Commonly applied keyhole microsurgery approaches
Supraorbital keyhole approach
Indication (
Table
1
)
Table 1
Indications of supraorbital keyhole approach
Anterior communicating artery aneurysms | Level II [ 53, 79]; Level III [ 7, 11, 23, 29, 31, 35, 38, 55‐ 59, 62, 75, 77, 87] |
Internal carotid artery aneurysms | Level II [ 53, 79]; Level III [ 11, 23, 35, 38, 55, 56, 58, 59, 77, 87] |
Middle cerebral artery aneurysms Posterior communicating artery aneurysms | |
Basilar artery aneurysms | |
Posterior cerebral circulation aneurysms | |
Ophthalmic artery aneurysms | |
Proximal superior cerebellar artery aneurysms | |
Anterior cerebral artery aneurysms
ACA A1 and A2 proximal aneurysms
| |
Anterior choroidal artery aneurysms | |
Pituitary adenoma | |
Tuberculum sellae meningioma | |
Olfactory groove meningioma Fronto-basal region meningioma | |
Suprasellar craniopharyngioma | |
Lesions located around the sellar region and central skull base are suitable for application of supraorbital keyhole approach.
Exposure zones (
Table
2
)
Table 2
Exposure zones of supraorbital keyhole approach
The frontal lobe base | Anterior communicating artery | Posterior clinoid process |
Anterior clinoid process | Anterior cerebral artery (A1 and A2 proximal) | Basilar artery apex |
Optic canal | Internal carotid artery | Posterior cerebral artery (P1 segment) |
Olfactory sulcus | Middle cerebral artery (M1 and M2 segments, part of M3 segment) | Superior cerebellar artery proximal |
Olfactory tract | Anterior choroidal artery | |
Optic nerve | Posterior communicating artery | |
Optic chiasm | The contralateral carotid artery medial surface | |
Oculomotor nerve | Anterior cerebral artery A1 and A2 proximal | |
Pituitary stalk | Middle cerebral artery M1 and M2 proximal | |
Diaphragm sellae | | |
Dorsum sellae | | |
Anteromedial temporal lobe | | |
Anterior upper pontine | | |
Interpeduncular cistern | | |
Head position
With the patient in a supine position, the Mayfield head holder is applied to fix the patient’s head. The head is rotated about 10°–60° to the contralateral side depending on the site and size of the lesion. The head position could be modified according to the requirement of the exposure areas.
Surgical procedures
-
A 3–4-cm skin incision is made within the eyebrow, and the subcutaneous and frontal fascia are separated. The frontalis muscle is cut parallel to the frontal skull base approximately 1 cm above its insertion into the orbicularis muscle, and the temporalis fascia is cut along the superior temporal line for approximately 2 cm. The frontal periosteum is incised in a semicircle shape from the superior temporal line, and then stripped and flipped. The base is located in the superior orbital rim. The temporalis muscle is bluntly separated, pushed 1.0–1.5 cm behind the superior temporal line.
-
A bone hole of about 3–5 mm diameter behind the frontal bone zygomatic process (keyhole position) is milled out. The milling cutter runs along the orbital roof from the hole and back to create a bone flap of approximately 2.0 cm × 2.5 cm. The dura is opened in a flap shaped with the base at the orbital rim. The frontal lobe is gently lifted from the base using a paddy or brain retractor and cerebrospinal fluid (CSF) constantly absorbed, aiming to decrease the intracranial pressure.
-
When finishing the intracranial procedure, the dura should be tightly sutured without a drainage tube. A skull lock or connector is used to fix the bone flap. The periosteum and myofascia are sutured layer by layer. The skin flap can be closed with continuously intradermal absorbable stiches or using metal cosmetic sutures, surgical stapler, to reduce scar.
Key points for recommendations
The skin incision is placed within an eyebrow for pleasing cosmetic outcome.
The head is extended about 20°–25°, so that the frontal lobe moves backward because of gravity and leaves the anterior skull base to reduce intraoperative traction.
Slightly rotation of the head about 5°–15° to the contralateral side is helpful for the contralateral approach surgery, and it also offers the surgeon a more comfortable operation direction.
