Basic information
- Allows exploration of the entire cerebello-pontine angle (CP angle):
- cranio-caudally – from the tentorium to the VII and VIII nerves
- medial to lateral – from the middle cerebellar peduncule to lateral aspect of the cerebellar hemisphere
- to the posterior petrous bone, internal auditory canal
To keep in mind
- Consider whether the patient (location of the lesion are suitable for this type of approach)
- Study carefully available images (CT, MR) to be prepared for anatomical rarities, use different section and projection
- Try to find the best route to the lesion with an avoidance of the excessive exposure and the retraction of brain tissue, otherwise use a different approach to reach the pathology
- Keep in mind the general anatomy (skin layers, muscle, skull bones, transverse-sigmoid sinus junction, CP angle, cranial nerves, brainstem, cerebellum)
- Be aware of possible slight distortion of the anatomy (patient is in the different position and the head is rotated – effect of the gravity)
- Try to project the anatomy on the skin surface
Highlights
Transverse-sigmoid junction, CP angle
Indications
Pathologies which are located in the CP angle.
Anatomy and important anatomical landmarks
- Occipital artery
- Skull bones – temporal bone, parietal bone, occipital bone, tip of the mastoid
- Superior nuchal line (indicates the superior end of the skin incision)
- Position of the sigmoid sinus – digastric groove (reference to the position of the sigmoid sinus, also indicates where to place the inferior limit of the skin incision)
- Position of the transverse sinus – a line connecting the root of the zygoma and the inion
- Asterion (can be palpated as a little depression, it is fundamental reference for placing the burr hole)
- CP angle
- Brainstem
- Cranial nerves
- Cerebellum, arteries (PICA, AICA, SCA)
Positioning
- Supine-lateral park bench position, 3/4 prone position or supine-lateral position
- Put the Mayfield fixation with sparing of the skin incision location
Body positioning: supine lateral (45°), lateral (90°), lateral oblique (135°), semi-sitting
Alternatively, the semi-sitting position is also widely used
The choice is dictated mainly by surgeon’s habits and preferences
Head positioning: contralateral rotation 0° lateral tilt 5° flexion -10°
Rotation
The degree of rotation depends on several factors:
▪ position of the pathology
▪ surgeon ergonomics and preferences (seated or standing surgical position)
Flexion
This aids in putting the occipital bone (which is rather flat) in a plane more parallel to the surgeon’s viewing position.
Contralateral bending
It helps in widening the space between the neck and the mastoid.
Red Flags
Avoid any site of compression by using multiple gel pads. Control the armpit and the position of the dependent arm (it should not be too stretched but neither too “pushed up”). The distance between the chin and the sternum should be at least 1 cm.
Surgery
1. Skin incision N°22 scalpel blade
The skin incision should be placed approximately 2 cm behind the ear. In practice, the “two fingers” rule works very well. Incision starts in a linear fashion behind the ear and at the level of the digastric groove turns gently anteriorly. Many surgeons prefer a C-shaped incision.
Skin and subcutaneous can be incised one layer and separated from the muscular fascia.
2. Dissection of muscular layers with monopolar cautery or scalpel
This can be achieved either with a scalpel or with a monopolar cautery in a single layer. Use an instrument to dissect the muscles from the bone, and then place a retractor. In this phase the occipital artery can be inadvertently cut; it should be ligated or coagulated carefully. Furthermore, blood from the emissary vein can come out – bone wax is often sufficient to stop the bleeding.
At the end of the dissection one should have a view of the root of the mastoid tip, the digastric groove, the asterion and the parietomastoid, occipitomastoid and lambdoid sutures.
3. Burr hole with perforator
The asterion is the ideal site to place the burr hole. The asterion is the joint of parietomastoid, occipitomastoid and lambdoid sutures. It serves as an approximation for the underlying transverse-sigmoid junction. As in approximately 20% of the subjects this correspondence is not reliable, many centers prefer to use a neuronavigation system.
4. Preparation of the burr hole with dura separator
This step is crucial to detach the dura from the inner bone; this mitigates the risk of the footplate of the craniotome tearing the dura of the sinus.
5. Craniotomy with high-speed drill
The craniotomy is performed with a high-speed drill or a craniotome. Usually the flap measures around 1.5 x 1.5 cm.
6. Bone drilling with high-speed drill
It is widely preferred not to pass the drill onto the transverse and sigmoid sinus (where the dura is strictly adherent to inner bone). It is better to leave a fine margin of bone beside the sinus and complete the craniotomy with a diamond burr and rongeurs.
7. Dura incision with N°10 scalpel blade and scissor
Dura is incised in a curvilinear fashion and reflected toward the sigmoid sinus. Stitches are placed to move the dura away from the surgical field.
Microexploration
SCA: Superior Cerebellar Artery
AICA: Anterior Inferior Cerebellar Artery
CN V: Cranial Nerve V, Trigeminal Nerve
CN VII: Cranial Nerve VII, Facial Nerve
CN VIII: Cranial Nerve VIII, Vestibulocochlear nerve
CN IX: Cranial Nerve IX, Glossopharyngeal Nerve
CN X: Cranial Nerve X, Vagus Nerve
CN XI: Cranial Nerve XI, Accessory Nerve
CN VI: Cranial Nerve VI, Abducens Nerve
SPV: Superior Petrosal Vein
SS: Sigmoid Sinus
TS: Transverse Sinus
MCP: Middle Cerebellar Peduncle
FLOC: Flocculus
Closure
1. Dura reconstruction
A water-tight closure of the dura is always needed, particularly in the posterior fossa or in case of cisternal and ventricular opening. In case of dura retraction, several types of synthetic substitutes or a galea flap can be used to complete the approximation.
2. Bone fixation with microscrews
At least 3 microplates are used to guarantee the stability of the bone flap and osteosynthesis. Generally, for smaller retrosigmoid craniectomies (1-2 cm), stringent bone reconstruction may be omitted.
3. Muscle reconstruction
A multi-layer occipital muscle reconstruction improves the water-tight closure.
4. Subcutaneous/skin suture
Aesthetic skin reconstruction is always mandatory.
