Ear and brainstem

Question 1

Anatomy of the external ear

Answer 1

• Auricle/Pinna - outer part of the ear that functions to capture and direct sound waves towards external acoustic meatus.
  
  • Mostly cartilaginous (lobule is only part that is not)
  • Helix - outer curve
  • Antihelix - inner curve
  • Conchea - hallow depression in middle of auricle
  • Tragus - strucutre immediately anterior to beginning of external acoustic meatus
  • Antitragus - opposite the tragus
• External acoustic meatus - sigmoid shaped tube that extends from deep part of conchea to tympanic membrane
  
  • Outer 1/3rd = cartilage
  • Inner 2/3rds = temportal bone
• Tympanic membrane - starts at end of external acoustic meatus
  
  • Connective tissue strucutre, covered with skin on outside and mucous membrane on inside
  • Membrane is connected to temportal bone via fibrocartilaginous ring

Question 2

Anatomy of Middle Ear

Answer 2

Middle ear is within the temporal bone. It extends from tympanic membrane to lateral wall of inner ear. Functions to transmit vibrations from tympanic membrane to inner ear through auditory ossicles.

• Tympanic cavity - found medial to the tympanic membrane. It contains the auditory ossicles (malleus, incus, stapes) that transmits sound vibrations through middle ear.
• Audtiroy ossicles - link tympanic membrane to oval window of internal ear. Sound vibrations move tympanic membrane which cahse auditory ossicles to oscilate.
  
  • Malleus - largest bone and is attached to the tympanic membrane via handle of malleus. Malleus articulates with the next auditory ossicle, the incus.
  • Incus - body articulates with malleus, short limb attaches to posterior wall, and long limb articulates with the stapes.
  • Stapes - articulates with incus and connections to oval window of inner ear.
• Epitympanic recess - space superior to tympanic cavity, and anterior to mastoid air cells. Part of malleus and incus extend into epitympanic recess.
• Mastoid air cells - found posterior to epitympanic recess. It is a collection of air-filled spaces in mastoid process of temporal bone. Air cells are found within a cavity known as the mastoid antrum that communicates with middle ear via aditus. The air cells act as a buffer system and release air into tympanic cavity when pressure is too low.
• Muscles of middle ear:
  
  • Tensor tympanic and stapedius serve a protective function - they contract in response to loud noise, inhibiting vibrations of auditory ossicles to reduce sound transmission to inner ear = acoustic reflex.
• Auditory Eustachian Tube:
  
  • Cartilaginous and bony tube that connects middle ear to nasopharynx. Acts to equalize pressure of middle ear to external auditory meatus.

Question 3

Anatomy of Inner Ear - Bony Labyrinth

Answer 3

Series of bony cavities, made up of the vestibule, cochlea, and semi-circular canals, each of which are lined with periosteum and contain perilymph fluid.

• Cochlea - contains cochlea duct of membranous labyrinth. Cochelar duct creates 2 perilymph filled chambers - the superior scala vestibuli and inferior scala tympani. (Purpose of these chambers is the transduction of movement into fluid movement in the organ of corti.
• Vestibule - Central part of bony labyrinth. Separated by middle ear via oval window. Communicates with cochlea anteriorly and semi-circular canals posteriorly. Two parts of the membranous labyrinth -saccule and utricle are located within vestibule.
• Semi-circular canals - 3 semi-circular canals - anterior, lateral, and posterior. Contain semi-circular ducts, which are responsbile for balance (along with utricle and saccule).

Question 4

Anatomy of Inner Ear Membranous Labyrinth - Hearing Structures

Answer 4

Membranous labyrinth - a continuous system of ducts filled with endolymph. it is found within the bony labyrinth and is surrounded by perilymph. Composed of the cochear duct (hearing); 3 semi-circular ducts, saccule, and utricle (balance).

• Cochlear duct - Located within bony structure of cochlea. The duct creates two canals - scala vestibuli and scala tympani. The floor of the cochlear duct is the basilar membrane, which separates the cochlear duct from the scala tympani.
  
  • The basilar membrane contrains the organ of corti which contrains 4 rows of hair cells on the surface. Above them the tectoral membrane moves in response to pressure variations in fluid-filled tympanic and vestibular canals. Movement of hairs produce electrochemical signals that can then be sent through auditory nerve and into auditory cortex of the brain.

