Acoustic neuroma is a benign neoplasm of the VIII cranial nerve, consisting of cells of the Schwann shell. It is clinically manifested by hearing loss, noise and ringing in the ear, vestibular disorders on the affected side, symptoms of compression of the facial, trigeminal, abductor nerves, brain stem and cerebellum, signs of intracranial hypertension and hydrocephalus. Acoustic neuroma is diagnosed by X-ray of the temporal bones, MRI or CT of the brain. Depending on the size of the formation, its surgical and radiosurgical removal, radiotherapy is possible. In some cases, it is advisable to monitor the tumor in dynamics and resolve the issue of treatment tactics only if progressive growth of the formation is detected.
General information
The VIII pair of PMN — the vestibular-cochlear, or auditory, nerve consists of the vestibular and auditory parts. The first carries information to the cerebral centers from the vestibular receptors of the cochlea, the second — from the auditory. In the vast majority of cases, neurinoma develops in the vestibular part of the vestibular cochlear nerve, and the symptoms of damage to the auditory department are associated with its compression by a tumor. Near the vestibular-cochlear nerve pass: the trunk of the facial nerve, the trigeminal nerve, the abductor, the lingopharyngeal and vagus nerves. As the neurinoma grows in the clinical picture, symptoms of compression of these nerves, as well as adjacent structures of the brain stem, may occur.
Acoustic neuroma originates from the Schwann cells surrounding the axons of nerve fibers. In this regard, in practical neurology, it is also known as vestibular (acoustic) schwannoma. The incidence of the tumor is approximately 1 case per 100 thousand people. At the same time, acoustic neuroma accounts for 12-13% of all cerebral tumors and about 1/3 of the tumors of the posterior cranial fossa. It develops mainly at the age of 30-40 years. There were no cases of the disease in children of the pre-puberty period.
Etiopathogenesis
The etiology of the auditory nerve neurinomas remains uncertain. Unilateral neurinomas are sporadic in nature, their direct connection with any etiofactors is not traced. Bilateral neurinomas are observed in many patients with neurofibromatosis type II — a genetically determined disease, for which benign tumor processes of various tissue structures of the nervous system (neurofibromas, gliomas, meningiomas, neurinomas) are typical. Neurofibromatosis is inherited in an autosomal dominant way, the risk of offspring disease in the presence of a pathological gene in both parents is 50%.
Pathogenetically, there are 3 stages of development of vestibular schwannoma. In the first, the small size of the formation (up to 2.5 cm) leads to hearing impairment (hearing loss) and vestibular disorders. In the second, the neoplasm grows to the size of a walnut, exerts pressure on the brain stem, which leads to the appearance of nystagmus, discoordination of movements and balance disorders. In the third stage, when the neurinoma reaches the size of a chicken egg, it causes a sharp compression of cerebral structures, hydrocephalus, visual disturbances and swallowing. At this stage, irreversible changes occur in the brain tissues, the tumor is inoperable and is the cause of death.
Morphology
Macroscopically, the neoplasm is a rounded or irregularly shaped dense node with a bumpy surface. From the outside it has a connective tissue capsule, inside there may be local or diffuse cystic cavities filled with brownish liquid. The color of the formation on the slice depends on its blood supply: usually pale pink with rusty areas, with venous stagnation – cyanotic, with hemorrhages in the tissue of the neurinoma — brownish brown.
Microscopically, acoustic neuroma consists of cells whose nuclei resemble rods. These cells form structures like palisades, between which there are areas consisting of fibers. As the tumor grows, fibrosis processes and the formation of hemosiderin deposits are observed in it.
Acoustic neuroma symptoms
The slow growth of vestibular schwannoma causes the presence of some asymptomatic period and the gradual development of the clinic. In 95% of patients, the first sign is a gradually progressive deterioration of hearing. In some cases, hearing loss occurs abruptly and suddenly. In 60% of patients, the first complaint is the appearance of noise or ringing in the ears. In its absence and the unilateral nature of the lesion of the auditory nerve, patients often do not notice the ongoing hearing impairment for a long time. Vestibular disorders are noted in 2/3 of cases. They are characterized by a feeling of instability or dizziness when turning the trunk and head, the appearance of nystagmus. Sometimes there are vestibular crises, in which nausea occurs against the background of dizziness and there may be vomiting. Acoustic neuroma in the initial stage can be mistaken for cochlear neuritis, Meniere’s disease, otosclerosis, labyrinthitis.
