Introduction

   In recent years intracranial dissecting aneurysms have been reported occasionally. The number of patients identified as this type of aneurysm has been increasing according to the recent spread of the use of cerebral angiography and 3 dimensional computerized Tomography (3D CT).²⁾ However, dissecting aneurysms arising merely in anterior cerebral artery (ACA), expecially in A1 segment, are found rarely excluding dissecting aneurysm which extend from the proximal intracranial carotid artery and the aneurysm afflicted the vertebral and basilar arteries.²⁾⁴⁾⁶⁾ The characteristics of the A1 aneurysm in the literature include frequency in young male adolescents, fragility of aneurysms, frequent ganglionic hemorrhage, vascular anomaly, fusiform shape and/or multiplicity of aneurysms.¹⁰⁾²³⁾ Dissecting aneurysm should be considered in diagnosis when young individuals developed cerebrovascular accident and If the initial symptom was subarachnoid hemorrhage (SAH), surgical treatment might be beneficial.
   We report here two cases of dissecting aneurysms of the A1 segment of the ACA, which was treated surgically, with the focus on the diagnosis and the surgical considerations.

Case Reports

1. Case 1：
   A 50-year-old male was admitted to other local hospital on June 9, 2005. The patient complained sudden bursting headache and his mental status was alert with a Glasgow coma Score (GCS) of 15/15. Then he was transferred to our hospital. On admission, he had no focal neurological deficit. CT revealed diffuse subarachnoid hemorrhage (SAH) which was classified as Fisher's group III (Fig. 1). Preoperative cerebral angiographic study was performed as follow (Fig. 2). Firstly, left carotid angiography revealed a diffuse dilatation of left A1 segment with pooling of contrast medium. Secondly, right carotid angiography with digital compression of the left common carotid artery didn't demonstrate dissecting aneurysm at the left A1 segment, which suggested poor collateral flow through the anterior communicating artery (AcomA).
   Left pterional craniotomy was performed on the day of admission. The left ICA, the left A1 and A2 segments were identified. The wall of the A1 showed blood blister appearance (dark red color) which suggested the formation of subadventitial hematoma (Fig. 3A). A perforating branch was observed arising from the A1 segment proximal to the dissecting aneurysm. This dissecting aneurysm involving A1 segment could not be wrapped by usual fashion because of the risk of injury to the nearby perforating arteries¹⁰⁾ and also could not be done trapping surgery because the collateral blood circulation in that territory of the distal right ACA seemed to be poor. However, this aneurysm was wrapped with a trousers shaped artificial dura, fixed with an aneurysmal clip and coated with fibrin glue (Fig. 3B).
   Despite of our care, CT performed after the operation revealed a subtle low density on the head of the left caudate, but there were no symptoms. Cerebral angiography obtained 2 weeks after the surgery showed that the dilated A1 segment was made attenuated and the dissecting aneurysm had disappeared (Fig. 4). He was discharged 15 days after operation without neurological deficit.

2. Case 2：
   A 44-year-old female who has hypertension developed sudden bursting headache and lethargy. In a emergency room, patient's mental status was drowsy with a GCS of 12/15 and motor weakness was not checked. CT scan demonstrated diffuse subarachnoid hemorrhage at basal cisterns and both sylvian cistern (Fig. 5). 3D CT showed fusiform dilatation of right A1 segment, focal severe stenosis of proximal A1 segment of ACA, AcomA within normal shape and no laterality of A1 dominance (Fig. 6).
   Trapping of the dissecting aneurysm was performed via the right pterional approach on the day of admission and subsequently craniectomy was also done for the control of the increased intracranial pressure. The right internal carotid artery (ICA) was identified and the A1 segment of the right ACA was exposed from the bifurcation of the ICA to its distal portion. A reddish fusiform dilatation was seen distal to the bifurcation. The vessel walls were dark purple in color, which is a typical finding of dissecting aneurysm (Fig. 7). We used intraoperative Transcranial Doppler (TCD) to confirm the patency of distal ACA flow when the temporary clipping was applied on contralateral A1 segment. After the A1 segment of the right ACA including the AcomA was identified, aneurysmal clips were applied to the A1 segment just proximal and distal portion of the dissecting aneurysm.
   The postoperative course was uneventful. The next day after the operation, CT revealed low density in the area supplied by the right Heubner artery, but there were no symptoms. Cerebral angiography obtained 4 weeks after the surgery showed that the dissecting aneurysm had disappeared (Fig. 8) and the right A2-A4 segments of ACA were visualized via collateral flow through the AcomA. After cranioplasty, she had no neurological deficit at discharge.

