What Are the Semicircular Ear Canals?
The Semicircular Canals
Where are the Semicircular Canals Located?
The semicircular canals part of the inner ear, and are located in the bony labyrinth of the mastoid bone. The bony labyrinth contains the cochlea (responsible for hearing), the vestibule (between the cochlea and canals), and the semicircular canals (responsible for balance).
There are three semicircular canals in humans:
- The lateral semicircular canal, located on a horizontal axis.
- The superior semicircular canal, positioned in a vertical plane that intersects with the posterior canal at a 90° angle.
- The posterior semicircular canal, positioned in a vertical plane that intersects with the superior canal at a 90° angle.
The Purpose of Semicircular Canals
- The semicircular canals detect movement and acceleration, which are interpreted by the brain to create a sense of balance.
Organs of Balance
What is the Purpose of the Semicircular Canals?
The inner ear is responsible for two senses: hearing and balance. The cochlea is the hearing organ of the ear, and the semicircular canals are the balance organ of the ear. The semicircular canals are filled with a fluid called endolymph. When the head changes position, movement of the endolymph fluid moves past tiny hair cells called cilia in the canals. Tiny, floating particles called otoliths are suspended in the endolympathic fluid, and the movement of these "crystals" enhance the stimulation of the cilia in the canals.
The semicircular canals are oriented in opposite directions on each side of the head. The lateral canals are responsible for detecting motion on a horizontal plane. When a person is spun in a chair, the lateral canals are stimulated to allow the body to know it is moving. The posterior and superior canals detect vertical head motion and rolling sensations.
Vestibular Sensory Illusion in Pilots
Aviation Graveyard Spiral
Vestibular Function and Balance
The entire vestibule and semicircular canal system is sometimes referred to as the vestibular system. This system connects to the auditory nerve and sends information about motion and acceleration to the brain. The brain, in turn, uses this information to create a sense of balance.
This system allows humans to stand upright, walk, and correct for changes in elevation. We know when we are spinning, dropping, or falling over. Sometimes, the vestibular system can lead humans astray, as there can be a perception of "leaning" or "turning," even when the human body is headed in a straight direction.
In aviation, there is a phenomenon known as the Graveyard Spiral - the result of a vestibular illusion. Pilots will turn an aircraft and will sense the craft banking into the turn. After a period of time, the inner ear adjusts to this sensation and the "leaning" sensation is lost. When the pilot straightens out the aircraft, it feels like the plane is banking again. If the pilot is not reading the gauges and is flying by sensation alone, he or she will pull back on the yoke and the plane will turn more (which will feel like "going straight" to the pilot). If this cycle continues, the plane will describe tighter and tighter circles, while descending at an ever-increasing rate.
The Graveyard Spiral is often the result of poor weather, as there is no visual indicator of the horizon to aid the pilot's spatial awareness. Pilots must complete "instrument only" training to prevent this phenomenon - when the weather is poor, pilots must fly by the instruments to prevent sensory illusions from occurring.
The Semicircular Canals and Balance
Superior Canal Dehiscence
Rotational Chair Testing
There are many vestibular disorders that cause dizziness and imbalance in humans. The primary disorders of the balance system in the inner ear are:
- Ménière's Disease: in Ménière's Disease, an abnormal amount of endolymphatic fluid accumulates in the inner ear. People with this condition will experience bouts of vertigo, may have tinnitus, vomiting, a rapid heart rate with anxiety, and trembling. The late stage of the disease results in hearing loss, which affects the low frequencies first and may progress to significant levels of hearing loss.
- Benign Peroxysmal Positional Vertigo is not a disease or disorder, but will cause a spinning sensation occasionally with head movement. This happens when the otoliths (otoconia) are dislodged and send a disproportionate signal for movement. Symptoms should resolve over time.
- Labyrinthitis occurs when there is an infection of the inner ear. This may cause balance problems, hearing loss, and visual disturbances.
- Superior Canal Dehiscense is caused by an opening in the bone above the semicircular canals. This condition will create the perception that an object is moving, even when it is stationary. A person with SCD may hear their own voice as abnormally loud. This problem can be surgically corrected if severe, though hearing loss may result from the operation.
