66 Brainstem Cavernous Malformations
Abstract
Brainstem cavernous malformations (BSCMs) account for 20% of all intracranial cavernous malformation (CM). Rates of hemorrhages and rehemorrhages have been estimated higher for BSCM than for CMs in other locations. Patients can present with headaches, cranial neuropathy, sensory and motor deficits, ataxia, hydrocephalus, coma, or even death. Symptoms are maximal at onset with recurrent episodes, and they may resolve after initial hemorrhage. Recurrent hemorrhages likely produce permanent deficits. CT and MRI are the preferred imaging diagnostic modalities. In experienced hands, BSCMs can be safely removed with surgery. Timing of surgery is important; we operate within 4 to 6 weeks of the last hemorrhage unless the patient is rapidly deteriorating. We favor aggressive surgical resection in symptomatic patients or of lesions that abut a pial or an ependymal surface; otherwise we manage them conservatively. The benefit of radiosurgery for BSCMs is questionable and not entirely recommended. Intraoperative neuronavigation should be used in all patients. Multiple skull base approaches have been described to access the brainstem with minimal morbidity. Overall, the surgical outcome of BSCMs is good. A combination of proper patient selection, adequate surgical approach and equipment, and surgeon experience is the best strategy for minimizing complications. Clinical and radiological long-term follow-up is mandatory in all patients.
Introduction
Cavernous malformations (CMs) are clusters of abnormal thin-walled blood vessels—mostly capillaries and veins—that can occur anywhere in the central nervous system. Because the vessels are leaky, multiple small hemorrhages accumulate over time. Depending on the location of the lesion and the extent of bleeding, CMs may result in focal deficits, seizures, and even death. Brainstem CMs (BSCMs) account for 20% of all intracranial CMs. The most common site is the pons. Compared to other intracranial CMs, BSCMs are less likely to remain clinically silent. However, not all patients require treatment; when making treatment decisions, one should weigh the natural history against the risks of surgery and the experience of the surgeon.
Major controversies in decision making addressed in this chapter include:
Whether treatment is indicated.
Safety and efficacy of surgery for BSCMs.
Timing for surgery.
Ideal surgical approach based on location.
Whether To Treat
Patients who present with BSCMs may be symptomatic or asymptomatic ( 1 in algorithm ). The natural history of BSCMs is a matter of dispute because of biases relating to the question of which CMs require medical attention. For patients with lesions who present with symptomatic hemorrhages, estimated annual rehemorrhage rates range from 4.6 to 6.5%, although rehemorrhage rates up to 30% annually have been reported. Rates of rebleeding have been estimated to be higher for BSCMs than for CMs in other locations. However, this difference may reflect presentation bias; because BSCMs are located in a highly eloquent region of the brain that is exquisitely sensitive to disruption, all hemorrhagic events are likely to be symptomatic and thus captured. The mortality rates for patients with BSCMs with recurrent hemorrhages range from 0 to 17%.
Factors predisposing to CM rupture include history of previous rupture, lesion size, location in the posterior fossa, and presence of a developmental venous anomaly. A history of previous rupture is associated with a 7-fold increase in the risk of a future rupture. As noted, patients with BSCMs have a higher rate of symptomatic hemorrhage and rehemorrhage than those with CMs in other locations. In our recent study of adult patients with BSCMs, the hemorrhage rate before surgical resection was 4.6% per year of life per patient.
Because of the highly eloquent nature of their location, both symptomatic and asymptomatic BSCMs have a high risk of deterioration ( 2, 3 in algorithm ). However, there is also a high risk of neurological deficits or complications with surgery because of the highly complex and inaccessible location of many brainstem regions. In general, surgical resection should be considered only for lesions that abut an accessible pial or ependymal surface ( 4–6 in algorithm ). The corollary to this rule is that the surgeon must exercise restraint if, at initial presentation, the lesion does not abut an appropriate surface. In this case, close observation is advocated, especially after a first-time hemorrhage ( 7, 8 in algorithm ). For lesions with confirmed rehemorrhaging over time, surgeons should consider more aggressive manipulation of eloquent structures, because the risk of further hemorrhage is high and such structures may already be dysfunctional ( 9 in algorithm ).
