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Surgical treatment of anterior circulation aneurysms
Surgical Treatment of Anterior Circulation Aneurysms Simo Valtonen, MD, PhD
With few exceptions, aneurysm surgery is preventive surgery; the aim is to prevent a subarachnoid hemorrhage from the aneurysm--rebleeding after the first hemorrage or possibly first bleeding from a previously unruptured aneurysm. Consequently, surgical mortality and morbidity must not exceed natural mortality and morbidity. The natural risk of death from rebleeds after aneurysmal subarachnoid hemorrhage is about 35% immediately after the bleeding, and decreasing quickly to a level of approximately 2%/y. The annual bleeding rate of previously unruptured aneurysms is traditionally considered to be about 1.4% but can be much lower. For good-grade patients, the ideal time of surgery after a subarachnoid hemorrhage is the first days after bleeding. For previously unruptured aneurysms, the timing of surgery is not critical. Of the open, intracranial procedures, clipping of the aneurysm neck is the only widely used procedure today. Intravascular treatment is a new and possibly good alternative to open operations for intracranial aneurysms; however, surgery is still the standard aneurysm therapy. To minimize operative mortality and morbidity, the best possible perioperative conditions must be created. Preoperatively, if the patient has had a recent subarachnoid hemorrhage, standard care includes maintenance of stable and normal blood pressure and nimodipine to prevent possible ischemic complications in addition to good general care. Perioperatively, it is important to prevent ischemic complications and to create the best possible operative conditions with respect to intracranial space. The most common anterior circulation aneurysm locations are internal carotid artery, anterior communicating artery, middle cerebral artery, and pericallosal artery. The approach to all these aneurysm locations, with the exception of pericallosal aneurysms, is the pterional one, with possible small variations. The general principle of dissecting an aneurysm is to follow the parent vessel and identify the aneurysm neck. The aneurysm neck always must be dissected free from surrounding arteries and clipped with one or more aneurysm clips. Techniques used vary according to the location, but common for all is avoiding a lesion to any important artery and achieving a complete closure of the aneurysm neck. It is particularly difficult and important to avoid closure of the socalled perforating arteries close to anterior communicating artery and internal carotid artery aneurysms. Copyright 9 2000 by W.B. Saunders Company
Introduction neurysm surgery is generally preventive surgery. Conse-
A ,quently, it must be performed with mortality and morbid-
ity below expected levels demonstrated by the natural history of
From the Department of Neurosurgery, Turku University Central Hospital, Turku, Finland. Address reprint requests to Simo Valtonen, MD, PhD, Department of Neurosurgery, PO Box 52, Turku University Central Hospital, 20521 Turku, Finland. Copyright 9 2000 by W.B. Saunders Company 1092-440X/00/0304-0002535.00/0 doi:l 0.1053/otns.2000.18514
the disease. The choice between aneurysm surgery and its alternative, endovascular surgery, must be considered.
Indications With few exceptions (eg, giant aneurysms, internal carotid aneurysm causing a third nerve palsy), surgery for an aneurysm is a prophylactic measure to prevent a potential bleed. Consequently the expected benefit in this sense must be weighed against the risks of surgery. A patient with an aneurysmal subarachnoid hemorrhage has a risk of about 35% of fatal rebleed on the day of the bleeding. This decreases about 5% weekly during the first month, 1 decreases more slowly afterward, and gradually reaches a level of 2% to 3% annual rebleeds? Expected operative mortality at any time must not be higher than these figures. The annual bleeding rate of previously unruptured aneurysms is reported to be about 1.4%, 3 with mortality of about 50%, even though much lower bleeding rates have been reported lately, especially for small aneurysms. 4 Consequently, surgery is indicated if the expected mortality and severe morbidity combined are considerably less than these numbers. Operative mortality and morbidity are not constants. They are influenced by the condition of the patient (the most important factor), age, and possible concurring diseases. 5 Because the mortality for aneurysm surgery for Hunt and Hess grade 5 patients is about 50% to 80%, surgery is not indicated in these cases. Generally, patients with a technically clippable aneurysm and who are grade 1 to 3 after an aneurysmal subarachnoid hemorrhage are candidates for surgery. The decision can be difficult to make when grade 4 patients are concerned because strict guidelines are hard to give. If surgery for incidental aneurysms cannot be performed with nearly zero mortality, it should not be performed at all. Surgery for nonruptured aneurysms is not risk-free. 6 Open intracranial operation is the standard of aneurysm treatment. Endovascular treatment (ie, coiling) is a new and attractive alternative, without proof of effectiveness or superiority over operation in randomized studies. 7 The choice between these two methods depends on experience, local habits, location of aneurysm, and the patient's preference. For example, in Finland, about half of all aneurysms are treated with coiling. 8 As a result of small reported series, complication and mortality rates of endovascular procedures in different situations are not well known, but in 8 different series, with a total of 1377 patients, the procedure-related mortality was 0 to 5%. 8
Preoperative Care and Surgical Timing Diagnostic Studies After subarachnoid hemorrhage, a 4-vessel angiographic study is mandatory for patients who are possible candidates for oper-
Operative Techniques in Neurosurgery, Vol 3, No 4 (December), 2000: pp 231-238
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ation. Most aneurysms can be detected with a good magnetic resonance angiography study, but the fresh blood disturbs the detection, and the accuracy rate is not high enough. After accidental discovery of an aneurysm, 4-vessel angiography is the gold standard. It is possible, however, to operate on a nonruptured aneurysm in the anterior circulation without it, after a high-quality magnetic resonance angiography study, but on all occasions when intravascular therapy is considered, anglographic studies are mandatory.
Medical Care The aim of medical care after aneurysmal subarachnoid hemorrhage is to prevent additional ischemic brain damage and to decrease the risk of a rebleed. Nimodipine has been proved to decrease the number of ischemic complications if used in connection with an aneurysmal subarachnoid hemorrhage, 9 and its use is a part of normal care. The clinician must be aware of the hypotensive effect of this drug. Dosage must be adjusted individually to keep the systolic blood pressure level at about 140 mmHg. If this blood pressure level is not possible, triple-H therapy (hypervolemia, hypertension, and hemodilution) or catecholamines must be added to the regimen or nimodipine discontinued. Antifibrinolytic agents decrease the risk of rebleeding before definite aneurysm surgery, 5 but because they also increase the risk of ischemic complications, they are not part of routine preoperative care. A patient in good clinical grade and with stable blood pressure can stay in a regular neurosurgical ward; all other patients should be cared for in an intensive care (or observation) unit.
Technical Requirements There are no specific requirements for the operative table, but attachment of a skeletal 3-point or 4-point head fixation device must be possible. Use of one of these devices (eg, Mayfield or Sugita) is mandatory. Intracranial aneurysms must not be operated on without an operative microscope. Because aneurysm surgery requires frequent microscope movements, a balanced microscope with magnetic locks is recommended strongly as well as one with zoomable optics. Neuronavigation is not necessary for aneurysm surgery; orientation is achieved by bony and vascular landmarks and intracranial vessels. Intraoperative angiography has been found to be useful and cost-effective in some situations. 12 h is an exclusive tool, and not available in most neurosurgical departments, and it is not requisite for aneurysm surgery. The aneurysm clip is the specific tool of aneurysm surgery. Several types exist; the best-known differ little from each other. To provide familiarity and to prevent confusion in critical situations, neurosurgeons should use one brand of clips only. Clips come in a variety of different shapes and sizes. It is advisable to have a good selection of clips, separated clearly from each other on the instrument table, even if nearly all aneurysms are ligated with a small selection (eg, straight clips of different lengths, bayonet clips, angled clips and curved clips). Temporary clips are necessary. Clip applicators (bayonet and pistol types) and a clip remover are also necessary. Self-retaining brain retractors are appropriate for aneurysm surgery even if there is a tendency today to avoid them. They help to visualize structures at the skull base and to protect the brain.