Attention should be focused on protecting the supraorbital nerve to avoid the risk of frontal numbness. The bone hole could be lateral to the superior temporal line, and the location may not be too low to avoid milling through the orbital wall.
When designing the bone window, we should avoid opening the frontal sinus. However, when this occurs, a tight repair is needed.
The inner plate of the supraorbital bone window edge is drilled out to gain greater visual space. Some bone ridge protrusions in the anterior skull base could be extradurally removed to increase the volume of the surgical corridor.
After opening the dura mater, the skull base is explored further to expose and open the chiasmatic and carotid artery cisterns to further release CSF, relaxing the frontal lobe and providing an optimal trajectory towards the skull base.
Endoscope and tube-shaft instruments can improve visualization and are particularly useful in selected cases, such as low-lying olfactory groove meningiomas, in which the lesion is not entirely within the surgeon’s direct line of sight.
It is recommended that the incision area could be compressed for several minutes after closing the skin flap for hemostasis, so as to reduce the incidence of subcutaneous hemorrhage and swelling.
The anterior clinoid process or posterior clinoid process may be drilled to provide a wide manipulation space for clipping of basilar artery aneurysms, if necessary.
In this approach, there are few venous effects on surgical access. Sometimes, we need to dissect sylvian fissure to reduce the traction between frontal and temporal lobes, or to increase the exposure of the internal part of sylvian fissure and temporal lobe.
Limitations (
Table
3
)
Table 3
Limitations and solutions of supraorbital keyhole approach
Limited exposure view and narrow surgical corridor | Reducing intraoperative ICP by draining CSF to increase the intracranial operating space ( Level III [ 23, 35, 38, 64]). |
| Preoperative lumbar puncture or ventricular drainage ( Level III [ 35, 38]) |
| The neuronavigation system ( Level III [ 62, 64]) Endoscopic-assisted keyhole surgery ( Level III [ 23, 26, 30, 38, 62, 64]) |
Limited microinstruments | The invention of special keyhole-adapted microinstruments could solve these problems, including Gun-type rod-shaped, slim and tube shaft-designed tools ( Level III [ 62, 64]) |
Postoperative palsy | Percutaneous mapping of the frontal branch of the facial nerve ( Level III [ 58]). Small skin incision, respecting the anatomical pathway of the supraorbital nerve route ( Level III [ 23, 26]) |
Unfamiliar with keyhole surgery | Performing anatomic dissection practice, and trained under the supervision of experienced senior neurosurgeons ( Level III [ 64]) |
Influence on the appearance of eyebrow | Meticulous wound closure, particularly of the brow skin incision. Closing the skin layer with a running subcuticular stitch (e.g., 5-0 Prolene, Prolene, noninvasive metal sutures) without any suture knots ( Level III [ 35, 52]). The tape can be used to further close the skin incision. If patient with light eyebrows, the eyebrow incision should be avoided. |
Pterional keyhole approach
Indication (
Table
4
)
Table 4
Indication of pterional keyhole approach
Anterior Communicating Artery Aneurysms | |
Internal carotid artery aneurysms | |
Middle cerebral artery aneurysms | |
Posterior communicating artery aneurysms | |
Ophthalmic artery aneurysms | |
Posterior cerebral artery aneurysms | |
Anterior choroidal artery aneurysms | |
Sphenoid wing meningiomas | |
Parasellar meningiomas | |
The pterional keyhole approach provides early visualization of the optic nerves and anterior circulation vessels while minimizing exposure and risk to unneeded parts of the frontal lobe and temporal lobe. In addition, selected patients with posterior circulation aneurysms, tumors around sellar region, and sphenoid wing meningiomas are suitable for application of pterional keyhole approach.