Discussion

1. Incidence
   In spite of the relatively rare incidence of dissecting aneurysms of the cerebral arteries, they have been increasingly reported in the past two decades.^{1)2)4)5)6)8)9)10)11)12)14)15)16)17)18)19)20)24)25)} Concerning the anterior circulation, however, most of them occur in the internal carotid artery (ICA) and the middle cerebral artery (MCA) although dissecting aneurysms have been unusually reported.^{1)2)6)9)12)13)14)} The ACA may be involved when the ICA dissection extends distally, but dissection confined to the ACA is extremely rare.
   Most initial symptoms of posterior circulation dissecting aneurysms are those of SAH and only a few patients are those of cerebral ischemia.¹¹⁾¹⁷⁾ In contrast, dissecting aneurysms in the anterior circulation usually cause cerebral ischemic attacks；the patients often complain of sudden headache with or without associated neurologic deficits, and onset with SAH is rare.¹²⁾

2. Cause
   The cause of the dissection in mostly reported cases is unknown. Several factors have been proposed as the possible cause of dissecting aneurysm of the ACA, including atherosclerosis, periarteritis nodosa, moyamoya disease, fibromuscular dysplasia, syphilitic angiopathy, homocystinurea, cystic necrosis of the media, Guillain-Barre syndrome, hypertension, migraine and trauma.^{1)3)4)6)7)8)20)22)24)}

3. Diagnosis
   The diagnosis of dissecting aneurysm requires a high index of suspicion. Many investigators have emphasized that cerebral dissecting aneurysms should be considered as the differential diagnosis in case of acute cerebrovasuclar accidents, especially when they occur to young patients.⁶⁾⁸⁾ Cerebral arterial dissections are diagnosed by the pathological and/or angiographic findings or autopsy.²⁾⁶⁾ Definitive diagnosis of intracranial dissecting aneurysm is occasionally difficult. No aneurysms is detected in 15 to 20% of spontaneous SAH patients angiographically.²⁰⁾ Angiographic features are "double lumen (false and true lumens)", "pearl and string signs", "string sign (tapered narrowing)", "retention of contrast material(pooling)" and "rosette signs",^{1)12)13)17)21)} but not always definitive since such signs are also seen in atherosclerotic vascular disease.⁶⁾ The only pattern that can be regraded reliably as a diagnostic sign is the "double lumen" which both true and false lumens are seen.¹⁾⁶⁾ The angiographical finding in our case was a smooth stenosis and post-stenotic dilatation with aneurysmal pouch of right A1. Dissecting aneurysm always should be considered even if the angiogram demonstrated no aneurysm. In our cases, the diagnosis of dissecting aneurysm was not definitive angiographically, and confirmed as dissecting aneurysm intraoperatively. Therefore surgical inspection is the only method to confirm the dissecting aneurysm in some cases including ours.¹²⁾¹⁹⁾