Surgical Timing Surgical timing for ruptured aneurysms is crucial because the risk of a rerupture is maximal immediately after rupture and decreases with time. ~,5 The earlier surgery is performed, the greater the expected prophylactic gain. The value of early surgery (within a few days) has been documented in some studies. l~ An operation within 3 days is the rule for grade l to 3 patients. Aneurysm clipping is major surgery, unavoidably attached to high risks in many cases. One should expect to have maximally good conditions, and for a major operation, these usually do not exist during the night. It is wise to sacrifice a few hours or days in favor of optimal operative conditions. The theoretic risk of a rerupture during this short period is slight, and there is no proof of benefits for surgery performed during the first 3 days compared to days 4 to 7,1~ and there are fewer benefits of immediate surgery. The situation is different with previously unruptured aneurysms. Some patients with unruptured aneurysms can be operated on in the same session as a ruptured aneurysm (see multiple aneurysms). Patients with an unruptured aneurysm not operable together with a ruptured one should not have a second operation immediately after the first one (and after the hemorrhage). There are no exact guidelines regarding how long one should wait, but at least 1 month can be recommended, to allow all immediate effects of the bleeding to disappear. With aneurysms found incidentally, there is no timing problem. Surgery can be performed on any suitable day shortly after the detection of the aneurysm; there is no real hurry, but the patient's nervousness and fear must be taken into account. 232
Perioperative Care One of the main goals of perioperative care is to produce intracranial space (slack brain). Mannitol decreases brain volume quickly and is an integral part of aneurysm surgery. The normal dose is 1 g/kg of 20% mannitol. It is important is to give mannitol early because the peak effect occurs about 30 minutes after infusion. Spinal drainage is another way to create intracranial space and is used widely. In patients whose condition is poor the brain is swollen, the ventricles are small, and there is a risk of tentorial herniation. In patients whose condition is good, there are no space problems anyway. Another important goal is to prevent hypoxic brain damage. Traditionally, to ease aneurysm clipping, controlled hypotension has been used. It is known to be potentially hazardous, ~3 and because ischemic complications are common after aneurysm surgery, controlled hypotension cannot be recommended, especially because it really does not help much. Temporary vascular occlusion is another way to make clipping easier. There are conflicting opinions about the maximal safe occlusion time and the effect of mannitol on it. 14 It can be considered reasonably safe to occlude a vessel for 4 minutes. This is a short time for aneurysm dissection, however, and can help only in the actual application of the clip. Longer times are potentially risky, and protective measures must be considered. Hypothermia can be used in difficult situations. ~ After about 2 decades of absence from aneurysm surgery, hypothermia has gained some popularity again. Its position and value remain to be seen. So-called neuroprotective drugs (thiopental and etoSIMO VALTONEN
midate) have been used to prevent perioperative ischemic brain damage, but definite proof of their effectiveness is lacking.
Postoperative Care and Controls Every patient is transferred to the intensive care unit immediately after aneurysm clipping. Patients with a preoperative grade 1 to 3 can be extubated immediately after the operation, but patients with a worse grade cannot. The possible complications, which must be detected early and treated, are postoperative clot (uncommon with aneurysm operations), infection, and ischemic complications. Blood pressure must be kept at a reasonably high level (eg, 150/90 mmHg) for the entire immediate postoperative period. If necessary, this pressure can be achieved with triple-H therapy or ionotropic drugs. Opinions differ about the length of nimodipine administration postoperatively; 10 to 14 days is common. The only way to be certain of a good anatomic result for aneurysm clipping is postoperative angiographic examination, but its cost-versus-benefit value is questionable. 16 Partially clipped aneurysms are seldom reoperated. Because postoperative angiography is an invasive investigation, its use cannot be considered mandatory in cases when reoperation (or alternatively coiling) will not be done, regardless of the finding of angiography.
Vascular Anatomy Aneurysm surgery is mainly vascular surgery. Good knowledge of vascular anatomy is a prerequisite. Intracranial vascular anatomy is full of variations, and a neurosurgeon cannot master them all. It is important to know the specifically problematic vessels, to respect every abnormal situation, to be certain not to clip any arteries, and at times to consult good textbooks. 17
Specific Operative Techniques Internal Carotid Artery Aneurysms Aneurysms situated proximally from the origin of the ophthalmic artery are difficult to reach and to clip. Consequently, with these aneurysms, endovascular treatment is a strong alternative. Open operation of these aneurysms is not described; the text refers only to aneurysms above the origin of the ophthalmic artery. Patient positioning. The patient lies supine, the head tilted to the opposite side from the aneurysm and backward (Fig 1). The lateral end of the superior margin of the orbita is the uppermost point of the head. It is possible to reach internal carotid artery aneurysms from the opposite side, but surgery from the same side is the rule. Approach and exposure. The skin incision is extended from a point just over the zygoma and 0.5 cm anteriorly from the earlobe, behind the hair line, a bit over the midline to the hair line. To protect the facial nerve, the lateral point must not be farther down or anteriorly (Fig 1). The incision must be long enough to reach the lateral part of the orbital rim without overly stretching the skin. After the skin incision, there are 3 ways to turn the bone flap. The choice depends on personal habit and departmental policies. It is important that the bone flap reaches anteriorly near ANTERIOR CIRCULATION ANEURYSMS
Fig 1. Position of the patient and skin incision for all anterior circulation aneurysms except pericallosal aneurysms. The lateral end of the skin incision is just in front of the ear and above the zygomatic arch. Because the skin incision must lie behind the hairline for cosmetic reasons, is it farther back than usual.
the orbital rim and laterally as far as easily possible. The proper size of the bone flap is about 4 cm (anterior-posterior) • 5 cm (medial-lateral) (Fig 2). The lateral part of the sphenoid ridge should be nibbled or drilled away. In the first method, skin is dissected free from the temporal facia. Then, temporal muscle is divided with a cutting diathermy just behind the orbital rim from medially to the sphenoid ridge. Another cut is made along the skin incision to the zygomatic arch. Three bur holes are used: the first just medially from the sphenoid ridge behind the orbital rim, the second just above the zygoma, and the third about 4 cm behind the orbital rim. These are connected with a high-speed drill, and the temporal bone is broken to leave the bone flap attached to the muscle. A little bit of the temporal bone is nibbled away. The lateral part of the sphenoid ridge is nibbled or drilled away. In the second method, skin is not dissected free from muscle, but both together are dissected free from bone, and a free bone flap is turned with bur holes placed as in the first method. In the third method, called by its inventor interfascial pterional crani23;3
Fig 2. Temporal bone flap for internal carotid and middle cerebral artery aneurysm operations. This also can be called the standard pterional bone flap. The anterior margin is near the orbital rim.
otomy, 17 skin is first dissected free from the temporal fascia. Afterward temporal muscle is divided, dissected free from bone, and turned downward. A free bone flap is turned as in the second method (for a detailed description, see Yasargi117). Dissection of the aneurysm. After exposing the dura, the microscope is used. There are no bridging veins, and arachoid attachments seldom are found between the frontal lobe and the anterior fossa dura. A self-retaining brain retractor is advanced under visual control downward to the basal arachnoid just laterally from the optic nerve and gently pushed backward. A retractor to the temporal lobe is not necessary at this stage and can cause aneurysm rupture. The basal arachnoid is opened with a hook and microscissors from midline laterally as far as one reaches (Fig 3). At this stage, there usually is a brisk flood of cerebrospinal fluid, and space problems, if any, disappear. Afterwards, the arachnoidea in the basal part of sylvian fissure is opened until the carotid bifurcation is visible. Any clot is washed out, and veins are coagulated and cut, as necessary. If one sees that the aneurysm is not
Fig 3. Opening of the basal arachnoid. The optic nerve and internal carotid artery are seen, and a clot is seen behind the artery.
