A transcondylar approach to the arteriovenous malformation at the ventral cervicomedullary junction: report of three cases

Biomechanical effects of the transcondylar approach on the craniovertebral junction

The transcondylar variation of the far-lateral, retrosigmoid approach is intended for pathologies in the anterolateral portion of the foramen magnum. That area is more clearly visualized when a fraction of the ipsilateral occipital condyle is removed. In this study, the biomechanical effect of this approach on occiput-C2 rotation was investigated. Our hypothesis was that the biomechanical characteristics are significantly altered following the transcondylar approach. Five human cadaveric upper cervical spine specimens (occiput-C7) were used in the study. Torsional moments were applied from zero to a maximum of 1.5 N m to the left and to the right using a mechanical testing machine. The resulting rotational motions of the O-C1, C1-2, and O-C2 segments were measured in the intact specimen and after a simulated right-sided transcondylar approach with resection of 2/3 of the condyle, confirmed by CT scanning and visual inspection. After the posterior two-thirds of the occipital condyle were removed, the neutral zone (NZ) increased 1.3° to the left and 2° to the right at C0-C1, and 7.4° to the left and 6.2° to the right at C1-2. The cumulative increase in NZ between O and C2 was 8.7° to the left and 8.2° to the right. The transcondylar approach also resulted in significant increases in range of motion (ROM) in axial rotation to both sides in all segments. ROM increased 2.8° to the left and 2.4° to the right between C0 and C1, 7.3° to the left and 5.4° to the right between C1 and C2, and 10.1° to the left and 7.8° to the right between CO and C2. Upon inspection, the area of the occipital condyle where the alar ligament attaches had been completely removed in three of the five specimens. Removing the posteromedial two-thirds of one occipital condyle alters the normal axial rotational movements of the craniovertebral junction on both sides. The insertion of the alar ligament can be inadvertently removed during condylar resection, and this could contribute to atlanto-axial instability. There is a biomechanical substrate to cranio-cervical instability following a transcondylar approach; these patients may need to be followed over several years to ensure it does not progress and necessitate occipito-cervical fusion.

Brainstem arteriovenous malformations: anatomical subtypes, assessment of "occlusion in situ" technique, and microsurgical results

OBJECT

The surgical management of brainstem arteriovenous malformations (AVMs) might benefit from the definition of anatomical subtypes and refinements of resection techniques. Many brainstem AVMs sit extrinsically on pia mater rather than intrinsically in the parenchyma, allowing treatment by occluding feeding arteries circumferentially, interrupting draining veins after arteriovenous shunting is eliminated, and leaving the obliterated nidus behind. The authors report here the largest series of brainstem AVMs to define 6 subtypes, assess this "occlusion in situ" technique, and analyze the microsurgical results.

METHODS

Brainstem AVMs were categorized as 1 of 6 types: anterior midbrain, posterior midbrain, anterior pontine, lateral pontine, anterior medullary, and lateral medullary AVMs. Data from a prospectively maintained AVM registry were reviewed to evaluate multidisciplinary treatment results.

RESULTS

During a 15-year period, the authors treated 29 patients with brainstem AVMs located in the midbrain (1 anterior and 6 posterior), pons (6 anterior and 7 lateral), and medulla (1 anterior and 8 lateral). The nidus was pial in 26 cases and parenchymal in 3 cases. Twenty-three patients (79%) presented with hemorrhage. Brainstem AVMs were either resected (18 patients, 62%) or occluded in situ (11 patients, 38%). All lateral pontine AVMs were resected, and the occlusion in situ rate was highest with anterior pontine AVMs (83%). Angiography confirmed complete obliteration in 26 patients (89.6%). The surgical mortality rate was 6.9%, and the rate of permanent neurological deterioration was 13.8%. At follow-up (mean 1.3 years), good outcomes (modified Rankin Scale [mRS] score ≤ 2) were observed in 18 patients (66.7%) and poor outcomes (mRS score of 3-5) were observed in 9 patients (33.3%). The mRS scores in 21 patients (77.8%) were unchanged or improved. The best outcomes were observed with lateral pontine (100%) and lateral medullary (75%) AVMs, and the rate of worsening/death was greatest with posterior midbrain and anterior pontine AVMs (50% each).