Exposure zones (
Table
5
)
Table 5
Exposure zones of pterional keyhole approach
Suprasellar region | Ipsilateral posterior communicating artery | Basilar artery apex |
Parasellar region | Anterior choroidal artery | Superior cerebellar artery proximal |
Retrosellar region | Internal carotid artery | Posterior cerebral artery (P1 and P2a) |
Cavernous sinus superior wall | Ophthalmic artery | |
Frontal part of the lateral wall of the cavernous sinus | Middle cerebral artery (MCA M1–M4 segment) | |
Sphenoid ridge | Ipsilateral and contralateral anterior cerebral artery | |
Temporal pole | (A1 and A2 proximal) | |
Frontal pole Anterior cranial fossa | Contralateral MCA M1 and M2 proximal The medial and bifurcation part of internal carotid artery | |
Front end of the middle cranial fossa | | |
Interpeduncular cistern | | |
Prepontine cistern | | |
Head position
The patient’s head is leaned slightly back so that the frontal lobe inclines and leaves the orbital roof by gravity; the head is rotated about 30°–60° to the contralateral side based on actual need: a greater rotation angle to the contralateral side is needed when the lesion is closer to the frontal end. The head is tilted slightly about 15° to the contralateral side to compensate for the upward inclination angle along the middle skull base.
Surgical procedures
-
About 2 cm outside the keyhole (posterior to the temporal line at the level of frontal skull base), around the pterion, an anterior hairline incision about 4–5 cm long is made. The subcutaneous tissue and temporal fascia are incised parallel to the skin incision. The temporal muscle is incised along the direction of the muscle fibers through the pterion and expanded with the mastoid expander. An osseous depression may be seen in the exposed central skull bone, which is the mark of the sphenoid ridge on the surface of the skull. A bone hole about 3-5 mm diameter is drilled on the bottom of the sphenoid ridge, and from this location, a bone flap about 2.5 cm in diameter is milled out, and one- to two-thirds of the lateral sphenoid ridge is removed; if necessary, the edge can reach the lateral side of the supraorbital fissure. Centering on the sphenoid ridge, the dural flap is cut open and retracted forward to expose and then open the sylvian fissure.
-
Closure is basically as same as that for the supraorbital approach. The hairline incision can be sutured in a routine manner.
Key points for recommendations
Usually, the exposure area could be adjusted according to surgery requirements by moving up or down the incision location, thus the location of the bone window and exposure area of the frontal and temporal lobes.
For anterior communicating artery aneurysms, the sylvian fissure is usually located at the lateral third of the bone window; two-thirds of the brain tissue exposed under the bone window is the frontal lobe, and the remaining third is the temporal lobe.
For middle cerebral artery aneurysms or lateral posterior communicating artery aneurysms, the sylvian fissure is usually placed in the center of the bone window so that the temporal lobe can be slightly retracted after opening the sylvian fissure.
For the aneurysm pointing towards the dorsal part of the parent artery, the pterional keyhole approach was more favorable for exposing the aneurysm neck.
While clipping the anterior communicating artery aneurysm via the pterional keyhole approach, olfactory nerve damage caused by raising the frontal base could be reduced through this lateral approach.
For middle cerebral artery aneurysms, the pterional keyhole approach could be used to evacuate hematoma in the temporal lobe simultaneously.
If necessary, the terminal part of the sylvian vein converge into the sphenoparietal sinus could be cut in order to retract the temporal lobe.
