oto disorders of the inner ear and lateral skull base

Question 1

What is the difference between stereotactic radio-

surgery and stereotactic radiotherapy?

Answer 1

Stereotactic radiosurgery implies a single fraction treatment
(all at once), whereas stereotactic radiotherapy implies
delivery of mutliple fractions.

Question 2

For what does the NF-2 gene code?

Answer 2

The NF-2 gene is a tumor suppressor gene located on
chromosome 22q12.2, and it codes for the schwannomin
or Merlin protein.

Question 3

What are the diagnostic criteria for NF-2?

Answer 3

● Bilateral vestibular schwannomas
● First-degree relative with NF-2 and the occurrence of a
unilateral vestibular schwannoma or any two of the
following: meningioma, neurofibroma, schwannoma,
glioma, or posterior subcapsular lenticular opacities
● Unilateral vestibular schwannoma and any two of the
following: meningioma, neurofibroma, schwannoma,
glioma, or posterior subcapsular lenticular opacities
● Multiple meningiomas and a unilateral vestibular
schwannoma or any two of the following: schwannoma,
glioma, neurofibroma, cataract

Question 4

What percent age of patients with NF-2 have a

family history of disease?

Answer 4

Approximately 50% of cases are inherited, and 50% develop

as a result of a new spontaneous mutation.

Question 5

What are the two subtypes of NF-2?

Answer 5

● Gardner (mild) phenotype: Development of a limited
number of intracranial tumors at a later age with slow
progression
● Wishart (severe) phenotype: Development of innumer-
able intracranial and spinal tumors early in life with rapid progression

Question 6

What are current treatment options for patients

with NF-2?

Answer 6

Observation with serial imaging, stereotactic radiation
(focused radiation), microsurgical resection (gross total, near total or subtotal), or bevacizumab (monoclonal anti-
body targeting vascular endothelial growth factor)

Question 7

What are the three surgical approaches commonly

used during vestibular schwannoma resection?

Answer 7

● Translabyrinthine
● Middle fossa
● Retrosigmoid

Question 8

What are the common indications for a translabyrinthine approach for resection of a vestibular schwannoma?

Answer 8

Patients with nonserviceable hearing (American Academy of
Otolaryngology–Head and Neck Surgery [AAOHNS] class C
or D), or with tumors > 2 cm where the prospect of hearing
preservation is low. The primary advantages include
minimal brain retraction, less risk of prolonged headache,
and reliable early facial nerve identification at the fundus.

Question 9

Describe the Trautmann triangle.

Answer 9

The dura of the posterior fossa located between the bony
labyrinth anteriorly, the sigmoid sinus posteriorly, the superior
petrosal sinus superiorly, and the jugular bulb inferiorly

Question 10

What is the initial treatment of a venous air
embolism supplied by a large rent in the sigmoid
sinus?

Answer 10

Irrigate the wound, and place wet Gelfoam over the
opening to block “air sucking.” Place the patient in the left
lateral decubitus (Durant maneuver) and Trendelenburg
position to trap the air pocket in the right heart. Administer
100% oxygen. Perform cardiopulmonary resuscitation if
cardiorespiratory collapse ensues.

Question 11

Describe the orientation of the seventh to eighth

nerve bundle at the fundus of the IAC.

Answer 11

● Facial nerve (VII): Anterior superior
● Superior vestibular: Posterior superior
● Cochlear: Anterior inferior
● Inferior vestibular: Posterior inferior

Question 12

Describe Mike’s dot.

Answer 12

The origin of the superior vestibular nerve in the medial wall of the vestibule

Question 13

What are the common indications for a middle
fossa approach for resection of a vestibular
schwannoma?

Answer 13

Patients with serviceable hearing who have intracanalicular
tumors or no more than 0.5 to 1 cm extension into the CPA.
Advantages include ~ 50% chance of hearing preservation;
disadvantages include a potentially unfavorable location of
the facial nerve with respect to the tumor and temporal
lobe retraction.

Question 14

In middle cranial fossa surgery, what landmarks

are used to locate the IAC?

Answer 14

The IAC is typically oriented parallel to the axis of the EAC,
approximated by the line bisecting the angle between the
greater superficial petrosal nerve and the arcuate eminence
(superior semicircular canal).

Question 15

To minimize greater superficial petrosal nerve
injury during a middle fossa approach, in which
direction should dural elevation proceed?

Answer 15

From posterior to anterior

Question 16

What are the common indications for a retrosigmoid approach for resection of a vestibular schwannoma?

Answer 16

It may be considered for patients with serviceable hearing
and medial-based tumors. Tumors extending to the fundus
of the internal auditory canal may be difficult to remove
given restricted access around the posterior semicircular
canal and vestibule. Larger, medial-base tumors can also be
removed, even if hearing preservation is not attempted.
Disadvantages include cerebellum retraction, headache,
and potentially limited access into to the lateral IAC when
hearing preservation is attempted.

Question 17

Describe the superficial landmarks for approximating the transverse and sigmoid sinus.

Answer 17

The asterion approximates the location of the junction of
the transverse sinus and sigmoid sinus and is often used
during retrosigmoid craniotomy. Asterion is defined by the
junction between the lambdoid, parietomastoid, and
occiptomastoid sutures.

Question 18

What percentage of patients with a vestibular
schwannoma and “useful” hearing will ultimately
lose their hearing after stereotactic radiosurgery?

Answer 18

Less than 25% of patients will retain useful hearing after

stereotactic radiosurgery at 10 years after treatment.

Question 19

What percentage of vestibular schwannoma are

associated with NF-2?

Answer 19

5%

Question 20

What percentage of intracanalicular vestibular
schwannomas will remain stable in size for the first
5 years after diagnosis?