CONCLUSIONS

Brainstem AVMs can be differentiated by their location in the brainstem (midbrain, pons, or medulla) and the surface on which they are based (anterior, posterior, or lateral). Anatomical subtypes can help the neurosurgeon determine how to advise patients, with lateral subtypes being a favorable surgical indication along with extrinsic pial location and hemorrhagic presentation. Most AVMs are dissected with the intention to resect them, and occlusion in situ is reserved for those AVMs that do not separate cleanly from the brainstem, that penetrate into the parenchyma, or are more anterior in location, where it is difficult to visualize and preserve perforating arteries (anterior pontine and lateral medullary AVMs). Although surgical morbidity is considerable, surgery results in a better obliteration rate than nonoperative management and is indicated in highly selected patients with high rerupture risks.

Radiosurgical techniques and clinical outcomes of gamma knife radiosurgery for brainstem arteriovenous malformations

Objective

Brainstem arteriovenous malformation (AVM) is rare and radiosurgical management is complicated by the sensitivity of the adjacent neurological structures. Complete obliteration of the nidus is not always possible. We describe over 20 years of radiosurgical procedures for brainstem AVMs, focusing on clinical outcomes and radiosurgical techniques.

Methods

Between 1992 and 2011, the authors performed gamma knife radiosurgery (GKRS) in 464 cerebral AVMs. Twenty-nine of the 464 patients (6.3%) reviewed had brainstem AVMs. This series included sixteen males and thirteen females with a mean age of 30.7 years (range : 5-71 years). The symptoms that led to diagnoses were as follows : an altered mentality (5 patients, 17.3%), motor weakness (10 patients, 34.5%), cranial nerve symptoms (3 patients, 10.3%), headache (6 patients, 20.7%), dizziness (3 patients, 10.3%), and seizures (2 patients, 6.9%). Two patients had undergone a previous nidus resection, and three patients had undergone a previous embolization. Twenty-four patients underwent only GKRS. With respect to the nidus type and blood flow, the ratio of compact type to diffuse type and high flow to low flow were 17 : 12 and 16 : 13, respectively. In this series, 24 patients (82.8%) had a prior hemorrhage. The mean target volume was 1.7 cm³ (range 0.1-11.3 cm³). The mean maximal and marginal radiation doses were 38.5 Gy (range 28.6-43.6 Gy) and 23.4 Gy (range 18-27 Gy), and the mean isodose profile was 61.3% (range 50-70%).

Results

Twenty-four patients had brainstem AVMs and were followed for more than 3 years. Obliteration of the AVMs was eventually documented in 17 patients (70.8%) over a mean follow-up period of 77.5 months (range 36-216 months). With respect to nidus type and blood flow, the obliteration rate of compact types (75%) was higher than that of diffuse types (66.7%), and the obliteration rate of low flow AVMs (76.9%) was higher than that of high flow AVMs (63.6%) (p<0.05). Two patients (6.9%) with three hemorrhagic events suffered a hemorrhage during the follow-up period. The annual bleeding rate of AVM after GKRS was 1.95% per year. No adverse radiation effects or delayed cystic formations were found.

Conclusion

GKRS has an important clinical role in treatment of brainstem AVMs, which carry excessive surgical risks. Angiographic features and radiosurgical techniques using a lower maximal dose with higher isodose profiles are important for lesion obliteration and the avoidance of complications.

[Partial transcondylar approach - analysis of the surgical technique in cadaver simulation]

The aim of the study was to present consecutive stages of the partial transcondylar approach. Six simulations of the partial transcondylar approach were performed on non-fixed human cadavers without any known pathologies in the head and neck. The consecutive stages of the procedure were documented with photographs and diagrams. The starting point for the partial transcondylar approach is a posterior repositioning of the suboccipital segment of the vertebral artery. The approach is achieved by partial removal of the occipital condyle and lateral mass of the atlas as well as by suboccipital craniectomy. Elevation of the cerebellar hemisphere presents an important supplement of the approach. The partial transcondylar approach is a reproducible technique, which provides surgical penetration of the anterior part of the cranio-cervical junction and related regions. This approach is particularly useful in the treatment of intradural tumours localized ventrally to the medulla.

