Small tumor
The small tumor is one in which the tumor is confined to the internal auditory canal (bony canal extending from the inner ear toward the brain). Contained in this canal are the hearing nerve (cochlear nerve), 2 balance nerves (superior and inferior vestibular nerves), as well as the facial nerve, which enables the face to move on that side. The surgical approach to a small tumor is usually the middle fossa approach, if the hearing is worth preserving, or the translabyrinthine approach, if the hearing is poor. The middle fossa approach involves shaving the side of the head and making an approach in front and above the ear. The translabyrinthine approach uses an incision behind the ear, and usually less hair is shaved.
In the vast majority of cases of small tumors, the tumor is totally removed. If the middle fossa approach is utilized in very small tumors, 50-65% of cases can retain useable or serviceable hearing post-operatively.
Medium tumor
In a medium sized tumor, there is extension from the bony canal towards the brain, but it has not yet put any pressure on the brain. A translabyrinthine or suboccipital approach is utilized. Also either the retrolabyrinthine or partial translabyrinthine approaches can be utilized with the suboccipital approach. If a translabyrinthine approach is selected, the incision is made behind the ear, over the mastoid bone. The mastoid and inner ear structures are removed to expose the tumor. This approach sacrifices the hearing and balance nerves in the operated ear, and the patient is made permanently deaf on that side. This approach allows complete removal of the tumor and has an advantage of providing maximum safety to the facial nerve. The balance mechanism of the opposite ear usually provides stability for the patient in approximately 4-6 weeks.
Large tumor
The large acoustic tumor has extended out of the bony canal, and pushes into the brainstem and cerebellum. The approach to a large acoustic tumor requires more extensive removal of the bone, to expose more of the tumor, and be able to control blood vessels. Generally, the translabyrinthine and/or suboccipital approach is utilized. The suboccipital approach uses a more posterior approach with a disadvantage of requiring more cerebellar retraction than the translabyrinthine technique. For those tumors which are quite large and have an anterior extension, the transcochlear approach is performed. A post-auricular incision is done and the external auditory canal is closed. All of the inner ear structures are removed. It does generally involve re-routing of the facial nerve, which will result in some facial weakness post-operatively, which may be minimal.
Partial vs. total removal of acoustic tumors
The goals of acoustic tumor surgery are preservation of life, removal of the entire tumor and protect as much cranial nerve and neurologic function as possible. Partial tumor removal is sometimes done, if the small remaining tumor is on the facial nerve or on an important blood vessel. Leaving a small portion of tumor in an older patient may prevent them from having a post-operative facial weakness, because further dissection around the facial nerve will likely traumatize the nerve. Also if a patient is older and has some cardiac or pulmonary problems, it would be desirable to get the patient out of the operating room as soon as possible. For example, surgery could be shortened, leaving some remaining tumor, if the patient’s response to surgery indicates a disturbance of any vital brain centers, such as respiration, blood pressure, or heart function. Decisions regarding leaving some tumor can sometimes be made pre-operatively, but usually are made at the time of surgery by the surgery team, and depend on the tumor characteristics and relationships to vital structures, patient age, patient’s medical status, and length of surgery. If termination of the operation is necessary without removal of the entire tumor, the remaining portion of the tumor may slowly enlarge to again produce symptoms. A subsequent operation can often then be accomplished without a significant change in vital signs, as there is a change in the relationship of the tumor to the brain. On occasion, there may be no further significant growth or symptoms related to the residual tumor, and it may not require any further surgery, but only observation.
Skull base tumors
The term "skull base tumors" refers to a group of tumors that have a tendency to grow along different regions of the bottom part of the skull, mostly on the inside but occasionally also on the outside of the skull. Several tumors that occur throughout the skull base include meningiomas, schwannomas, chordomas, glomus tumors, chondrosarcomas, and metastatic tumors, including malignant head and neck tumors. Skull base tumors are relatively rare. The reported frequency of the more commonly encountered tumors are as follows:
- Fewer than 10% of all primary intracranial tumors are schwannomas
- Approximately 0.1% of all intracranial tumors are chondrosarcomas
- Approximately 0.1% of all intracranial tumors are chordomas
Although these are more common types of skull base tumors, different tumors can exist and each surgical approach and procedure is different.