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Fig 4. Inspecting for posterior communicating and anterior choroidal arteries. The posterior communicating artery is seen behind the dissector, and the anterior choroidal artery is seen above the aneurysm.
attached to the temporal lobe, and if it is necessary, a second retractor can be used to retract and protect the temporal lobe. At this stage, one must identify the posterior communicating and anterior choroidal arteries (Fig 4). These are not the only arteries arising from internal carotid artery, but they are the only important ones. Ligation of the posterior communicating artery is often disastrous, and ligation of the anterior choroidal artery nearly always disastrous. The caliber of the posterior communicating artery is highly variable. It arises from the internal carotid artery a few millimeters above the clinoid process and courses posteriorly and laterally. The anterior choroidal artery is the first branch of the internal carotid artery distally after the posterior communicating artery, usually < 5 m m above it. Its direction is also posteriorly and laterally, and it is far smaller in caliber than the posterior communicating artery. Internal carotid artery aneurysms can point in any direction, although most of them point posteriorly or laterally. The neck is usually attached to the posterior communicating artery, but exceptions exist. The neck is dissected free from the arachnoidea and carotid, posterior communicating, and anterior choroidal arteries. Generally, this dissection is best done at the lateral side of the internal carotid artery. At this stage, at the latest, one must see the oculomotor nerve. It seldom needs any dissection, but it must be identified to be saved. Clipping. It is easy to apply the clip incorrectly to an internal carotid artery aneurysm after a perfect dissection. The main problem is the backward direction of the aneurysm; perfect visualization of the neck from all aspects is not always possible. The most common mistakes are a clip parallel to the aneurysm and vertical to the artery, not occluding the aneurysm completely, or inclusion of one of the aforementioned important arteries. Depending on the direction of the aneurysm, one must choose the direction of applying the clip. Generally, it is done best between the temporal lobe and the internal carotid artery, SIMO VALTONEN
higher up to have better visibility of the bifurcation. There is no need to identify posterior communicating or anterior choroidal arteries. There are rarely important small artery branches so that it is enough to identify and dissect the anterior and middle cerebral arteries free from the aneurysm neck. Dissecting the neck of these aneurysms generally is easy but so is displacing the clip. Applying the clip parallel to the neck and not occluding it completely is a common mistake. Positive identification of the placement tips of the clip is necessary after clipping.
Anterior Communicating Artery Aneurysms
Fig 5. Medially pointing internal carotid artery aneurysm clipped. For large and broad-necked aneurysms, the use of a booster clip may be necessary.
(ie, the tips coming from a superior and lateral direction), but there are exceptions (Fig 5). If there is space enough to apply it, a curved clip is generally best. A clip with an open ring for the internal carotid artery and tips parallel to it is easy to apply, but the important artery branches are in danger. After the clip is placed one should see the tips of the clip near the artery to ensure the right direction of the clip, the posterior communicating and anterior choroid arteries outside the clip, and no arteries (because errors in identification occur) inside it. Temporary clips are useful in difficult clipping situations. Problems occur with temporary clips because of lack of space below the aneurysm and because one clip below the aneurysm seldom stops all blood flow to the aneurysm, because blood still flows from the posterior communicating artery and through the anterior communicating artery from the opposite side. Rupture of the aneurysm during dissection or clipping can be a major disaster if the rupture occurs at the aneurysm neck. This situation is more prone to happen with internal carotid artery aneurysms, compared to aneurysms in other locations, because the internal carotid artery often has arteriosclerotic plaques. In this situation, the first step is to create visibility by applying a temporary" clip to the carotid artery below and above the aneurysm and, if possible, to the posterior communicating artery. Alter that, one has three choices: (1) Apply 2 clips to the internal carotid artery, below and above the rupture, and ensure saving at least the anterior choroid artery; (2) place a curved clip, with a booster clip on it, closing a part of the internal carotid artery and the rupture site inside it; and (3) suture the rupture. The result from the first method can be disastrous, but not necessarily so if there is a good crossflow from the opposite side through the anterior communicating artery. The third method is extremely difficult, especially if the carotid artery is arteriosclerotic. Aneurysms of the internal carotid artery bifurcation. Clipping these aneurysms is in some respects different from the operation described earlier for aneurysms originating at the posterior communication artery. The first part of the dissection is done in the same way, but the sylvian fissure is opened a bit ANTERIOR CIRCULATION ANEURYSMS
Patient positioning. Positioning the patient for anterior communicating artery aneurysms is the same as for internal carotid artery aneurysms (Fig l). These midline aneurysms can be reached from both sides. Because the right-sided approach is more comfortable for a right-handed surgeon, it is used for most anterior communicating artery aneurysms. For me, the exception is an aneurysm that points from left to right or a highly dominant left anterior cerebral artery, but for many neurosurgeons there are no exceptions at all; the choice is a matter of preference. Approach and exposure. The approach and exposure are the same as for internal carotid artery aneurysms, with some exceptions. It is not necessary to reach as far laterally; the bone flap can be limited to the sphenoid ridge (Fig 6). The median edge of the bone flap is extended slightly more medially for easier access to the midline. Removal of the sphenoid ridge is a matter of preference. Dissection of the aneurysm. The dura is opened in a semicircular manner. The basal arachnoidea is opened in the same way as described for the internal carotid artery aneurysms. It is important to open the arachnoidea laterally at least to the sylvian fissure. If, at this stage, part of the anterior cerebral artery is readily visible, the dissection can be continued by following it toward midline. If not, it is advisable to open the arachnoidea over the sylviarl fissure up to the internal carotid artery bifurcation, then follow the anterior cerebral artery to the midline
Fig 6. Bone flap for anterior communicating aneurysms. It is extended a bit farther medially than the standard pterional bone flap shown in Fig 2. 2;35
and anterior communicating artery complex. To go directly to this complex, not following the anterior cerebral artery, is possible but risky. When approaching the midline, one must if possible identify three important small arteries: frontopolar, fronto-orbital, and Heubner's arteries. Their anatomy and origin are highly variable, 17 and they must not be sacrificed. Near the midline, it is best to follow the anterior cerebral artery to the anterior communicating artery complex and to the origin of the ipsilateral pericallosal artery. It often is better to resect a small ( < 1 cm 3) part of the basal part of the frontal lobe than to overuse retraction or to be satisfied with poor visibility. After this resection, one must identify positively the aneurysm neck, the contralateral pericallosal artery (often difficult), and the contralateral anterior cerebral artery (usually easily visible). The aforementioned small arteries that are visible must be identified, but invisible arteries need not be sought actively. As the last part of the dissection, the aneurysm neck is dissected free from the surrounding arteries. If the aneurysm points forward or down, this dissection usually is easy, in contrast to the case with backward and upward pointing ones. Clipping. The clip is placed across the aneurysm neck. This is one of the few situations in which a fenestrated clip is often useful. The clip must not include even the smallest artery. Sometimes, it is easier and tempting to apply the clip so that the anterior communicating artery is included. Because of the frontopolar, fronto-orbital, and Heubner's arteries, which can originate anywhere in the anterior communicating artery complex and are often are overlooked, the clipping of incidental arteries must be avoided. For the same reason, so-called trapping (2 clips on the anterior communicating artery enclosing aneurysm neck between them) is not allowed. If temporary clips are used to ease clipping or in case of aneurysm rupture, 2 are always needed, one on both anterior cerebral arteries. If the aneurysm ruptures during manipulation of its neck, trapping often is the only possibility, with its possible disastrous consequences. Rupture at the aneurysm fundus is gener-
ally not a disaster at all; the clip is applied to the neck in the usual manner.