Question 5

Anatomy of Inner Ear Membranous Labyrinth - Balance

Answer 5

Membranous labyrinth - a continuous system of ducts filled with endolymph. it is found within the bondy labyrinth and is surrounded by perilymph. Composed of the cochear duct (hearing); 3 semi-circular ducts, saccule, and utricle (balance).

• Saccule and Utricle - mambranous sacs located in vestibule. They are organs of balance that detect movement of the head.
  
  • Utricle - largers, receives 3 semicircular canals
  • Saccule - globular in shape, receives cochlear duct
  • endolymph drains from saccule and utricle into endolymphatic duct, which travels through vestivular aquaduct to posterior aspect of petroud part of temporal bone, where the duct extends into sac where endolymph can be secreted and absorbed.
• Semi-circular ducts - located within semi-circular canals. With head movement endolymph changes speed and direction. Sensory receptors in ampulla of canals can detect these changes, allowing processing for balance.

Question 6

Innervation of inner ear

Answer 6

• Vestibulocochlear nerve (CN VIII) - enters inner ear via internal acoustic meatues where it divides into:
  
  • Vestibular nerve - forms vestibular ganglion, which then splits into superior and inferior parts to supply the utricle, saccule, and 3 semi-circular canals ducts -> responsible for balance
  • Cochlear nerve - enters at base of modiolus and branches pass through lamina to supply organ or corti -> responsible for hearing.

Question 7

Physiology of hearing

Answer 7

• Auricle “catches” incoming sound waves and funnels them down external auditory canals → sounds waves vibrate the tympanic membrane → results in motion of ossciles → postion like motion of stapes pushes inner ear fluid around the two and one-half turns of the cochlea (stapes transmit movement to oval window. As oval window moves, the round window moves in the opposite direction, compensating for changes in cochlear pressure, and allowing movement of fluid). → frequency specific movement sets up fluid waves with cochlea → results in movement of organ corti → movement of organ of corti bends stereocilia, deploarizing subsequent action of the auditory message traveling through hair cells via auditory nerve to brain → brain interprets the sounds

Question 8

Hearing Loss Classification

Answer 8

• Sensorineural - involves inner ear, cochlea, or auditory nerve. Tends to be due to inner ear problems.
• Conductive - any cause that limits external sounds from getting “into” inner ear i.e., cerumen impaction. Tends to be due to outer or middle ear problems.
• Mixed loss - combination oof sensorineural and conductive
• Central (involving higher brain centers) and auditory dyssynchrony (also called auditory neuropathy or auditory neuropathy spectrum disorder).

Question 9

Conductive Causes of Hearing Loss Children

Outer ear

Answer 9

• Congenital - External auditory canal (EAC) develops from 8-28 weeks gestation, so problems can occur at any time in this period.
  
  • Microtia - absence or malformation of aurcicle (mild to moderate hearing loss)
  • Atrexia or significant EAC stenosis (moderate to maximal hearing loss)
  • Accessory aurcile appendage/ preauricular tag - accessory appendages made of skin, subsutaneous fat, and/or cartilage may occur near auricle or anywhere along anterior border of stenocleiodmastoid. Children with these may have unilateral hearing loss.
• Obstruction - EAC may be obstrucuted by cerumen and bony growths.
  
  • Osteomas - benign solitary smooth - round osseous lesions occuring at tympanosquamous and tympanomastoid suture lines inside EAC (usually present in middle age, but can occur in children).
• Infection - Can result in bloackage of EAC due to debris, edema, or inflammation.
  
  • Otitis externa - may develop after local EAC trauma or impacted cerumen becomes contraminated by bacteria. Symptoms: Otalgia, pruritus, discharge, hearing loss.
• Trauma - Penetrating trauma to EAC or meatus.

Question 10

Conductive Causes of Hearing Loss Children

Middle ear

Answer 10

• Congenital - malformation of ossicles can result in hearing loss. Ossicular and other middle ear malformation occur as part of syndromes and occasionally as isolated events.
  