Limitations (
Table
6
)
Table 6
Limitations of pterional keyhole approach
Surgical view is obstructed by Sylvian vein | Dissecting the Sylvian fissure, placing the Sylvian vein behind the brain retractor and retracing the temporal lobe, opening the carotid cisterns to release CSF ( Level II [ 16]; Level III [ 7, 12, 81]). |
Narrow surgical corridor | The sphenoid ridge is removed with a diamond drill to provide wide manipulation space of the skull base area. The sphenoid ridge is the most important landmark to precisely position the scheduled keyhole mini-craniotomy ( Level III [ 20, 48, 72]). Introducing neuroendoscope ( Level III [ 12]) |
Postoperative temporal muscle atrophy | Making a vertical incision along the muscle fibers on the temporal muscle, or dissecting the temporal muscle by the retrograde dissection method. Minimal preparation and dissection of bones and muscles can decrease iatrogenic surgical trauma, cranial deformities and temporal muscle atrophy significantly ( Level II [ 16] Level III [ 12]). |
Injury to the temporal branch of the facial nerve | The incision of pterional keyhole approach is made between the branches of the facial nerve. Cutting the temporal muscle near the edge of its insertion to the temporal bone. Applying the interfascial or subfascial technique ( Level II [ 16]; Level III [ 12, 47, 48, 76, 81]). |
Subtemporal keyhole approach
Indication (
Table
7
)
Table 7
Indications of subtemporal keyhole approach
Tumors confined in the petroclival and suprasellar region | |
Hypothalamic gliomas
Retrochiasmatic craniopharyngiomas
Trigeminal neuromas
Pituitary adenomas
Petroclival meningiomas
Pontine cavernous hemangioma
| |
Posterior cerebral artery (P2-P3 segment) aneurysms | |
Middle fossa arachnoid cysts | |
Lesions located within the petroclival and suprasellar region are suitable for application of subtemporal keyhole approach. In addition, the subtemporal keyhole approach can be applied to some posterior cerebral circulation aneurysms
Exposure zones (
Table
8
)
Table 8
Exposure zones of subtemporal keyhole approach
Temporobasal skull base Anterior clinoid process | Anterolateral midbrain Anterolateral pons | Tentorium Upper clivus |
Posterior clinoid process | Anterosuperior cerebellar | Superior cerebellar artery |
Internal carotid artery | | Basilar artery |
Posterior communicating artery | | Posterior cerebral artery |
Anterior choroidal artery | | CN V, CN VI, CN VII, CN VIII |
Pituitary stalk | | |
CN II, CN III, CN IV | | |
Optic tract | | |
Tentorium | | |
Head position
The patient is laid in the supine position with shoulders elevated, and the head is rotated about 90° to the contralateral side of the lesion, keeping the zygomatic arch in the horizontal position. The head is extended back about 15° so that the trachea is not oppressed; the head is lateroflected 15° to compensate for the upward incline angle along the middle skull base. This head position can tilt the temporal lobe away from the skull base by gravity.
Surgical procedures
A skin incision is made about 1 cm in front of the tragus. Starting from the superior zygomatic arch, a vertical incision approximately 4 cm above the zygomatic arch is made. A Y-shaped temporalis fascia incision is made and retracted to expose the surgical field. The temporal muscle is incised longitudinally and retracted. A hole is drilled posterosuperior to the zygomatic arch base, and a bone window about 2.0–2.5 cm in diameter is created using the milling cutter. The dura is incised in a flap shape and flipped inferiorly. The temporal base is lifted gently to gradually release CSF to reduce the intracranial pressure (ICP). The edge of the tentorial incisura is exposed gradually deeper in.
Key points for recommendations
The subcutaneous tissue is separated, making sure to avoid damaging the frontal branch of the facial nerve and superficial temporal artery.
Individualized keyhole approaches for posterior cerebral circulation artery aneurysms are safe and effective.
Incision of the tentorium could further resect lesions in the posterior fossa, and the petrous apex can be removed with a diamond drill to provide enough space to deal with the lesions in the petroclival region. Generally, the surgical manipulating corridor is limited to the top of the internal auditory canal.
A preoperative lumbar drain may be helpful, since access to the CSF spaces will be able only when reaching the tentorial edge.
Although the Labbe' vein is not visible directly below the bone window, it may be damaged by traction when lifting the base of the temple. If necessary, the arachnoid membrane on the venous surface can be separated to increase the degree of dissociation and make the operation space larger.
Limitations (
Table
9
)
Table 9
Limitations and solutions of subtemporal keyhole approach
Deep-seated location and limited manipulating surgical corridor | Releasing CSF and reducing ICP through cisterna ambiens opening ( Level III [ 36, 60]) Lumbar drainage ( Level III [ 60]) |
| Introducing neuroendoscope ( Level III [ 39, 66, 73]) |
Insufficient exposure of lesions in posterior cranial fossa The tentorial sinus affects the incision of the tentorium | The tentorium is subsequently divided, lengthening the incision. Opening the Meckel’s cave to reach the internal auditory canal ( Level III [ 60, 73]) A tentorial incision perpendicular to the superior petrosal sinus is made by coagulation or clipping. Hemostasis with electrocoagulation and hemostatic materials packing ( Level III [ 60, 73]) |
Injury of the trochlear nerve | Dissecting the cerebellar tentorial segment of the trochlear nerve, and then cut the tentorium cerebelli to enlarge the surgical manipulating corridor ( Level III [ 60]) |
Postoperative temporal atrophy | Making a short vertical incision at the posterior margin of the temporal muscle ( Level III [ 60]). |
Indication (
Table
10
)
Table 10
Indication of median suboccipital keyhole approach
Posterior inferior cerebellar arteries aneurysm | |
Posterior cranial fossa tumors | |
Medulloblastoma
| |
Ependymoma
| |
Capilliary hemangioblastoma
| |
Cerebellar metastases
| |
Brainstem glioma | |
The median suboccipital keyhole approach is appropriate for lesions located in the cerebellar vermis, fourth ventricle, dorsum of pons, and medullary dorsal parts (e.g., Distal posterior inferior cerebellar artery (PICA) aneurysms).