Progressive tumor growth eventually leads to complete deafness on the side of the lesion, the addition of symptoms of damage to nearby structures. However, it should be remembered that the severity of symptoms does not always correlate with the size of the tumor. Depending on the localization of the neurinoma and the direction of its growth, with small sizes it can give a more severe picture than a large neoplasm, and vice versa.
First of all, acoustic neuroma causes compression of the trigeminal nerve, which is accompanied by facial pain and paresthesia on the side of the tumor. Facial pains have a dull, aching character; at first they occur as paroxysms, and then they become permanent. Sometimes they are mistaken for toothache or trigeminal neuralgia. A little later or simultaneously with facial pain, symptoms of peripheral damage to the facial nerve occur (paresis of facial muscles and associated facial asymmetry, violation of salivation, loss of taste on the anterior 2/3 of the tongue) and the abductor nerve (diplopia, converging strabismus). If acoustic neuroma is located in the internal auditory canal, then the symptoms of compression of the facial nerve may manifest in the early period of the disease. In such cases, it is necessary to exclude neuritis of the facial nerve.
A further increase in neurinoma leads to damage to the vagus and pharyngeal nerves with impaired phonation, dysphagia, loss of taste on the posterior 1/3 of the tongue and extinction of the pharyngeal reflex. When the cerebellum is compressed, cerebellar ataxia appears. Even in advanced cases, with compression of the brain stem, sensory and motor conduction disorders are extremely poorly expressed; paresis is noted in exceptional cases.
In the third stage, acoustic neuroma is characterized by signs of intracranial hypertension. There is a headache in the back of the head and frontal area, accompanied by vomiting. During ophthalmoscopy, stagnant discs of the optic nerves are noted. Perimetry can reveal individual scotomas or hemianopsia, which is associated with compression of the chiasm and optical tracts.
Diagnostics
The diagnosis is carried out by an otoneurologist, in his absence, a neurologist together with an otolaryngologist. In some situations, an additional consultation of a vestibulologist, ophthalmologist and dentist is required. The patient is undergoing neurological examination, audiometry, otoscopy, electrocochleography, electronystagmography, auditory VP examination, vestibulometry, stabilography.
Radiography and neuroimaging methods help to more accurately establish the diagnosis of “auditory nerve neurinoma”. In the early stages, with a small size of a neurinoma (up to 1 cm), it is usually not visualized using a CT scan of the brain. Therefore, an X-ray of the skull is performed with a targeted image of the temporal bone. The diagnosis of vestibular schwannoma is confirmed by the expansion of the internal auditory canal. Since neurinomas absorb contrast well, it is possible to use CT with contrast. At the same time, the formation with clear smooth contours is visualized.
MRI of the brain in the case of neurinoma reveals hypo- or isointensive on T1-weighted images, and hyperintensive on T2-weighted images. Tumors with a size of 3 cm or more are characterized by signal heterogeneity associated with the presence of cystic areas in them. It is possible to visualize the deformation of the brain stem and cerebellum. When contrast MR tomography is performed, heterogeneous accumulation of contrast is observed in 70%.
Treatment
A radical method of treatment is the removal of a neurinoma, which can be performed by open surgery or radiosurgery. Surgical removal is advisable for a large tumor, if an increase in its size is detected in the dynamics of observations, with the growth of a neuroma after radiosurgical intervention. Deafness and paresis of the facial nerve are often the result of surgery. Stereotactic radiosurgical removal is possible for neurinomas smaller than 3 cm. It is also performed in elderly patients with prolonged growth after subtotal resection and in cases where the risk of surgery is significantly increased due to somatic pathology.
Radiation therapy of neurinoma has indications similar to indications for radiosurgery. Irradiation is not a way to remove the formation, but prevents its further growth and avoids surgery. Patients with neurinoma accidentally detected by CT or MRI without clinical symptoms, patients with long-term hearing disorders and elderly patients with mild symptoms are shown a wait-and-see tactic with constant monitoring of the size of the formation and dynamics of clinical symptoms.
Forecast
The outcome of a neurinoma largely depends on the timeliness of diagnosis and the size of the tumor. The prognosis is favorable with adequate treatment of vestibular schwannoma in stages I and II. With radiosurgical removal in the early stages, 95% of the growth is stopped and the patient’s ability to work is fully restored. With surgical intervention, the risk of hearing loss and damage to the facial nerve is high. In stage III of neurinoma, the prognosis is unfavorable: the patient may die when the vital cerebral structures are compressed by an increasing tumor.