Anatomical Considerations
The brainstem is located in the center of the cerebrum, surrounded by vital neurovascular structures. The entire brainstem is considered eloquent brain, and some neurosurgeons still consider BSCMs to be inoperable. In our centers (Mayo Clinic Rochester and Barrow Neurological Institute), we routinely use several standard approaches to reach lesions in the brainstem (▶ Fig. 66.1 ). When these approaches are used in conjunction with appropriate technological advances, we have been able to remove BSCMs successfully with acceptable morbidity rates. With the exception of the middle cerebellar peduncle (MCP), the surgical approaches that are used do not require brain transgression at all before making an entry into the brainstem. However, the MCP has been shown to tolerate manipulation even if the lesion does not present to the surface. In contrast, areas such as the floor of the fourth ventricle, although near an accessible surface, may nonetheless be treacherous. Various surgical approaches can expose the brainstem safely. (See the Cerebrovascular Management—Operative Nuances section below).
Classification
Multiple CM grading systems have been developed as decision aids. Although not specific to BSCMs, a grading scheme by Hernesniemi et al is based on data from 303 patients with cavernomas of the spine and brain. Using the Glasgow Outcome Scale, they classified the condition of patients at last follow-up as favorable if the score was 5 or unfavorable if the score was 1–4. Factors associated with an unfavorable outcome included infratentorial, basal ganglia, or spinal location and preexisting neurological deficit. On the basis of these data, the authors formulated a grading system to score lesions from 1 to 3; rates of long-term unfavorable outcome were 13% for grade 1, 22% for grade 2, and 55% for grade 3.
Lawton and colleagues developed a brainstem-specific grading scheme based on 104 patients with surgically treated BSCMs. Using the modified Rankin Scale, they classified scores of 0–2 as favorable and scores >2 as unfavorable, and they constructed a grading system by assigning points according to relative weight for lesion size, depth (crossing the axial midpoint of the brainstem), associated venous anomaly, age, and time from last hemorrhage. BSCM grades may range from 0 to 7 points, and the grades predict outcomes with high accuracy. Lawton et al found that patients with BSCM grades of 6 and 7 experienced the highest rates of poor neurological outcomes (50% each).
The Hernesniemi and the Lawton grading schemes both aid in differentiating patients who might reasonably expect favorable surgical outcomes. These schemes can thus help to guide neurosurgeons in counseling patients on the relative risks of surgical resection (▶ Table 66.1 ).
Workup
Clinical Evaluation
Patients with BSCMs can have a host of hemorrhage-related symptoms, ranging from mild to devastating effects that may include coma or even death. Symptoms from BSCMs may include cranial neuropathy (63%), sensory deficit (53%), motor symptoms (37%), headaches (39%), double vision (33%), ataxia (25%), vertigo (25%), nausea/emesis (17%), and dysarthria (12%). BSCM of the midbrain can cause cerebral aqueduct occlusion or can extend into the third ventricle, causing hydrocephalus. Similarly, medullary lesions can extend caudally into the spinal cord, causing myelopathy or cervical radicular symptoms. In general, symptoms are typically maximal at onset and tend to gradually resolve as blood is resorbed. Symptoms may resolve completely after an initial hemorrhage. However, recurrent episodes, if left untreated, are likely to result in progressively more severe deficits and permanent impairment.
A thorough neurological examination helps guide surgical decision making. All neurological deficits should be documented. New or progressive deficits may substantiate the need for intervention. Furthermore, an understanding of which areas may have been compromised by hemorrhage can aid assessment of the viability of tissues in the neighborhood of the BSCM and can guide surgical approach decisions.
Imaging
Computed tomography (CT) demonstrates acute hemorrhage in patients with sudden-onset symptoms. However, the sensitivity of CT is less than 50%. Magnetic resonance imaging (MRI) is the gold standard imaging modality. T1-weighted MRI sequences provide the most accurate anatomical detail and are the best choice when considering surgical intervention. Gadolinium-enhanced T1-weighted MRI sequences are useful in identifying associated developmental venous anomalies. On T2-weighted sequences, lesions have a focal, reticulated “salt-and-pepper” pattern and are surrounded by a black ring of hypointensity consistent with hemosiderin deposition. Gradient-echo images exhibit marked sensitivity for hemosiderin deposition and frequently reveal small incidental CMs not visible with other sequences (▶ Figs. 66.1 – 66.3 ).