  • Most common abnormalities are fixation of malleus or incus, incudostapedial discontinuity, and stapes fixation.
  • Osteogenesis imperfecta - autosomal dominant condition associated with fragile bones that fracture easily.
• Infection - Acute otitis media (AOM) is most common childhood disorder associated with conductive hearing loss. Hearing loss is due to fluid filling middle ear space, preventing tympanic membrane from vibrating adequately and thus dimishing vibration of ossicular chain. Hearing loss lasts as long as fluid fills the middle ear spaces.
• Tympanic membrane perforation
• Trauma - blunt trauma can result in temporal bone fracture which can cause conductive hearing loss. Often also associated with perforation of the tympanic membrane.
• Tumour - malignant tumours can cause conductive hearing loss. However, benign cholesteatoma is more common.

Question 11

Sensorineural causes of hearing loss children

Answer 11

• Congenital
  
  • Infection - congential infection caused by cytomegalovirus, toxoplasmosis, rubella, or syphilis is associated with SNHL
  • Malformations of middle ear - hereditary causes tends to be associated with malformations of semicircular canals, cochlea, and interal auditory canal.
  • Perilymph fistula - leak of inner ear fluid through defect in otic capsule. Permits communication between midle and inner ear. Can be due to trauma or congenital defect of stapes foot plate.
  • Hereditary - hereditary hearing loss accounts for 50% of cases of SNHL
    
    • Autosomal recessive - most common inheritance pattern
    • Autosomal dominant - Wardenburg syndrome types I and II, neurofibromatosis I and II< and brachio-oyo-renal syndrome
    • X-linked - Hunter syndrome, Alport syndrome, X-linked congenital SNHL, and early onset progressice SNHl.
• Acquried SNHL - Causes include prematurity, infection, hyperbilirubinemia (bilirubin is toxic to cochlear nuclei and central auditoru pathway), ototoxins, noise, tumours

Question 12

Central Causes of Hearing Loss Children

Answer 12

• Central auditory processing disorder (CAPD) and auditory dyssynchrony (auditory neuropathy) are defects in perception or the analysis of auditory information associated with dysfunction of central auditory nervous system or auditory nerve.

Question 13

Vestibular System

Answer 13

• End organs of vestibular system - semicircular canals and otoliths - sense angular and linear motion, respectively.
• Each semicircular canals is reponsible for a different axis (each is orthogonal to each other). Each canal is filled with a fluid known as endolymph, as we move are head along a certain plane, it causes the endolymph to shift within that particular semi-circular canal allowing us to sense what plane our head is rotating along.
• Otolithic organs include the utricle and saccule which help us detect linear acceleration and head position. Within the otolithic organs are calcium carbonate crystals attached to hair cells within a gel-like substance. As we move, the crystals move, because they are heavier than the surrounding gel. When they move, they pull the hair to which they are attached, triggering an action potential.
• The CNS receives signals from both right and left labyrinths and compares signals - during motion, the right and left and alternately excited and inhibited - resulting in differences in eightj nerve activity between left and right. Differences in right and left result from unilateral peripheral vestibular disorder and is interpreted as vertigo.
• Information for vestibular labyrinth is relayrd via CN VIII to the brainstem vestibular nuceli → cerebellum, ocular motor nuclei, and spinal cord. Vestibuloocular connections are responsbile for coordinated eye movements during head motions, while vestibulospinal pathways helps maintain upright posture. The cerebellar connections hep modulate these activities.

Question 14

Neurofibromatosis Type II

Answer 14

• Dominantly inherited syndrome that predisposes people to multiple tumours of the nervous system.
  
  • Most common - bilateral vestibular schwannomas; intracranial and spinal meningiomas; spine tumours

Question 15

Neurofibromatosis Type II

Etiology

Answer 15

• Due to abnormalities in NF2 gene, located in chromosome 22
• NF2 gene produces merlin/schwannomin, which is a cell-membrane-related protein that acts as a tumour suppressor.
• Development of schwannomas and other tumors requries inactivation of both NF2 alleles

Question 16

Clinical Featues Neurofibromatosis type II

Answer 16

• Neurologic lesions
  
  • Bilateral vestibular schwannomas - cause tinnitus, hearing loss, and balance dysfunction
  • Schwannomas of other cranial nerve
  • Intracranial meningiomas
  • Spinal tumours - can cuase pain, muscle weakness, or paresthesias
  • Peripheral neuropathy - often involves facial nerve
• Eye lesion
  
