Perspectives Commentary on: Exposure of Wide Cerebellomedullary Cisterns for Vascular Lesion Surgeries in Cerebellomedullary Cisterns: Opening of Unilateral Cerebellomedullary Fissures Combined with Lateral Foramen Magnum Approach by Matsushima et al. pp. E615-E621.
Michael T. Lawton, M.D. Professor, Neurological Surgery Vice Chairman, Department of Neurological Surgery Chief, Cerebrovascular and Skull Base Surgery Tong-Po Kan Endowed Chair University of California-San Francisco
Cerebellomedullary Fissure Dissection and Tonsillar Mobilization: A Gateway to Lesions Around the Medulla Adib A. Abla and Michael T. Lawton
S
ubarachnoid spaces are the vascular neurosurgeon’s gateway to pathology: aneurysms around the circle of Willis, cavernous malformations in deep locations, and some margins of arteriovenous malformations. Subarachnoid dissection accesses these lesions without violating brain structures, and the process of deconstructing arachnoid membranes and disconnecting interlobular trabeculae and intercommunicating cisterns relaxes the brain for easier retraction. Some subarachnoid spaces such as the sylvian fissure are transformative, and a wide fissure split opens corridors to the carotid cistern, insula, and basilar apex. There is no fissure elsewhere in the brain as versatile as this one, but the cerebellomedullary fissure (CMF) is the most comparable fissure in the posterior fossa. In this issue of WORLD NEUROSURGERY, Matsushima et al. quantify the exposure that is obtained by combining the far lateral or suboccipital approach with extensive dissection of the cisterna magna, cerebellomedullary cistern (CMC), and CMF.
The tonsil stands in the middle of this anatomy. It is an ovoid structure on the inferomedial aspect of the cerebellar hemisphere that attaches superolaterally through the tonsillar peduncle. The other tonsillar surfaces are free, with the inferior pole and posterior surface in the cisterna magna; the anterior surface facing the CMF; the lateral surface in the CMC; and the medial surface in the vallecula, a cleft or upward connection between the cisterna magna and fourth ventricle. Subarachnoid dissection through these planes detaches the tonsil such that it can be mobilized and retracted in various directions to enhance
Key words Cerebellomedullary cistern - Glossopharyngeal neuralgia - Subtonsillar approach - Telovelar approach - Transcerebellomedullary fissure approach - Vertebral artery aneurysm -
Abbreviations and Acronyms CMC: Cerebellomedullary cistern CMF: Cerebellomedullary fissure PICA: Posterior inferior cerebellar artery
WORLD NEUROSURGERY 82 [5]: e591-e592, NOVEMBER 2014
exposure significantly: tonsillar retraction medially to widen the CMC and reach up into cerebellopontine angle, tonsillar retraction laterally to enter the fourth ventricle or veer into the lateral recess or middle cerebellar peduncle, and tonsillar retraction superiorly to access the posterior medulla and CMF. We routinely dissect these planes around the tonsil during our various procedures, rarely taking the time to define regions of exposure, the way Matsushima et al. have done. Their efforts produced some valuable lessons. Their article provides an excellent description of the CMF, dividing it into the supratonsillar, uvulotonsillar, and medullotonsillar spaces. The supratonsillar space, as defined by the authors, does not refer to space above the tonsil in the tonsillobiventral fissure (which we sometimes use to approach cavernous malformations in the inferior cerebellar peduncle) (2), but rather the space between the inferior medullary velum and the tonsil, which is deep rather than superior to it. The authors also define regions around the CMF, including region A, inside the CMF and outside the fourth ventricle; region B, inside the fourth ventricle; region C, inside both the CMF and the CMC, around the lateral recess; and region D, only in the CMC. These definitions may be helpful in conceptualizing the approaches and determining how best to mobilize the tonsil. Heros (1) first reported the far lateral approach in 1986 as an approach to aneurysms of the vertebral artery and vertebrobasilar junction. This simple variation of the suboccipital craniotomy
Department of Neurological Surgery, University of California San Francisco, San Francisco, California, USA To whom correspondence should be addressed: Michael T. Lawton, M.D. [E-mail: [email protected]] Citation: World Neurosurg. (2014) 82, 5:e591-e592. http://dx.doi.org/10.1016/j.wneu.2014.06.017
www.WORLDNEUROSURGERY.org
e591
PERSPECTIVES