Stereotactic radiosurgery for arteriovenous malformations, Part 5: management of brainstem arteriovenous malformations

Object

In this paper, the authors' goal was to define the long-term outcomes and risks of stereotactic radiosurgery (SRS) for arteriovenous malformations (AVMs) of the medulla, pons, and midbrain.

Methods

Between 1987 and 2006, the authors performed Gamma Knife surgery in 996 patients with brain AVMs; 67 patients had AVMs in the brainstem. In this series, 51 patients (76%) had a prior hemorrhage. The median target volume was 1.4 cm3 (range 0.1–13.4 cm3). The median margin dose was 20 Gy (range 14–25.6 Gy).

Results

Obliteration of the AVMs was eventually documented in 35 patients at a median follow-up of 73 months (range 6–269 months). The actuarial rates of documentation of total obliteration were 41%, 70%, 70%, and 76% at 3, 4, 5, and 10 years, respectively. Higher rates of AVM obliteration were associated only with a higher margin dose. Four patients (6%) suffered a hemorrhage during the latency period, and 2 patients died. The rate of AVM hemorrhage after SRS was 3.0%, 3.0%, and 5.8% at 1, 5, and 10 years, respectively. The overall annual hemorrhage rate was 1.9%. Permanent neurological deficits due to adverse radiation effects (AREs) developed in 7 patients (10%) after SRS, and a delayed cyst developed in 2 patients (3%). One patient died at an outside institution with symptoms of AREs and unrecognized hydrocephalus. Higher 12-Gy volumes and higher Spetzler-Martin grades were associated with a higher risk of symptomatic AREs. Ten of 22 patients who had ocular dysfunction before SRS had improvement, 9 were unchanged, and 3 were worse due to AREs. Eight of 14 patients who had hemiparesis before SRS improved, 5 were unchanged, and 1 was worse.

Conclusions

Although hemorrhage after obliteration did not occur in this series, patients remained at risk during the latency interval until obliteration occurred. Thirty-eight percent of the patients who had neurological deficits due to prior hemorrhage improved. Higher dose delivery in association with conformal and highly selective SRS is required for safe and effective radiosurgery.

Spinal arteriovenous shunts presenting as intracranial subarachnoid haemorrhage

Background

In approximately 5% of patients with intracranial subarachnoid haemorrhage (SAH), the cause is another than a ruptured aneurysm or perimesencephalic haemorrhage. One of these causes is a spinal arteriovenous shunt (SAVS). The aim of this study was to investigate the characteristics of patients with SAVS who present with intracranial SAH without symptoms and signs suggesting a spinal cause.

Methods

We systematically reviewed the literature and searched the SAH database of the University Medical Center Utrecht, The Netherlands, for patients with SAVS presenting with intracranial SAH and studied the characteristics of patients with SAVS whose clinical presentation mimicked intracranial SAH caused by rupture of a saccular aneurysm.

Results

Thirty-five patients were identified after a review of the literature. In our SAH database, comprising 2142 patients included in the period 1985–2004, we found one patient (0.05%, 95 % CI 0.006–0.3%). SAH due to SAVS occurred at any age (4–72 years). The SAVS was located at the craniocervical junction in 14 patients, at the cervical level in 11, and at the thoracolumbar level in the remaining 11 patients. The majority of patients (n = 26, 72%) had no disabling deficits at discharge or follow-up.

Conclusion

Rupture of a SAVS presenting as intracranial SAH is rare and can occur at any age. The SAVS can be located not only at the craniocervical junction or cervical level but also in the thoracolumbar region. Most patients with SAVS presenting as intracranial SAH have a good recovery.

Jugular tubercle: Morphometric analysis and surgical significance

OBJECT

Maximizing intradural exposure via the extreme lateral infrajugular transcondylar-transtubercular exposure (ELITE) approach depends on understanding the fundamental anatomy of the jugular tubercle (JT). Drilling the JT can maximize the extent of exposure achieved with the ELITE approach. Removing the JT is critical for optimizing access to the inferior and midclival areas, vertebrobasilar artery junction, and ventral pons and medulla.