4. Treatment
   It is controversial about the necessity of surgical intervention because the natural history of intracranial dissecting aneurysm without ischemia and/or hemorrhage is still unclear.⁵⁾¹⁷⁾ With this regard, surgery or conservative treatment for dissecting aneurysm of intracranial arteries should be selected carefully whether it is ischemic or hemorrhagic.¹⁴⁾²⁰⁾ If the initial symptom was SAH, surgical inspection and treatment such as trapping, wrapping or proximal ligation with or without extracranial to intracranial bypass should be considered even if the angiography does not demonstrate any abnormal finding.¹⁾⁵⁾ Some authors reported that the dissecting aneurysm of the right A1 was confirmed by microsurgery, and the A1 segment was trapped to prevent rebleeding.⁷⁾ On the contrary, in the case of ischemia, serial angiographic observation should be checked with the administration of anti-coagulant or anti-platelet agent.^6)17)18)
   In our first case, we chose wrapping surgery for the patient with SAH, because angiography showed poor collateral flow through the AcomA. Generally speaking, dissecting aneurysms of the cerebral circulation have poor outcome, because primary clip reconstruction with preservation of the involved blood vessels and perforators is very difficult.¹⁰⁾ However, we selected wrapping surgery with trousers shaped artificial dura for preventing injury of perforators. The outcome of surgery was satisfied.
   In our second case, after 3D CT was checked, trapping surgery was done successfully because of sufficient collateral flow through the AcomA, which was confirmed by intraoperative TCD. The patency of AcomA could be only confirmed by 4 vessel angiography, but it could be replaced by 3D CT and intraoperative TCD when the diagnostic angiography was not available.

Conclusion

   On the basis of our cases and review of the literature, it is recommended that dissecting aneurysm should be considered in diagnosis when young individuals develop cerebrovascular accident and if the initial symptom was SAH, surgical inspection and treatment should be considered. The determination which operation technique will be done should be based on the patency of AcomA and the laterality of A1 dominance which were shown by 3D CT or cerebral angiography. Therefore, it is estimated that the proper diagnostic tools and surgical skills avoiding the injury of perforators may give rise to successful surgical outcome.

REFERENCES

1. Amagasa M, Sato S, Otabe K. Posttraumatic dissecting aneurysm of the anterior cerebral artery: Case report. J neurosurg 23:221-5, 1988

2. Araki T, Ouchi M, Ikeda Y. [A case of anterior cerebral artery dissecting aneurysm]. No Shinkei Geka 24:87-91, 1996(Jpn, with Eng. abstract)

3. Gherardi J, Lee HY. Localized dissecting hemorrhage and arteritis: Renal and cerebral manifestations. JAMA 199:187-8, 1967

4. Guridi J, Gallego J, Monzon F, Aguilera F. Intracerebral hemorrhage caused by transmural dissection of the anterior cerebral artery. Stroke 24:1400-2, 1993

5. Hasegawa S, Manabe H, Takemura A, Nagahata M. Surgical treatment for dissecting aneurysm of the anterior cerebral artery presenting only with headache: A case report. No Shinkei Geka 56:789-93, 2004 (Jpn. with Eng. abstract)

6. Hashimoto H, Lida J, Shin Y, Hironaka Y, Sasaki T. Subarachnoid hemorrhage from intracerebral dissecting aneurysm of the anterior circulation: Two case reports. Neurol Med Chir(Tokyo) 39:442-6, 1999

7. Hatayama K, Karasawa H, Naito H, Hirota N, Sugiyama K, Ueno J, et al. Anterior cerebral artery dissecting aneurysm associated with Fibromuscular dysplasia(FMD): A case report. No Shinkei Geka 29:451-6, 2001(Jpn. with Eng. abstract)

8. Hayashi N, Fukuda O, Endo S, Takaku A. Intracerebral hemorrhage secondary to dissecting aneurysm of the anterior cerebral artery. No To Shinkei 48:1053-6, 1996(Jpn. with Eng. abstract)

9. Hirao J, Okamoto H, Watanabe T, Asano S, Teraoka A. Dissecting aneurysms at the A1 segment of the anterior cerebral artery: Two case reports. Neurol Med Chir(Tokyo) 41:271-8, 2001

10. Hino A, Fujimoto M, Iwamoto Y, Oka H, Echigo T. Surgery of proximal anterior cerebral artery aneurysms: Neurosurgical techniques. Acta Neurochir 144:1291-6, 2002

11. Ishikawa R, Sunagawa S, Itoh I, Iwashita K. An experience of dissecting cerebral aneurysm of the anterior cerebral artery. No Shinkei Geka 21:355-9, 1993

12. Kidooka M, Okada T, Sonobe M, Nakazawa T, Handa J. Dissecting aneurysm of the anterior cerebral artery: