Residency · Residency · Neurosurgery
Posterior Fossa Surgical Approaches
Overview
The posterior fossa contains the brainstem, cerebellum, and critical neurovascular structures within a confined space. Surgical approaches must balance adequate exposure with the imperative to avoid injury to cranial nerves, the venous sinuses, and the brainstem. The major approaches include retrosigmoid, suboccipital midline, far-lateral, and telovelar corridors.
Relevant Surgical Anatomy
Bony Landmarks
The asterion is the junction of the lambdoid, parietomastoid, and occipitomastoid sutures and approximates the transverse-sigmoid sinus junction. The mastoid process serves as the attachment point for the sternocleidomastoid muscle, with the sigmoid sinus lying medial and anterior to it. The inion (external occipital protuberance) overlies the confluence of sinuses (torcular Herophili). The foramen magnum marks the transition from the posterior fossa to the spinal canal, and the occipital condyles lie laterally to it, with the hypoglossal canal positioned above them.
Venous Sinuses
The transverse sinus runs along the tentorium attachment to the occipital bone. The sigmoid sinus is its S-shaped continuation, descending to the jugular bulb. The confluence of sinuses (torcular) is the junction of the superior sagittal sinus, straight sinus, and bilateral transverse sinuses, and its anatomy is highly variable. The mastoid emissary vein connects the sigmoid sinus to extracranial veins and can bleed briskly when encountered.
Key Neural Structures
Cranial nerves IV through XII traverse the posterior fossa. The cerebellopontine angle (CPA) contains CN V, VII, VIII, and the AICA. The lower cranial nerves (IX, X, XI, XII) exit the medulla and course toward the jugular foramen and hypoglossal canal.
Retrosigmoid (Lateral Suboccipital) Approach
Indications
This approach is used for cerebellopontine angle tumors (vestibular schwannomas, meningiomas, epidermoids), microvascular decompression (for trigeminal neuralgia, hemifacial spasm, and glossopharyngeal neuralgia), petroclival meningiomas (lateral approach), and AICA and PICA aneurysms.
Positioning
The patient may be placed in the lateral decubitus (park bench) position or the sitting position, though the latter requires precautions for air embolism. The head is flexed and rotated slightly away from the operative side, with the mastoid process as the highest point.
Technique
A retroauricular curvilinear or linear incision is made 2 finger-breadths behind the mastoid. Subperiosteal muscle dissection exposes the occipital bone. A craniotomy or craniectomy is performed posterior to the sigmoid sinus and inferior to the transverse sinus. Identifying the transverse-sigmoid junction before opening bone is essential; while the asterion provides an approximation, it can be inaccurate, so neuronavigation or Doppler confirmation is recommended. The dura is opened with a flap based on the sinuses, and CSF is released from the cerebellomedullary cistern to achieve cerebellar relaxation. Dissection then proceeds along the petrous surface to the CPA.
Advantages
This is a versatile approach providing access to the CPA, tentorium, and petrous surface. Hearing preservation is possible for vestibular schwannomas, unlike with the translabyrinthine approach. It also affords direct access to cranial nerve root entry zones for microvascular decompression.
Complications
Venous sinus injury can cause life-threatening hemorrhage, so the craniotomy should be planned with imaging confirmation of sinus anatomy. Air embolism risk is increased in the sitting position and is monitored with precordial Doppler and end-tidal CO2. CSF leak is the most common complication and requires meticulous dural closure and thorough waxing of mastoid air cells. Cerebellar injury or swelling may result from retraction or venous infarction, and hearing loss may follow CN VIII manipulation or loss of the labyrinthine artery.
Suboccipital Midline Approach
Indications
This approach is used for cerebellar tumors (hemangioblastomas, metastases, pilocytic astrocytomas), fourth ventricular tumors (medulloblastoma, ependymoma), vermian and paravermian lesions, pineal region tumors (via the supracerebellar infratentorial variant), and Chiari malformation decompression.
Positioning
The patient is positioned prone with the head in a Mayfield clamp and the neck flexed. At least 2 finger-breadths of space must be maintained between the chin and chest to avoid airway compromise and venous congestion. The concorde position (modified prone with the head slightly elevated) is an alternative.
Technique
A midline vertical incision extends from just above the inion to C2-C3. Subperiosteal muscle dissection exposes the suboccipital bone bilaterally. A suboccipital craniotomy or craniectomy is performed below the transverse sinuses. A C1 laminectomy may be added for additional inferior exposure in Chiari decompression or foramen magnum tumors. The dura is opened in a Y-shaped fashion. For fourth ventricular tumors, the telovelar approach (described below) avoids splitting the vermis.
Vermian Splitting
Traditionally, the fourth ventricle was accessed by splitting the inferior vermis. However, this technique is associated with posterior fossa (cerebellar) mutism syndrome in children and has been largely replaced by the telovelar approach when possible.
Telovelar Approach
Concept
The telovelar approach accesses the fourth ventricle through the cerebellomedullary fissure without splitting the vermis. The tela choroidea and inferior medullary velum are opened to enter the ventricle, preserving cerebellar midline structures and potentially reducing the risk of cerebellar mutism.
Technique
After a standard suboccipital midline exposure, the cerebellomedullary fissure is dissected by gently separating the tonsils bilaterally. The tela choroidea and inferior medullary velum are identified and opened, providing excellent exposure of the floor and lateral recesses of the fourth ventricle. Because PICA runs within and around the tonsils, careful identification of the artery is critical throughout.
Advantages over Vermian Splitting
The telovelar approach preserves midline cerebellar structures, may reduce the incidence of cerebellar mutism syndrome, and provides equivalent or superior exposure of the fourth ventricle compared with vermian splitting.
