Tumor Types > Acoustic Neurinomas
Acoustic neurinomas are benign tumors which start in the vestibular portion of the VIIIth cranial nerve. The term "acoustic neuroma" is actually a misnomer since it this a primary intracranial tumor of the myelin forming cells called "Schwann cells" (schwannoma).
Acoustic neurinomas usually start in the porus acousticus, the internal auditory canal. The VIIIth cranial nerve and the facial nerve (nerve that activates muscles of the face) run from the brainstem and pass through the porus accousticus, forming a connection to the inner ear (by way of the VIIIth nerve) and to the facial muscles (by way of the facial - or VIIth - nerve). From there, the VIIth and VIIIth nerves grow into the posterior fossa, an area of the skull's interior where the brainstem and cerebellum are housed.
The VIIIth nerve has two major parts: the auditory nerve (nerve associated with hearing) and the superior and inferior vestibular nerves. As the acoustinc neurinoma grows, patients will occasionally feel dizzy. More significantly, they will start to notice a reduction of hearing on the affected side. This usually starts with a reduction of speach discrimination. Frequently, these patients will give a history of moving the telephone from one ear to the other because of difficulties in speech comprehension in that ear. As the tumor increases in mass, it compresses the auditory nerve further, and hearing continues to be diminished until the patient reaches the point of total deafness.
Untreated, acoustic neurinomas will expand all the way into the posterior fossa, potentially affecting other cranial nerves such as the trigeminal nerve, which carries sensation from that side of the face. This would explain why some patients may notice numbness of one side of the face along with the hearing loss. If the tumor extends downward toward the base of the skull, the lower cranial nerves can be affected and this may influence swallowing. Pressure on the brainstem can result in clumsiness of the hands and arms. Eventually, this pressure will cause an obstruction of the 4th ventricle which carries ventricular fluid from the interior of the brain to the outside of the brain. Ventricular fluid is still being produced and so it builds up within the ventricular system. This condition is called hydrocephalus, and is fatal if left untreated.