3．脳動脈瘤：Anterior Circulation─Pterional Approach

Operative Neurosurgery: Personal View and Historical Backgrounds (3) Anterior Circulation - Pterional Approach

はじめに

　もう25星霜が経過したが，1982年にanterior communicating artery（AcomA）aneurysmをpterional approachで根治手術をすることに関して，当時のstatusを日本脳神経外科コングレスの『Neurosurgeons』に発表する機会を得た．その後も，今もなお同じようなことを行っている．本稿はそれ以来のlearning process Lernprocessを示すものである．整理のために当職就任1993年以来の1,100例あまりの動脈瘤自執刀例をExcelにまとめているが，個々の症例を思い出すと，手術の仕方，管理の仕方は当時と比べてあまり変わっていないと言えなくもないが，やはり各ステップでの自分で納得した手技が残ってきたとも言える．

　近年，endovascular techniqueの勃興によりconventional microsurgical techniqueの活用の場が狭まってきているのは確かではあるが，依然として必要不可欠なtechniqueであることも事実である．こちらに着任して間もなく，それを象徴するような出来事があった．AcomA aneurysmの患者さんで，Prof. Yasargilからinterventional neuroradiologistのProf. Valavanisに紹介されcoilingがなされ，当科にV-P shuntだけのために入院してきた症例があった．しかし治療としてのcompletenessの立場から言えば，neck clippingを今の段階で去ることはできないし，endovascular method で解決できない場合にもどうしても必要なstandardなtechniqueである．私がいつもstaffに言うことは，

　1）一般的に言って，convemntional microsurgical methodで難しい手術はendovascular methodでも難しい．これらの技術は多くは互いに相補うのではなくて，むしろ競合するものであるが，その競合が相互技術の発展につながる．

　2）ある技術を完成に近づけることができたと考え始めると，必ずそれに対抗する良い技術が出現してくる．脳神経外科関連の分野で言えばmicrosurgery，Gamma-knife，endovascular surgery などがそれぞれ発展の歴史をもち，他の技術に対抗しながら，個々の中でも競争しながら洗練されてゆくものである．

　日本の実情にそぐわないかもしれないがconventional microsurgical method の発展の中に身をおいてきたものとして，本稿では，conventionalか endovascularかの議論には深入りしない．当科に13年前に赴任して，それまで行われていなかったくも膜下出血急性期重症例の治療を始めた．幸い，病態生理の解明の進展に対応してその治療にふさわしいneuroresuscitationを標榜した脳神経外科集中治療室を，スタッフを含めて整備することができたように思う^1,12)．今回は，ごくスタンダードなpterional approachとanteriorcirculation aneurysmsについて私見とその歴史的背景を述べてみたい．

　Under the title of anterior circulation aneurysms and the pterional approach, followings are presented and emphasized along with mentioning their historical development in our present performance status.

　Pterional approach: head positioning with Mayfield-Kees fixation apparatus using one pin around the processus matoideus and the other 2 pins on the contralateral side behind the hair line the head turned 30 degrees and reclined chin-up15-20 grade. Skin incision beginning just in front of the tragus ending up at the midline hair line in a curvilinear fashion always including the superficial temporal artery STA in its frontal branch and the facial nerve (frontal branch) in the skin flap. A muscle fascial preparation is so fashioned such that a strip of myofascial cuff is left at the linea temporalis and the temporal musculature is reflected and retracted towards the postero-basal direction in order to expose the pterion, for which a small short myofascial incision is added parallel to the skin incision towards the tragus up to several cm above it to prevent postoperative trismus. Bone flap is sawed out usually using three burr holes, at the key hole just at the proximal part of the linea temporalis, frontomedially on the squama frontalis and on the sutura squamosa, so that the Sylvian fissure and the superior temporal gyrus are exposed enough for further procedure. The sphenoid ridge is drilled away until the most lateral corner of the superior orbital fissure comes into view. Thus one has drilled away enough to do surgery in question at the skull base even at the time of “angry brain” due to subarachnoid hemorrhage SAH. This procedure can be followed by selective extradural anterior clinoidectomy SEAC in case of necessity. The dura is opened in a curvilinear fashion so that the dura can be reflected over the drilled sphenoid wing and so that the Sylvian fissure and the superior temporal gyrus is exposed for the treatment of aneurysms of the internal carotid artery ICA, of the anterior communicating artery AcomA and the middle cerebral artery MCA. The dural incision is suitable also for doing other surgeries e.g. basilar bifurcation aneurysms and amygdalohippocampectomy AHE.