Exposure zones
The posterior circumference of the foramen magnum, marginal sinus, occipital sinus, the cerebellar tonsil and inferior vermis, cervicomedullary junction, posterior inferior cerebellar artery, and the fourth ventricle, pons and medullary dorsal parts.
Head position
The patient is laid in a prone position and the head flexed forward to fully extend the craniocervical junction with the tentorium in a perpendicular plane.
Surgical procedures
An upward suboccipital median incision from 1 cm below the foramen magnum and approximately 4-cm long is made. The scalp is incised and separated sharply along the midline. A bone window about 2.5 cm in diameter is created from the trailing edges of the foramen magnum in the upward direction. The dura is incised in a X or Y shape and the occipital sinus closed with cautery or suture. The cisterna magna is opened, the cerebellar tonsil lifted, the arachnoid adhesion separated, and the fourth ventricle exposed through the cerebellomedullary fissure approach.
Key points for recommendations
This approach can clearly expose the entire fourth ventricle from the foramen magnum to the lower aqueduct. Dissecting more laterally within the ventricular chamber, the vestibular area and the foramen of Luschka of the fourth ventricle can be observed.
The procedures of the suboccipital “open-door” keyhole craniotomy require two paramedian bone holes and removal of the internal occipital crest.
Stages of craniotomy: (1) two paramedian burr hole trephinations; (2) median suboccipital craniotomy; (3) Due to partial removal of the posterior arch of the atlas without laminectomy, the exploration of the cervicomedullary junction can be extended.
Distal PICA aneurysm clipping via median suboccipital keyhole approach could obtain satisfactory results.
The occipital sinus should be treated with sutures or bipolar coagulation. It is usually necessary to repair the dura to prevent cerebrospinal fluid leakage.
Limitations (
Table
11
)
Table 11
Limitations and solutions of median suboccipital keyhole approach
Cerebrospinal fluid leakage | The dural opening should be closed watertight using interrupted or running sutures ( Level III [ 36]). |
Limited surgical view from inferior to superior | Drainage of CSF from cistern magna, cerebellomedullary, and prepontine cisterns aids in relaxation of cerebellum and obtaining adequate exposure ( Level III [ 36]). Careful preoperative positioning of patients and modifying the position during the operation, which provide an excellent overview of the target ( Level III [ 36]) |
Hydrocephalus | Endoscopic third ventriculostomy can be performed prior to tumor resection as tumor removal could be proposed as an effective method for reducing the probability of postoperative shunting ( Level III [ 65]) |
Limited visualization of fourth ventricle | After the arachnoid membrane is opened sufficiently and the cerebellum is relaxed well, the cerebellar tonsils can be retracted bilaterally, and the inferior cerebellar vermis can be easily elevated upward to expose the whole fourth ventricle region ( Level III [ 36]). |
Retrosigmoid keyhole approach
Indication: (
Table
12
)
Table 12
Indication of retrosigmoid keyhole approach
Hypoglossal neurinoma Vestibular schwannomas Facial nerve neuromas Cochlear nerve neuromas Petroclival, tentorial, CPA meningiomas | |
Deep pontine lesion (glioma, cavernous angioma etc.) | |
CPA Cholesteatoma | |
MVD of HFS*
trigeminal neuralgia
glossopharyngeal neuralgia
| |
The retrosigmoid keyhole approach provides a safe and effective route to the cerebellopontine angle, upper and middle clivus, and with minor variation may provide an avenue to the lower clivus and foramen magnum.