Middle Cerebral Artery Aneurysms Patient positioning. Positioning is the same as for the previously described aneurysm locations. Positioning is not influenced by the location of the aneurysm along the middle cerebral artery. Approach and exposure. The approach and exposure are similar to that for internal carotid artery aneurysms. Dissection of the aneurysm. The dura is opened in a semicircular manner and turned forward in the same way as for internal carotid artery aneurysms. Opening the basal arachnoidea also is done in the same manner. Some of the more distal aneurysms can be seen already at this stage, just behind the arachnoidea in the sylvian fissure, and can be approached directly. Most aneurysms are not visible, however, and it is best to start dissecting from the median end of the sylvian fissure. By gentle pressure with 2 retractors, the frontal and temporal lobes are separated. Small veins can be sacrificed, but large ones should be dissected to either side as well as all arteries. With a direct approach to the aneurysm and dissection along the sylvian fissure, it is important to secure an easily reachable segment of the middle cerebral artery for eventual application of a temporary clip. Middle cerebral artery aneurysms typically are situated at the origins of the major branches. There are no hidden or difficultto-detect small and important arteries, but often one of the main branches is much smaller than the 2 other ones. One should identify positively the major trunk of the middle cerebral artery and all branches, originating at the neck of the aneurysms and recognizable on angiography. The neck must be dissected free from all of these. Clipping. Middle cerebral artery aneurysms tend to be broad-necked far more often than aneurysms in other locations. Consequently, using 1 clip, one or more branches are often
Fig 7. Positioning of the patient and skin incision for pericallosal artery aneurysms. Marking of the midline is mandatory because orientation is lost after dressing the head.
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clipped. It is better to clip the aneurysm using 2 or, if necessary, 3 clips, generally straight ones, to occlude the aneurysm without including any of the artery branches. Especially with meddle cerebral artery aneurysms, temporary clipping can be useful. One clip on the main trunk of the artery effectively slackens the aneurysm, and if the neck is already dissected free, 4 minutes is ample time to apply clips properly, avoiding occlusion of artery branches. After clipping, it is possible to puncture the aneurysm with a long puncture needle. This practice ensures the completeness of the occlusion and can save a postoperative angiogram. Especially in the case of a middle cerebral artery aneurysm, applying an extra clip is generally easy. If a middle cerebral artery aneurysm ruptures during dissection or clipping, it is seldom a disaster. One must create enough visibility with suction to see the proximal trunk of the artery and apply a temporary clip. Afterward, the aneurysm can be clipped, and the temporary clip can be removed.
Middle cerebral artery aneurysm and intracerebral hematoma. Patients with middle cerebral artery aneurysms often have intracerebral hematomas. Such hematomas nearly invariably lie in the temporal lobe just inferior to the sylvian fissure. On these occasions, it is best, after opening the dura, to go first to the hematoma to create space. The arachnoidea is opened on the temporal side of the sylvian fissure; an arachnoidal opening of about 3 cm is usually enough. The hematoma is entered and evacuated and the anterior wall of the hematoma is inspected. If the aneurysm is visible, one can go directly to its proximal end and omit the dissection from basal cisterns, as described earlier. If the aneurysm is not clearly visible, however, it is safer to do the dissection.
Pericallosal Artery Aneurysms Patient positioning. Pericallosal artery aneurysms are always situated nearly exactly in midline. Consequently, these aneurysms can all be reached from the right side, regardless of whether they originate from the right or left pericallosal artery. The patient is positioned supine, with the head tilted slightly to the left and backward (Fig 7).
Fig 8. Bone flap for pericallosal artery aneurysms. ANTERIOR CIRCULATION ANEURYSMS
Fig 9. Some of the anterior circulation aneurysms can be reached from the opposite site, as with this anterior cerebral artery aneurysm.
Approach and opening. The skin incision is placed just behind the hairline, extending more to the right than the left (Fig 7). It is easy to place a craniotomy too far back, so the skin incision must be long enough to allow the front edge of the bone flap to be about 3 cm from the orbita. It is advisable to mark the midline on the skin before turning the skin flap and while turning it to scratch a mark on the periosteum or the bone. A bone flap of approximately 4 X 3 cm is turned so that the left margin is exactly at midline and the front margin is 3 cm from the orbital rim (Fig 8). The dura is opened in a circular manner with the flap attached to midline or in an H-shaped fashion, with the horizontal bar parallel to midline and about 1.5 cm from it. Dissection of the aneul~ysm. Bridging veins generally are not a problem in this region of the sagittal sinus. A space of 2 to 3 cm can usually can be found between them; if not, sacrificing a small vein is acceptable. A broad self-retaining retractor is placed on the interhemispheral surface of the right hemisphere and gently pushed laterally. Arachnoidea is first dissected free from the falx, then from the left hemisphere. At this stage, it is advisable to work straight downward adjacent to the corpus callosum and not to follow the cortical arteries. The corpus callosum is recognized by its perfect white color and arteries running on its surface. Pericallosal arteries can lie parallel to or on top of each other. It is possible for the left artery to lie slightly to the right. With the approach described, one usually first sees the pericallosal arteries distal to the aneurysm. They are followed proximally to the aneurysm. At this stage, cortical arteries are helpful for orientation. Clipping. Pericallosal arteries tend to be narrow necked, and with almost no exceptions, they have only 2 distal arteries. Hidden or difficult-to-detect perforating arteries do not exist. A single clip is always enough, and generally there is enough free space between the neck and arteries. A temporary clip is effective in allowing the aneurysm to become slack, but is seldom necessary.
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Multiple Aneurysms The accepted policy is to clip all aneurysms that clippable from the same approach are at the same time. In practical terms, this means that all aneurysms on the ipsilateral side and in the midline as well as some selected aneurysms, mainly internal carotid aneurysms, on the contralateral side (Fig 9). In one well-documented study, is every extra aneurysm operated on during the same session increased the risk of a poor outcome so that it may be better not to clip an excessive n u m b e r at one time.
Conclusions In case of potentially hazardous preventive surgery, one must be careful with indications, have a good knowledge of intracranial vascular anatomy and general neurosurgical technique, and optimal operative conditions. Careful preparation of the intracranial vessels with m i n i m u m damage to the brain is the important principle of aneurysm surgery.