METHODS

In this cadaveric study, the individual structural variations in the JT were evaluated in 100 split occipital bones. The mean length of the JT was 1.65 +/- 0.36 cm (range 1.2-3 cm); its mean width was 1.15 +/- 0.16 cm (range 0.7-1.7 cm); and its mean thickness was 0.61 +/- 0.15 cm (range 0.2-1 cm). The authors analyzed the difference in morphometric data with regard to right and left sides and found no statistically significant difference between the two sides. Furthermore, data from the morphometric study were compared with the results of 20 measurements obtained from three-dimensional computed tomography (3D CT) scans. Accordingly, the mean length of the JT was 1.35 +/- 0.15 cm (range 1-2.8 cm); the mean width, 1.10 +/- 0.12 cm (range 0.8-1.3 cm); and the mean thickness, 0.51 +/- 0.18 cm (range 0.2-1 cm).

CONCLUSIONS

Morphometric data on the JT contribute significantly to the neurosurgeon's task of skull base drilling. The 3D CT scans were useful in the preoperative planning.

Quantitative description of the far-lateral transcondylar transtubercular approach to the foramen magnum and clivus

OBJECT

The purpose of this study was to evaluate the far-lateral transcondylar transtubercular approach (complete FLA) based on quantitative measurements of the exposure of the foramen magnum and petroclival area obtained after each successive step of this approach.

METHODS

The complete FLA was reproduced in eight specially prepared cadaveric heads (a total of 15 sides). The approach was divided into six steps: 1) C-1 hemilaminectomy and suboccipital craniectomy with unroofing of the sigmoid sinus (basic FLA); 2) partial resection of the occipital condyle (up to the hypoglossal canal); 3) removal of the jugular tuberculum; 4) mastoidectomy (limited to the labyrinth and the fallopian canal) and retraction of the sigmoid sinus; 5) resection of the lateral mass of C-1 with mobilization of the vertebral artery; and 6) resection of the remaining portion of the occipital condyle. After each successive step, a standard set of measurements was obtained using a frameless stereotactic device. The measurements were used to estimate two parameters: the size of the exposed petroclival area and the size of a spatial cone directed toward the anterior rim of the foramen magnum, which depicts the amount of surgical freedom available for manipulation of instruments. The initial basic FLA provided exposure of only 21 +/- 6% of the petroclival area that was exposed with the full, six-step maximally aggressive (complete) FLA. Likewise, only 18 +/- 9% of the final surgical freedom was obtained after the basic FLA was performed. Each subsequent step of the approach increased both petroclival exposure and surgical freedom. The most dramatic increase in petroclival exposure was noted after removal of the jugular tuberculum (71 +/- 12% of final exposure), whereas the least improvement in exposure occurred after the final step, which consisted of total condyle resection.

CONCLUSIONS

The complete FLA provides wide and sufficient exposure of the foramen magnum and lower to middle clivus. The complete FLA consists of several steps, each of which contributes to increasing petroclival exposure and surgical freedom. However, the FLA may be limited to the less aggressive steps, while still achieving significant exposure and surgical freedom. The choice of complete or basic FLA thus depends on the underlying pathological condition and the degree of exposure required for effective surgical treatment.

Far lateral transcondylar approach: dimensional anatomy

OBJECTIVE

The far lateral extension of the classic suboccipital craniectomy has been found to reduce the depth of the field and improve the angle of surgical perspective to the ventrolateral clivus. The goal of the present study is to determine and compare the dimensions of the far lateral transcondylar approach with the suboccipital craniectomy.

METHODS

Ten cadaveric specimens were used to study the anatomy at the foramen magnum (FM), occipital condyle (OC), and vertebral artery. The distances from the posterior midline of the FM to the medial and lateral borders of the OC were measured. The distance of the vertebral artery from the transverse foramen of C1 to its dural entry and the distance from the dural entry to the posteroinferior cerebellar artery were measured. The amount of OC removal that was necessary to expose the contralateral jugular tubercle was determined. A reference line was constructed from the posterior margin of the FM to the border of the OC. From this line, the angle of surgical approach provided by each exposure was measured.