turned into the workhorse approach to posterior inferior cerebellar artery (PICA) aneurysms. His original descriptions of the positioning (lateral decubitus), incision (a diagonal incision oriented from inferomedial to superolateral), C1 laminectomy, lateral suboccipital craniotomy, condylar resection, and dural flap (flapped toward the sigmoid) have been embraced without significant modification (1). With this approach, mobilization of the tonsil medially and superiorly enhances the working window. Some dissection of the CMF and inferior pole of the tonsil is beneficial. We do not find it necessary to dissect medial to the tonsil except to open the foramen of Magendie, evacuate intraventricular hematoma if present, and communicate the ventricle with the cisterna magna. Our dissection typically is focused more laterally in the CMC. Dissection with region D is directed at the triangles defined by the lower cranial nerves, specifically the vagoaccessory triangle, infrahypoglossal triangle, and suprahypoglossal triangle, because these anatomic spaces create windows of exposure through which PICA aneurysms are dissected and clipped (4). Tonsillar mobilization is an important first step, but dissection through the labyrinthine of cranial nerves is the crux of the operation. The same is true for microvascular decompression for glossopharyngeal neuralgia, which requires careful dissection of the PICA deep to cranial nerves IX and X, delicate handling of the lower cranial nerves, and transposing the artery without causing cranial nerve morbidity. Mussi and Rhoton (3) first reported the telovelar approach in an anatomic study in 2000 as an approach to lesions of the fourth ventricle, especially lesions in the superior and lateral recesses. This exposure uses a standard suboccipital approach (prone
REFERENCES 1. Heros RC: Lateral suboccipital approach for vertebral and vertebrobasilar artery lesions. J Neurosurg 64:559-562, 1986. 2. Lawton MT, Quinones-Hinojosa A, Jun P: The supratonsillar approach to the inferior cerebellar peduncle: anatomy, surgical technique, and clinical application to cavernous malformations. Neuro-
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www.SCIENCEDIRECT.com
position, a midline incision, C1 laminectomy, and a midline craniotomy) but differs with its more lateral dissection trajectory. The tonsil is mobilized laterally, and the inferior medullary velum and tela choroidea of the fourth ventricle are cut, unroofing the ventricle. In contrast to the limited dissection used with PICA aneurysms, we extensively dissect the CMF with the telovelar approach to maximize the lateral exposure. Aggressive release along the medial aspect of the tonsil opens the uvulotonsillar space, improving visualization and minimizing disruption of the vermis, which can have untoward effects. The upward reach of the telovelar approach is usually greater than that of the far lateral approach, which requires this extra CMF dissection. This approach is ideal for cavernous malformations that manifest to the ependymal surface of the brainstem in the lateral or superolateral aspect of the floor of the fourth ventricle. In summary, dissection in the cisterna magna, CMF, and CMC is valuable in defining medullary and lower cranial nerve anatomy, releasing the tonsil for retraction, and visualizing a wide variety of pathology. The tonsil should be viewed as a mobile structure that can be safely retracted to open important corridors into subarachnoid and ventricular spaces in the posterior fossa. Similar to the sylvian fissure in the supratentorial compartment, the CMF is an important gateway in the infratentorial compartment that can be opened strategically to optimize exposure of PICA aneurysms, compressive arterial loops, tumors, and cavernous malformations. We congratulate the authors for refining our appreciation of this valuable space and for their use of natural anatomic corridors that allow us to avoid brain transgression during surgery.
surgery 59:ONS244-ONS251, ONS251-242].
2006
[discussion
3. Mussi AC, Rhoton AL Jr: Telovelar approach to the fourth ventricle: microsurgical anatomy. J Neurosurg 92:812-823, 2000. 4. Rodriguez-Hernandez A, Lawton MT: Anatomical triangles defining surgical routes to posterior inferior cerebellar artery aneurysms. J Neurosurg 114:1088-1094, 2011.
Citation: World Neurosurg. (2014) 82, 5:e591-e592. http://dx.doi.org/10.1016/j.wneu.2014.06.017 Journal homepage: www.WORLDNEUROSURGERY.org Available online: www.sciencedirect.com 1878-8750/$ - see front matter ª 2014 Elsevier Inc. All rights reserved.