References 1. Pakarinen S: Incidence, aetiology and prognosis of primary subarachnoid hemorrhage. Acta Neurol Scand 43(supp129):1-128, 1967 2. Winn HR, Richardson AE, Jane JA: The long-term prognosis in untreated cerebral aneurysms: I. The incidence of late hemorrhage in cerebral aneurysm: a 10-year evaluation of 364 patients. Ann Neurol 1:358-370, 1977 3. Juvela S, Porras M, Heiskanen O: Natural history of unruptured intracranial aneurysms: a long-term follow-up study. J Neurosurg 79:174-182, 1993 4. International Study of Unruptured Intracranial Aneurysms Investigators: Unruptured intracranial aneurysms--risk of rupture and risks of surgical intervention. N Engl J Med 339:1725-1733, 1998
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5. Kassell NF, Torner JC, Haley EC, et al: The International Cooperative Study on the timing of aneurysm surgery: Part I. Overall management results. J Neurosurg 73:18-36, 1990 6. Raaymakers TW, Rinkel GJ, Limburg M, et al: Mortality and morbidity of surgery for unruptured intracranial aneurysms: a meta-analysis. Stroke 29:1531-1538, 1998 7. Vanninen R, Koivisto T, Saari T, et al: Ruptured intracranial aneurysms: acute endovascular treatment with electrolytically detachable coils--a randomized study. Radiology 211:325-236, 1999 8. van Rooij WJ: Endovascular Treatment of Cerebral Aneurysms. Thesis, University of Utrecht, 1998 9. 0hman J, Heiskanen O: Effect of nimodipine on the outcome of patients after aneurysmal subarachnoid hemorrhage and surgery. J Neurosurg 69:683-686, 1988 10. 0hman J, Heiskanen O: Timing of operation for ruptured supratentorial aneurysms: a prospective, randomized study. J Neurosurg 70:55-60, 1989 11. Ljunggren B, S&veland H, Brandt L, et al: Early operation and overall outcome in aneurysmal subarachnoid hemorrage. J Neurosurg 62: 547-551, 1985 12. Kallmes DF, Kallmes MH: Cost-effectiveness of angiography performed during surgery for ruptured intracranial aneurysms. AJNR Am J Neuroradiol 18:1453-1462, 1997 13. Farrar JK, Gamache FW, Ferguson GG, et al: Effects of profound hypotension on cerebral blood flow during surgery for intracranial aneurysms. J Neurosurg 55:857, 1981 14. Suzuki J, Yoshimoto T: The effect of mannitol in prolongation of permissible occlusion time of cerebral artery--clinical data of aneurysm surgery. Neurosurg Rev 1:13-19, 1979 15. Aebert H, Brawanski A, Philipp A, et al: Deep hypothermia and circulatory arrest for surgery of complex intracranial aneurysms. Eur J Cardiothorac Surg 13:223-229, 1998 16. Kallmes DF, Kallmes MH, Lanzino G, et al: Routine angiography after surgery for intracranial aneurysms: a cost versus benefit analysis. Neurosurgery 41:629-639, 1997 17. Yasargil MG: General operative techniques, in Yasargil MG (ed): Microneurosurgery I. Stuttgart, Georg Thieme Verlag, 1984 18. Rinne J, Hernesniemi J, Niskanen M, et al: Management outcome for multiple intracranial aneurysms. Neurosurgery 36:31-38, 1995
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With few exceptions, aneurysm surgery is preventive surgery; the aim is to prevent a subarachnoid hemorrhage from the aneurysm--rebleeding after the first hemorrage or possibly first bleeding from a previously unruptured aneurysm. Consequently, surgical mortality and morbidity must not exceed natural mortality and morbidity. The natural risk of death from rebleeds after aneurysmal subarachnoid hemorrhage is about 35% immediately after the bleeding, and decreasing quickly to a level of approximately 2%/y. The annual bleeding rate of previously unruptured aneurysms is traditionally considered to be about 1.4% but can be much lower. For good-grade patients, the ideal time of surgery after a subarachnoid hemorrhage is the first days after bleeding. For previously unruptured aneurysms, the timing of surgery is not critical. Of the open, intracranial procedures, clipping of the aneurysm neck is the only widely used procedure today. Intravascular treatment is a new and possibly good alternative to open operations for intracranial aneurysms; however, surgery is still the standard aneurysm therapy. To minimize operative mortality and morbidity, the best possible perioperative conditions must be created. Preoperatively, if the patient has had a recent subarachnoid hemorrhage, standard care includes maintenance of stable and normal blood pressure and nimodipine to prevent possible ischemic complications in addition to good general care. Perioperatively, it is important to prevent ischemic complications and to create the best possible operative conditions with respect to intracranial space. The most common anterior circulation aneurysm locations are internal carotid artery, anterior communicating artery, middle cerebral artery, and pericallosal artery. The approach to all these aneurysm locations, with the exception of pericallosal aneurysms, is the pterional one, with possible small variations. The general principle of dissecting an aneurysm is to follow the parent vessel and identify the aneurysm neck. The aneurysm neck always must be dissected free from surrounding arteries and clipped with one or more aneurysm clips. Techniques used vary according to the location, but common for all is avoiding a lesion to any important artery and achieving a complete closure of the aneurysm neck. It is particularly difficult and important to avoid closure of the socalled perforating arteries close to anterior communicating artery and internal carotid artery aneurysms. Copyright 9 2000 by W.B. Saunders Company
Introduction neurysm surgery is generally preventive surgery. Conse-
A ,quently, it must be performed with mortality and morbid-
ity below expected levels demonstrated by the natural history of
From the Department of Neurosurgery, Turku University Central Hospital, Turku, Finland. Address reprint requests to Simo Valtonen, MD, PhD, Department of Neurosurgery, PO Box 52, Turku University Central Hospital, 20521 Turku, Finland. Copyright 9 2000 by W.B. Saunders Company 1092-440X/00/0304-0002535.00/0 doi:l 0.1053/otns.2000.18514
the disease. The choice between aneurysm surgery and its alternative, endovascular surgery, must be considered.
Indications With few exceptions (eg, giant aneurysms, internal carotid aneurysm causing a third nerve palsy), surgery for an aneurysm is a prophylactic measure to prevent a potential bleed. Consequently the expected benefit in this sense must be weighed against the risks of surgery. A patient with an aneurysmal subarachnoid hemorrhage has a risk of about 35% of fatal rebleed on the day of the bleeding. This decreases about 5% weekly during the first month, 1 decreases more slowly afterward, and gradually reaches a level of 2% to 3% annual rebleeds? Expected operative mortality at any time must not be higher than these figures. The annual bleeding rate of previously unruptured aneurysms is reported to be about 1.4%, 3 with mortality of about 50%, even though much lower bleeding rates have been reported lately, especially for small aneurysms. 4 Consequently, surgery is indicated if the expected mortality and severe morbidity combined are considerably less than these numbers. Operative mortality and morbidity are not constants. They are influenced by the condition of the patient (the most important factor), age, and possible concurring diseases. 5 Because the mortality for aneurysm surgery for Hunt and Hess grade 5 patients is about 50% to 80%, surgery is not indicated in these cases. Generally, patients with a technically clippable aneurysm and who are grade 1 to 3 after an aneurysmal subarachnoid hemorrhage are candidates for surgery. The decision can be difficult to make when grade 4 patients are concerned because strict guidelines are hard to give. If surgery for incidental aneurysms cannot be performed with nearly zero mortality, it should not be performed at all. Surgery for nonruptured aneurysms is not risk-free. 6 Open intracranial operation is the standard of aneurysm treatment. Endovascular treatment (ie, coiling) is a new and attractive alternative, without proof of effectiveness or superiority over operation in randomized studies. 7 The choice between these two methods depends on experience, local habits, location of aneurysm, and the patient's preference. For example, in Finland, about half of all aneurysms are treated with coiling. 8 As a result of small reported series, complication and mortality rates of endovascular procedures in different situations are not well known, but in 8 different series, with a total of 1377 patients, the procedure-related mortality was 0 to 5%. 8
Preoperative Care and Surgical Timing Diagnostic Studies After subarachnoid hemorrhage, a 4-vessel angiographic study is mandatory for patients who are possible candidates for oper-
Operative Techniques in Neurosurgery, Vol 3, No 4 (December), 2000: pp 231-238
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ation. Most aneurysms can be detected with a good magnetic resonance angiography study, but the fresh blood disturbs the detection, and the accuracy rate is not high enough. After accidental discovery of an aneurysm, 4-vessel angiography is the gold standard. It is possible, however, to operate on a nonruptured aneurysm in the anterior circulation without it, after a high-quality magnetic resonance angiography study, but on all occasions when intravascular therapy is considered, anglographic studies are mandatory.
Medical Care The aim of medical care after aneurysmal subarachnoid hemorrhage is to prevent additional ischemic brain damage and to decrease the risk of a rebleed. Nimodipine has been proved to decrease the number of ischemic complications if used in connection with an aneurysmal subarachnoid hemorrhage, 9 and its use is a part of normal care. The clinician must be aware of the hypotensive effect of this drug. Dosage must be adjusted individually to keep the systolic blood pressure level at about 140 mmHg. If this blood pressure level is not possible, triple-H therapy (hypervolemia, hypertension, and hemodilution) or catecholamines must be added to the regimen or nimodipine discontinued. Antifibrinolytic agents decrease the risk of rebleeding before definite aneurysm surgery, 5 but because they also increase the risk of ischemic complications, they are not part of routine preoperative care. A patient in good clinical grade and with stable blood pressure can stay in a regular neurosurgical ward; all other patients should be cared for in an intensive care (or observation) unit.