- Ototoxicity is caused by drugs that are harmful to the inner ear. Some drugs cause hearing loss, some result in vestibular damage, and others affect both the hearing (cochlea) and balance systems. Drugs like Gentimicin and other aminoglycosides are known to affect the hearing and balance system of the inner ear.
- Acoustic Neuroma: a tumor that grows on the auditory nerves connected to the cochlea and/or balance system. These tumors are benign (non-malignant), but pose a serious threat to hearing and balance. Surgery may be recommended to remove the tumor, or radiation may be used to try to shrink the tumor.
- Perilymph Fistula occurs when there is a tear in the fine membrane between the middle ear and inner ear. This tear allows air pressure to affect the endolymphatic fluid and cause severe vertigo and hearing loss. Bed rest to allow the fistula to heal and surgery are options for treatment.
- Secondary Endolymphatic Hydrops has symptoms similar to Ménière's Disease, but is caused by another underlying disorder. Autoimmune disorders or head trauma may cause an excessive amount of endolymphatic fluid to collect in the inner ear.
- Mal de Débarquement is the feeling of motion one experiences on dry land after being at sea for an extended period of time. This problem is most likely caused by the brain rather than the inner ear - the brain's balance centers adjust for the continuous motion of being at sea, and continue to misread signals once the motion has stopped for a significant period of time.
- Enlarged Vestibular Aqueducts is a congenital disorder caused by a wider-than-normal aqueduct between the vestibular system and the cochlea. This condition is associated with a fluctuating, progressive hearing loss and balance disorders. Children with this condition may walk at a later age, as they must learn to visually compensate for the lack of balance information from the inner ear.
- Autoimmune Inner Ear Disorder is caused when the immune system attacks the inner ear. Byproducts of autoimmune diseases like Cogan's Disease can circulate through the body and end up in the endolymphatic fluid of the inner ear. This debris can cause vertigo and balance problems.
- Allergies and Middle Ear Infections may cause temporary balance problems and vertigo.
Testing the Vestibular System
What the Test Measures
How the Test is Performed
This test can determine if balance issues are due to vestibular or neurological problems.
The patient is spun in a chair and eye movements are tracked. Other versions of this test include fixation testing (fixating on a dot that moves with the spinning person) and optokinetic testing, where the patient tracks moving stripes with his or her eyes.
ENG/VNG testing will determine if a balance problem is vestibular in nature.
Eye movements and object tracking are performed first. Then the eyes are observed as the head is moved, to determine if nystagmus is present. Caloric testing is then performed with hot and cold air in the ear, while monitoring for nystagmus.
Computerized Dynamic Posturography
This test determines the source of a balance problem (motor, vestibular, or central)
The patient stands on a plate that moves, and the computer records the patient's efforts to compensate.
auditory brainstem response (ABR), otoacoustic emissions, and vestibular evoked myogenic potentials may detect the source of a vestibular problem.
These tests are objective, and require no cooperation from the patient. Brainwaves, otoacoustic responses, and muscle action potentials are measured.
The patient listens to pure tones at controlled volues in a sound booth.
Dynamic Visual Acuity Testing
Visual acuity when then the head is in motion.
The patient reads a standard eye chart while moving.
Vestibular System Sensory Issues
Development of the Sense of Balance
The vestibular system is the first sensorimotor system to become fully myelinated as a fetus develops. The fetal vestibular system receives stimulation from the mother's movements. Babies are typically born with a fully developed system and begin to hone their sense of balance over time.
Occasionally, children do not have a functional or well-integrated vestibular system. Pediatric balance problems can stem from enlarged vestibular aqueducts or other inner ear anomalies. Some children have sensory integration problems and may seek vestibular stimulation by spinning or swinging frequently.
Children with sensory integration problems in the vestibular pathway will often "zone out" when they are forced to sit still. These children often need to rock, jump, or move to concentrate on tasks. Adults might see these children as "hyper" when these children are simply trying to stimulate their vestibular systems.
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