The Anatomy of the Inner Ear

The inner ear is the innermost part of the ear that plays an important role in hearing and balance. The inner ear consists of tiny bony structures filled with fluid. As sound waves travel from the outer to the inner ear, they create waves in the fluid of the inner ear, which in turn moves the tiny hairs in the ear that send sound or movement signals to the brain.

Problems with this part of the ear can result in hearing loss and balance issues. Inner ear problems are one of the primary causes of vertigo.

Anatomy

Structure

The ear is made up of the outer ear, middle ear, and inner ear. The inner ear consists of the bony labyrinth and membranous labyrinth. The bony labyrinth comprises three components:

• Cochlea: The cochlea is made of a hollow bone shaped like a snail and divided into two chambers by a membrane. The chambers are full of fluid, which vibrates when sound comes in and causes the 30,000 tiny hairs lining the membrane to vibrate and send electrical impulses (sound signals) to the brain. The cochlea is about 9 millimeters wide at its widest point, and about 5 millimeters tall. If it could be uncoiled, the cochlea would be about 30 millimeters long.
• Semicircular canals: Also known as the labyrinthine, the semicircular canals rest on top of the cochlea, connected by the vestibule. There are three of them, and they line up at 90-degree angles to one another, which allows the the brain to know which direction the head is moving. Like the cochlea, these canals are filled with fluid. They also contain small calcium crystals and tiny hairs that sense the movement of the fluid. The size of the canals is difficult to measure since they are surrounded by bone
• Vestibule: The vestibule is the central part of the bony labyrinth. It is separated from the middle ear by the oval window, and communicates anteriorly with the cochlea and posteriorly with the semicircular canals

Inside the bony labyrinth lies the membranous labyrinth, which is also made up of three parts:

• Cochlear duct: This triangle-shaped duct is located inside the bony labyrinth and creates two canals that sit above and below it. These two canals—the scala vestibuli above the duct and the scala tympani below it—are separated from the main duct by membranes. The membrane between the cochlear duct and the scala tympani—also known as the basilar membrane—is where the primary hearing organ, the Organ of Corti, is located. The upper membrane is called Reissner's membrane, which helps control the flow of fluid from the duct to the scala vestibuli
• Semicircular ducts: This is where fluid, called endolymph, changes speed and direction when you move your head. Sensory receptors in these ducts detects this change and send information to your brain to help you maintain balance
• Utricle and saccule: These sacs are within the membranous labyrinth and work to detect vertical and horizontal positioning. Endolymph enters and exits these sacs

The final structural pieces of the inner ear are the seventh and eighth cranial nerves, which carry information about sound and balance to the brain:

• Cranial nerve 7: Also called the facial nerve, this nerve provides information to the brain on sensory and motor function. One section of the nerve controls the motor function of some part of the face, and another controls sensory information related to taste and hearing. Specifically, this nerve carries information to the brain from the tympanic membrane.
• Cranial nerve 8: Another cranial nerve, the eighth cranial nerve is also called the vestibulocochlear nerve. It combines vestibular and cochlear nerve fibers. The vestibular nerve fibers provide information to the brain about balance, while the cochlear nerve fibers carry information about sound. Both are sensory nerve fibers. Damage to this nerve could impact both hearing and balance

Location

The inner ear lies directly next to the middle ear. It is encased by the temporal bone, or the part of the skull at surrounds the ear at each side of the head. The portion of the temporal bone that houses the inner hear is the most dense portion of this bone.

Above the inner ear, also housed in the temporal bone, is the temporal lobe of the brain. This is the portion of the brain that processes sound and speech, as well as some vision, emotion, and memory.

Anatomical Variations

Some problems with the inner ear are congenital defects, problems present at birth. About 80% percent of congenital hearing loss is caused by problems with the formation of membranes in the inner ear, usually involving the tiny hairs that line the cochlea. A smaller number of congenital hearing loss is caused by malformations of the bony labyrinth in the inner ear.