WORLD NEUROSURGERY, http://dx.doi.org/10.1016/j.wneu.2014.06.017
Michael T. Lawton, M.D. Professor, Neurological Surgery Vice Chairman, Department of Neurological Surgery Chief, Cerebrovascular and Skull Base Surgery Tong-Po Kan Endowed Chair University of California-San Francisco
Cerebellomedullary Fissure Dissection and Tonsillar Mobilization: A Gateway to Lesions Around the Medulla Adib A. Abla and Michael T. Lawton
S
ubarachnoid spaces are the vascular neurosurgeon’s gateway to pathology: aneurysms around the circle of Willis, cavernous malformations in deep locations, and some margins of arteriovenous malformations. Subarachnoid dissection accesses these lesions without violating brain structures, and the process of deconstructing arachnoid membranes and disconnecting interlobular trabeculae and intercommunicating cisterns relaxes the brain for easier retraction. Some subarachnoid spaces such as the sylvian fissure are transformative, and a wide fissure split opens corridors to the carotid cistern, insula, and basilar apex. There is no fissure elsewhere in the brain as versatile as this one, but the cerebellomedullary fissure (CMF) is the most comparable fissure in the posterior fossa. In this issue of WORLD NEUROSURGERY, Matsushima et al. quantify the exposure that is obtained by combining the far lateral or suboccipital approach with extensive dissection of the cisterna magna, cerebellomedullary cistern (CMC), and CMF.
The tonsil stands in the middle of this anatomy. It is an ovoid structure on the inferomedial aspect of the cerebellar hemisphere that attaches superolaterally through the tonsillar peduncle. The other tonsillar surfaces are free, with the inferior pole and posterior surface in the cisterna magna; the anterior surface facing the CMF; the lateral surface in the CMC; and the medial surface in the vallecula, a cleft or upward connection between the cisterna magna and fourth ventricle. Subarachnoid dissection through these planes detaches the tonsil such that it can be mobilized and retracted in various directions to enhance
Key words Cerebellomedullary cistern - Glossopharyngeal neuralgia - Subtonsillar approach - Telovelar approach - Transcerebellomedullary fissure approach - Vertebral artery aneurysm -
Abbreviations and Acronyms CMC: Cerebellomedullary cistern CMF: Cerebellomedullary fissure PICA: Posterior inferior cerebellar artery
WORLD NEUROSURGERY 82 [5]: e591-e592, NOVEMBER 2014
exposure significantly: tonsillar retraction medially to widen the CMC and reach up into cerebellopontine angle, tonsillar retraction laterally to enter the fourth ventricle or veer into the lateral recess or middle cerebellar peduncle, and tonsillar retraction superiorly to access the posterior medulla and CMF. We routinely dissect these planes around the tonsil during our various procedures, rarely taking the time to define regions of exposure, the way Matsushima et al. have done. Their efforts produced some valuable lessons. Their article provides an excellent description of the CMF, dividing it into the supratonsillar, uvulotonsillar, and medullotonsillar spaces. The supratonsillar space, as defined by the authors, does not refer to space above the tonsil in the tonsillobiventral fissure (which we sometimes use to approach cavernous malformations in the inferior cerebellar peduncle) (2), but rather the space between the inferior medullary velum and the tonsil, which is deep rather than superior to it. The authors also define regions around the CMF, including region A, inside the CMF and outside the fourth ventricle; region B, inside the fourth ventricle; region C, inside both the CMF and the CMC, around the lateral recess; and region D, only in the CMC. These definitions may be helpful in conceptualizing the approaches and determining how best to mobilize the tonsil. Heros (1) first reported the far lateral approach in 1986 as an approach to aneurysms of the vertebral artery and vertebrobasilar junction. This simple variation of the suboccipital craniotomy
Department of Neurological Surgery, University of California San Francisco, San Francisco, California, USA To whom correspondence should be addressed: Michael T. Lawton, M.D. [E-mail: [email protected]] Citation: World Neurosurg. (2014) 82, 5:e591-e592. http://dx.doi.org/10.1016/j.wneu.2014.06.017
www.WORLDNEUROSURGERY.org
e591
PERSPECTIVES