Answer 20

50 to 80%

Question 21

Where along the eighth nerve are vestibular

scwhannomas hypothesized to arise from?

Answer 21

Obersteiner-Redlich zone, near the porus acousticus, 1 cm

from the brainstem

Question 22

What is the Hitselberger sign?

Answer 22

Loss of sensation in the posterosuperior part of external
auditory meatus and conchal bowl supplied by the sensory
division of CN VII

Question 23

What is the most common histologic classification

for vestibular schwannomas?

Answer 23

Antoni type A or B. Type A tissue demonstrates dense
cellularity with nuclear palisading and associated Verocay
bodies, whereas type B has a loose arrangement with a less
cellular reticular pattern in a myxoid stroma.

Question 24

How are CPA meningiomas radiographically dif-

ferentiated from vestibular schwannomas?

Answer 24

Meningiomas are sessile, demonstrating a broad base along
the petrous ridge, and they have dural “tails.” They often
cause hyperostosis of the adjacent underlying bone and
may have intratumoral calcifications. In contrast, vestibular
schwannomas have a globular appearance and are centered
over the IAC.

Question 25

From what meningeal layer do meningiomas

originate?

Answer 25

The arachnoid cap cells of the outer surface of the

arachnoid mater

Question 26

What percentage of paragangliomas will prove to

be malignant?

Answer 26

Approximately 4%

Question 27

From what cells do glomus tympanicum tumors

originate?

Answer 27

Cells of neural crest origin arising from the tympanic branch
(Jacobson nerve) of the glossopharyngeal nerve

Question 28

Describe a positive Brown’s sign.

Answer 28

● Blanching of a middle ear mass when performing
pneumatic otoscopy
● May be seen with glomus tympanicum or glomus
jugulare tumors

Question 29

What is the most appropriate management of a
growing glomus tympanicum tumor that is limited
to the promontory?

Answer 29

Transcanal tumor removal

Question 30

What is the most appropriate management of a
glomus tympanicum that fills the middle ear and
extends into the mastoid air cells?

Answer 30

Tympanomastoidectomy with extended facial recess ap-

proach and tumor removal

Question 31

How frequently would you expect a patient with a
glomus tumor to have elevated catecholamine
levels on laboratory workup?

Answer 31

About 1 to 3% of glomus tumors will have a secretory

component with elevated catecholamine levels.

Question 32

Describe the classic radiographic findings of a

glomus jugulare on temporal bone CT.

Answer 32

Homogeneous soft tissue mass centered over an enlarged

jugular foramen with “moth-eaten” irregular bony margins.

Question 33

What nuclear medicine scans can be used to

evaluate for head and neck paragangliomas?

Answer 33

Metaiodobenzylguanidine scan (MIBG), PET scan, and
111 In-pentetreotide scintigraphy

Question 34

What is the general approach to treatment of

petrous apex cholesterol granulomas?

Answer 34

Rather than excision, cholesterol granulomas are treated by

drainage procedures. Transcanal (infracochlear), transmastoid (retrolabyrinthine, infralabyrinthine, subarcuate), and
transsphenoidal endoscopic approaches are all methods of
drainage that spare inner ear function.

Question 35

How can a cholesterol granuloma be differentiated

from an epidermoid cyst on imaging?

Answer 35

Classically, cholesterol granulomas appear as T1 and T2
hyperintense expansile lesions of the petrous apex that do
not enhance with gadolinium. Epidermoids, on the other
hand, most commonly demonstrate low intensity on T1 and
high intensity on T2. Furthermore, diffusion-weighted MRI
will reveal diffusion restriction with epidermoids. CT results
are often similar.

Question 36

What should be considered when determining the
best surgical approach for drainage of a petrous
apex cholesterol granuloma?

Answer 36

The location of the lesion in relationship to the jugular bulb,
sphenoid sinus, eustachian tube, otic capsule, and most
importantly the ICA should be considered. The patient’s
preoperative hearing status is also to be considered.
Extended endonasal approaches are now being used with
increasing frequency, but lesions in the posterior inferior
petrous apex (between the carotid foramen and jugular
foramen) are usually best approached by a transcanal
infracochlear approach.

Question 37

What is the key radiologic feature of a petrous

apex epidermoid?

Answer 37

The key distinguishing feature between epidermoids and
other lesions is the high-signal intensity of diffusion-weighted
images. Epidermoids exhibit markedly restricted diffusion
and therefore are bright on diffusion-weighted MRI.

Question 38

How can you radiographically differentiate an IAC/
CPA lipoma from a vestibular schwannoma or
meningioma?

Answer 38

A lipoma is hyperintense on precontrast T1-weighted MRI

and will subtract with fat-supression techniques.

Question 39

Describe the management approach for a CPA/IAC

lipoma?

Answer 39

Most CPA/IAC lipomas do not grow, and generally a “watch-
and-scan” approach is used. Surgical resection is reserved for patients with intractable symptoms that are clearly
attributable to mass effect.

Question 40

From where do skull-base chondrosarcomas most commonly arise?

Answer 40

Chondrocytes of the foramen lacerum at the petroclival

synchondrosis

Question 41

What is the most common location from which a

skull base chordoma arises?

Answer 41

Chordomas are tumors that arise from remnants of the
notochord. These can occur anywhere along the axial
skeleton, but at the skull base, they are almost invariably
found at the clivus.

Question 42

What percentage of patients with Von Hippel-Lindau disease will develop endolymphatic sac tumors?

Answer 42

Approximately 10%

Question 43

What is the primary management strategy for

endolymphatic tumors?

Answer 43

Complete surgical excision