　Following points are emphasised in aneurysm surgery at the acute stage of SAH:

1) Artificial hypotension of the systolic pressure down to 100mmHg. 2) Use of temporary clipping (or trapping) method not only for the management of premature rupture but for complete dissection of aneurysms and for optimal neck clipping. 3) Opening of the lamina terminalis to gain slackness of “angry brain” and to insert an external ventricular drainage at the end of surgery for the intracranial pressure ICP registration and its regulation. 4) Importance of subpial approximation (“subpiale Annäherung”) at the time of aneurysm dissection to take advantage of the strength of the pia mater over (the arachnoid) and the dome of aneurysm. 5) Use of oxycellulose Tabotamp^® insertion technique between the aneurysm neck and neighbouring perforators to avoid inclusion of the latter (“Mitklippen”) at the time of neck clipping. 6) Double clipping method at the time of incomplete neck clipping due to athromatous plaque. Combination of fenestrated clip and standard clip or “puncture and collapse” method at the time of aneurysm incorporation of parent and /or branching arteries. 7) Use of microDoppler to confirm patency of the parent arteries and their branches.

　For doing surgery of aneurysms of main cerebral arteries following points are discussed:

1) AcomA aneurysms; a) Aneurysms located lower than 1.5cm above the level of the anterior clinoid process in the lateral view on digital subtraction angiography DSA can be managed with conventional pterional approach. b) Usually from the right side (non dominant hemisphere) but in the following situations from the left side: hematoma on the left side, concomitant unruptured aneurysms on the left side, left A1 dominancy at the acute stage of SAH and its dissection is considered to be difficult from the right side. c) Always rectal gyrus approach to the aneurysm with any direction. This also enables safety subpial approximation “subpiale Annaeherung” to aneurysms. d) No use of coagulation as much as possible.

2) ICA aneurysms; a) Performance of selective extradural anterior clinoidectomy SEAC for paraclinoid aneurysms not only for aneurysm dissection but for proximal control. b) Neither sacrifice nor Mitklippen of the posterior communicating artery at the time of ICA-PcomA aneurysms, not only for the prevention of the PCA infarction in case of its embryonal type but also for the prevention of tuberothalamic infarction due to compromise of the anterior thalamoperforating arteries originating from the PcomA. c) Dissecting aneurysms of the ICA: difficulties of diagnosis or differentiation from paraclinoid anurysms. Difficulties of optimal direct clipping and resort to trapping or proximal ligation plus bypass. Its predilection site is at the medial anterior wall of the C1 portion but may occur also in any other portion of the ICA.

3) Middle cerebral artery MCA aneurysms; a) For dissection, superior temporal gyrus ablation is not necessary but opening of the Sylvian fissure by retrograde tracing of a cortical artery on the surface of the temporal lobe. This cortical artery can be used as recipient when a bypass surgery turns out to be necessary with the use of a donor artery from the frontal branch of the STA. b) Importance of interpreting DSA or 3DCT angiography in regard to whether the aneurysm dome is outside (lateral) or inside (medial) of the MCA bifurcation. This helps in anticipating whether the aneurysm dome or MCA branches come into view first at the time of intraSylvian dissection. c) In case of presence of aneurysms at the contralateral MCA, accessibility from the ipsilateral side depends on the distance form the midline (for example early bifurcation), the relationship to the sphenoid wing (not below the level of the sphenoid wing) and no strong Sylvian vein darining into the sphenoparietal sinus being in the way.