Technical Requirements There are no specific requirements for the operative table, but attachment of a skeletal 3-point or 4-point head fixation device must be possible. Use of one of these devices (eg, Mayfield or Sugita) is mandatory. Intracranial aneurysms must not be operated on without an operative microscope. Because aneurysm surgery requires frequent microscope movements, a balanced microscope with magnetic locks is recommended strongly as well as one with zoomable optics. Neuronavigation is not necessary for aneurysm surgery; orientation is achieved by bony and vascular landmarks and intracranial vessels. Intraoperative angiography has been found to be useful and cost-effective in some situations. 12 h is an exclusive tool, and not available in most neurosurgical departments, and it is not requisite for aneurysm surgery. The aneurysm clip is the specific tool of aneurysm surgery. Several types exist; the best-known differ little from each other. To provide familiarity and to prevent confusion in critical situations, neurosurgeons should use one brand of clips only. Clips come in a variety of different shapes and sizes. It is advisable to have a good selection of clips, separated clearly from each other on the instrument table, even if nearly all aneurysms are ligated with a small selection (eg, straight clips of different lengths, bayonet clips, angled clips and curved clips). Temporary clips are necessary. Clip applicators (bayonet and pistol types) and a clip remover are also necessary. Self-retaining brain retractors are appropriate for aneurysm surgery even if there is a tendency today to avoid them. They help to visualize structures at the skull base and to protect the brain.
Surgical Timing Surgical timing for ruptured aneurysms is crucial because the risk of a rerupture is maximal immediately after rupture and decreases with time. ~,5 The earlier surgery is performed, the greater the expected prophylactic gain. The value of early surgery (within a few days) has been documented in some studies. l~ An operation within 3 days is the rule for grade l to 3 patients. Aneurysm clipping is major surgery, unavoidably attached to high risks in many cases. One should expect to have maximally good conditions, and for a major operation, these usually do not exist during the night. It is wise to sacrifice a few hours or days in favor of optimal operative conditions. The theoretic risk of a rerupture during this short period is slight, and there is no proof of benefits for surgery performed during the first 3 days compared to days 4 to 7,1~ and there are fewer benefits of immediate surgery. The situation is different with previously unruptured aneurysms. Some patients with unruptured aneurysms can be operated on in the same session as a ruptured aneurysm (see multiple aneurysms). Patients with an unruptured aneurysm not operable together with a ruptured one should not have a second operation immediately after the first one (and after the hemorrhage). There are no exact guidelines regarding how long one should wait, but at least 1 month can be recommended, to allow all immediate effects of the bleeding to disappear. With aneurysms found incidentally, there is no timing problem. Surgery can be performed on any suitable day shortly after the detection of the aneurysm; there is no real hurry, but the patient's nervousness and fear must be taken into account. 232
Perioperative Care One of the main goals of perioperative care is to produce intracranial space (slack brain). Mannitol decreases brain volume quickly and is an integral part of aneurysm surgery. The normal dose is 1 g/kg of 20% mannitol. It is important is to give mannitol early because the peak effect occurs about 30 minutes after infusion. Spinal drainage is another way to create intracranial space and is used widely. In patients whose condition is poor the brain is swollen, the ventricles are small, and there is a risk of tentorial herniation. In patients whose condition is good, there are no space problems anyway. Another important goal is to prevent hypoxic brain damage. Traditionally, to ease aneurysm clipping, controlled hypotension has been used. It is known to be potentially hazardous, ~3 and because ischemic complications are common after aneurysm surgery, controlled hypotension cannot be recommended, especially because it really does not help much. Temporary vascular occlusion is another way to make clipping easier. There are conflicting opinions about the maximal safe occlusion time and the effect of mannitol on it. 14 It can be considered reasonably safe to occlude a vessel for 4 minutes. This is a short time for aneurysm dissection, however, and can help only in the actual application of the clip. Longer times are potentially risky, and protective measures must be considered. Hypothermia can be used in difficult situations. ~ After about 2 decades of absence from aneurysm surgery, hypothermia has gained some popularity again. Its position and value remain to be seen. So-called neuroprotective drugs (thiopental and etoSIMO VALTONEN
midate) have been used to prevent perioperative ischemic brain damage, but definite proof of their effectiveness is lacking.
Postoperative Care and Controls Every patient is transferred to the intensive care unit immediately after aneurysm clipping. Patients with a preoperative grade 1 to 3 can be extubated immediately after the operation, but patients with a worse grade cannot. The possible complications, which must be detected early and treated, are postoperative clot (uncommon with aneurysm operations), infection, and ischemic complications. Blood pressure must be kept at a reasonably high level (eg, 150/90 mmHg) for the entire immediate postoperative period. If necessary, this pressure can be achieved with triple-H therapy or ionotropic drugs. Opinions differ about the length of nimodipine administration postoperatively; 10 to 14 days is common. The only way to be certain of a good anatomic result for aneurysm clipping is postoperative angiographic examination, but its cost-versus-benefit value is questionable. 16 Partially clipped aneurysms are seldom reoperated. Because postoperative angiography is an invasive investigation, its use cannot be considered mandatory in cases when reoperation (or alternatively coiling) will not be done, regardless of the finding of angiography.
Vascular Anatomy Aneurysm surgery is mainly vascular surgery. Good knowledge of vascular anatomy is a prerequisite. Intracranial vascular anatomy is full of variations, and a neurosurgeon cannot master them all. It is important to know the specifically problematic vessels, to respect every abnormal situation, to be certain not to clip any arteries, and at times to consult good textbooks. 17
Specific Operative Techniques Internal Carotid Artery Aneurysms Aneurysms situated proximally from the origin of the ophthalmic artery are difficult to reach and to clip. Consequently, with these aneurysms, endovascular treatment is a strong alternative. Open operation of these aneurysms is not described; the text refers only to aneurysms above the origin of the ophthalmic artery. Patient positioning. The patient lies supine, the head tilted to the opposite side from the aneurysm and backward (Fig 1). The lateral end of the superior margin of the orbita is the uppermost point of the head. It is possible to reach internal carotid artery aneurysms from the opposite side, but surgery from the same side is the rule. Approach and exposure. The skin incision is extended from a point just over the zygoma and 0.5 cm anteriorly from the earlobe, behind the hair line, a bit over the midline to the hair line. To protect the facial nerve, the lateral point must not be farther down or anteriorly (Fig 1). The incision must be long enough to reach the lateral part of the orbital rim without overly stretching the skin. After the skin incision, there are 3 ways to turn the bone flap. The choice depends on personal habit and departmental policies. It is important that the bone flap reaches anteriorly near ANTERIOR CIRCULATION ANEURYSMS
Fig 1. Position of the patient and skin incision for all anterior circulation aneurysms except pericallosal aneurysms. The lateral end of the skin incision is just in front of the ear and above the zygomatic arch. Because the skin incision must lie behind the hairline for cosmetic reasons, is it farther back than usual.