Function

The purpose of the inner ear is to sense and process information about sound and balance, and send that information to the brain. Each part of the inner ear has a specific function.

• Cochlea: The cochlea is responsible for hearing. It is made up of several layers, with the Organ of Corti at the center. This is the portion of the cochlea that is lined with tiny hairs called cilia. Cilia move as sound waves pass through the fluid in the cochlea, and this movement is translated into an electric impulse that is passed from the inner ear to the brain through the eighth cranial nerve
• Semicircular canals: The semicircular canals are a three-loop system that uses the shifting of fluid to give the brain information about balance, motion, and how the head is positioned relative to surroundings. The first canal is responsible for sensing up-and-down movement. The second canal senses side-to-side movements, and the third canal signals to the brain when the head is tilted. Each canal contains fluid and tiny hairs. As fluid shifts through the canals with movement of the head, these tiny hairs send messages to the brain through the seventh cranial nerve
• Vestibule: The main function of the vestibule is to detect changes in gravity and up-and-down movement. For example, it is responsible for sensing if you are standing up straight or upside-down

Associated Conditions

A number of conditions can result from problems within the middle ear. Most of these problems involve hearing loss or a disruption of balance.

When hearing loss is the problem, the symptoms should be clear. Hearing can be reduced or even gone completely. When inner ear problems cause balance issues, the symptoms may include:

• Dizziness
• Vertigo
• Motion sickness
• Nausea or vomiting
• Unsteadiness or frequent falls
• Lightheadedness
• Headaches

Specific conditions that can result from problems in the inner ear are:

• Acoustic neuroma: An acoustic neuroma is a slow-growing, noncancerous tumor that forms on the nerve that runs from the inner ear to the brain. In some cases, the tumor may grow large enough that it puts pressure on the nerve and causes symptoms like dizziness, hearing loss, balance problems, or tinnitus (ringing in the ear). Since these tumors are usually slow-growing and don’t spread to other organs, your healthcare provider may just monitor its growth and your symptoms. If the tumor begins to cause problems, radiation and surgical removal of the tumor may be recommended.
• Benign paroxysmal positional vertigo (BPPV): This condition is also known as positional vertigo. It is the most common form of vertigo, and can result in intense dizziness with a change in the position of the head. There is no real known cause for this condition outside of a head injury. Symptoms rarely become serious, and are more annoying or uncomfortable. Your healthcare provider may use repositioning techniques called canalith repositioning to reduce your symptoms, or a plug can be surgically placed to block fluid flow in the area causing the problem. Surgical plugging is about 90% effective at treating this condition.
• Hearing loss: In general, problems with the inner ear’s function or structure can cause hearing loss or reduction. If the problem lies in the structure of the cochlea, a cochlear implant may be able to help. In other cases of hearing loss, treatments vary, but hearing aids can help reduce the impact of the problem.
• Ménière's disease: Ménière's disease involves the pressure of the fluid in the inner ear. Although the exact cause of this disease isn’t known, the development of Ménière's disease has been linked to poor fluid drainage, an abnormal immune response, family history, or viral infections. Ménière's disease usually begins in one ear, but can sometimes spread to both ears. Symptoms include vertigo and dizziness, hearing loss, tinnitus, and intense feelings of pressure or fullness in the ear. Treatments center on symptom management, such as using medications to control nausea or dizziness. Your healthcare provider may also use physical therapy, hearing aids, diuretics, or positive pressure therapy to help correct to alleviate your symptoms. In severe cases, surgery may be used to reduce the level of fluid in the inner ear and relieve pressure.
• Vestibular neuritis and labyrinthitis: This condition occurs when the nerve that sends signals to the brain about motion and balance becomes inflamed or swollen. Thought to originate from viral infections in the ear or other body parts, this swelling and inflammation make it difficult to send information about balance to the brain. Symptoms include problems with balance, dizziness, and concentration. Treatment for this problem involves treating the underlying viral cause if there is one, managing symptoms like nausea or dizziness, and physical therapy to help restore balance.
• Superior semicircular canal dehiscence (SSCD): This is a rare condition where the bony area that covers the semicircular canal is reduced or absent altogether. Symptoms of this condition include vertigo, hearing loss, a feeling of pressure in the ears, and even problems blinking or breathing. This condition can be repaired with surgery to fill and resurface the area of missing bone.
• Ménière's disease: Ménière's disease involves the pressure of the fluid in the inner ear. Although the exact cause of this disease isn’t known, the development of Ménière's disease has been linked to poor fluid drainage, an abnormal immune response, family history, or viral infections. Ménière's disease usually begins in one ear, but can sometimes spread to both ears. Symptoms include vertigo and dizziness, hearing loss, tinnitus, and intense feelings of pressure or fullness in the ear. Treatments center on symptom management, such as using medications to control nausea or dizziness. Your healthcare provider may also use physical therapy, hearing aids, diuretics, or positive pressure therapy to help correct to alleviate your symptoms. In severe cases, surgery may be used to reduce the level of fluid in the inner ear and relieve pressure.
• Vestibular neuritis and labyrinthitis: This condition occurs when the nerve that sends signals to the brain about motion and balance becomes inflamed or swollen. Thought to originate from viral infections in the ear or other body parts, this swelling and inflammation make it difficult to send information about balance to the brain. Symptoms include problems with balance, dizziness, and concentration. Treatment for this problem involves treating the underlying viral cause if there is one, managing symptoms like nausea or dizziness, and physical therapy to help restore balance.
• Superior semicircular canal dehiscence (SSCD): This is a rare condition where the bony area that covers the semicircular canal is reduced or absent altogether. Symptoms of this condition include vertigo, hearing loss, a feeling of pressure in the ears, and even problems blinking or breathing. This condition can be repaired with surgery to fill and resurface the area of missing bone.

