Suboccopital retrosigmoid craniotomy is an accepted approach for the vestibular schwannoma surgery. It provides wide exposure of the cerebellopontine angle and gives an opportunity to resect large tumors and preserve hearing in some cases. Lateral sinus thrombosis is a known complication after the posterior fossa surgery and has been noted in 4.7-11.6% of cases [2–8]. However, there is limited data about that complication exclusively in the vestibular schwannoma surgery. There is a series of studies [1, 4, 10], however, they have some limitations. In most of them non-contrast CT was used as a postoperative control , the analyzed cohort included various surgical approaches [1, 3, 10] or translabyrinthine approach . There are no large cohort studies with well-documented postoperative radiological investigation on the retrosigmoid approach in the vestibular schwannoma surgery.
The risk factors of deep venous thrombosis after skull base surgery are well described . The main concern it to prevent pulmonary embolism, which could be fatal. On the other hand, in thrombosis of the dural sinuses hyperemia and venous hypertension, which could cause intracranial hypertension and cerebral hemorrhage, are more problematic. What is more, the risk factors and management of the lateral dural venous sinuses are less evident than in the case of deep venous thrombosis.
Various symptomatology can be presented in the venous sinus thrombosis: headaches, seizures, intracranial hypertension, intraparenchymal hemorrhage [1, 6, 8]. Some cases of fatal course of sinus thrombosis have even been presented . However, most studies emphasize mild or even asymptomatic course of lateral sinus thrombosis [1, 2, 3, 5, 8]. In our study, most of the patients, except one, were completely asymptomatic. In one case, a massive cerebellar edema developed, which could be explained by the large sigmoid sinus thrombosis. What is more, all of the patients remain asymptomatic after long follow-up. Our results support the conclusions of other authors [1, 2, 3, 5, 8] that postoperative DST is rarely symptomatic in such cases. This is likely due to collateral venous drainage, making thrombosis well-tolerated.
Duration of the surgery, the tumor volume, side, the patient’s age or sex were not risk factors of postoperative thrombosis. Moreover, we did not observe the correlation between thrombosis and smaller sinus diameter observed by other authors [1, 3]. It is an interesting fact, conflicting with the assumption that lower caliber of the vessel and slower flow could increase the risk for thrombosis in that group of patients. The most important risk factor seems to be an intraoperative injury of the sinus, which was observed in 23% of cases in the thrombosis group and only in 3% of cases in the non-thrombosed group. It means that sinus exposure is the main risk factor of thrombosis and the main prophylactic activities should focus on minimizing the sinus manipulation. Some authors theorized that even heat conduction during bone drilling, could facilitate the thrombotic cascade . In our group, the most common moment of sinus injury was the moment of passing the footplate parallel to the sigmoid sinus. We have noticed that in most of such cases we injured not the sigmoid sinus itself, but the mastoid emissary vein which connects the venous drainage of the brain with the superficial scalp venous network. During craniotomy, the vein was usually ruptured and avulsed, causing also bleeding from the sigmoid sinus. We manage that complication by changing the strategy of craniotomy. Nowadays, we perform craniotomy to the point where the vein exits the skull through the foramen. The remaining part of the bone is drilled using the high-speed bone cut and diamond drills, to the level of the medial margin of the sinus, which is sufficient for the vestibular schwannoma surgery. After such modification of the craniotomy technique, we observed a markedly lower incidence of the postoperative sinus thrombosis. However, further analysis on that topic is needed.
Previously, the incidence of DST after posterior fossa surgery was estimated at 11.6% . Recent studies have noted more frequent diagnoses of thrombosis – 32.4% , 38.9% , however, there are studies with lower frequency – 6% , 6.7% . The reported incidence depends on the character of the study – retrospective or prospective, the imaging modality – contrast enhanced or not and on the surgical approach. In our retrospective study, where every analyzed case was investigated using contrast enhanced MRI, we noted postoperative changes in 22%. It must be noted that in 7 cases the sinus was narrowed by the hemostatic agent used for the dural closure, which does not mean thrombosis at all. True thrombosis was noted in 19 cases, which account for 16% of the studied patients. As mentioned above, the studies analyzed lesions of various surgical approaches, different pathologies, non-contrast postoperative examinations. To the best of our knowledge, our study is the largest study focusing on vestibular schwannoma operated on using the retrosigmoid approach with postoperative contrast enhanced MRI.