Meningiomas
Meningiomas are tumors that arises from the meninges — the membranes that surround your brain and spinal cord. The majority of meningioma cases are noncancerous (benign), though rarely a meningioma can be cancerous (malignant). Meningiomas can arise in the sphenoid bone and optic sheath. In the middle cranial base, meningiomas occur in the olfactory groove and planum tuberculum. Posterior fossa meningiomas often involve the petrous bone. Meningiomas can be divided into en plaque and en masse tumors. En plaque tumors are flatter and grow along the dura. En masse tumors bulge into the intracranial compartment and often have a dural tail on contrast-enhanced images.
Schwannomas
Schwannomas are also referred to as neuromas, neurinomas, and neurilemomas. These tumors almost always develop from sensory nerves. Because the olfactory and optic nerves do not have a Schwann cell layer, they do not develop these tumors. The most common intracranial schwannoma develops from the vestibular nerve and occupies the posterior cranial fossa. It is referred to as a "vestibular schwannoma" or "acoustic neuroma." The second most common intracranial schwannomas develop from the trigeminal nerve and account for fewer than 8% of intracranial schwannomas. Trigeminal schwannomas usually arise from the root or ganglion and occupy the middle fossa and, sometimes, the posterior fossa. These schwannomas also may occupy both the middle and posterior fossa with a dumbbell shape. Schwannomas of the other cranial nerves are rare.
Chordomas
These tumors come from remnants of a fetal spinal cord that dissolves as the fetus develops. The clivus is the second most common location, after the sacrococcygeal region, for chordomas. They may develop in persons of any age, but they manifest most commonly in persons aged 20-40 years. A slight male predominance exists. Brain or dural invasion is possible, but usually occurs late in the course with aggressive or recurrent tumors if a dural defect was left after the initial operation. Chordomas rarely metastasize, but they often invade local structures; therefore, patients may die from the regional spread of the tumor. Approximately 10% of chordomas show malignant histologic features, which may be related to previous irradiation.
Glomus tumors
Another benign tumor found in the posterior skull base is the "glomus tumor" or paraganglioma. The paraganglia are small aggregates of cells derived from embryonic neuroepithelium that is distributed throughout the body. These tumors are closely associated with the sympathetic system (fight or flight nervous system). Most of the paragangliomas of the skull base are found along the course of the tympanic branch of cranial nerve IX, in the adventitia of the jugular bulb, in the tympanic canaliculus, or along the lesser petrosal nerve. These tumors are slow-growing hypervascular tumors that usually occur in the temporal bone. Patients usually present with gradual hearing loss, unilateral pulsatile tinnitus, and lower cranial nerve deficits. Approximately 1-3% of paragangliomas produce catecholamines, and they may be locally invasive but rarely metastasize.
Chondrosarcomas
Chondrosarcomas can occur anywhere in the skeletal system, including intracranially at the petrosphenoclival junction. Most commonly, patients are diagnosed with chondrosarcomas during the third or fourth decade of life. Males are affected more often than females. Chondrosarcomas can be divided into classic, dedifferentiated and mesenchymal tumors.
Classic chondrosarcomas are composed of large cells with single or multiple nuclei. Microscopically, on low power, abundant cartilaginous matrix is seen. These tumors can be subdivided into 3 grades (I-III). The lower-grade tumors are less aggressive, have minimal malignant potential, and act clinically similar to chordomas. These tumors occur most commonly as a subtype of chondrosarcomas, and they usually carry a good prognosis.
The dedifferentiated and mesenchymal subtypes occur less frequently and are both more aggressive tumors. The dedifferentiated variety has features in common with an anaplastic sarcoma. The mesenchymal subtype has islands of undifferentiated mesenchymal cells and islands of cartilage.
Symptoms of skull base tumors
The symptoms are completely dependent on the size and location of the tumor. When the tumors are smaller, patients may have only headaches, dizziness or hearing changes. However as the tumors grow symptoms increase in severity such as a complete loss of vision, total hearing loss, facial paralysis, or diminished cognitive function. Only through a thorough evaluation and appropriate diagnostic tests can these tumors be effectively evaluated.
Diagnosis
As always, the first steps in diagnosis is a careful history and physical examination. A CT and/or MRI scan may be performed without and with intravenous contrast enhancement to define these tumors. Additional testing includes: sophisticated audiometric testing, balance testing, and vision testing.
Treatment
Surgery is generally required with a team of physicians including neurosurgeons, ophthalmologists, internists and others. There are numerous approaches utilized, depending on the size and location of the tumor, as well as the health and neurologic status of the patient. In addition, consideration is made regarding the hearing status in both ears. Lastly the approach is based on the skill and experience of the surgical team. Further treatment can also include radiation therapy and/or stereotactic radiosurgery