The Endoscope-Assisted Minimally Invasive Retro-Sigmoid Approach (EAMIRSA): A Combination of Techniques for Improving Middle Skull Base Surgery

Summary

The endoscope-assisted minimally invasive retro-sigmoid approach (EAMIRSA) is a surgical technique that can be used for the treatment of the cerebellopontine angle (CPA) and internal auditory canal (IAC) disease. Here, we describe the material necessary to perform EAMIRSA and illustrate the technique using a step-by-step cadaver dissection.

Abstract

EAMIRSA is a surgical technique that can be used for the treatment of the cerebellopontine angle (CPA) and internal auditory canal (IAC) diseases, as well as for treating schwannoma, for decompression surgeries (e.g., loops of the anterior inferior cerebellar artery (AICA)), and for the vestibular neurectomy in patients with invalidating vertigo. This technique combines the use of an endoscope and a microscope; the former allows a perfect view of the surgical area (CPA, IAC, and brain structures) and the latter ensures safety of the surgery maneuvers. The use of a minimally invasive approach reduces post-surgery headaches and the risk of a Cerebrospinal Fluid (CSF) leak. Our group successfully used EAMIRSA during decompression procedures for treating a hemi-facial spasm and tinnitus. Results were satisfactory in terms of function recovery. Sequelae and surgical complications were observed in less than 1% of patients. In acoustic schwannoma surgery, facial nerve damage was observed in less than 1% of cases and the recurrence rate was 0.3%. This article describes the material necessary for performing EAMIRSA and illustrates the technique using a step-by-step cadaver dissection.

Introduction

Surgical approaches to the cerebellopontine angle (CPA) and internal auditory canal (IAC) are associated with high risks due to a limited view of the target areas. Techniques that totally rely on endoscopy have been proposed^1,2, but the high risk of facial nerve sequelae¹, significant learning curves, and the need for a third hand for maintaining the endoscope in position² have limited their applicability.

The endoscope-assisted minimally invasive retro-sigmoid approach (EAMIRSA), originally proposed in 1976³, was further developed in 1993⁴ by Magnan, who used this approach for endoscope-assisted surgery. The technique was improved by combining a microscope and an endoscope to safely treat neurovascular conflicts in the CPA/IAC⁵. Furthermore, different from the totally endoscopic transpromontorial approach¹, the retrosigmoid route allows a total preservation of the cochlea and labyrinth^2,3,4,5,6; this combined approach reduces the risks that are typically associated with approaches that rely on an endoscope only or a microscope only. EAMIRSA's major advantage lies in the use of the endoscope, which allows a 360° view and thus a complete visualization of the structures' relative positions^6,7,8. Additionally, the endoscope allows a minimally invasive approach that reduces the risk of persistent headaches and cerebrospinal fluid (CSF) leak, which are common major complications of this type of surgeries⁹.

The endoscope improves visibility of the structures' relative positions as the instrument goes inside the surgical area, thereby allowing the surgeon to investigate CPA anatomy via careful movements of the surgical instruments. This improved visualization eliminates the need for the extended craniotomy (commonly performed in CPA surgery to increase visibility and eliminate "blind" angles).

In EAMIRSA, the endoscope is first used to view and inspect the target area, which is then dissected with two hands under microscope vision^6,7. EAMIRSA can be used in the treatment of different CPA and IAC diseases, schwannoma removal¹⁰, and surgical decompression of cochlear⁶ or facial nerves⁷. It has been proven to increase both the short and long-term surgical outcomes with a very low incidence (<1%) of major sequelae^5,6,7,8.

In this article, we describe the material necessary for performing EAMIRSA and illustrate the technique using a step-by-step cadaver dissection.

Protocol

This study was approved by the Human Ethic Committee of University Hospital of Perugia and was conducted in accordance with the ethical principles outlined in the Declaration of Helsinki.

The sample was represented from subject over 18 years, with equal sex distribution.