During the analysis of the postoperative imaging we found various presentations of the lateral sinuses after retrosigmoid craniotomy. In 7 cases we did not found any loss of contrast filling inside the sinus lumen, however, the flow was compromised by compression caused by the hemostatic agent. None of the patients from that group developed any symptoms. Long-term postoperative control revealed complete resolution of compression in 5 cases, partial resolution in 1 case. One patients was lost to follow-up. Such presentation seems to be benign and asymptomatic. To the best of our knowledge, such radiological symptom was never described nor analyzed in the neurosurgical literature.
In 19 cases, we managed to visualize true thrombosis of the sinus. In 31% of cases, the extent of the thrombus was compromising less than a half of the sinus diameter. In the rest of the cases, the thrombus size was larger. In that group, we managed to highlight the differences in the radiological pattern. In 23% of cases, thrombosis was filling more than a half of the sinus diameter. In 8%, it filled almost all cross-section of the sinus, leaving marginal flow of the contrast around the thrombus. In another 8% of cases, we noted complete obstruction of the sinus, however, without engorgement of the veins draining into the sinus. Moreover, in 3% of all cases we found a complete thrombosis with such engorgement. To the best of our knowledge, there is no classification of postoperative dural sinus thrombosis in the literature. The authors of previous studies managed the topic noting only the fact of presence or absence of the thrombus.
Anticoagulation and management
The main concern in dural sinus thrombosis is prevention of the thrombus propagation, obstruction of the blood drainage and increased intracranial pressure due to hyperemia or intracerebral hemorrhage. Such a situation could be fatal, as reported . The role of anticoagulation is to prevent such thrombus progression. However, the usage of anticoagulants in the early postoperative course is always a dilemma. What is more, there are no guidelines managing that topic. Despite the relative clinical safety of the low-molecular-weight heparine (LMWH) in the prophylactic dosage, the use of higher, therapeutic doses seems to be more risky and thus is avoided postoperatively. However, even low dosage could cause hemorrhagic complications, as reported by other authors . In our group, we did not observe any complications which could be associated with the used anticoagulation. We manage thrombosis basing on its extent. The sinus compression or a small thrombus was managed only by administering IV fluids and clinical observation. In more prominent cases, we administered LMWH in prophylactic dosage and fluids. Anticoagulation was administered in 10 cases (38%), never earlier than 24-48 hours after the surgery, and was usually discontinued before the discharge. In all cases we used enoxaparin. Based on our results, it is debatable if anticoagulation is necessary in such cases. Almost all of them were asymptomatic. In some of the cases, the thrombus was noticed retrospectively and such patients had been managed without anticoagulation. Only one case, described earlier, could be associated with venous thrombosis. However, in that particular case, the superior petrosal vein was closed during the surgery, so the explanation of that case is complex. On the other hand, we did not observe any hemorrhagic complications in that group of patients. Using our thrombosis classification, we hypothesize that the cases where a thrombus is causing the engorgement of other draining veins (grade VI) are the most important indication for the administration of anticoagulants in that group of patients. However, such a situation was observed only in 1 patient (4% of all thrombotic cases).
In our study we have analyzed the largest group of the vestibular schwannomas operated on using the suboccipital retrosigmoid approach. What is more, it is the largest group examined with the postoperative MRI, which is superior in the diagnostics of thrombosis to routine non-contrast CT. We attempted to classify the postoperative changes in the lateral venous sinuses and develop a classification which could help further investigators. Additionally, we performed a clinical and radiological follow-up of most of the patients, giving the natural history of postoperative dural sinus thrombosis.
However, it is the experience of one institution only. Moreover, the presented study is retrospective and based only on contrast enhanced MRI (DSA was not performed). A large, multicenter prospective trial is required for the high-level guidelines for management of postoperative dural sinus thrombosis after vestibular schwannoma resection.