  • Cataracts
  • Epiretinal membranes
  • Retinal hamartomas
• Skin lesion
  
  • Cutaneous tumor
  • Skin plaques
  • Subcutaneous tumors
• Children diagnosed have atypical but more severe presentation

Question 17

Diagnosis Neurofibromatosis Type II

Answer 17

Clinical diagnosis of NF2 is based on presence of any ONE of the following:

• Bilateral vestibular schwannomas before 70
• Unilateral vestibular schwannomas before 70 and first degree relative with NF2
• Any two of the following - meningioma, non-vestibular schwannoma, neurofibroma, giloma, cerebral calcification, cataract AND
  
  • First-degree relative with NF2 of unilateral vestibular schwannomas + negative C2TRI testing
• Mulitiple meninigomas AND:
  
  • Unilateral vestibular schwannoma or any 2 of the following: non-vestivular schwannom, neurofibroma, giloma, cerebral calcification, cataract
• Constitutional or mosaic pathogenic NF2 gene mutation from blood or by identification of an identical mutation of 2 separate tumors in same person.

Question 18

Management Neurofibromatosis Type II

Answer 18

• Tumor surveillance and follow up for those with known MF2 mutation
  
  • Audiology, opthalmologic, and cutaneous examination
  • Annual brain MRI starting at 10
  • Surveillance spinal MRI every 2 years starting at 10

Question 19

Brain Tumor

Answer 19

• Diverse group of neoplasms arising from different cells within CNS or metasis (lung, melanoma, brest).
  
  • Brains tumours can produce symptoms and signs via:
    
    • Local brain invasion
    • Compression of adjacent strucutres
    • Increased intracranial pressure
    • Based on function of invovled areas of the brain
• Clinical manifestations: May present with generalized and/or focal signs, or may be asymptomatic
  
  • Generalized: headache, seizures, nausea/vomiting, depressed level of consciousness, neurocognitive dysfunction
  • Focal: Seizure, weakness, sensory loss, aphasia, visual spatial dysfunction
• Headaches - usually dull and constand, but occasionally throbbing. Bifrontal and worse on side with tumor. Severity tends to progress over time.
• Seizures - Most common symptoms of primary and metastic brain tumor. Tumor related seizures are typically repetitve and stereotypes in a given patient.
• Focal deficits - vary based on tumor location. Symptoms may be caused by local tissue disruption, mass effect on nearby strucutres, or vasogenic edema.
• Cognitive dysfunction - includes memory problems, mood or personalities changes
• Increased intracranial pressure - can be result of large mass or from restriction of CSf causing hydrocephalus. Classic triad - headach, nausea, and papilledema

Question 20

Cerebellum Structure

Answer 20

• Cerebellum is the “mini brain” - consists of 2 hemispheres connected by the vermis
• Cerebellar zones:
  
  • Midline: vermis
  • Lateral to vermis: intermediate zone
  • Katerak to intermediate zone: lateral hemisphere

Question 21

Cerebellar Function

Answer 21

Functional Division of Cerebellum

• Cerebrocerebellum - formed by lateral hemisphere:
  
  • Involved in planning movements, motor learning, regulating coordination of muscle activation, and contributes to visually guided movements
  • Receives input from - cerebral cotex and pontine nuclei
  • Sends output to thalamus and red nucleus
• Spinocerebellum - formed by vermis and intermediate zone
  
  • Involved in regulating body movements by allowing for error correction and received proprioceptive information
• Vestibulocerebellum
  
  • Invovled in balance and ocular reflexes (fixation on a target)
  • Receives input from vestibular system and sends output back to vestibular nuclei

Question 22

Mindbrain Structure

Answer 22

Most superior part of the brainstem. It can be furture divided into 2 parts:

• Tectum - located posterior to cerebral aqueduct. Contains 4 rounded strucutres known as colliculi (2 superior and 2 inferior) that sit inferiorly to the pineal gland.
• Pair cerebral peduncles - located anteriorly and laterally. Further separated by substania nigra into crus cerebri (anterior) and tegmentum (posterior).