1. Preparation of the Patient

1. Set up the operating room as shown in Figure 1. Place the patient on the surgical table and connect the anesthesia monitoring system.
2. Anesthesia
   1. Pre-medicate with 1-2 mg of Lorazepam Oral intravenously or with 2-4 mg of Lorazepam Oral if using oral suspension.
   2. 5 to 15 min after premedication, induce general anesthesia by combining drugs for total intravenous anesthesia (TIVA) such as Propofol, short-acting opioid, and low- to-moderate levels of inhalational agents¹¹.
3. Intubation
   1. When the patient sleeps, open his/her mouth with the dominant hand by placing a thumb on the lower jaw and the middle finger on the upper jaw. Extend the patient’s neck.
   2. Open the laryngoscope blade and insert it into the patient’s mouth. Slowly advance the blade along the tongue dorsal surface until the tip of the epiglottis and the vocal folds is seen.
   3. Under the direct vision of the glottic plane, insert an armed endotracheal tube for intubation.
   4. To conclude this procedure, attach the endotracheal tube to the automatic respiratory machine¹².
      NOTE: This armed endotracheal tube is preferred in order to avoid possible occlusions and narrowing caused by inappropriate jaw contractions by the patient due to stimulation of the cranial nerves in the surgical area¹³.
4. Surgical Preparation of the Patient
   1. Cut the patient’s hair 6-8 cm posterior to the ear with a sterile scalpel.
   2. Turn the patient’s head to the side contralateral to the surgical site. Disinfect the skin of the surgical area with a sterile gauze imbibed with iodine-based scrub or chlorhexidine solution and grabbed with sterile gloves; repeat this step at least twice.
   3. Turn the patient head from the lateral position to a frontal position and stabilize him/her in supine position.
      NOTE: The head must be turned to the side opposite to the surgical side (45°-60°), with a small flection of his/her head and chin near the clavicle.
   4. Protect the patient’s eyes with a lacrimal gel and close the eye with a bilateral ocular bandage. Place electrodes for facial monitoring 1 cm from lateral cantus and 1-2 cm from the mouth angle (Figure 2a).
   5. Change the sterile gloves with new ones. By using 4 sterile drapes prepare a sterile area that includes the posterior part of the ear and the mastoid area following the best practice guidelines^14,15.
      1. Start by placing the first drape covering the face, the second one covering the patient’s hair, the third covering the neck and the last one posteriorly to the mastoid area.
      2. Finally cover the field with transparent sterile adhesive surgical patch (Figure 2b).

