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The internal ear (also known as the inner ear, Latin: auris interna) is an essential part of the auditory and vestibular system consisting of cavities and canals. The inner ear is where eventually sound waves are converted into electrical impulses.
The internal ear structures are located within the petrous part of the temporal bone. The inner ear consists of two parts - the bony labyrinth and the membranous labyrinth.
The bony labyrinth of the internal ear is comprised of the vestibule, semicircular canals, and cochlea. The membranous labyrinth, which is enclosed by the bony labyrinth, forms tiny sacs (saccule and utricle) and tubules (semicircular ducts and cochlear duct).
Endolypmh and perilymph
The cochlea and its components contain two different types of fluid: endolymph and perilymph. Endolypmh has a specific and unique composition that is not found anywhere else in the body, while perilymph is similar to the extracellular fluid.
Endolymph is found in the scala media or the cochlear duct. Endolymph is rich in potassium and is created from perilymph. Its potassium concentration is higher than its sodium ion concentration. How endolymph is formed is not yet understood.
It is known that endolymph may be created from perilymph resulting in selective ion transport through the Reissner's membrane (separates endolymph from perilymph). The stria vascularis, found in the lateral wall of the cochlear duct, is responsible for maintaining the high ratio of potassium ions to sodium ions. It is believed that the endolymphatic sac is reabsorbing endolymph.
Perilymph fills the space between the bony labyrinth and the membranous labyrinth. Compared to endolymph, perilymph has a higher sodium ion concentration than potassium. Perilymph is created from the blood plasma.
Vasculature and innervation of the inner ear
Blood supply and venous drainage
The bony labyrinth is supplied by:
- The anterior tympanic artery, a branch of the maxillary artery
- The stylomastoid artery, a branch of the posterior auricular artery
- The petrosal artery, a branch of the middle meningeal artery from the maxillary artery
The membranous labyrinth is supplied by the labyrinthine artery, a branch of the basilar artery.
Venous drainage happens through the vestibular and cochlear veins. These veins connect and create the labyrinthine vein that drains into the sigmoid or the inferior petrosal sinus.
The lymph from the inner ear is drained into the parotid, mastoid, and superficial cervical lymph nodes.
The vestibulocochlear nerve (CN VIII) is responsible for sending sensory information from the inner ear to the brainstem. The vestibulocochlear nerve is formed by two nerves - the cochlear nerve and the vestibular nerve. The vestibulocochlear nerve forms the auditory pathway.
The cochlear nerve arises from the organ of Corti, which lies in the cochlea of the inner ear. The receptor cells (hair cells) in the organ of Corti receive the primary stimuli. The information is then transmitted to pseudounipolar neurons located in the spiral ganglion, which lies in the modiolus (center) of the cochlea. The axons of these neurons (special somatic afferent fibers) form the cochlear nerve.
The cochlear nerve connects with the vestibular nerve within the internal acoustic meatus and forms the vestibulocochlear nerve. Then the vestibulocochlear nerve continues through the internal acoustic opening and ends up in the posterior fossa of the cranium. The nerve ends on the border between the pons and the medulla oblongata, where it contacts with the cochlear nuclei. The cochlear nerve is involved in the sense of hearing.
The vestibular nerve arises from the receptors of the maculae of the utricle and saccule and from the receptors from the ampullary crest of the membranous labyrinth. The receptor cells receive the primary stimuli, and the neurons of the vestibular ganglion transmit the information further from the receptors via their dendrites.
The axons arising from the neurons in the vestibular ganglion form the vestibular nerve, which joins the cochlear nerve in the internal auditory meatus to form the vestibulocochlear nerve. The vestibular nerve is associated with the sense of equilibrium, spatial orientation, and motion. The fibers of the vestibular nerve reach the vestibular area of the brainstem, where they synapse with the vestibular nuclei.
Axons from neurons of the vestibular nuclei travel in various directions: to the motor neurons of the anterior horn of spinal cord via the vestibulospinal tract, to the inferior olivary nucleus via the vestibulo-olivary tract, to the cerebellum via the vestibulocerebellar tract, and to the cerebral cortex via the ventral posterolateral nucleus of the thalamus. These axons also join the ascending fibers of the medial longitudinal fascicle.
Common internal ear disorders
Every disorder of the internal ear is associated with hearing impairment or balance problems.
Meniere's disease is an internal ear disorder causing vertigo and hearing loss. The exact reason why the disease occurs is unknown. The pathogenesis is connected to an abnormal amount of the endolymph within the internal ear. The risk factors for this happening are viral infection, genetic predisposition, abnormal immune response, and affected fluid drainage due to the blockage or anatomical abnormality.
Symptoms of Meniere's disease are
- Hearing loss that may come and go;
- Recurring episodes of vertigo that can last from 20 minutes to several hours but usually no longer than 24 hours;
- Ringing in the ear or tinnitus;
- Feeling fullness in the ear that feels like pressure.
Meniere's can be treated by medications for vertigo or diuretics to reduce the fluid. Noninvasive procedures like rehabilitation, hearing aid, and positive pressure therapy can also be used. Sometimes surgery may be needed.
Labyrinthitis and vestibular neuritis
Labyrinthitis happens when an infection affects branches of the vestibulocochlear nerve, also known as an inflammation of the labyrinth, while vestibular neuritis is caused by an inflammation of the vestibular nerve.
Both of the disorders can be caused by whether bacterial or viral infections. Viral infections are more common than bacterial. The most common viruses are herpes viruses, influenza, measles, rubella, mumps, Epstein-Barr virus.
The symptoms of neuritis are nausea, vomiting, imbalance, affected concentration, and vision. In the case of labyrinthitis, the symptoms are mostly the same, but the patients can also experience difficulties in standing up or walking as well as tinnitus and hearing loss. Both of the disorders have acute and chronic phases.
The acute phase is characterized by being sudden, having severe dizziness. The chronic phase has chronic dizziness. The patients do not feel well in general but are unable to describe their symptoms precisely.
During the acute phase, people may have to use medications for nausea or suppress dizziness, sometimes steroids, antivirals, or antibiotics. In chronic cases, vestibular rehabilitation may be needed.
Benign paroxysmal positional vertigo
Benign paroxysmal positional vertigo is the most common vertigo reason. Benign paroxysmal positional vertigo can be idiopathic (no known reason) or can be related to head trauma or migraines. The risk factors are age (usually affects people above the age of 50), gender (more in women), head injury.
The dizziness is caused by calcium particles known as otoliths that are displaced into the semicircular canals. Due to the fact that otoliths are dense, the head movement causes the otoliths to tilt the hair cells.
Symptoms usually are:
- A loss of balance
- A sense of spinning or moving (vertigo)
The symptoms usually come and go and last no longer than one minute.
Even though benign paroxysmal positional vertigo can stop on its own, sometimes treatment is needed. One of the options is canalith repositioning when a doctor performs simple maneuvers for positioning the head. During this procedure, the aim is to move particles from the semicircular canals into the vestibule.
In rare cases, surgery may be needed to block the portion of the internal ear that causes the problem.
Superior semicircular canal dehiscence
Superior semicircular canal dehiscence happens when there is no bone overlying the superior semicircular canal. The reason for this could be development abnormality, head trauma, major pressure-altering activities (scuba diving, flying), or thinning of the temporal bone covering the superior semicircular canal over a lifetime.
Symptoms of the superior semicircular canal dehiscence are vertigo, sound-induced vertigo, pressure in the ear, and autophony (hearing internal noises louder than normal), hearing loss.
The most common way to treat the disease is surgery to close the opening.