65 Thalamic and Basal Ganglia Cavernous Malformations
Abstract
Thalamic and basal ganglia cavernous malformations (CMs) are relatively rare but can cause devastating neurological deficits. They have a notable higher rate of bleeding (>10% per patient per year) than more superficial lesions. Clinically, patients can present with headaches, intracerebral or intraventricular hemorrhage, hydrocephalus, cranial nerve dysfunction, and/or coma. Computed tomography (CT) and magnetic resonance imaging (MRI) are the preferred imaging diagnostic modalities. In experienced hands, thalamic and basal ganglia CMs can be safely removed with surgery. We favor surgical resection of lesions in patients who are symptomatic or of lesions that abut a pial or ependymal surface; otherwise, we manage them conservatively. Six different surgical approaches have been well described for thalamic CM resection based on their location within the thalamus, including transsylvian orbitozygomatic, contralateral interhemispheric transcallosal, ipsilateral interhemispheric transcallosal, posterior interhemispheric transcallosal, parietooccipital transventricular, and supracerebellar infratentorial. The benefit of radiosurgery for deep-seated CMs is questionable and not entirely recommended. Overall, the surgical outcome of patients with thalamic and basal ganglia CMs is good. The risk of transient neurological morbidity is 7 to 21%, but the risk of permanent neurological disability is 1.3 to 3.2%. Clinical and radiological long-term follow-up is mandatory in all patients.
Introduction
Cavernous malformations (CMs) are more common than generally realized. Two large postmortem neuropathology studies and two large institutional reviews of magnetic resonance imaging (MRI) scans demonstrated a prevalence of 0.34 to 0.53% (mean 0.49%). These findings suggest that CMs affect nearly 1 in 200 persons. CMs occur throughout the central nervous system: 80% are supratentorial, 15% are in the brainstem and basal ganglia, and 5% are in the spinal cord. CMs of the thalamus and basal ganglia are relatively rare but can nevertheless cause devastating neurological deficits.
Major controversies in decision making addressed in this chapter include:
Whether treatment is indicated.
Safety and efficacy of surgery for thalamic and basal ganglia CMs.
Timing of surgery.
Role of radiosurgery.
Whether to Treat
The natural history of CMs varies considerably as reported in the medical literature. Retrospective studies report hemorrhages rates of 0.25 to 2.3% per patient per year; however, these studies underestimated the risk of bleeding, as their authors assumed that the lesions were present from birth. Prospective studies have shown that symptomatic lesions in patients with a history of prior hemorrhage have a higher rebleeding rate (4.5% per year) than that of incidental lesions (0.6% per year; 1 –3 in algorithm ). The lesion location has long been considered the most important factor in predicting future bleeding events, with deep-seated lesions in the basal ganglia having a notably higher rate (10.6% per patient per year) than that of more superficial lesions (0% per patient per year; 4 in algorithm ). Lesions that rupture outside the lesion capsule (overt or extralesional hemorrhage) can have rehemorrhage rates as high as 25.2% per patient per year.
Timing of Surgery
Thalamic, basal ganglia, and brainstem CMs all cause repetitive hemorrhages. A rupture displaces rather than invades surrounding structures, including tracts, cranial nerves, and nuclei, and descending motor tracts or ascending sensory tracts. Hemorrhaging from CM causes temporary neurological deficits that eventually improve over time. Because of this improvement, some surgeons argue that treatment should be delayed, because surgical entry into the CM may mimic the symptoms of a prior bleed. Other surgeons argue that immediate surgical resection after a bleed should be performed, because blood and breakdown products may be harmful to nuclei, whereas surgery minimizes compression of critical tracts. Our preference is to operate within 4 to 6 weeks of the last episode of hemorrhaging. If the condition of the patient declines rapidly, a more urgent intervention may be indicated ( 3 in algorithm ). We favor aggressive surgical resection of lesions in patients who are symptomatic and of lesions that abut a pial or an ependymal surface ( 3, 4 in algorithm ). For patients with minimal symptoms, smaller lesions (< 5 mm), or lesions that do not arise to a pial or an ependymal surface, we favor conservative management ( 6, 8 in algorithm ).
