Indications
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In general, most focal cervicomedullary tumors are considered benign in pathology. Common tumors of the cervicomedullary region include low-grade astrocytomas, gangliogliomas and ependymomas. They are typically well-delineated and benign tumors, typically amenable to gross total resection.
Contraindications
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Anaplastic astrocytomas with a diffuse growth pattern may also be encountered and are challenging to resect (biopsy can be done to establish a diagnosis).
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Lesions that are stable clinically and radiographically, without signs of malignancy, may be initially managed in a non-operative manner with serial imaging and neurological evaluation.
Preoperative Considerations
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The difficulty of surgical removal is correlated with the tumor’s growth pattern, pathology and involvement of surrounding structures.
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Cervicomedullary tumors often extend from the lower two-thirds of the medulla to the superior aspect of the spinal cord. Some anatomical barriers (e.g. pyramidal decussation, medial lemniscus, etc.) confine these benign tumors in the cervicomedullary region making them more accessible and resectable ( Figure 7.1 ).
Figure 7.1
Classification of brainstem lesions based on CT and MRI by Choux et al. (2000). Type I: diffuse, Type II: focal intrinsic, Type III: focal exophytic, Type IV: cervicomedullary.
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Intraoperative monitoring and assessment of the functional integrity of neural pathways is typically recommended during surgery. This includes somatosensory evoked potentials, brainstem auditory evoked potentials, spontaneous EMG of the lower cranial nerves and motor evoked potentials.
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A cartouche stimulation probe can be used over the dorsal medulla to find a non-eloquent safe entry zone to access the cervicomedullary lesion. This allows for detection of functional neural tissue overlying the lesion and the proximity of functional tissue to the tumor.
Surgical Procedure
Patient Positioning
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The patient is placed in the prone position with the head flexed and fixed in a Mayfield three-point fixation headholder.
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For young children aged less than 3 years or patients with a very thin skull, a cerebellar headrest padded with soft gel rolls is typically recommended.
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A semi-sitting position can be used while keeping in mind its potential complications, including venous air embolisms.
Skin Incision
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A standard midline skin incision is performed.
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If the tumor is significantly eccentric, the skin incision is placed laterally as in a far lateral approach.
Craniotomy
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Cervicomedullary tumors are usually approached through a midline suboccipital craniotomy combined with C1 laminectomy. If the lesion extends caudally, the opening is complemented with additional necessary cervical laminectomies.
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Laterally and/or ventrally located tumors are accessed through a dorsolateral approach (far lateral or retrosigmoid approaches). It is typically not necessary to perform extensive bone removal of the occipital condyle and/or the lateral mass of C1. Adequate exposure is obtained by the craniotomy and a C1-hemilaminectomy. Only when necessary, partial drilling of the posterior third of the occipital condyle can be performed.
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Often the cervical lamina can be replaced at the end of the procedure, and therefore cervical laminotomies can be considered instead of laminectomies.
Dural Opening
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The dura mater is typically opened in a Y-shape. In the cervical portion a midline longitudinal incision is done under magnification with the operative microscope. Tenting sutures are used for dural traction.
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Intraoperative ultrasound can be used to localize the lesion before or after dural opening.
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A cartouche stimulation probe can be used over the dorsal medulla to find a non-eloquent safe entry zone.
Intradural Dissection
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The surgical technique for tumor removal is similar to the methods used for intramedullary spinal cord tumors.
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Less than 1 cm longitudinal myelotomy is typically performed at the surface of the cervicomedullary junction ( Figure 7.2 ). This opening is placed over the tumor. The best approach is that which allows exposure of the tumor with the least risk of injury to surrounding structures. Anatomical knowledge, preoperative imaging and intraoperative mapping can be used to help select the safe entry zone ( Figure 7.3 ).
