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Retrolabyrinthine approach to petrous apex

Introduction:

This approach is considered to be the unsung hero of skull base surgery. This technique is ideally suited for patients with pathologies involving the posterior cranial fossa with retained hearing. Directly accessing the Cerebellopontine angle through temporal bone and avoiding neural structures preserves hearing. This approach allows for mobilization of the sigmoid sinus posteriorly and access to the posterior fossa through the presigmoid space. This approach provides excellent exposure laterally from the 4th cranial nerve to the upper border of the jugular tubercle. There is only limited access to the ventral brain stem and clivus. Factors that limit this approach:

1. Poorly pneumatized mastoid

2. Forward lying sinus

3. High jugular bulb

4. Low lying tegmen

This approach can be used by itself for small tumors or in conjunction with other techniques to gain greater exposure. These combined approaches include:

Translabyrinthine approach Infratemporal approach Transcochlear approach Combined transtemporal approaches Retro sigmoid craniotomies

 

Indications:

1. Resection of CP angle tumors

2. Resection of petrous ridge tumors

3. Vestibular neurectomy

4. Partial resection of the sensory root of 5th cranial nerve

5. Fenestration of symptomatic arachnoid cysts

6. Meningiomas

7. Metastatic lesions

8. Biopsy of brainstem lesions

9. In conjunction with other approaches in extensive skull base surgeries

 

Procedure:

This surgery is performed under general anesthesia. Patient is placed supine. Surgeon should be seated comfortably during surgery. The patient’s head is rotated 70° away from the surgeon. Hair is removed about 4 cms superiorly and post auricularly in order to site the incision.

Facial nerve monitoring electrodes should be placed and verified for its function. Abdomen is also prepared to harvest abdominal fat. Preoperative antibiotics are also administered on the table. Before starting the surgery, Intravenous mannitol and frusemide are administered to bring down the intracranial tension.

Incision:

A C shaped incision is made with a 15-blade scalpel 3-4 cm posterior to the post aural crease extending up to the mastoid tip.

 

 

Image showing incision used in this procedure

 


Skin and subcutaneous tissue flap is elevated anteriorly up to the external acoustic meatus. Next an offset incision is created through the temporalis muscle, fascia and periosteum. This helps later during wound closure as the wound can be closed in layers. This layered closure helps in prevention of CSF leak.

A periosteal elevator is used to elevate the periosteum away from the cranium exposing the mastoid cortex.

The following bony landmarks need to be identified:

Root of the zygoma

External auditory meatus

Linea temporalis

Mastoid emissary foramen

Asterion

Henle’s spine

The following triangles should be identified before actual drilling starts:


Fukushima outer mastoid triangle:


Three points of this triangle include:

• Posterior root of zygoma

• Asterion

• Mastoid tip


Fukushima Inner triangle (Trautmann’s triangle)


• Anterior – Superior (anterior) semicircular canal

• Superior – Superior petrosal vein

• Lateral – Sigmoid sinus

• Inferior – Jugular bulb

McEwen’s triangle:

• Flat triangle behind the external auditory canal

 

 

Figure showing the various triangles that must be marked during the procedure

 

Bone over the Fukushima’s outer triangle is drilled out using a cutting burr. Under magnification a complete mastoidectomy is performed. Proper size diamond burr bit is used to remove bone overlying middle cranial fossa dura, sigmoid sinus and posterior fossa dura. Maximal exposure of dura could be obtained by skeletonizing the sigmoid sinus and jugular bulb completely. The lateral semicircular canal and posterior semicircular canal should be well defined.

The entire course of mastoid segment of facial nerve should also be deroofed. Sinodural angle dura should also be exposed by careful drilling in the area. Bone over the posterior fossa dura between the posterior semicircular canal and the sigmoid sinus should be removed with blunt dissection. Care must be taken to protect the underlying endolymphatic duct and sac. Using gelfoam aditus and mastoid antrum is packed. The mastoid cavity should be copiously irrigated with bacitracin solution in order to remove any bone dust that may be present there.

 

 

Image showing the various landmarks used to identify post fossa dura

 

 

Image showing endolymphatic sac


A 11 blade and micro scissors is used to open the dura anterior to sigmoid sinus. It is opened with an anteriorly based C shaped flap as shown below.

 

 

 

 

 

The endolymphatic sac would be visualized inferior to the posterior canal as a thickened area of dura. The dural flap is secured with stay sutures for better exposure. A neurosurgical cottonoid patty is placed over the brainstem. This produces a small amount of tension between the cerebellum and the petrous ridge. The arachnoid adhesions in this location are transected and CSF is released. This causes the cerebellum to fall away from the petrous ridge allowing better visualization of the CP angle. Posterior face of petrous ridge, and cranial nerves 7 and 8 in the center of the field. In addition, this exposure provides access to the Cranial nerve 5 anteromedially. Cranial nerves 9, 10, and 11 lie inferolaterally. It should be noted that the rostral division of the anteroinferior cerebellar artery is associated with the 7th and 8th cranial nerves. After completion of the procedure, meticulous hemostasis is secured. The dural flap is approximated with 4-0 braided suture. The aditus, antrum, facial recess and retrofacial air cells are covered with temporalis fascia. The entire mastoid cavity is obliterated using abdominal fat graft to prevent CSF leak. The wound is then closed in layers.

Complications:


1. Bleeding from dural venous sinuses

2. Cerebellar oedema

3. Injury to cochlear nerve

4. Injury to facial nerve

5. Injury to intracranial blood vessels

6. CSF leak

7. Post op head ache

8. Conductive hearing loss if bone dust is not properly removed by irrigation, or if the abdominal fat graft herniates into the middle ear cavity.

 

 

 

 

Balasubramanian Thiagarajan

Professor Balasubramanian Thiagarajan

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