AVS CANS Flashcards
How is the BM tonotopically organized?
high frequencies to low frequencies (base to apex)
How are the CN VIII tonotopically organized?
high on external part of bundle - lows wrapped around the cores
How is the A1 tonotopically organized?
Lows laterally to highs medially
what is the main thing we know about the Cochlear Nucleus?
The CN is the first station for processing auditory information in the brainstem (monoaural/ipsilateral at this level)
What is the gold standard for symptoms of Schwanomas?
MRI with enhancement agent (gadalidium)
Is CN VIII a part of the peripheral nervous system
yes
What is a bundle of myelinated axons ascending in the brainstem
lateral leminiscus
Are there more fibers ipsilaterally and or more going contralaterally?
Contralaterally (stronger of the two pathways)
why do we say as have built in redundancy?
info comes in from one cochlea to go to both sides and crosses over in different places and a lesion in one spot and a tone is played, the information from that cochlea would make it up to the brain because it has other ways to go up and bypass the lesion
so many pathways it can take that it will still reach the cortex for perception due to so many commissures and ipsilateral and contralateral pathways for it to get to the cortex through one of those pathways.
Why is redundancy important?
knowing our environment and for our safety
communication
helps us do really well
causes problem as audiologist because a simple test is not possible because the signal can still get to the brain so it is hard to find lesions or tumors
in order to assess a lesion or understand by audiogram have to do speech testing
if words or sentences are too easy, we could do CAP (put sentences in noise or split the sentences up between the two ears)
How do vestibular Schwanoma’s form?
from myelin (Schwann cells) of the superior and inferior vestibular nerves (of cranial nerve VIII)
Where do Schwanoma’s form?
In the medial internal auditory canal (IAC) or lateral cerebellopontine angle (CPA)
Schwanoma’s make up _____ of CPA tumors and _______ of all intracranial tumors
80%, 7-8%
What is the hallmark symptom of Schwanoma?
Slowly progressive unilateral sensorineural hearing loss
What are the other symptoms of Schwanoma’s?
include unilateral tinnitus, dizziness and dysequilibrium, headache, sensation of pressure or fullness in the ear, otalgia, trigeminal neuralgia, and numbness or weakness of the facial nerve
How can we test for Schwanoma’s and what do they show?
An audiogram is the first test performed (see ). It usually reveals an asymmetric sensorineural hearing loss and a greater impairment of speech discrimination than would be expected for the degree of the hearing loss. Acoustic reflex decay, the absence of waveforms, and increased latency of the 5th waveform in auditory brain stem response testing are further evidence of a neural lesion. Although not usually required in the routine evaluation of a patient with asymmetric sensorineural hearing loss, caloric testing demonstrates marked vestibular hypoactivity (canal paresis).
What are the 3 major divisions of the cochlear nucleus?
anterior ventral cochlear nucleus (AVCN)
Posterior ventral cochlear nucleus (PVCN)
Dorsal cochlear nucleus
What part of CANS is the only auditory brainstem nuclei to receive only ipsilateral input?
cochlear nucleus
Is the Cochlear Nucleus Tonotopically Organized?
yes
what are stria and the 3 divisions of cochlear nucleus?
dorsal stria, intermediate, ventral acoustic stria
DCN, PVCN and AVCN
the first place to get binaural info is here (ISPI AND CONTRA)
SOC
What are the two major divisions of SOC?
lateral superior olive
medial superior olive
describe the LSO
Largest of the nuclei of the SOC in cats, smaller in humans
S-shaped in cat and more like a boxing glove in other species
Receives input from the ipsilateral AVCN and contralaterally from the AVCN and PVCN (some fibers via the medial nucleus of the trapezoid body)
timing is disrupted
describe the MSO
Smaller
Disc-shaped
Receives direct fibers from the CN of both sides
timing and integrity is maintained because of no extra synapse
what are the functions of the LSO
Predominantly receives and processes high frequency information
Localization of sounds based on interaural intensity differences
Why is sound intensity different at the two ears? (interaural attenuation)
Head shadow
Body baffle
Intensity decreases with distance
cannot rely on time fully because some of the neurons run through synapse and reaches ears differently based on presentation
sound decreases over distance and head shadow (sound doesn’t get around head as well and absorbed on body)
what are the functions of MSO
Receives and processes predominantly low-frequencies
Localization of sounds based on temporal cues
Stimuli reach the ear at different times and at different phases
what receives direct input from both CN with no other synapses that would interfere with analysis of timing information?
MSO
what is the first level of the CANS to receive binaural input
SOC
Why is would low frequencies be used for temporal cues (phase) and high frequencies for intensity cues
Low frequency temporal cues = ITD (will hit the eardrum of each ear at different phases of a waveform) but in high frequencies is unable to do this
what allows for the analysis of interaural differences in the stimuli?
binaural input at the SOC
Is the SOC
Tonotopically Organized?