the orbital rim and laterally as far as easily possible. The proper size of the bone flap is about 4 cm (anterior-posterior) • 5 cm (medial-lateral) (Fig 2). The lateral part of the sphenoid ridge should be nibbled or drilled away. In the first method, skin is dissected free from the temporal facia. Then, temporal muscle is divided with a cutting diathermy just behind the orbital rim from medially to the sphenoid ridge. Another cut is made along the skin incision to the zygomatic arch. Three bur holes are used: the first just medially from the sphenoid ridge behind the orbital rim, the second just above the zygoma, and the third about 4 cm behind the orbital rim. These are connected with a high-speed drill, and the temporal bone is broken to leave the bone flap attached to the muscle. A little bit of the temporal bone is nibbled away. The lateral part of the sphenoid ridge is nibbled or drilled away. In the second method, skin is not dissected free from muscle, but both together are dissected free from bone, and a free bone flap is turned with bur holes placed as in the first method. In the third method, called by its inventor interfascial pterional crani23;3
Fig 2. Temporal bone flap for internal carotid and middle cerebral artery aneurysm operations. This also can be called the standard pterional bone flap. The anterior margin is near the orbital rim.
otomy, 17 skin is first dissected free from the temporal fascia. Afterward temporal muscle is divided, dissected free from bone, and turned downward. A free bone flap is turned as in the second method (for a detailed description, see Yasargi117). Dissection of the aneurysm. After exposing the dura, the microscope is used. There are no bridging veins, and arachoid attachments seldom are found between the frontal lobe and the anterior fossa dura. A self-retaining brain retractor is advanced under visual control downward to the basal arachnoid just laterally from the optic nerve and gently pushed backward. A retractor to the temporal lobe is not necessary at this stage and can cause aneurysm rupture. The basal arachnoid is opened with a hook and microscissors from midline laterally as far as one reaches (Fig 3). At this stage, there usually is a brisk flood of cerebrospinal fluid, and space problems, if any, disappear. Afterwards, the arachnoidea in the basal part of sylvian fissure is opened until the carotid bifurcation is visible. Any clot is washed out, and veins are coagulated and cut, as necessary. If one sees that the aneurysm is not
Fig 3. Opening of the basal arachnoid. The optic nerve and internal carotid artery are seen, and a clot is seen behind the artery.
234
Fig 4. Inspecting for posterior communicating and anterior choroidal arteries. The posterior communicating artery is seen behind the dissector, and the anterior choroidal artery is seen above the aneurysm.
attached to the temporal lobe, and if it is necessary, a second retractor can be used to retract and protect the temporal lobe. At this stage, one must identify the posterior communicating and anterior choroidal arteries (Fig 4). These are not the only arteries arising from internal carotid artery, but they are the only important ones. Ligation of the posterior communicating artery is often disastrous, and ligation of the anterior choroidal artery nearly always disastrous. The caliber of the posterior communicating artery is highly variable. It arises from the internal carotid artery a few millimeters above the clinoid process and courses posteriorly and laterally. The anterior choroidal artery is the first branch of the internal carotid artery distally after the posterior communicating artery, usually < 5 m m above it. Its direction is also posteriorly and laterally, and it is far smaller in caliber than the posterior communicating artery. Internal carotid artery aneurysms can point in any direction, although most of them point posteriorly or laterally. The neck is usually attached to the posterior communicating artery, but exceptions exist. The neck is dissected free from the arachnoidea and carotid, posterior communicating, and anterior choroidal arteries. Generally, this dissection is best done at the lateral side of the internal carotid artery. At this stage, at the latest, one must see the oculomotor nerve. It seldom needs any dissection, but it must be identified to be saved. Clipping. It is easy to apply the clip incorrectly to an internal carotid artery aneurysm after a perfect dissection. The main problem is the backward direction of the aneurysm; perfect visualization of the neck from all aspects is not always possible. The most common mistakes are a clip parallel to the aneurysm and vertical to the artery, not occluding the aneurysm completely, or inclusion of one of the aforementioned important arteries. Depending on the direction of the aneurysm, one must choose the direction of applying the clip. Generally, it is done best between the temporal lobe and the internal carotid artery, SIMO VALTONEN
higher up to have better visibility of the bifurcation. There is no need to identify posterior communicating or anterior choroidal arteries. There are rarely important small artery branches so that it is enough to identify and dissect the anterior and middle cerebral arteries free from the aneurysm neck. Dissecting the neck of these aneurysms generally is easy but so is displacing the clip. Applying the clip parallel to the neck and not occluding it completely is a common mistake. Positive identification of the placement tips of the clip is necessary after clipping.
Anterior Communicating Artery Aneurysms
Fig 5. Medially pointing internal carotid artery aneurysm clipped. For large and broad-necked aneurysms, the use of a booster clip may be necessary.
(ie, the tips coming from a superior and lateral direction), but there are exceptions (Fig 5). If there is space enough to apply it, a curved clip is generally best. A clip with an open ring for the internal carotid artery and tips parallel to it is easy to apply, but the important artery branches are in danger. After the clip is placed one should see the tips of the clip near the artery to ensure the right direction of the clip, the posterior communicating and anterior choroid arteries outside the clip, and no arteries (because errors in identification occur) inside it. Temporary clips are useful in difficult clipping situations. Problems occur with temporary clips because of lack of space below the aneurysm and because one clip below the aneurysm seldom stops all blood flow to the aneurysm, because blood still flows from the posterior communicating artery and through the anterior communicating artery from the opposite side. Rupture of the aneurysm during dissection or clipping can be a major disaster if the rupture occurs at the aneurysm neck. This situation is more prone to happen with internal carotid artery aneurysms, compared to aneurysms in other locations, because the internal carotid artery often has arteriosclerotic plaques. In this situation, the first step is to create visibility by applying a temporary" clip to the carotid artery below and above the aneurysm and, if possible, to the posterior communicating artery. Alter that, one has three choices: (1) Apply 2 clips to the internal carotid artery, below and above the rupture, and ensure saving at least the anterior choroid artery; (2) place a curved clip, with a booster clip on it, closing a part of the internal carotid artery and the rupture site inside it; and (3) suture the rupture. The result from the first method can be disastrous, but not necessarily so if there is a good crossflow from the opposite side through the anterior communicating artery. The third method is extremely difficult, especially if the carotid artery is arteriosclerotic. Aneurysms of the internal carotid artery bifurcation. Clipping these aneurysms is in some respects different from the operation described earlier for aneurysms originating at the posterior communication artery. The first part of the dissection is done in the same way, but the sylvian fissure is opened a bit ANTERIOR CIRCULATION ANEURYSMS
Patient positioning. Positioning the patient for anterior communicating artery aneurysms is the same as for internal carotid artery aneurysms (Fig l). These midline aneurysms can be reached from both sides. Because the right-sided approach is more comfortable for a right-handed surgeon, it is used for most anterior communicating artery aneurysms. For me, the exception is an aneurysm that points from left to right or a highly dominant left anterior cerebral artery, but for many neurosurgeons there are no exceptions at all; the choice is a matter of preference. Approach and exposure. The approach and exposure are the same as for internal carotid artery aneurysms, with some exceptions. It is not necessary to reach as far laterally; the bone flap can be limited to the sphenoid ridge (Fig 6). The median edge of the bone flap is extended slightly more medially for easier access to the midline. Removal of the sphenoid ridge is a matter of preference. Dissection of the aneurysm. The dura is opened in a semicircular manner. The basal arachnoidea is opened in the same way as described for the internal carotid artery aneurysms. It is important to open the arachnoidea laterally at least to the sylvian fissure. If, at this stage, part of the anterior cerebral artery is readily visible, the dissection can be continued by following it toward midline. If not, it is advisable to open the arachnoidea over the sylviarl fissure up to the internal carotid artery bifurcation, then follow the anterior cerebral artery to the midline
Fig 6. Bone flap for anterior communicating aneurysms. It is extended a bit farther medially than the standard pterional bone flap shown in Fig 2. 2;35
and anterior communicating artery complex. To go directly to this complex, not following the anterior cerebral artery, is possible but risky. When approaching the midline, one must if possible identify three important small arteries: frontopolar, fronto-orbital, and Heubner's arteries. Their anatomy and origin are highly variable, 17 and they must not be sacrificed. Near the midline, it is best to follow the anterior cerebral artery to the anterior communicating artery complex and to the origin of the ipsilateral pericallosal artery. It often is better to resect a small ( < 1 cm 3) part of the basal part of the frontal lobe than to overuse retraction or to be satisfied with poor visibility. After this resection, one must identify positively the aneurysm neck, the contralateral pericallosal artery (often difficult), and the contralateral anterior cerebral artery (usually easily visible). The aforementioned small arteries that are visible must be identified, but invisible arteries need not be sought actively. As the last part of the dissection, the aneurysm neck is dissected free from the surrounding arteries. If the aneurysm points forward or down, this dissection usually is easy, in contrast to the case with backward and upward pointing ones. Clipping. The clip is placed across the aneurysm neck. This is one of the few situations in which a fenestrated clip is often useful. The clip must not include even the smallest artery. Sometimes, it is easier and tempting to apply the clip so that the anterior communicating artery is included. Because of the frontopolar, fronto-orbital, and Heubner's arteries, which can originate anywhere in the anterior communicating artery complex and are often are overlooked, the clipping of incidental arteries must be avoided. For the same reason, so-called trapping (2 clips on the anterior communicating artery enclosing aneurysm neck between them) is not allowed. If temporary clips are used to ease clipping or in case of aneurysm rupture, 2 are always needed, one on both anterior cerebral arteries. If the aneurysm ruptures during manipulation of its neck, trapping often is the only possibility, with its possible disastrous consequences. Rupture at the aneurysm fundus is gener-
ally not a disaster at all; the clip is applied to the neck in the usual manner.
