Anatomy
Nerves
- C1:
- Motor only nerve
- Rectus capitis muscles, omohyoid, thyrohyoid and geniohyoid.
- Suboccipital nerve Aka posterior ramus of the first cervical spinal nerve (C1)
- C2:
- Motor and sensory nerve
- Rectus capitis muscles, omohyoid, sternohyoid, sternothyroid and longus colli.
Positioning
- Patient position and skin incisions for a far-lateral approach using the park-bench position.
- A straight incision (a) is now preferred over the hockey stick incision (b,c).
- The straight incision reduces operative time, blood loss, and muscle atrophy;
Technique
- Lateral positioning is preferred for the far lateral craniotomy, with the superior sagittal sinus oriented parallel to the floor.
- The scalp flap is planned using a hockey stick-style incision, with the major limb in the midline from the C2 spinous process to the superior margin of the transverse sinus, the superior edge paralleling the transverse sinus, and the minor limb descending along the flat part of the mastoid to its tip.
- After the skin is incised and the posterior occipital musculature identified, the epicranial aponeurosis is incised with a 1cm muscle cuff at the superior margin for closure.
- Subperiosteal dissection can then be carried superior to inferior to elevate the single-layer myocutaneous flap, carefully exposing the deep bony and soft tissue structures until the mastoid tip and C2 lamina are reached.
- The flap is covered and padded with cotton sponges, retained in place with fish hook retractors, and bony landmarks are used to approximate the location of the transverse-sigmoid junction,
- Where a large burr hole is fashioned, exposing the full margins of the sinus.
- The posterior margin of the descending sigmoid sinus is exposed by drilling a bony trough, a large bone flap is turned extending at least 1cm past the midline and down to the foramen magnum
- and the dura is carefully stripped off the inner table of the skull as the bone flap is elevated.
- With the bone flap completed, attention is turned to C1, where a wide, bilateral laminectomy is performed.
- However, care must be exercised in the region of the suboccipital triangle to prevent injury to the vertebral artery (VA).
Suboccipital triangle
- Borders
- Superior oblique
- Which passes from the C1 transverse process to the occipital bone
- Inferior oblique
- Which extends from the transverse process of C1 to the spinous process of C2
- Rectus capitis posterior major
- Which extends from the occipital bone below the inferior nuchal line to the spinous process of C2.
- Contents
- Vertebral artery
- As it passes behind the superior facet of C1 and across the upper edge of the posterior atlantal arch.
- Suboccipital nerve
- Clinically
- Used for far lateral approaches
The left semispinalis and longissimus capitis have been removed to expose the suboccipital triangle
- The C1 posterior arch and lateral mass are exposed; the lateral mass houses the V2 and V3 segment of the VA.
- The craniotomy and C1 osteotomy are performed.
- The occipital condyle is partially drilled and the VA can be mobilized.
- Each pathology will demand a different tailoring of the far-lateral approach with regard to size of the lateral suboccipital craniotomy, C1 posterior arch and lateral mass osteotomy, and occipital condyle drilling
- Exposure of the cerebellomedullary fissure and its main neurovascular content. It is possible to see the anterolateral sulcus (ALS).
- Anterolateral view of the brainstem stressing the theoretical area of exposure (shaded area) provided by the far-lateral approach. Optimal neurotomies for both the olivary zone (OZ) and the ALS safe entry zone can be seen.
- Posterolateral view of brainstem also stressing the theoretical area of exposure provided by the approach (shaded area). The ideal incisions for the lateral medullary zone and the posterior intermediate sulcus are depicted here
Exam answer
- This tumour was removed by a posterior lateral approach. An appropriately investigated, consented and marked patient is placed prone in pins with the neck extended and the head flexed.
- A midline incision (inion to spinous process of C2) is made. Dissect along the median raphe to expose the occiput and posterior arch of C1. Elevate muscles from inion to foramen magnum.
- Small paramedian craniotomy/craniectomy is performed. Work laterally across C1, dissecting muscles free and exposing vertebral artery in its grove on superior surface of C1.
- Remove the posterior arch of C1, dissecting the vertebral artery free from the bone to give proximal control. Once exposed; open dura in a “reverse 7” shape to expose right cerebellar tonsil and upper cervical cord. Identify spinal accessory nerve branches, in this case overlying the tumour, and dissect these free as far as possible. Use a combination of CUSA and microdissectors to free tumour from cord and debulk it until capsule can be completely excised. Haemostasis. Dural closure (where possible) ± graft. Closure in layers
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