EXERCISE 17

ENDOSCOPIC ENDOMEATAL DISSECTION OF INTERNAL AUDITORY CANAL

STEP I – MODIOLUS AND COCHLEAR NERVE.

Cochlear nerve enters the cochlea from IAC at the wall between 1st and the 2nd turn of cochlea through tractus spiralalis foraminosus of the IAC.
The tractus spiralis foraminosus present at the antero inferior wall of the fundus of IAC

STEP J – NERVES OF INTERNAL AUDITORY MEATUS
All the four nerves are visualised endoscopically in the internal auditory canal

STEP K – PORUS OF IAC
Insinuation of 0 degree endoscope medially visualises the porus of IAC particularlly in cadevers.
Here all the nerves are displaced anteriorly

STEP L – TRANSMEATAL VISUALISATION OF IAC AND ITS RELATIONS WITH ICA AND JUGULAR BULB “FRONT DOOR” TO ENDOMEATAL ENDOSCOPIC DISSECTION OF LATERAL SKULL BASE.
Once cocheactomy is done, skeletonization of IAC and jugular bulb is performed.
The front door to lateral skull base is seen through this door majority of the lateral skull base procedures are performed including petreousectomy, type B, ITF approaches, exposure of jugular bulb endoscopic transclival approaches and complete exposure of facial nerve in temporal bone.

Labelling:
CN – Cochlear nerve
FN – Facial nerve
SVN – Superior vestibular nerve
IVN – Inferior vestibular nerve
P – Porus of IAC
F – Fundus
HFN – Horizontal FN
IAC – Internal auditory canal
PA – Petrous apex
ICA – Internal carotid artery
JB – Jugular bulb

ENDOMEATAL ENDOSCOPIC DISSECTION OF INTERNAL AUDITORY CANAL

STEP E – COCHLEAR NERVE ENTERING THE MODIOLUS
In endomeatal dissection dissection of IAC the first nerve to be identified is cochlear nerve. It is the thickest of all the nerves of IAC.
we can see clearly the entry of cochlear nerve in to modiolus.

STEP F – TRANSVERSE CREST
Once the bone over the fundus is removed the transverse crese is visible seperating cochlear nerve to facial nerve anteriorly.
The relation of nerves are observed

STEP G – IDENTIFICATION OF TRIANGULAR SPACE BETWEEN COCHLEAR AND FACIAL NERVES.
Once the postero inferior wall of IAC is dissected and superior and inferior vestibular nerves are cut surgeon can observe a triangular gap between cochlear and facial nerves.

STEP H – DISSECTION OF POSTERIOR WALL OF IAC 270 DEGREES.
This dissection allows visualisation of all 4 nerves of IAC. In endomeatal approach the facial nerve position is far away to surgeon

Labelling:
CN – Cochlear nerve
S – saccule
U – utricle
2nd C – 2nd turn of cochlea
MO – Modiolus
TC – Transverse crest
1 – cochlear nerve
2 – Facial nerve
3 – Superior vestibular nerve
IVN – Inferior vestibular nerve
IAC – Internal auditory canal

Fig G
3 – Apex of cochlea
4 – Vestibule
5 – Basal turn of cochlea



ENDOMEATAL ENDOSCOPIC DISSECTION OF INTERNAL AUDITORY CANAL


Normally internal auditory canal can be approached 1 Translabyrinthine approach
2 Middle fossa approach
3 Trans otic approach
4 Trans cochlear approch
For endoscopic surgeon endoscopic endomeatal approach is very essential The endoscopic dissection of IAC requires good anatomy in that region. These are the pictures of endomeatal endoscopic dissection of IAC


STEP..A
LOCATION OF IAC IN DRY BONE ON MEDIAL WALL OF MIDDLE EAR..After cochleactomy and exposure of the medial wall of the oval window the location of iac is situated below the anterior half of the oval window


STEP..B
IAC LOCATION ON MEDIAL WALL OF THE MIDDLE EAR


STEP..C
DISSECTION OF BENZ WHEEL…
After cochleactomy the BENZ wheel is formed between oval window superiorly ,the 2nd turn of cochlea antero superiorly and basal turn inferiorly The center of wheel is entry point of cochlear nerve in to modiolus from iac .The coclear nerve is identified


STEP D..
COMPLETE EXPOSURE OF COCHLEAR NERVE….Careful removal of bony wall between 1st and 2nd turns of cochlea exposes cochlear nerve completely in fundus of iac


SURGICAL IMPORTANCE
For exposure of IAC the completecochleactomy is not necessary . For small acoustic tumour removal in total hearing loss limited exposure of Benz wheel with maximal preservation of cochlea ,tumour removal from iac and cochlear implant are the present concepts of the treatment
HFN..horizontal facial nerve OW..Oval window IAC..Internal auditory canal 1..Benz wheel 2 spherical recess 3..elleptical recess 4..Wall between oval window and cochlea CN..Cochlear nerve


STEP A
COCHLEACTOMY…Cochleostomy is the initial step of the dissection
C..Basal turn of cochlea OW..Oval window
STEP B..EXPOSURE OF COCHLEAR AQUEDUCT
After cochleactomy, the aqueduct is completely exposed.T he duct connects the scala tympani to the subarachnoid space in the pyramidal fossa. It is very thin duct admits the hair only
1..Internal auditory canal 2..Cochlear aqueduct 3..Pyramidal fossa 4.Jugular bulb


STEP B1
THE INNER OPENING OF COCHLEAR AQUEDUCT IN SCALA TYMPANI
The inner opening of the duct is present inside the round window membrane behind the posterior part of CRISTA SEMILUNARIS
1..The inner opening of cochlear aqueduct 2.. vertical crest 3..crista semilunaris 4..Scala tympani 5..Scala vestibuli 6..Area of rwm attachment


STEP C
COCHLEAR AQUEDUCT AND INTERNAL AUDITORY CANAL
Cochlear aqueduct after coming out from scala tympani it runs below the internal auditory canal parallel to its floor to enter the pyramidal fossa to open into subarachnoid space. Transmeatally once the sac is removed the pyramidal tip of the petrous bone (pyramidal fossa border) is seen
1..Cochlear aqueduct 2..Transmeatal exposure of IAC 3..Internal carotid artery