Question 23

Nuclei of the midbrain

Answer 23

• Substantia Nigra - distincitive, pigments band of cells located in tegmentum of midbrain dorsal to peduncles. It is divided into pars compacta and pars reticulars. The pars compacta sends efferent fibers to putamen and caudate nucleus and is a source of dopaminergic transmission. Together, the pars compacta and reticularis forms a feedback loop with basal gangli and create a feedback loop that influences motor activity.
• Red nuclei - found in tegmental section in rostal portion of midbrain, dorsal to substantia nigra. Thought to provide an important motor feedback loop involving cortex, cerebellum, and spinal cord.
• Superior and inferior colliculi - make up a majority of tectum. Superior is the primary projection for optic tracts; inferipr receives input from auditory system.
• Reticular formation - Has many connections. Influence thought to be vaired and involved in a number of organ system. Thought to contribute to axial musculature and muscle tone, attentiveness, and arousal.
• Cranial nerve nuclei of midbrain - nuclei for CN III (oculomotor) an CN IV (trochlear) are found in tegmentum. Both provide innervation for motor movement of the eyes.

Question 24

Pons Stucture

Answer 24

• Largest part of brainstem, located above medulla and below midbrain. Functions as a connection between cerebrum and cerebellum. Its posterior border is separated from the cerebellum by the aqueduct of Sylvius, and more inferiorly, by the fourth ventricle. Motor and sensory tracts traverse the anterior surface of the pons. The sensory fibers are located behind the motor fibers.
• Several cranial nerves originate from the ventral surface of the pones - CN V, CN VI, CN VII, CN VIII (exits at the junction between pons and medulla).
• Pons is composed of 2 major componetns:
  
  • Ventral pons - contain pontine nuclei that are reponsible for coordinating movements.
  • Tegmentum - Forms part of recitular formation (found thoughout the brainstem).

Question 25

Nulcei of Pons

Answer 25

• Locus cerulens - thin, pigmented nuclear group that lies near floor of fourth ventricle. Major source of norepinephrine.
• Pontine nucleus - corticopontine fibers synapse here providing means through which cortex influences cerebellum.
• Superior olivary nuclei - lie in lateral aspect of tegmental portion of pons. Receive bilateral input from dorsal and ventral cochlear nuclei.
• Cranial nerve nuclei - CN V (sends motor messages to the jaw and receives sensory messages from the teeth, tongue, and parts of the face); CN VI (responsible for control of lateral rectus msucles that abduct eyes); Motor nucleus of cranial nerve VII (responsible for muscles of facial expression).

Question 26

Medulla Structure

Answer 26

• Most inferior part of brainstem. Inferior border is marked by orgin of first pair of cervical spinal nerves that occur as medulla leaves skull through foramen magnum.
• Anterior structures:
  
  • Anterior median fissure - continuous along length of spinal cord
  • Pyramids - paired swelling found between anterior medial fissure and ventrolateral sulcus.
  • Olives - another pair of swellings found laterally to the pyramides - between ventrolateral and posterolateral sulci.
• Cranial nerves CN IX, CN X, CN XI, CN XII exit the medulla.

Question 27

Nuclei of Medulla

Answer 27

• Inferior olivary nuclei - most predominant nuclear mass in medulla. Plays role in motor coordination and learning.
• Nuclei gracilis and cuneatus - synpase with tracts that carry position and vibratory sense from lower and upper extremities.
• Crainal nerve nuclei
  
  • Dorsal and ventral cochlear nuceli - vestibulocochlear (VIII) CN
  • Solitary nuceli - Sensory fibers for CN VII, CN IX, and CN X
  • Nucleaus ambiguus - CN IX and X motor
  • Dorsal motor nuclei - CN V
  • Hypoglossal nuclei - CN XII

Question 28

Reticular Formation

Answer 28

The reticular formation is a set of interconnected nuclei that are located throughout the brain stem. Its dorsal tegmental nuclei are in the midbrain while its central tegmental nuclei are in the pons and its central and inferior nuclei are found in the medulla.

The reticular formation has two components:

• The ascending reticular formation is also called the reticular activating system. It is responsible for the sleep-wake cycle, thus mediating various levels of alertness. This part of the reticular system projects to the mid-line group of the thalamus, which also plays a role in wakefulness. From there, information is sent to the cortex.
• The descending reticular formation is involved in posture and equilibrium as well as autonomic nervous system activity. It receives information from the hypothalamus. The descending reticular formation also plays a role in motor movement.