Exposure zones
The retrosigmoid keyhole approach can expose the following anatomic structures: the trigeminal nerve, facial nerve, acoustic nerve, lower cranial nerves, lateral and anterior lateral pons, lateral cerebellar hemisphere, vertebral artery, and posterior inferior cerebellar artery. For treating lower cranial nerve lesions, the surgical incision and bone window position may descend accordingly.
Head position
The patient is placed in the lateral park bench position. The patient’s head is rotated about 10°–20° to the contralateral side from the lateral position to provide a direct view angle with less retraction to the cerebellar hemisphere and can open the cerebellopontine angle cistern successfully. During the operation, based on the location of the lesion, the view angle can be adjusted by changing the degree of inclination (left and right) of the operating bed.
Surgical procedures
Starting at the junction line of the external occipital protuberance and mastoid roots, about 1.5–2.0 cm posterior to the mastoid roots, a vertical, oblique (along the hairline) incision, or a horizontal incision of about 4 cm long is made. In the case of craniectomy, a burr hole about 2.5 cm in diameter is created behind the mastoid. In the case of craniotomy, a burr hole of about 5 mm is made at the junction between the transverse and the sigmoid sinus junction. Then, a craniotomy of about 2.5 cm in diameter is performed using a craniotome. The top of the bone window is near the horizontal inferior margin of the transverse sinus, and its lateral edge is on the posterior edge of the sigmoid sinus. Mastoid air cells are closed with bone wax. The dura is incised in a flap shape with its base located at the sigmoid sinus. The lower lateral cerebellum is gently lifted from the petrous bone, and the lateral cerebellomedullar cistern is opened gradually to release CSF. The cerebellopontine angle cistern and lateral cerebellomedullar cistern are dissected to expose the anatomical structures.
Key points for recommendations
Removal of the anterior inner edge of the craniectomy over the sigmoid sinus can significantly increase the angle for visualization.
Watertight dural closure and sealing of mastoid air cells are important to prevent postoperative CSF leakage.
The petrous vein should be preserved as far as possible to prevent cerebellum or brain stem swelling caused by dysfunction of venous drainage.
Neuroendoscope could help recognize hidden and deep-lying structures like small arteries and veins responsible for vascular compression on the surface of cranial nerves.
Diamond drill could be used to remove the posterior wall of the internal auditory canal to resect the tumor inside. Introducing the neuroendoscopy could clear observe the internal auditory canal and reduce the opening area of the internal auditory canal.
Limitations (
Table
13
)
Table 13
Limitations and solutions of retrosigmoid keyhole approach
CSF leakage | Watertight dural closure and packing of mastoid air cells using bone wax, glue or free fascial tissue. Repairing the defect of internal acoustic meatus with muscle mixed glue and gelfoam sponge graft. Perform lumbar CSF drainage and compression on the region of the operative wound ( Level III [ 15, 36]). |
High ICP | Continuous drainage of cerebrospinal fluid after release the CSF by lumbar cistern drainage during operation. Release CSF by opening the lateral cerebellomedullary cistern ( Level III [ 15, 36]) |
Obstructed surgical view by nerve and vessels | Introducing neuroendoscope or modifying the view angle of microscope during the operation ( Level III [ 40, 42]) |
Surgery-related cranial nerve injury | Intraoperative electrophysiological monitoring ( Level III [ 15, 54]) |
Interhemispheric transcallosal keyhole approach
Indication (
Table
14
)
Table 14
Indication of interhemispheric keyhole approach
Lateral ventricular tumors | |
Third ventricular tumors | |
Interhemispheric transcallosal hemispherotomy | |
Anterior cerebral artery aneurysms (A2, A3) | |
Anterior communicating artery aneurysms | |
The interhemispheric keyhole approach has been applied for the lesions between the medial side of the brain and cerebral falx, the corpus callosum, the central lateral ventricle, the anterior cerebral artery along the surface of the corpus callosum, and the suprasellar region.