Conservative Management and Radiosurgery
Gamma Knife (Elekta Instruments, Inc.) radiosurgery has been promoted as an alternative option for all CMs, especially for deep-seated lesions that are more surgically challenging. Radiosurgery has been shown to reduce the risk of rebleeding in patients who have repeated pretreatment hemorrhaging (from 30% per year before treatment to 15% per year over a 2-year period and 2.4% thereafter). In contrast, the benefit to patients with a single hemorrhagic episode is less clear (from 2.2% per year before treatment to 5.1% in the first year and 1.3% thereafter). Minor adverse effects from radiation have been observed in 7.3% patients.
There is no evidence that radiosurgery protects patients with CMs against future bleeding. The presumed protective effect of irradiation may simply reflect the natural biological evolution of CMs. Radiotherapy is not absolutely safe, especially when applied to lesions in eloquent brain areas, such as the thalamus and basal ganglia. Since the risk of hemorrhage after radiosurgery treatment is not eliminated and the rate of radiation-related complications can be as high as 13%, the benefit of this treatment is questionable.
Anatomical Considerations
The initial surgical approach and the resection of thalamic CMs are both directly associated with the risk of creating a new neurological deficit or worsening an existing neurological deficit. The benefit of surgery must be weighed against these risks. Some neurosurgeons may deem CMs to be inoperable, or as requiring much deliberation before selection of the best surgical approach. The thalamus is located in the center of the lateral ventricles rostral to the brainstem, where it is surrounded by vital neurovascular structures, including the internal capsule, midbrain, foramen of Monro, thalamostriate veins, and internal cerebral veins. Different surgical approaches can safely expose the thalamus. As described later in the Treatment section, six different microsurgical approaches are commonly used to reach certain regions of the thalamus ( 7 in algorithm ).
Pathophysiology
How a CM develops is unknown. Some lesions are clearly acquired, including those in patients with familial lesions and those in patients where the CM has developed in areas of previously irradiated brain tissue. The lesions are lobulated and dark red or purple. Inside the lesion is a honeycomb of thin-walled vascular spaces. Small hemorrhages may occur within the lesion or adjacent to the lesion. These hemorrhagic episodes displace normal brain tissue, causing symptoms. It is uncommon for a hemorrhage to infiltrate brain tissue. Gliotic tissue surrounds the mass, and it is usually stained yellow from hemosiderin. No intervening tissue or neural tissue is present within the malformation. At its periphery, gliotic tissue contains hemosiderin-packed macrophages.
Workup
Clinical Evaluation
Patients with CMs can have a host of symptoms ranging from mild to devastating effects related to hemorrhage, including coma and death. Patients with thalamic CMs can present with headaches (41%), hemorrhage (59% [intraparenchymal 78%, intraventricular 22%]), hydrocephalus (11%), sensory problems (22%), and cranial nerve III, IV, VI, or VII dysfunction (10%). Factors predisposing CMs to rupture are deep-seated location, large size, previous hemorrhage, and associated developmental venous anomalies. Thalamic lesions can extend caudally into the upper brainstem, and patients may present with motor, sensory, and/or cranial nerve deficits, and with cerebral aqueduct obstruction. Symptoms, which are typically maximal at onset, tend to gradually resolve as blood is resorbed. Thus, after an initial hemorrhage, symptoms may resolve completely. Recurrent hemorrhagic episodes are likely to result in progressively more severe deficits and in permanent impairment, if the CM is left untreated.
Imaging
Computed tomography can help demonstrate acute hemorrhage in patients who present with sudden-onset symptoms. However, the sensitivity of computed tomography is less than 50%. MRI is by far the gold standard for imaging of CMs ( 5 in algorithm ). In T2-weighted MRI sequences, the lesions have a focal, reticulated “salt-and-pepper” pattern, and they are surrounded by a ring of hypointensity consistent with hemosiderin deposition (▶ Figs. 65.1 – 65.3 ). Gradient echo images exhibit marked sensitivity for hemosiderin deposition and will frequently reveal small, incidental CMs not visible in other sequences. T2-weighted and gradient echo sequences both tend to overestimate the size of the lesion. T1-weighted sequences provide the most accurate anatomic detail, and thus are the best choice for surgical intervention. Gadolinium-enhanced T1-weighted sequences are useful in identifying cases of associated developmental venous anomaly. Preoperative identification of developmental venous anomalies can aid planning for surgical resection.
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