Ch 123 - Middle and Inner ear

Question 1

List the components of the tympanic cavity

Answer 1

• Epitympanum (dorsal component) - Smallest, largely occupied by incus and part of malleus
• Mesotympanum (middle component) - True tympanic chamber. Bound laterally by the tympanic membrance and posteriorally by the cochlear membrane. Promontory on medial aspect
• Hypotympanum (ventral component) - Largest, sitting within the tympanic bulla

Question 2

What are the main components of the tympanic membrance?

Answer 2

• Pars flaccida dorsally
• Pars tensa ventrally, much larger. Manubrium of malleus is embedded in dorsal pars tensa

Question 3

Where is the aural opening of the auditory (eustachian) tube located?

Answer 3

Rostral mesotympanic chamber

Question 4

List the ossicles of the middle ear

Answer 4

• Malleus - articulates with the pars tensa laterally and the incus medially
• Incus - articulates with the malleus laterally and stapes medially
• Stapes - articulates with the incus medially and the fibrocartilaginous ring around the oval window medially

Question 5

What ligaments hold the ossicles in place?

Answer 5

• Lateral ligament of the malleus
• Dorsal ligament of melleus and incus
• Annular ligament of the stapes

Question 6

What muscles are responsible for movement of the ossicles?

Answer 6

• Tensor tympani muscle - pulls malleus medially, increasing the tension and convexity of the tympanic membrane
• Stapedius muscle - Tenses the stapes to limit its movement

Act to dampen sound vibrations

Question 7

What is the major anatomical difference of a cats bulla as compared to a dog?

Answer 7

Double chamber:
- Larger ventrocaudomedial chamber of hypotympanum
- Smaller rostrolateral compartment of mesotympanum and epitympanus

Bony promontory is more exposed, or more sensitive, to iatrogenic trauma

Question 8

What muscle causes opening of the auditory tube during swallowing?

Answer 8

Tensor veli palatini

Question 9

What is the bony labrinth?

Answer 9

A perilymph filled cavity in the temporal bone that communicates with the middle ear through the vestibular and cochlear windows

Question 10

What are the three sections of the membranous labyrinth?

Answer 10

Vestibule
- Middle portion containing the saccule and utricle

Cochlea
- Bony spiral ‘seashell’ structure containing the cochlear coil
- Coil originates at the cochlear window and is divided by the cochlear duct into the scala vestibuli and the scala tympani
- Floow of the duct contains the Organ of Corti

Semicircular canals
- Anterior, posterior and lateral
- Each has an ampulla arranged at right angles to each other
- The saccule and utricle are found at the confluence of the canals

Question 11

How does the inner ear function in relation to sound perception?

Answer 11

Ossicles transmit sound waves to inner ear causing movement of endolymph within the cochlea. Soundwaves are converted to nerve impulses by the hair cells in the organ of Corti and transmitted to the brain via the cochlear nerve fibres

Question 12

What structures within the inner ear contribule to vestibular function?

Answer 12

• Semicircular canals
• Saccule
• Utricle

Question 13

What are the three routes of infection of the middle ear?

Answer 13

• Extension from external ear through tumpanum
• Extension from nasopharyn through auditory tube
• Haematogenous

Question 14

List inflammatory diseases of the middle ear

Answer 14

• Polyps
• Topical agents causing a sterile inflammation (antiseptics, ceruminolytics)
• Cholesteatoma and cholesterol granuloma

Question 15

What breed is overrepresented with secretory (serous) otitis media

Answer 15

CKCS

Question 16

List forms of congenital inner ear abnormalities that result in deafness

Answer 16

• Collapse of scala media in Dalmatians
• Neuroepithelial degeneration in Rottweilers
• Changes in the organ of Corti in Pointers

Question 17

What radiographic view of the bulla is most uselful for identifying fluid changes?

Answer 17

10-degree ventrocaudodorsal view of tympanic chambers

Question 18

How may CSF be helpful in work-up of otitis?

Answer 18

• If there is extension into the inner ear, changes on CSF consistent with bacterial meningitis can be seen

Question 19

What is BAER testing?

Answer 19

Brainstem Auditory Evoked Responses

Question 20

How long should ABx be administered for otitis media/interna?

Answer 20

At least 4-6 weeks

Question 21

What are some appropriate ABx choices while awaiting culture results?

Answer 21

• Aminoglycosides (gentamicin) topically (systemically can cause ototoxicity)
• Fluoroquinolones topically and/or systemically

Question 22

What is the reported recurrence of polyps with traction?

Answer 22

• Nonendoscopic traction 57%
• Endoscopic traction, transtympanic curettage and post-op corticosteroids - 13.5%
• Traction of nasopharyngeal polyp with prednisolone - 10%

Question 23

What are the indications for surgical management of middle ear disease?

Answer 23

• Failure of medical management
• Neuro signs requiring tympanic decompression
• Neoplasia

Question 24

What muscles need to be dissected during the approach to a VBO?

Answer 24

• Platysma
• Sphincter colli
• Digastricus
• Myelohyoid
• Hyoglossus
• Styloglossus

Question 25

What nerve and artery should be avoided during a VBO?

Answer 25

- Hypoglossal nerve - Lingual artery

Question 26

What can be palpated externally to aid in the ventral approach to the bulla in a dog?

Answer 26

The paracondylar (jugular) process - Bulla is 5-10mm rostral and medial to this prominence

Question 27

What is the recommended surgery for a cholesteatoma? What is the prognosis?

Answer 27

- Removal of the entire epidermal cyst with meticulous stripping of the epithelium and additional margins which include the bulla wall - Prognosis guarded, 45-50% recurrence

Question 28

What factors are assoc with higher risk of recurrence with cholesteatomas?

Answer 28

- Inability to open mouth - Neuro signs - Lysis of temporal bone