Exposure zones
The interhemispheric transcallosal keyhole approach can expose the following anatomic structures: corpus callosum, cerebral falx, the distal segment of the anterior cerebral artery, body of the lateral ventricle, third ventricle, and thalamus.
Head position
The patient is placed in supine position, and the head is flexed approximately 30° to 45° angle with the horizontal line to allow the surgical dissection near to the perpendicular plane.
Surgical procedures
A linear incision about 4-cm long is made 1 cm beside and parallel to the midline or perpendicular to it with two thirds of the incision on the side of the craniotomy. The location of the lesion determines the specific incision points. A burr hole about 3–5 mm in diameter is drilled immediately beside the midline, and a small bone flap about 2–2.5 cm is milled out to expose the sagittal sinus at one edge of the craniotomy. The dura is opened in a curvilinear fashion with base towards the midline. The head is rotated 10° to 30° and lateroflected to the craniotomy side, tilting the hemisphere away from the midline. The arachnoid membrane near the sagittal sinus is separated carefully without damaging the large draining veins. If necessary, a section of the vein is freed up by separating the arachnoid on its surface to increase the degree of displacement. Dissection is performed along the lateral cerebral hemisphere descending gradually to the corpus callosum, which is incised longitudinally for about 1.5–2 cm to reach the lateral ventricle. Caution has to be taken to avoid injure of the pericallosal arteries.
Key points for recommendations
This approach can provide a direct midline orientation with symmetrical access to both lateral ventricles and both walls of the third ventricle.
The contralateral interhemispheric transcallosal approach can effectively expose the lesions located in the lateral part of lateral ventricle.
With this approach, the third ventricular chamber can be approached via an interforniceal, subchoroidal, transchoroidal, or transforaminal path.
Place the bone window to anterior coronal suture could avoid larger bridging veins through surgical corridor. A 1.5- to 2.0-cm callosal incision is beneficial to avoid the occurrence of permanent interhemispheric disconnection.
Neuronavigation is helpful to localize and to plan a targeted and tailored interhemispheric approach
Limitations (
Table
15
)
Table 15
Limitations and solutions of interhemispheric transcallosal keyhole approach
Interhemispheric retraction | The patient's head is fixed in a horizontal position and tilted approximately 45°–60° with sufficient gravitational retraction of the hemisphere; applying the osmotic solutions or releasing cerebral spinal fluid to reduce the ICP ( Level III [ 1, 34]) The contralateral approach may minimize the need for retraction of the dominant frontal lobe for treatment of dominant-hemisphere lesions. If required, a retractor is usually placed superiorly to gently lift the falx, which protects the ipsilateral hemisphere ( Level III [ 1]). |
Venous injury | Applying special instruments and precise navigation to place the skin incision and craniotomy in a position free of bridging veins for a precise craniotomy ( Level III [ 1]) Small burr hole may help to limit the extent of brain retraction; full free of the bridging veins can prevent venous injuries effectively ( Level III [ 34]). |
Callosal incision | Only for small tumors, and the length of the callosal incision should be limited to 1.5 cm ( Level III [ 34]) |
Limited operation space or high ICP | Releasing cerebral spinal fluid ( Level III [ 34, 84]) The angled endoscopes can be helpful to allow a panoramic view of the resection cavity and to avoid leaving residual tumor behind ( Level III [ 28]). |
Intraoperative bleeding | Bipolar coagulation on the walls of the ventricle is minimized to prevent scarring, and most bleeding stopped with the application of small pieces of Surgicel ( Level III [ 1]) |
Limited lateral exposure | Only performing for lesions located closer to the midline ( Level III [ 34]) Using contralateral approach to better expose the contralateral deep structures ( Level III [ 34]) |
Infratentorial supracerebellar keyhole approach
Indication (
Table
16
)
Table 16
Indications of infratentorial supracerebellar keyhole approach
Pineal region tumors
Pinealomas
Cholesteatoma
Germinomas
Glioblastoma multiforme
Pineocytoma
Pineoblastoma
Medulloblastoma
| |
Meningiomas of pineal region Meningiomas of the lateral ventricles | |
Tumors in the posterior part of third ventricular | |
The infratentorial supracerebellar keyhole approach is a common strategy used to access midline and paramedian lesions located beneath the deep venous system in the pineal-tectal region.