Differential Diagnosis
The differential diagnosis includes primary neoplasms of the brainstem, metastatic tumors, and other vascular lesions, such as arteriovenous malformations. The classical salt-and-pepper (“popcorn”) appearance on MRI is almost pathognomonic of CM; it is rarely seen in patients with other pathological conditions.
Treatment
Conservative Management
Conservative management should always be discussed with the patient, and caveats regarding the risk of further bleeding and neurological deterioration should be clearly explained. In cases with no pial or ependymal presentation for the lesion, conservative management should be the first line of treatment ( 7 and 8 in algorithm ). Advanced age or medical unsuitability for extensive surgical procedures should also be considered relative contraindications to surgery. Factors that may contraindicate conservative management include multiple documented hemorrhages, progressive neurological deterioration, and patient preference ( 2, 5, 9 in algorithm ).
Although other vascular and neoplastic lesions may be amenable to treatment with a variety of methods, the management of BSCM is essentially a decision either to observe or to surgically resect. There are no medical therapies, the lesions are angiographically occult, and stereotactic radiosurgery has yet to be proven to effectively prevent rebleeding. Thus, if treatment is advised, then surgery is the only current option ( 6, 9 in algorithm ). Surgical risk is largely determined by the precise location of the lesion and the potential for damaging surrounding structures, due either to surgical manipulation or to rebleeding of the lesion.
Cerebrovascular Management—Operative Nuances
Timing of Surgery
BSCMs cause repetitive hemorrhages. A hemorrhage displaces rather than invades surrounding structures (cranial nerves and nuclei; descending motor or ascending sensory tracts). Hemorrhage from a BSCM causes temporary neurological deficits that eventually improve over time. Considering this improvement, some surgeons argue that treatment should be delayed, because resection of BSCMs and manipulation of surrounding brainstem tissue may cause symptoms mimicking those of a prior bleed. However, other surgeons prefer to perform surgical resection immediately after a bleed to minimize compression on critical tracts because blood and breakdown products can be harmful to nuclei. The preference at Barrow Neurological Institute and the Mayo Clinic Rochester is to operate within 4 to 6 weeks of the last hemorrhagic episode ( 10 in algorithm ). If the patient exhibits rapid decline, more urgent intervention may be indicated ( 11 in algorithm ). We are in favor of aggressive surgical resection of lesions in patients who are symptomatic or of lesions that abut a pial or an ependymal surface ( 2–5 in algorithm ). For patients with minimal symptoms or those with lesions that do not abut a pial or an ependymal surface, we favor conservative management ( 7, 8 in algorithm ).
Operative Planning
MRI is essential to operative planning. CMs have a pathognomic popcorn appearance on T2-weighted sequences that often puts the diagnosis in little question. However, thin-slice MRI is essential to define exactly where the malformation is in the brainstem and what its relation is to the pia. MRI-guided intraoperative navigation can help tailor the craniotomy, select the appropriate surgical trajectory, minimize disruption to normal tissue, and confirm localization of the target lesion. On T2-weighted sequences, the lesion may appear to extend to the surface, implying that the surgeon will readily localize the lesion at the time of surgery. However, because of hemosiderin bloom artifact, the exact size and extent of the lesion may be difficult to ascertain preoperatively. For this reason, the surgeon should refer to T1-weighted sequences to best determine the thickness of the CM rim, and should have a low threshold for using intraoperative navigation to help identify lesions in delicate locations (▶ Figs. 66.1 – 66.3 ). At surgery, it is common to find that the location within the brainstem is not clear from inspection of the pia alone, and that a combination of image guidance and brainstem landmarks is necessary to locate the lesion. Because transgressing even a small rim of surrounding normal tissue may be clinically consequential, it is critical to use careful technique and safe entry zones. Intraoperative neuromonitoring is a useful surgical adjunct. For patients with BSCMs, routine monitoring should be conducted using somatosensory-evoked potentials, cranial nerve monitoring, motor-evoked potentials, and brainstem auditory-evoked potentials.
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