Acoustic Neuromas
What is an acoustic neuroma?
An acoustic neuroma is a benign tumor that can affect the balance and hearing nerves behind the inner ear. Tumors are classified as benign or malignant based upon their growth characteristics. Benign tumors generally grow slowly and do not spread throughout the body. When they grow, they usually invade or push adjacent structures aside. Unlike benign tumors, malignant tumors like breast cancer, colon cancer, lung cancer and prostate cancer generally grow faster and can spread to other regions of the body.
Where are these acoustic neuromas?
A brief description of the anatomy of this region is described. Sound entering the ear canal causes vibrations of the tympanic membrane (eardrum). The eardrum is attached to one of three ossicles (bones) found in the middle ear and it is called the malleus. The malleus bone transmits the mechanical vibrations to another ossicle, the incus, which in turn transmits the vibrations to the smallest of the three ossicles, the stapes. The stapes bone vibrates and sends the message to the inner ear. The inner ear is made of a bony labyrinth filled with fluid and membranes. As the stapes vibrates, the fluid in the inner ear is set into motion. The cilia on the ends of hair cells within the inner ear are bent and an electrical signal is generated and travels along the (cochlear) hearing nerve and then back to the brain. The inner ear is composed of the cochlea, which is responsible for hearing and the semicircular canals that convey balance information concerning angular acceleration of the head back to the brain. Two other organs, the saccule and utricle, found in the inner ear report on the linear acceleration of the head.
The internal auditory canal contains three different types of nerves, the hearing, balance and facial nerve. The facial nerve is responsible for conducting information concerning motion of the face on that side. This is the nerve that is responsible for our ability to raise our eyebrows, close our eyes, flare our nostrils and raise the corner of the mouth on that side. It is also responsible for conducting information regarding taste from the front 2/3 of the tongue and also tear production from the lacrimal glands in our eyes.
The superior and inferior vestibular nerves (or upper and lower balance nerves) convey information regarding balance function from a number of balance organs including the semicircular canals, utricle and saccule. Peripherally, i.e. closer to the inner ear, the superior and inferior vestibular nerves are coated by a layer of Schwann cells which produce a substance called myelin, which allow efficient conduction of nerve impulses. Centrally, i.e. closer to the brainstem, these nerves are coated by a layer of cells called oligodendroglia, which have the same function as Schwann cells. The junction where the Schwann cells meet the oligidendroglia cells is termed the Obersteiner-Redlich zone. Acoustic neuromas arise from Schwann cells, located near the Obersteiner-Redlich zone, and are more appropriately termed vestibular schwannomas.