Middle Cerebral Artery Aneurysms Patient positioning. Positioning is the same as for the previously described aneurysm locations. Positioning is not influenced by the location of the aneurysm along the middle cerebral artery. Approach and exposure. The approach and exposure are similar to that for internal carotid artery aneurysms. Dissection of the aneurysm. The dura is opened in a semicircular manner and turned forward in the same way as for internal carotid artery aneurysms. Opening the basal arachnoidea also is done in the same manner. Some of the more distal aneurysms can be seen already at this stage, just behind the arachnoidea in the sylvian fissure, and can be approached directly. Most aneurysms are not visible, however, and it is best to start dissecting from the median end of the sylvian fissure. By gentle pressure with 2 retractors, the frontal and temporal lobes are separated. Small veins can be sacrificed, but large ones should be dissected to either side as well as all arteries. With a direct approach to the aneurysm and dissection along the sylvian fissure, it is important to secure an easily reachable segment of the middle cerebral artery for eventual application of a temporary clip. Middle cerebral artery aneurysms typically are situated at the origins of the major branches. There are no hidden or difficultto-detect small and important arteries, but often one of the main branches is much smaller than the 2 other ones. One should identify positively the major trunk of the middle cerebral artery and all branches, originating at the neck of the aneurysms and recognizable on angiography. The neck must be dissected free from all of these. Clipping. Middle cerebral artery aneurysms tend to be broad-necked far more often than aneurysms in other locations. Consequently, using 1 clip, one or more branches are often
Fig 7. Positioning of the patient and skin incision for pericallosal artery aneurysms. Marking of the midline is mandatory because orientation is lost after dressing the head.
23,6
SIMO VALTONEN
clipped. It is better to clip the aneurysm using 2 or, if necessary, 3 clips, generally straight ones, to occlude the aneurysm without including any of the artery branches. Especially with meddle cerebral artery aneurysms, temporary clipping can be useful. One clip on the main trunk of the artery effectively slackens the aneurysm, and if the neck is already dissected free, 4 minutes is ample time to apply clips properly, avoiding occlusion of artery branches. After clipping, it is possible to puncture the aneurysm with a long puncture needle. This practice ensures the completeness of the occlusion and can save a postoperative angiogram. Especially in the case of a middle cerebral artery aneurysm, applying an extra clip is generally easy. If a middle cerebral artery aneurysm ruptures during dissection or clipping, it is seldom a disaster. One must create enough visibility with suction to see the proximal trunk of the artery and apply a temporary clip. Afterward, the aneurysm can be clipped, and the temporary clip can be removed.
Middle cerebral artery aneurysm and intracerebral hematoma. Patients with middle cerebral artery aneurysms often have intracerebral hematomas. Such hematomas nearly invariably lie in the temporal lobe just inferior to the sylvian fissure. On these occasions, it is best, after opening the dura, to go first to the hematoma to create space. The arachnoidea is opened on the temporal side of the sylvian fissure; an arachnoidal opening of about 3 cm is usually enough. The hematoma is entered and evacuated and the anterior wall of the hematoma is inspected. If the aneurysm is visible, one can go directly to its proximal end and omit the dissection from basal cisterns, as described earlier. If the aneurysm is not clearly visible, however, it is safer to do the dissection.
Pericallosal Artery Aneurysms Patient positioning. Pericallosal artery aneurysms are always situated nearly exactly in midline. Consequently, these aneurysms can all be reached from the right side, regardless of whether they originate from the right or left pericallosal artery. The patient is positioned supine, with the head tilted slightly to the left and backward (Fig 7).
Fig 8. Bone flap for pericallosal artery aneurysms. ANTERIOR CIRCULATION ANEURYSMS
Fig 9. Some of the anterior circulation aneurysms can be reached from the opposite site, as with this anterior cerebral artery aneurysm.
Approach and opening. The skin incision is placed just behind the hairline, extending more to the right than the left (Fig 7). It is easy to place a craniotomy too far back, so the skin incision must be long enough to allow the front edge of the bone flap to be about 3 cm from the orbita. It is advisable to mark the midline on the skin before turning the skin flap and while turning it to scratch a mark on the periosteum or the bone. A bone flap of approximately 4 X 3 cm is turned so that the left margin is exactly at midline and the front margin is 3 cm from the orbital rim (Fig 8). The dura is opened in a circular manner with the flap attached to midline or in an H-shaped fashion, with the horizontal bar parallel to midline and about 1.5 cm from it. Dissection of the aneul~ysm. Bridging veins generally are not a problem in this region of the sagittal sinus. A space of 2 to 3 cm can usually can be found between them; if not, sacrificing a small vein is acceptable. A broad self-retaining retractor is placed on the interhemispheral surface of the right hemisphere and gently pushed laterally. Arachnoidea is first dissected free from the falx, then from the left hemisphere. At this stage, it is advisable to work straight downward adjacent to the corpus callosum and not to follow the cortical arteries. The corpus callosum is recognized by its perfect white color and arteries running on its surface. Pericallosal arteries can lie parallel to or on top of each other. It is possible for the left artery to lie slightly to the right. With the approach described, one usually first sees the pericallosal arteries distal to the aneurysm. They are followed proximally to the aneurysm. At this stage, cortical arteries are helpful for orientation. Clipping. Pericallosal arteries tend to be narrow necked, and with almost no exceptions, they have only 2 distal arteries. Hidden or difficult-to-detect perforating arteries do not exist. A single clip is always enough, and generally there is enough free space between the neck and arteries. A temporary clip is effective in allowing the aneurysm to become slack, but is seldom necessary.
237
Multiple Aneurysms The accepted policy is to clip all aneurysms that clippable from the same approach are at the same time. In practical terms, this means that all aneurysms on the ipsilateral side and in the midline as well as some selected aneurysms, mainly internal carotid aneurysms, on the contralateral side (Fig 9). In one well-documented study, is every extra aneurysm operated on during the same session increased the risk of a poor outcome so that it may be better not to clip an excessive n u m b e r at one time.
Conclusions In case of potentially hazardous preventive surgery, one must be careful with indications, have a good knowledge of intracranial vascular anatomy and general neurosurgical technique, and optimal operative conditions. Careful preparation of the intracranial vessels with m i n i m u m damage to the brain is the important principle of aneurysm surgery.
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