turned into the workhorse approach to posterior inferior cerebellar artery (PICA) aneurysms. His original descriptions of the positioning (lateral decubitus), incision (a diagonal incision oriented from inferomedial to superolateral), C1 laminectomy, lateral suboccipital craniotomy, condylar resection, and dural flap (flapped toward the sigmoid) have been embraced without significant modification (1). With this approach, mobilization of the tonsil medially and superiorly enhances the working window. Some dissection of the CMF and inferior pole of the tonsil is beneficial. We do not find it necessary to dissect medial to the tonsil except to open the foramen of Magendie, evacuate intraventricular hematoma if present, and communicate the ventricle with the cisterna magna. Our dissection typically is focused more laterally in the CMC. Dissection with region D is directed at the triangles defined by the lower cranial nerves, specifically the vagoaccessory triangle, infrahypoglossal triangle, and suprahypoglossal triangle, because these anatomic spaces create windows of exposure through which PICA aneurysms are dissected and clipped (4). Tonsillar mobilization is an important first step, but dissection through the labyrinthine of cranial nerves is the crux of the operation. The same is true for microvascular decompression for glossopharyngeal neuralgia, which requires careful dissection of the PICA deep to cranial nerves IX and X, delicate handling of the lower cranial nerves, and transposing the artery without causing cranial nerve morbidity. Mussi and Rhoton (3) first reported the telovelar approach in an anatomic study in 2000 as an approach to lesions of the fourth ventricle, especially lesions in the superior and lateral recesses. This exposure uses a standard suboccipital approach (prone
REFERENCES 1. Heros RC: Lateral suboccipital approach for vertebral and vertebrobasilar artery lesions. J Neurosurg 64:559-562, 1986. 2. Lawton MT, Quinones-Hinojosa A, Jun P: The supratonsillar approach to the inferior cerebellar peduncle: anatomy, surgical technique, and clinical application to cavernous malformations. Neuro-
e592
www.SCIENCEDIRECT.com
position, a midline incision, C1 laminectomy, and a midline craniotomy) but differs with its more lateral dissection trajectory. The tonsil is mobilized laterally, and the inferior medullary velum and tela choroidea of the fourth ventricle are cut, unroofing the ventricle. In contrast to the limited dissection used with PICA aneurysms, we extensively dissect the CMF with the telovelar approach to maximize the lateral exposure. Aggressive release along the medial aspect of the tonsil opens the uvulotonsillar space, improving visualization and minimizing disruption of the vermis, which can have untoward effects. The upward reach of the telovelar approach is usually greater than that of the far lateral approach, which requires this extra CMF dissection. This approach is ideal for cavernous malformations that manifest to the ependymal surface of the brainstem in the lateral or superolateral aspect of the floor of the fourth ventricle. In summary, dissection in the cisterna magna, CMF, and CMC is valuable in defining medullary and lower cranial nerve anatomy, releasing the tonsil for retraction, and visualizing a wide variety of pathology. The tonsil should be viewed as a mobile structure that can be safely retracted to open important corridors into subarachnoid and ventricular spaces in the posterior fossa. Similar to the sylvian fissure in the supratentorial compartment, the CMF is an important gateway in the infratentorial compartment that can be opened strategically to optimize exposure of PICA aneurysms, compressive arterial loops, tumors, and cavernous malformations. We congratulate the authors for refining our appreciation of this valuable space and for their use of natural anatomic corridors that allow us to avoid brain transgression during surgery.
surgery 59:ONS244-ONS251, ONS251-242].
2006
[discussion
3. Mussi AC, Rhoton AL Jr: Telovelar approach to the fourth ventricle: microsurgical anatomy. J Neurosurg 92:812-823, 2000. 4. Rodriguez-Hernandez A, Lawton MT: Anatomical triangles defining surgical routes to posterior inferior cerebellar artery aneurysms. J Neurosurg 114:1088-1094, 2011.
Citation: World Neurosurg. (2014) 82, 5:e591-e592. http://dx.doi.org/10.1016/j.wneu.2014.06.017 Journal homepage: www.WORLDNEUROSURGERY.org Available online: www.sciencedirect.com 1878-8750/$ - see front matter ª 2014 Elsevier Inc. All rights reserved.
WORLD NEUROSURGERY, http://dx.doi.org/10.1016/j.wneu.2014.06.017