2. Surgical Technique

1. Skin and Muscle Time
   1. With a dermographic pen, draw the proposed incision line in the retro-auricular area (Figure 2c, and Figure 3).
   2. Inject 5-7 mL of 2% lidocaine with 1: 100,000 epinephrine solution using a 5 mL syringe.
   3. With a cold scalpel (No 11), perform a skin incision of 6-8 cm with an arciform shape. Make the incision with a convex side facing backwards, about 1 cm behind the supposed posterior edge of the craniotomy and 2 fingers behind the helix projection on the retro-mastoid region.
   4. With an electric scalpel, elevate an anterior skin flap. Perform a muscle-periosteal incision with posterior convexity with a monopolar scalpel (Figure 4a).
   5. Insert a Beckman orthostatic retractor. Expose the posterior edge of the mastoid in the posterior part of the surgical field, the digastric groove that is under the ear lobe, and the mastoid emissary vein, which bleeds more than the surrounding areas. Coagulate the mastoid emissary vein (Figure 4b and 4c).
      NOTE: From now on, the surgical procedures are performed under microscope vision adjusting the magnification (from smaller to higher) to clearly see the anatomical structures.
   6. Grab the sterile microscope for otoneurosurgery with two hands and place it close to the operatory area. Focus the microscope by using 1X magnification and adjust it until the operatory area is perfectly in the center. Then use 1.6X magnification to start the bone time.
2. Bone Time (Figures 5a, 5b, 5c, 5d)
   1. Take the drill with a large cutting burr and perform a circular craniotomy of 1.5- 2 cm in diameter posterior to the sigmoid sinus using its emissary vein as a landmark.
      NOTE: The sigmoid sinus emissary vein is a synonymous of mastoid emissary vein.
   2. Start the dissection with a large cutting burr; then, use a diamond burr when the dura and the sigmoid sinus is approached to preserve their surfaces. Perform the drilling until the sigmoid sinus is completely exposed, leaving a very thin bony shell over it to preserve the venous structure.
      NOTE: The sigmoid sinus is the posterior limit of the mastoidectomy area. One can identify it by listening to the sound of the drill during the procedure: the sound of the cutting burr during drilling will become metallic when in contact with the sinus. Change the cutting burr with a diamond one when the sound changes.
   3. After exposing the sinus, remove the hard bone from its posterior surface. When only a thin bony shell remains, use a micro bone-dissector to remove the bone without injuring the dura.
   4. Preserve the bone dust that comes out from drilling by collecting it with a spatula and then by putting it in metallic bowl until the closure time.
3. Dura Time (Figures 6a, 6b, 6c)
   1. Before this step, ask all the members of the surgical team to change their sterile gloves.
      NOTE: During this surgical step, ask the anesthesiologist to increase the dose of general sedation for increasing the depth of the patient’s sleep.
   2. With a micro-scalpel open the dura with a V shape incision, starting just behind the sigmoid sinus to reduce the need for cerebellar mechanical retraction during the access to the IAC.
      NOTE: The dura incision needs to be performed without touching the brain and 1-2 mm from the craniotomy edges in order to facilitate dura re-suturing at the end of surgery.
   3. Perform the dura hemostasis by holding the bleeding point with a bipolar coagulator forceps and then coagulating by 1-2 shots. After that, fasten the dura to the adjacent tissues with a silk suture.
   4. Ask the anesthesiologist to gradually dehydrate the patient by administering 1 g/kg mannitol and by hyperventilation leading to an arterial pCO₂ between 25 to 32 mmHg, which reduces CSF intra-cranial pressure and causes a spontaneous retraction of the cerebellum.
   5. Place a thin neurosurgical micro-cotton or surgical substitute of dura (1.5 cm x 5 cm) over the cerebellum.
4. Cerebellopontine Angle (CPA) Approach by Using a Microscope (Figure 6d, 6e, 6f)
   1. Grab a small piece of cotton patch with a micro clamp without teeth and gently compress the cerebellum posteriorly.
   2. With micro-scissors, open the cisterna magna to complete the access to the CPA. Use an aspirator with hand-suction control.