YES
Bundles of axons leave the SOC and go where?
some go contralaterally
some go straight to the lateral leminiscus
some go straight up to IC
What is the lateral leminiscus?
fiber tract of ascending descending crossed and uncrossed fibers
where do the lateral leminiscus run to and from?
from cn to ic
pons to midbrain
ventral and dorsal
what gives the pathway more strength and integrity and connects the DNLL?
Commissure of probst
How is the LL tonotopically organized?
lows dorsal, highs ventral
what is the largest of the brainstem nuclei?
IC
What is the obligatory relay station of the ascending auditory pathway?
IC
Can the IC be bypassed like the other stations in CANS?
No
what location receives input directly or indirectly from all lower centers
ic
the central nucleus of IC processes what?
auditory only
the pericentral nucleus of IC does what
contains somatosensory and auditory fibers
taking info and orients our body to where sound is (helps with directing attention and head to where the sound is)
Large tract of axons projecting ipsilaterally to the medial geniculate body in the thalamus
brachium of IC
Is the IC Tonotopically Organized?
yes
Group of nuclei in the thalamus
medial geniculate body
Last station in the CANS prior to areas in the cortex
medial geniculate body
3 divisions of MGB and describe each one
Ventral - Transmits directly to the primary auditory cortex (A1)
medial and dorsal - Responsive to somatosensory and acoustic stimulation
Is the MGB Tonotopically Organized?
yes
part of the interthalamic adhesion
gudden’s commissure
Explain the process once it leaves MGB
fibers exit the thalamus as thalamocortical (from thalamus to cortex) projections (bundle of axons), traveling through the internal capsule and becoming auditory radiations (corona radiata - fibers radiating around the crown)
The auditory radiations go to the primary auditory cortex (A1)
connecting the frontal lobe to the occipital lobe. This fiber tract also connects the Wernicke’s area of the temporal lobe (understanding–comprehension) with Broca’s area in the frontal lobe (motor –fluency)
arcuate fasiculus
area for understanding and comprehension
wernicke’s
area for motor and fluency
broca’s
branches from AICA (anterior inferior cerebellar artery) or from the basilar artery
Labyrinthine artery
Draw out AVS vascular supply
see drawing
what supply the endolymphatic sac in the posterior fossa
meningeal arteries from the external carotid
dense portion of the temporal bone house the inner ear structures
optic capsule
How are fractures categorized?
according to the direction they travel in the bone
Traditional classification of bone fractures
longitudinal - 80%, Parallels the long axis of the petrous pyramid of the bone going from lateral to medial, Tears in skin of canal and TM
CHL (conductive hearing loss)—middle ear/ossicular disruption
Facial nerve injury 10-25%
transverse - 20%, Perpendicular to the petrous pyramid
SNHL
Vertigo or dizziness
Hemotympanum
Facial nerve injury 50%
What classification scheme does not predict symptoms or prognosis reliably
traditional classification
a high number of fractures are actually referred to as
mixed/oblique - hard to classify as either transverse or longitudinal
what are the new classifications of bone fractures
new classification scheme, based on whether the otic capsule, containing the inner ear structures is disrupted or spared.
Describe otic capsule sparing fractions (new classification)
90+%
temporoparietal blow
FN paralysis (6-13%)
CHL or mixed
CSF leak not likely
Otic capsule sparing fractures usually cause conductive or mixed hearing loss and rarely result in facial nerve paralysis. These fractures are more common and generally caused by a blow to the temporoparietal region, involving the squamous portion of the temporal bone, the mastoid air cells and the middle ear space. CSF leak is not likely with otic capsule sparing fractures.
Describe otic capsule disrupting fractures (new classification)
2.5-5.8%
occipital blow
FN paralysis 30-50%
SNHL
CSF Leak 2-4x more likely
damage to labyrinth, IAC, other
almost always cause sensorineural hearing loss and are associated with a higher incidence of facial nerve paralysis. These factures are usually caused by a blow to the occipital region,. and the line of the fracture runs from the foramen magnum across the petrous pyramid to the otic capsule, often passing through the jugular foramen, internal auditory canal and foramen lacerum where the internal carotid artery courses.
what is the shape for an epidural hematoma
lemon shaped
why is an eh lemon shaped?
because the dura connects at suture lines and the blood cannot continue to spread.
what is the shape for a subdural or subarachnoid hematoma?
banana shape
why is sh banana shaped?
because the blood can continue to spread and pool along the curve shaped of the subarachnoid space.
what is the diagnosis for bone fractures
Facial nerve function
Inspection of ear and temporal bone
Bloody otorrhea and hemotympanum are common
Aseptic examination, no irrigation or removal of blood from canal due to possible CSF leak with route for infection/meningitis
CSF leak warrants hospitalization
CT or High resolution CT scan
Bedside assessment of hearing with tuning forks, and check for vertigo, dizziness, presence of nystagmus
Commonly BPPV
what is the treatment for bone fractures
Surgery, decompression, corticosteroids, bed rest and not straining
Tx hearing loss and remaining balance issues later