Far-Lateral (Transcondylar) Approach
Indications
This approach is used for ventrolateral foramen magnum lesions (meningiomas, schwannomas), vertebral artery and PICA aneurysms, lower clivus tumors (chordomas, chondrosarcomas), and ventral brainstem lesions.
Positioning
The patient is placed in the lateral decubitus (park bench) position with the affected side up. The head is flexed and rotated to bring the mastoid tip superiorly.
Technique
A hockey-stick or lazy-S incision extends from behind the mastoid to the upper cervical spine. Subperiosteal dissection exposes the suboccipital bone, C1 posterior arch, and C2 lamina. The vertebral artery (V3 segment) must be identified and mobilized; it runs in the sulcus arteriosus on the superior surface of C1 and is released from the C1 arch to allow anterior mobilization. A suboccipital craniotomy extends to the foramen magnum, and partial resection of the occipital condyle (typically the posterior third) is performed to improve the angle of approach to ventral pathology. Preserving more than 50% of the condyle is essential to maintain occipitocervical stability. A C1 laminectomy completes the bony exposure, and the dural opening provides access to the ventrolateral foramen magnum and lower clivus.
Advantages
This approach provides direct access to ventral and ventrolateral brainstem pathology without brainstem retraction and avoids traversing the cerebellum.
Complications
Vertebral artery injury is the primary vascular risk and requires meticulous identification of the V3 segment. Occipitocervical instability may result if more than 50% of the condyle is resected or if the occipitocervical ligaments are disrupted. Lower cranial nerve injury (IX, X, XI, XII) is possible and these nerves must be identified and preserved during dissection. CSF leak is a concern, particularly with condylar drilling near the hypoglossal canal.
<image> Posterior view of the skull showing the planned craniotomy outlines for three posterior fossa approaches superimposed: retrosigmoid (lateral, posterior to the sigmoid sinus), midline suboccipital (centered on the midline below the inion), and far-lateral (extending laterally to the foramen magnum with condylar drilling shown). The transverse sinus, sigmoid sinus, and foramen magnum are labeled. Bony landmarks including the asterion, inion, and mastoid process are indicated. Clean anatomical illustration with color-coded approach outlines. </image>
<image> Intraoperative view through the telovelar approach to the fourth ventricle. The cerebellar tonsils are gently separated bilaterally, revealing the tela choroidea and inferior medullary velum being opened. The floor of the fourth ventricle (rhomboid fossa) is visible with the facial colliculus and stria medullaris labeled. The PICA branches course around the tonsils. Detailed microsurgical illustration with labeled structures. </image>
<image> Lateral view showing the far-lateral approach with the vertebral artery (V3 segment) mobilized from the C1 sulcus arteriosus. Partial condylar drilling is depicted with the dura opened to show the ventrolateral foramen magnum. CN IX, X, XI exiting the jugular foramen and CN XII exiting the hypoglossal canal are labeled. The PICA origin from the vertebral artery is shown. Medical illustration with surgical perspective and labeled neurovascular structures. </image>
Comparison of Posterior Fossa Approaches
| Feature | Retrosigmoid | Suboccipital Midline | Telovelar | Far-Lateral |
|---|---|---|---|---|
| Position | Lateral decubitus or sitting | Prone | Prone | Lateral decubitus |
| Target | CPA, petrous surface | Cerebellum, 4th ventricle, pineal | 4th ventricle floor | Ventrolateral foramen magnum |
| Key indications | VS, MVD, CPA tumors | Cerebellar tumors, Chiari | 4th ventricular tumors | FM meningiomas, VA/PICA aneurysms |
| Vermian splitting | No | Sometimes (traditional) | No | No |
| Critical venous structure | Transverse-sigmoid junction | Transverse sinus, torcular | None specific | Vertebral artery V3 |
| Major risk | Sinus injury, CSF leak, air embolism | Cerebellar mutism (vermian split) | PICA injury | VA injury, occipitocervical instability |
Clinical Pearls
The asterion should never be assumed to accurately mark the transverse-sigmoid junction; neuronavigation or intraoperative Doppler should be used to confirm sinus location before placing burr holes. In the retrosigmoid approach, aggressive waxing of opened mastoid air cells is essential to prevent postoperative CSF leak and rhinorrhea. The sitting position provides excellent cerebellar relaxation but carries the risk of venous air embolism, requiring precordial Doppler monitoring and a central venous catheter for air aspiration. The telovelar approach has largely replaced vermian splitting for fourth ventricular tumors and is associated with fewer complications. In the far-lateral approach, the vertebral artery V3 segment must be freed from C1 before any condylar drilling, as the artery is at highest risk where it pierces the dura. More than 50% of the occipital condyle should be preserved to avoid occipitocervical instability. PICA has a highly variable course with multiple loops around the tonsils and brainstem, and it must be carefully traced and preserved throughout posterior fossa surgery.
References
- Rhoton AL Jr. "The Posterior Fossa Cisterns." Neurosurgery. 2000;47(Suppl 3):S287-S297.
- Rhoton AL Jr. "The Far-Lateral Approach and Its Transcondylar, Supracondylar, and Paracondylar Extensions." Neurosurgery. 2000;47(Suppl 3):S195-S209.
- Mussi AC, Rhoton AL Jr. "Telovelar Approach to the Fourth Ventricle." Journal of Neurosurgery. 2000;92(5):812-823.
- Samii M, et al. "The Retrosigmoid Approach." Neurosurgery. 2004;55(Suppl 4):1-160.