Tests

Testing for inner ear problems will focus on checking for structural problems, hearing loss, or balance issues. When the structure of the inner ear is suspected to be causing a problem, imaging tests like computed tomography (CT) or magnetic resonance imaging (MRI) are most effective at identifying the origin of the issue.

An audiologist may perform a series of exams using different sounds and tones to test your hearing, including: 

• Pure-tone test: This test is also called an audiometry test. You will hear a variety of sounds played through headphones. This test helps find the quietest sounds you can hear at different pitches
• Tuning fork tests: A two-pronged metal fork is placed behind your ear or on the top of the head. The provider performing the test will strike the fork to make a tone. This test can show if there is hearing loss in one or both ears. It can also show which type of hearing loss you have

For balance, testing is a bit different and usually involves a series of tests. You may undergo one of the following exams:

• Vestibular testing: This includes a series of examinations to test how well your vestibular system is working, including electronystagmography or videonystagmography. Both of these tests record eye movements in different situations, and these movements can provide clues to the function of the inner ear. Most forms of vestibular testing could make your vertigo or dizziness worse. It’s a good idea to bring someone to drive you home after the test
• Computerized dynamic posturography: This test evaluates three senses—vestibular, somatosensory, and vision. When these senses are functioning well together, you have good balance. This test, combined with results of other tests, allows your healthcare provider to evaluate which sense is not functioning and target treatment. During this test, you will stand on a platform barefoot, secured with a safety harness. Your balance will then be tested in a variety of situations
• Vestibular evoked myogenic potential: Electrodes will be placed on your forehead, under your eyes, on your neck, and on your collarbone. Headphones are placed in your ear, and a loud clicking noise is played while you look up or over your shoulder. How strongly your respond to the sound will indicate how well your vestibular cranial nerves are functioning
• Video head impulse testing: Special goggles are placed on your face, and you will be asked to track targets with your eyes. An audiologist will then produce certain movements to stimulate different parts of the inner ear, measuring and comparing your response to the different movements

If you are concerned about your hearing or balance, you should call your healthcare provider. These tests can provide your practitioner with information to determine what your next steps should be.