Chapter 123 Middle and Inner Ear Flashcards
What is thought to be primary aetiology of OM in dogs?
And in cats?
Dogs = extension of OE
Cats = ascending cause (associated with viral nasopharyngeal infection) –> interrupted middle ear drainage
How is the tympanic cavity divided?
Dorsal = Epitympanum
Middle = Mesotympanum
Ventral = Hypotympanum
What structures are contained in eachpart of the tympanic cavity (aka tympanum)
Dorsal = Epitympanum
Middle = Mesotympanum
Ventral = Hypotympanum
Dorsal = Epitympanum:
- Incus and part of malleus
Middle = Mesotympanum
- Tympanic membrane
- Cochlear membrane i.e. covering of round (aka cochlear) window i.e. is separation between middle and inner ear
- Promontory (bone that contains cochlea)
- Opening of eustacean tube
Ventral = Hypotympanum
LAbel the diagram
What are the layers of the tympanic membrane/
Inner epithelium (derived from pharyngeal pouch)
Central fibrous (also derived from paryngeal pouch)
Outer stratitied squamous epithelium (from ectoderm)
How does the TM regenerate?
The outer epithelial cells originate at the attachment of the malleus and migrate peripherally from this point to the external auditory meatus; this continual centrifugal movement of cells clears debris from the surface of the membrane and is also responsible for its repair in the event of rupture.
What type of epithelium lines the tympanic cavity?
Respiratory epithelium
Label the diagram
What is the lenght and lumiinal diameter of auditory tube?
1-2cm length
1-2mm width
HOw could tympanic end of auditory tube be distinguished from pharyngeal side?
Mucous and goblet cells predominate at pharyngeal side
Other names for auditory tube
Eustacean or pharyngotympanic
WHat are the names of the ossicles and what is their orinetation
MIS
Malleus (hammer). Manubrium part of malleus imbedded in fibrous portion of tympanic membrane
Incus (anvil). In middle
Stapes (stirrup) contacts vestibular (oval) window
What are the ligaments of the ossicles?
- the lateral ligament of the malleus that attaches it to the tympanic notch
- the dorsal ligament of the malleus and incus that attaches them to the epitympanic recess
- the annular ligament of the stapes that attaches it to the vestibular window.
Name the muscles responsible for movement of the ossicles?
- Tensor tympani* (increases tension on TM)
- Stapedius* (tenses stapes)
What nerve does the facial nerve enter the iner ear with?
What foreman do the enter through
Vestibulocochlear
Internal acoustic meatus
Where does facial nerve exit tympanic cavity
Via stylomastoid foramen, caudal to external auditory meatus
Inside tympanic cavity, the facial canal is incomplete meaning facial nerve is exposed near vestibular (oval ) window i.e. near stapes
What vessel supplies the meddle ear
Tympanic artery (branch of maxillary artery)
How does the middle ear of cats differ to dogs
Distincs osseous septum between hypotympanum and meso- and epitympanum
Tympanic cavity lining contains more abundant cilliated and secretory cells than dogs
Label the diagram
When performign VBO, hypotympanum is entered. Where does the meso-/epitympanum sit relative to this?
Where should the septum be broken down and why?
Rostrolateral
MAke osteotomy of septum as (cranio) ‘lateral’ as possible to avid promontory to reduce risk of Horner’s and vestibular issues
Which inner ear structure is responsible for hearing and which for balance?
Cochlea (Organ or Corti) for hearing
Semicircular canals (maculae) for balance
List rads signs of OM
- loss of air density within the tympanic cavity
- thickening or irregularity of the bulla wall
- destruction of the bulla and otoliths
- bony proliferation extending to involve the temporomandibular joint and petrous temporal bone.
Comment on imaging
A, Magnetic resonance imaging (MRI) scan of middle ear polyp in a cat extending into the external meatus.
B, MRI scan of septic otitis media in a dog. The T2 transverse view shows the tympanic bulla is expanded and eroded.
comment on imaging
Magnetic resonance imaging of an 11-year-old male castrated Golden Retriever with abnormal mentation and posture, right-sided facial nerve paralysis, and right head tilt. The mass lesion extended from the right bulla through the inner ear and into the cranium, causing localized edema. Cholesteatoma was confirmed on histologic samples of tissue retrieved via myringotomy.
What is this and what is it for
Reuter Bobbin vent tube for tympanostomy drainage.
Myringotomy provides only temporary remission of clinical signs in dogs with secretory otitis media resulting from auditory tube dysfunction and obstruction. Placement of tympanostomy tube or “grommets” in the tympanic membrane in the pars tensa is reported to produce long-term resolution but is technically more complex, with insertion requiring microscopy. Total ear canal ablation may provide a radical but more permanent solution.