RESULTS

The mean distance of the vertebral artery from the transverse foramen of C1 to its dural entry was 22+/-3 mm (standard deviation), and the distance from the dural entry to the posteroinferior cerebellar artery was 17+/-8 mm. The distance from the posterior midline of the FM to the medial border of the OC was 27+/-0.5 mm; the distance from the posterior midline of the FM to the lateral border of the OC was 40+/-0.4 mm; and the long axis of the OC was 30+/-0.4 mm. The amount of OC removal to expose the contralateral jugular tubercle without brainstem retraction was 17+/-1 mm. The angle of surgical approach versus the reference line decreased from 88+/-2 degrees with the suboccipital craniectomy alone to 47+/-2 degrees for the far lateral transcondylar exposure (P < 0.001). This angle decreased an average of 2.4 degrees per millimeter of OC removal.

CONCLUSION

Understanding the dimensions of the craniovertebral junction has clear implications for surgery in this area. If a lesion may be approached through a perpendicular exposure, the suboccipital craniectomy alone may be sufficient. Additional exposure of the ventrolateral clivus without brainstem retraction requires condylar removal. A more limited condylar removal than the 17 mm described in this report may be adequate if the entire 47-degree angle is not needed.

Microsurgical anatomy of the transcondylar, supracondylar, and paracondylar extensions of the far-lateral approach

Despite a large number of reports of the use of the far-lateral approach, some of the basic detail that is important in safely completing this exposure has not been defined or remains poorly understood. The basic far-lateral exposure provides access for the following approaches: 1) the transcondylar approach directed through the occipital condyle or the adjoining portions of the occipital and atlantal condyles; 2) the supracondylar approach directed through the area above the occipital condyle; and 3) the paracondylar exposure directed through the area lateral to the occipital condyle. The transcondylar approach provides access to the lower clivus and premedullary area. The supracondylar approach provides access to the region of, and medial to, the hypoglossal canal and jugular tubercle. The paracondylar approach, which includes drilling of the jugular process of the occipital bone in the area lateral to the occipital condyle, provides access to the posterior portion of the jugular foramen and to the mastoid on the lateral side of the jugular foramen. In this study, the anatomy important to completing the far-lateral approach and these modifications was examined in 12 cadaveric specimens. In the standard posterior and posterolateral approaches, an understanding of the individual suboccipital muscles is not essential. However, these muscles provide important landmarks for the far-lateral approach and its modifications. Other important considerations include the relationship of the occipital condyle to the foramen magnum, hypoglossal canal, jugular tubercle, the jugular process of the occipital bone, the mastoid, and the facial canal. These and other relationships important to completing these exposures were examined in this study.

The suboccipital cavernous sinus

The authors studied the microsurgical anatomy of the suboccipital region, concentrating on the third segment (V3) of the vertebral artery (VA), which extends from the transverse foramen of the axis to the dural penetration of the VA, paying particular attention to its loops, branches, supporting fibrous rings, adjacent nerves, and surrounding venous structures. Ten cadaver heads (20 sides) were fixed in formalin, their blood vessels were perfused with colored silicone rubber, and they were dissected under magnification. The authors subdivided the V3 into two parts, the horizontal (V3h) and the vertical (V3v), and studied the anatomical structures topographically, from the superficial to the deep tissues. In two additional specimens, serial histological sections were acquired through the V3 and its encircling elements to elucidate their cross-sectional anatomy. Measurements of surgically and clinically important features were obtained with the aid of an operating microscope. This study reveals an astonishing anatomical resemblance between the suboccipital complex and the cavernous sinus, as follows: venous cushioning; anatomical properties of the V3 and those of the petrous-cavernous internal carotid artery (ICA), namely their loops, branches, supporting fibrous rings, and periarterial autonomic neural plexus; adjacent nerves; and skull base locations. Likewise, a review of the literature showed a related embryological development and functional and pathological features, as well as similar transitional patterns in the arterial walls of the V3 and the petrous-cavernous ICA. Hence, due to its similarity to the cavernous sinus, this suboccipital complex is here named the "suboccipital cavernous sinus." Its role in physiological and pathological conditions as they pertain to various clinical and surgical implications is also discussed.