Exposure zones (
Table
17
)
Table 17
Exposure zones of infratentorial supracerebellar keyhole approach
Confluence of sinuses | Splenium of the corpus callosum |
Inferior surface of the tentorium | Pineal gland |
Straight sinus | Posterior commissure |
Upper vermis | Pulvinar thalami |
Tentorial cerebellar surface | Crus fornicis |
Central cerebellar vein | Hippocampal commissure |
Great cerebral vein of Galen | Velum interpositum |
Head position
1.
The semi-sitting position: The patient is placed in the semi-sitting position with slight flexion of the neck. Although this position is associated with a risk for venous air embolism, it may provide maximal relaxation of the cerebellum, with limited use of cerebellar retraction and taking advantage of the effect of gravity.
2.
The concorde position: (A) Surgeon standing at the head of the patient: The patient is placed in prone position. Then, the upper body is lifted or raised around 15°–20° to promote appropriate venous circulation. Additionally, the head is flexed down to the external occipital protuberance-eyebrow line, which lies perpendicular to the floor. (B) Surgeon standing on the lateral side of the patient: The head could be anteroflected about 45° to bring the tentorium in a perpendicular plane. The neck flexed 30°–45° to the contralateral side, and the head tilted to the operative side 30°.
Surgical procedures
A 4-cm straight vertical incision is made from 5 mm above the external occipital protuberance in the midline or paramedian for the “paramedian infratentorial supracerebellar keyhole approach” (PISKA). A craniectomy or craniotomy located just inferior to the transverse sinus is carefully performed in keyhole fashion using a high-speed drill. Care has to be taken to protect the adjacent sinuses.
The dura is opened in a U-shaped fashion and reflected superiorly to expose the interface between the cerebellum and the tentorium. The positioning and the effect of gravity could be taken advantage of to obtain the maximum operating room [
22,
32]. The draining veins should be preserved as far as possible in case of edema and venous infarction of the cerebellum [
27,
68]. Some studies reported that the paramedian approach may allow working around the bridging veins to preserve them [
27]. It is better to sacrifice smaller thin walled veins and preserve large veins, keeping in mind that there it is difficult to predict which vein, when occluded, will result in serious complication [
32,
45].
The arachnoid overlying the vein of Galen is carefully dissected from lateral to medial. The basal veins of Rosenthal can be seen close to the tentorial notch, on both sides. Great care is needed at this point to avoid damage to the veins, since the basal veins of Rosenthal and the vein of Galen must always be preserved [
22,
32,
68].
Key points for recommendations
In cases of lesions located lateral to the pineal region, we could apply a surgical corridor between the basal vein of Rosenthal and the tentorial edge. The dissection comes from lateral to medial, dividing small feeders from the posterior choroidal and superior cerebellar arteries.
If necessary, the periosteal sheet of the external protuberance can be used for watertight closure.
Cerebellar swelling might be caused by incision of the precentral vein of the cerebellum, which led to dysfunction of venous drainage of the cerebellum. We could dissect arachnoid overlying the precentral vein of the cerebellum to make it nomadic so that the vein or part of its branches can be preserved.
Limitations (
Table
18
)
Table 18
Limitations and solutions of infratentorial supracerebellar keyhole approach
Limited exposure and visualization structures | The neuroendoscope can be introduce through limited surgical corridors ( Level III [ 4, 44, 71, 82]). Relaxation of the cerebellum by releasing cerebrospinal fluid and sufficient gravitational retraction of the cerebellum ( Level III [ 4, 34]) . The tentorium can be incised to expose supratentorial tumors ( Level III [ 44]). Minor bridging veins over the tentorium may be divided to allow further cerebellar slump ( Level III [ 44]). If necessary, the superior cerebellar vein and draining veins coming from the surface of the cerebellum can be coagulated and cut without postoperative deficits to provide a wider exposure ( Level III [ 22, 44]). |
Cerebellar swelling or dysfunction of venous drainage | Preserving the normal veins as much as possible (dissecting arachnoid overlying the precentral vein of the cerebellum) ( Level III [ 34]) |