   3. Under microscope view, dissect the arachnoid surrounding the cranial nerves (acoustic-facial bundle) and around the lower cranial nerves with a smooth micro-dissector.
5. Endoscopy Time
   NOTE: This surgery step lasts about 30 min. (Figure 7a-7b)
   1. Carefully insert a rigid 30° endoscope directing downwards to visualize the CPA.
   2. In the center of the surgical field, first identify the acoustic-facial bundle (central landmark of the CPA) as a group of nerves running from the cerebellum towards the IAC. Then identify the area where the vessel(s) is(are) direct contact with the bundle causing the neurovascular conflict (NVC).
   3. Inspect the area around the facial-acoustic bundle using the 360° view allowed by the endoscope (Figure 8a, 8b, 8c).
   4. During this procedure, that takes about 5 min, periodically take out the endoscope from the field and clean its tip by touching it with the cotton patch (this maneuver will help cool down the endoscope’s light); then re-introduce the endoscope into the surgical field.
6. Microvascular decompression under microscope with endoscope assistance (Figure 7c, 7d, 7e)
   1. Take a smooth round micro-dissector and gently separate the vessel (offending) that is attached to the nerve involved in the microvascular compression.
      NOTE: The most common offending vessel is the posterior inferior cerebellar artery (PICA) followed by the anterior inferior cerebellar artery (AICA).
   2. If the offending vessel juts in the first portion of the IAC, drill this bony wall with a very small diamond burr to free the nerve completely.
      NOTE: The drilling direction is from the CPA (behind) to the temporal bone (posterior to anterior). During the procedure, drill away only 1-3 mm of bone to prevent damage to the endolymphatic structures (sac and/or duct). It is important to follow the nerve direction to identify the IAC bony wall.
   3. If the loop causing the nerve compression is due to a vein, after vessel detachment, coagulate the vein by using 1-2 shots of bipolar forceps coagulator.
   4. Grab a polytetrafluoroethylene sponge with a micro-Hartman alligator forceps and insert the sponge between the offending artery(ies) and the nerve (facial/cochlear/ vestibular) to prevent impingement recurrences.
   5. Move the sponge using a micro-dissector until an appropriate and stable position is reached.
      NOTE: The position where no part of the vessel remains in contact with the nerve is considered as an appropriate position. The sponge needs to restore the normal facial-acoustic bundle anatomy, where vessels run close to the nerve(s) without crossing or touching it/them.
   6. At the end of the microvascular decompression, use the endoscope to check the sponge perfectly divides nerve(s) and vessel(s) and to double-check the result of the decompression by orienting the endoscope upwards so as to properly visualize the vertebral artery that may cause further impingement of the nerves.
7. Closure (Figure 9A-9E)
   1. With a micro-Hartman alligator forceps remove the neurosurgical micro-cotton from the surgical area and wash the CPA area with abundant saline solution with a syringe.
      Note: Abundant irrigation is needed to remove residual blood cloths.
   2. Hold a piece of dura substitute with micro-Hartman pliers and place it to fill the posterior cisterna.
   3. With a micro needle-holder, place 6/0 single re-absorbable stitches to close the dura. In order to obtain a waterproof closure, insert some muscular tissue between the stitches and fix it with fibrin glue.
   4. Use a micro-clamp without teeth to place a piece of dura substitute on the dura external surface, then add a slide of absorbable hemostat of adequate size to cover the dura.
   5. With a micro-spatula take the bone dust, preserved during the drilling time, and lay it on the craniotomy area to obtain a homogeneous cover. Then lay some fibrin glue on the top to totally recover and fix the area. Place 2/0 absorbable interrupted sutures to close the muscle-periosteal flap and 3/0 subcutaneous absorbable stitches.
   6. Suture the skin with 2/0 nonabsorbable stitches.
   7. Apply a compression bandage, which will be kept for 4 days.

