Superficial Cavernous Malformations

Keywords: cavernous malformation, craniotomy, vascular malformation

28.1 Patient Selection

Typical computed tomographic (CT) characteristics of these lesions include focal or nodular appearing lesions with mild to moderate increase in attenuation without mass effect. Calcification can be noted. Contrast-enhanced CT shows mild to moderate enhancement with heterogeneous mottled appearance and a rim of decreased attenuation consistent with the gliotic tissue surrounding the lesion ( ▶ Fig. 28.1). On magnetic resonance imaging (MRI), they appear as well-circumscribed popcorn-like lesions ( ▶ Fig. 28.2). Acute hematoma containing oxyhemoglobin is isointense on T1-weighted images and hypointense on T2-weighted images. Subacute hemorrhage will contain extracellular methemoglobin, which is hyperintense on both Tl- and T2-weighted images. There is usually a heterogeneous core surrounded by a low signal intensity hemosiderin rim on T1-weighted images. The hypointense rim becomes more prominent or “blooms” on T2-weighted and gradient echo sequences. Susceptibility-weighted imaging, which usually uses a gradient echo sequence, is the most sensitive for detection of CMs. ⁴^, ⁵ CMs are not usually associated with mass effect or edema unless there is recent hemorrhage. They are frequently associated with developmental venous anomalies and are generally angiographically occult ( ▶ Fig. 28.1). The diagnosis can usually be inferred from the MRI appearance and catheter angiography is rarely indicated except when an arteriovenous malformation must be ruled out. CT or MR angiography may help.

Fig. 28.1 (a) Contrast-enhanced computed tomographic scan demonstrating a right frontal hyperdensity associated with (b) a vascular structure. These characteristics are consistent with a superficial cavernous malformation and associated developmental venous anomaly.

Fig. 28.2 (a) Axial T2, (b) fluid attenuated inversion recovery images of left parietal cavernous malformation. (c) Axial T2 magnetic resonance imaging (MRI), (d) gradient echo MRI. MR images of right frontal superficial cavernous malformation.

28.2 Indications and Contraindications to Surgery

Indications for surgical treatment of superficial cavernous malformation (SCMs) include repeated hemorrhages or medically refractory seizures with electrophysiological localization to the region of the SCM. ⁴ The location of the lesion (eloquent vs. noneloquent cortex) has to be considered and balanced against the risk of resection. Asymptomatic, incidentally discovered lesions should almost always be left alone. In terms of timing of surgery, it is easiest to resect the lesion by waiting more than 6 weeks after a hemorrhage. The options for management include observation. The role of stereotactic radiosurgery remains controversial and surgical resection is generally favored over radiation.

28.3 Preoperative Preparation

Preoperative imaging should include an MRI with gradient echo sequences/susceptibility-weighted imaging. The main diagnostic issue here is that these extremely sensitive images can detect brain microhemorrhages that are basically the same as tiny CMs, so consideration needs to be given to what the etiology of any other abnormalities is. When operating in or near eloquent cortex, either or both functional MRI, diffusion tensor imaging and intraoperative electrophysiological monitoring are highly recommended. Awake craniotomy is another option for these cases. Frameless stereotactic image acquisition for preoperative craniotomy planning and intraoperative navigation may be considered mandatory for localization. Ultrasound should also be prepared for intraoperative use to help locate small subcortical lesions. General neurosurgical principles apply for all craniotomies performed for the removal of SCMs, including the administration of perioperative antibiotics. Consideration should be given to anticonvulsant usage in patients not already on these drugs. When the indication for surgery is medically refractory seizures, consultation with a seizure specialist should be considered to accurately assess the likelihood of obtaining improvement in seizure control when surgical resection is being considered. Removal of an SCM for amelioration of seizures should include the additional goal of excising the surrounding hemosiderin-stained tissue to decrease the relative excitability of the surrounding cortex that can contribute to seizure formation. The location of any associated developmental venous anomaly should be determined since this venous drainage should not be injured.

28.4 Operative Procedure

The patient is placed under general anesthesia and the head is placed in a pin fixation device. The patient’s images are registered using frameless stereotaxis for craniotomy flap planning. The positioning and incision will be determined by the location of the lesion ( ▶ Fig. 28.3). The scalp is then prepared and draped in a standard manner. The skin incision is made and scalp hemostasis maintained with Raney hemostatic clips. The scalp flap is reflected with fishhooks and a standard craniotomy is performed using a high-speed drill with one burr hole, or multiple burr holes if the dura is likely to be adherent to the bone or the craniotomy crosses a venous sinus. Blunt dissection of the underlying dura mater is performed using a Love–Adson dissector (Codman, Johnson & Johnson), and the bone flap is removed using the high-speed drill. Hemostasis following bone flap removal is controlled with bone wax and microfibrillar collagen. The high-speed drill is used to create holes circumferentially around the bone flap for dural tackup sutures. The dura is opened in a cruciate or curvilinear fashion and retracted using 4–0 braided nonabsorbable suture held by hemostatic clips or by suturing to the scalp flap.

Fig. 28.3 Positioning and incisions for approaches to superficial CMs. (a) Pterional approach for lesions in the vicinity of the anterior sylvian fissure. (b) Approach to the middle cranial fossa showing two types of incisions and the bone removal, including removal of bone down to the floor of the middle fossa (shaded area) necessary to approach lesions on the undersurface of the temporal lobe. This exposure would also access lateral temporal lobe CMs. (c) Occipital paramedian approach with various incisions and a bone flap to access the occipital lobe. (d) Approach to lesions over the frontoparietal convexity near the midline. (e) Frontal midline approach to the medial frontal lobe.

(Reproduced with permission from Mohsenipour I, Fischer J, Platzer W, Pomaroli A, eds. Approaches in Neurosurgery: Central and Peripheral Nervous System. New York: Thieme; 1994. Pp. 49, 56, 66, 74, 78.)

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