Describe VBO in cats
- The position of the bulla should be confirmed at this point by palpation of the ventral aspect of the bulla, which can be located within the triangular area bounded by the mandibular symphysis, the caudal border of the mandible, and the larynx
- A 3- to 5-cm paramedian skin incision, centered over the bulla, is made. Dissection is continued through the platysma and sphincter colli muscles. The mandibular salivary gland and bifurcation of the linguofacial and maxillary veins are retracted from the dissection field
- The digastricus and mylohyoid muscles are separated by blunt dissection, and the underlying hyoglossus and styloglossus muscles are retained by small Weitlaner retractors or two pairs of miniature Gelpi retractors. Care should be taken to identify and avoid manipulation of the hypoglossal nerve, which lies close the lingual artery and enters the base of the tongue.
- After the bulla has been located, an elevator may be used to strip away its periosteal covering. Osteotomy can be performed by perforating the bone with a drill or an enlarging series of Steinmann pins. Considerable care should be exercised during the osteotomy procedure because weakening of the tympanic bone by middle ear disease may allow collapse of the osteotomy site, resulting in the drill or pin’s perforation of the underlying bony promontory and deeper structures. To counter this risk, the osteotomy site is best enlarged by judicious use of rongeurs or by very controlled use of a burr, taking great care not to entrap any surrounding soft tissues.
- The bony septum separating the hypotympanic cavity laterally from the true middle ear should then be dismantled to provide access to the point from which the polyp originates. This is again achieved by pin or drill penetration of the bone and careful subsequent enlargement. The risk for damage to the tympanic plexus at this point, giving rise to Horner’s syndrome, may be minimized by locating the osteotomy site as far laterally as possible to avoid contact with the bony promontory.
- A small curette is used to gently scoop out polypoid material attached within the mesotympanic chamber, which may sometimes extend into the external auditory meatus. Curettage should never be performed over the promontory or near the round window or ossicles.
What anatomic feature is used as a landmark for VBO in dogs?
pointed paracondylar (jugular) process
The bulla is located 5 to 10 mm rostral and 5 to 10 mm medial to this bony prominence
(number 11 in schematic)
Label the diagram
Lateral view of the canine skull.
1, Orbital ligament (inset);
2, infraorbital foramen;
3, orbit;
4, pterygopalatine fossa;
5, optic canal, orbital fissure, and rostral alar foramen;
6, retroarticular process;
7, retroarticular foramen;
8, external acoustic meatus;
9, tympanic bulla;
10, stylomastoid foramen;
11, paracondylar process;
12, occipital condyle;
13, nuchal surface;
14, mastoid process;
15, zygomatic arch;
16, temporal fossa;
17, nuchal crest.
Describe VBO in cats (repeat card with different anatomy diagram)
- The position of the bulla should be confirmed at this point by palpation of the ventral aspect of the bulla, which can be located within the triangular area bounded by the mandibular symphysis, the caudal border of the mandible, and the larynx
- A 3- to 5-cm paramedian skin incision, centered over the bulla, is made. Dissection is continued through the platysma and sphincter colli muscles. The mandibular salivary gland and bifurcation of the linguofacial and maxillary veins are retracted from the dissection field
- The digastricus and mylohyoid muscles are separated by blunt dissection, and the underlying hyoglossus and styloglossus muscles are retained by small Weitlaner retractors or two pairs of miniature Gelpi retractors. Care should be taken to identify and avoid manipulation of the hypoglossal nerve, which lies close the lingual artery and enters the base of the tongue.
- After the bulla has been located, an elevator may be used to strip away its periosteal covering. Osteotomy can be performed by perforating the bone with a drill or an enlarging series of Steinmann pins. Considerable care should be exercised during the osteotomy procedure because weakening of the tympanic bone by middle ear disease may allow collapse of the osteotomy site, resulting in the drill or pin’s perforation of the underlying bony promontory and deeper structures. To counter this risk, the osteotomy site is best enlarged by judicious use of rongeurs or by very controlled use of a burr, taking great care not to entrap any surrounding soft tissues.
- The bony septum separating the hypotympanic cavity laterally from the true middle ear should then be dismantled to provide access to the point from which the polyp originates. This is again achieved by pin or drill penetration of the bone and careful subsequent enlargement. The risk for damage to the tympanic plexus at this point, giving rise to Horner’s syndrome, may be minimized by locating the osteotomy site as far laterally as possible to avoid contact with the bony promontory.
- A small curette is used to gently scoop out polypoid material attached within the mesotympanic chamber, which may sometimes extend into the external auditory meatus. Curettage should never be performed over the promontory or near the round window or ossicles.