3. Post Operatory care

1. Ask to the anesthetist to remove the endotracheal tube and wake up the patient.
2. Prescribe antibiotic treatment with cephalosporins, analgesic and glycerol (if necessary) then send him/her in the intensive care department for 24 h.
3. After the patient has come back to the otolaryngology department (24-48 h), test the spontaneous nystagmus by looking at his/her eyes during the finger-following test and evaluate if nerve deficits are present. If the patient is comfortable, ask him/her to walk with the person present in the room.
4. After 7 days suspend medical treatments if no complication occurs and discharge the patient to his/her house.

Results

EAMIRSA has been successfully used by several surgeons^4,5,6,7,9. Our group has used this technique on patients suffering from facial hemispasm (HFS) due to a vascular loop compression on the VII cranial nerve (facial) and has achieved satisfactory results in the short- and long-term follow-up, including short recovery time, sym...

Discussion

Loops in the CPA and IAC ultimately manifest with a number of symptoms/signs, including HFS and tinnitus. In our previous studies^6,7, we quantified the correlation between the loop presence and the occurrence of HFS (or tinnitus) on the ipsilateral side. We also found that in cases where tinnitus was due to a loop impingement involving the cochlear nerve, the minimum caliber of the impinging vessel was 0.8 mm⁶.

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Materials

Name	Company	Catalog Number	Comments
No Consumable
Facial nerve monitoring system	NIM (Medtronic Jacksonville (FL), USA)	8253002
Video Column	AIDA, Storz	S-1263
Video Camera	STORTZ	S-1263
Focal Operatory microscope	ZEISS OPMI SENSERA	S-0607
Rigid Endoscope 0°	STORZ	7229 AA
Rigid Endoscope 45°	STORZ	7229 FA
Drill Stilus Legend	MEDTRONIC	1898001
Electric Scalpel DIATERMO MB 380	GIMA	30692
Aspiration system: 1 continuous and 1 with finger-control	TOBI	28234
Instruments for the otoneurology approach:
- Stainless cups.
- Large cup warms serum.
Lempert elevator	MILTEX	19-1340
- Scalpel Handle No. 3, 4, and 7.
- Needle holder: 1 strong, 1 less strong.	MAYO HEGART	26531
- Two pairs of scissors Allaine: 1courbe, 1 right.	EMEA	Curved : PO144123 ; Straight: PO143282
-Dissection griffes, foam, De Bakey	SURGIWAY	14.1012T
- Aspirations:
o Three Fraziers No. 10.12.15.	INTEGRA
o Otoneurology suction set: 1 * 2.0, 1 * 1.8; 2 * 1.5; 3 * 1.2 (long); 2 * 1.0.	INTEGRA	3724218; 3724172; 3724210
- Retractors:
o Faraboeuf: 1pair thin, 1 thick pair 12-14 cm	INTEGRA
o Auto-static of Wulstein or Beckmann	INTEGRA	o-28-8
DRILL BURRS	MEDTRONIC
	Cutting	See on productor catalogue
	Diamonds	See on productor catalogue
Micro-instruments of otoneurology:
Fine bipolar forceps Magnan	INTEGRA	CP392BI and CP392BS
A pair of scissor Sterkers.	NOT AVAILABLE
Two pairs of scissors Yasargil (1 right, 1 curve).	SKLAR	98-2114
Two-surgical forceps (one right, one curve upward).	INTEGRA Metzbound	Curved: CP351-3; Straight: CP351-2
A dura scalpel	STORTZ	28164 EL
A dura elevator	STORTZ	663535
In case of IAC first: surgical scissors straight strong
Straight instruments:
A hook	INTEGRA	MCO31
A blunt hook.	INTEGRA	MCO813
Two Marquet periostal elevator (1 left, 1 right).	INTEGRA	MCO950
Bayonet instruments
House Hartman	INTEGRA	3721036
One pointed and sharp hook	INTEGRA Causse	MCO657
Four small periostal elevator (facing up, down, left, right)	STORTZ	223982
Four large periostal elevator (facing up, down, left, right)	INTEGRA	MCO224A-2-0
Four hooks (facing up, down, left, right)	STORTZ	223942; 223943
MORTINI Set	STORTZ	28164 MDA
Instruments for otoneurosurgery approach :
Scalpel n°23
Dissection griffes	INTEGRA MAC INDOE	CP 400 ; CP 407 ;CP 415
Wulstein auto-static	INTEGRA	o-28-8
Elevator Lampert	MILTEX	19-1340
Bipolair forceps	INTEGRA	CP392-2R
Scalpel for dura	STORTZ	28164 EL
Scalpel n°11
Dissection griff DeBakey	SURGIWAY	14.1012T
Scissor Allaine	EMEA	Curved : PO144123 ; Straight: PO143282
Needle holder	MAYO HEGART	26531
Consumable Material
Two suction tubes, with suction cannulas of different sizes.
Operating light handpiece.
Microscope case, camera cover, instrument pocket.
Xomed / Medtronic 2-channel electrode set for monitoring the facial nerve.
Scalpel blades: 22, 11.	Medtronic
Glass scalpel blades.
Surgical gloves and caps
Adhesive surgical (craniotomy pack).
Sterile woven and nonwoven compresses.
Syringe and needle infiltration.
Drugs: 0.5% adrenaline xylocaine, saline, sterile water
Neuropatch	B/Braun-Aesculap, Tuttlingen, Germany
Fibrillar Surgicel	Braun
Biological glue 2ml	TIssucol
Horsley Wax
Cottons of neurosurgery cut into various sizes that will be carefully counted (opposite)
Teflon cut into various sizes then crushed to be thinned	Codman
Lyophilized hardstock or Durapatch Ethisorb Polyglactin 910 / Polydioxanon	Bard PTFE
Re-absorbable suture	Ethicon
Not-resorbable suture	Vicryl
Urethane sponge	Novafil 2.0