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Endoscopy of the Cerebellopontine Angle: What did we learn?
Category : Skull Base Surgery » Acoustic neuroma
The cerebellopontine angle (CPA) is defined as the angle formed in the horizontal place by the pons and the cerebellum in which are inscribed the trigeminal and acoustico-facial nerve bundles

ENDOSCOPY OF THE CEREBELLOPONTINE ANGLE: WHAT DID WE LEARN?
Jacques MAGNAN
ORL, Hôpital 13915 MARSEILLE CEDEX 20 (F)


The cerebellopontine angle (CPA) is defined as the angle formed in the horizontal place by the pons and the cerebellum in which are inscribed the trigeminal and acoustico-facial nerve bundles. The angle is bordered antero-laterally by the posterior face of the petrous temporal bone.

Three distinct levels exist in the CPA, each with it own nerves and associated arteries and veins.
Level I: located superiorly with trigeminal nerve, superior cerebellar artery and superior cerebellar vein

Level II: middle section. The main landmark is the internal auditory canal ( IAC) in which enters the acoustico-facial nerve bundle; the anterior-inferior cerebellar artery and inconstant vein.

Level III: located inferiorly with lower cranial nerves, posterior-inferior cerebellar artery and vertebral artery.

It is important to note that the acoustico-facial nerve bundle in the IAC lies approximately 5.5 cm deep to the level of a retrosigmoid craniotomy.

The anatomical relationships of IAC are as follows:

1 – The labyrinth (posterior-semi-circular canal and vestibule); the average distance from the posterior lip of the porus to the labyrinth is between 6 – 8 mm.

2 – The jugular bulb: the height of the bulb can reach the floor of the IAC and, exceptionally, can extend even higher,
The preoperative CT scan assessment shows these anatomical relationships.


1 - Keyhole retrosigmoïd approach

1-1 The operating position is dorsal decubitus i.e.the usual otologic position. Head rotated away from the surgeon with moderate flexion of the neck.

1-2 Neuromonitoring is required, bipolar diathermy is indispensable and the operating microscope is used with 250 or 300 mm focal length.
Specially adapted microinstrumentation is useful. Conventional otologic instruments are too short and the usual neurosurgical instruments are too bulky. Specific acoustic neuroma set offers instruments with a bayonet handle, which facilitates visualization with the keyhole approach. Each instrument is designed for use in specific direction.

1-3 Surface landmarks are the posterior margin of the mastoid process and the Frankfort plane, which is adjacent to the superior occipital line.

1-4 Cranial landmark is the mastoid emissary vein which centers the craniotomy, close to and just behind the sigmoid sinus (15-20 mm diameter)

1-5 Requirements for a successful exposing of the CPA
1. Spontaneous retraction of the cerebellum thanks to anesthetic agents, hyperventilation and hypocapnia
2. U. shaped dural incision
3. Free drainage of cerebrospinal fluid (CSF)
4. Follow the posterior surface of the temporal bone to open the basal cistern to allow the complete emptying of CSF from the posterior fossa.
This allows the exposure of the neurovascular structures crossing the CPA without the use of retractors.
5. Protect the surface of cerebellum with cottonoïds. The goal is to obtain a safe « corridor » within which to operate. Having which is maintained spontaneously without the need for retractors provides this. The « corridor » has a safe bony wall superiorly and a protected cerebellar surface inferiorly.

1–6 we recommend rigid endoscopes for precise surgical maneuvers. The safest endoscope is 6 cm long, 4 mm in diameter, and 0°. This gives a panoramic view and the limited length prevents inadvertent contact with deeper structures. Other useful endoscopes are length 11 or 14 cm, 4 mm and 2.7 mm diameter, 0 and 30°.
Aseptic technique is essential and achieved by using autoclaved endoscopes and video-endoscopic control, protection camera and light cable with sterile covers.
Both the nursing staff and the surgeon should be fully trained in the use of endoscopic techniques.

2– Applications

1-6 Acoustic Neuroma
The concept of « minimally invasive » surgery must be synonymous with « minimal morbidity » surgery.
The challenge of contemporary acoustic neuroma surgery is to save facial nerve in all cases and hearing in 50 % of cases when the tumor size is less than 2 cm in the CPA.
At the beginning of the procedure, endoscopy allows mapping of the neurovascular structures adjacent to the tumor.
When the removal is considered to have been complete, endoscopy can check with certainty the eradication from the lateral extremity of the IAC this avoiding the necessity of drilling away excessive bone (which would put the labyrinth risk).
Results:
From 1994 to 1999: 168 Acoustic Neuromas using minimally invasive technique
(Grade I: 26 cases intracanalicular; Grade II: 77 cases, to 10 mm within the CPA; Grade III: 54, 11 to 22 mm within the CPA, Grade IV: 11 superior to 20 mm).


Total removal was performed in 163 cases, incomplete removal in 5 cases with 1 recurrence.
Facial nerve was preserved in 100 % of cases with 91 % of normal function.
Cochlear nerve was preserved in 76 % of cases with 46 % of hearing preservation.


1-7 Microvascular decompression in Trigeminal neuralgia and hemifacial spasm.
Per-operative endoscopy is the key for the safe and reliable identification of the offending vessels.
Endoscopy provides a dynamic and panoramic view of the entire neurovascular bundle without causing any distortion. This enables multiple sites of compression to be visualized, which would otherwise escape detection. The view is equally clear both outside and within CSF. While the vascular decompression at present is done with the microscope, the adequacy of the decompression can be checked by endoscopy at the end of the operation.
MRI scanning using a CISS (constructive interference in steady state) T2 sequence is indispensable in pre-operative assessment.

Results of microvascular decompression in 223 hemifacial spasms.
The most common offending vessel was the posterior inferior cerebellar artery associated with the vertebral artery in 45 % of cases, alone in 16 %. The result after 6 months follow-up: relief 91 %, failure 9 %. After revision surgery relief 96 %.
Post operative complications: facial paralysis: 0 Total hearing loss: 2 Partial hearing loss: 3

Results of microvascular decompression in 115 Trigeminal neuralgia.
The must common offending vessels were the superior cerebellar artery in 76 % alone in
50 % associated with aberrant venous structure in 26 %.
The results after 6 months follow up were complete relief in 80 %, partial relief in 6 %, and failure in 14 % with no morbidity.

1-8 Vestibular neurotomy
Minimally invasive retrosigmoïd approach is particularly suited to vestibular neurotomy to treat incapacitating vertigo in Meniere’s disease when residual hearing still exists. The retrosigmoïd approach is not technically difficult provides excellent access, and allows a sure and selective section of the vestibular nerve.

A short video highlighting these points will be shown.

References:
MAGNAN J., SANNA M.
Endoscopy in Neurotology
Thieme Verlag, Stuttgart 1999.
Prepared by : Prof. Jacques Magnan