颈静脉孔区的膜性解剖(下)
问题二:颈静脉球如何向颈内静脉过渡?
1、无法区分窦壁是骨膜层还是脑膜层来源,光镜下沿颅骨和沿脑表面分布的窦壁并无明显区别;
2、窦壁本身包含有动静脉血管(硬脑膜动静脉);
3、窦壁的腔内面和Chordae Willisii表面,都衬有一层内皮细胞(endothelial cells),即与颈内静脉的内皮细胞相延续;
4、Chordae Willisii的成分与各部位的窦壁无明显差别。
综上,颅内静脉窦的管壁只有内膜层是与外周静脉基本相符的,外膜层如果把硬膜壁算上的话勉强及格,但以平滑肌为主的中膜层是绝对缺如的。
图11:左,Balik文中后颅窝静脉窦壁的组织学切片;右,Liang文中冠状位(D)和矢状位(E)切片,箭头和三角所对处即为颈静脉球-颈内静脉过渡区域,但放大倍数无法显示膜性结构变化
1、颅内静脉窦的窦壁,大致分外膜层和内膜层;
2、外膜层即硬膜,但其具体来源,“贴着骨质的是骨膜层、贴着脑组织的是脑膜层”的说法只是推测或帮助理解的一种方式,实际上是难以辨别具体来源的,很可能是两种成分互相融合;
3、从颈静脉球到颈内静脉,外膜层从窦壁硬膜过渡到血管外膜,内膜层从窦壁内皮过渡到血管内皮,性质都没有实质改变,属于“顺延;
4、从颈静脉球到颈内静脉,增加了血管中膜层平滑肌,这是发生的实质性变化。
图12:致密纤维组织和颈动脉鞘,图I和B为Rhoton实验室早期理论,认为此层致密组织即为颈动脉鞘;E为Komune近期文献,证实颈动脉鞘来源复杂;黑白图摘自Hakuba早期文献。
“Within the carotid canal the adventitia and media of the internal carotid artery become gradually thinned by the loss of smooth muscle cells and collagen fibers”,“ abrupt changes in structure occur at the petrous curvature and the first knee of the carotid siphon”
问题三:颈静脉孔硬膜反折(jugular dural fold)到底指哪个?
再查其别称“plica occipitalis obliqua”的出处,出自1991年Lang的专著[25],对其的定义为“In about two thirds of the author's dissections, dura folds were found running from the posterior part of the foramen magnum, forwars and laterally in the direction of the jugular foramen”。原文没有配图,从命名“枕斜行皱襞”和描述来看,似乎范围和形态要比Rhoton定义的大得多。
图16:“Tubingen line”与下方的硬膜反折
综上,比较上述文字和图片,个人认为Lang和Silverstein定义的jugular dural fold与Rhoton等人定义的并不是同一个结构,前者范围更广,纵跨枕骨和岩骨,后者范围小得多,仅仅沿颈静脉孔神经部后上缘分布。
最后,在本节中看看内淋巴囊(endolymphatic sac)的膜性构成[27]。内淋巴液充满于膜迷路(membranous labyrinth)之中,后者的壁由一层上皮细胞围成。膜迷路的前庭发出内淋巴管(endolymphatic duct),又称前庭导水管(vestibular aqueduct),经岩骨后壁表面的内淋巴嵴(endolymphatic ridge)下方的骨性开口穿出,“包埋”于岩骨后壁骨膜层和脑膜层双层硬膜之间[28]。因此,内淋巴囊本身的上皮壁被“压扁”故呈双层膜性结构,整体又是个硬膜间结构,故在手术中若要贯穿此处膜性结构,都需经过四层:乙状窦前入路切开Trautmann三角的硬膜时[29],从外到内依次切开骨膜层-内淋巴囊外壁-内淋巴囊内壁-脑膜层;乙状窦后入路若要从硬膜内暴露此处岩骨骨质,需从内到外以相反顺序切开上述四层膜性结构。
图18:左,内淋巴囊的膜性构成,蓝色图片可见《上篇》中所说的延伸入耳蜗导水管内的脑膜层开口;中,内淋巴囊与岩骨后壁;右,内淋巴囊与乙状窦前入路
问题四:寰枕交界区的膜性层次和外科意义?
图19:Al-Mefty枕下海绵窦解剖,详见下述
1、通过外环,SCS向横突孔下方延续为包绕V3垂直段的静脉丛(venous plexus around the V3v,VAVP)。
2、POAM外侧部存在一个缺口(图19上左虚线圈),SCS由此通过吻合静脉(anastomotic vein)向浅层“漏”出,进入枕后肌群中层和深层(枕下三角肌肉)之间,这部分“漏”出去的硬膜间间隙包绕的静脉丛称为“枕下静脉丛”(suboccipital venous plexus,SVP),也通过其他肌肉间静脉吻合支与VAVP、VVP相沟通。
3、SCS和VAVP借一不完整的骨膜层分隔与深面的另一薄层静脉丛模糊分界,后者主要位于中线两旁,是颅内基底窦、枕窦、边缘窦的直接向下延伸,并向下贯穿整个椎管的硬膜间间隙,称为“椎静脉丛”(vertebral venous plexus,VVP)。上述不完整的骨膜层分隔,即图19中下组织切片的“m”,个人理解为来自枕骨大孔缘、C1后弓、C2后弓立体骨面之间不同深浅交错的骨膜层,类似于海绵窦内的骨膜层分隔或韧带,因此,VVP和SCS、VAVP终究是在同一大范围的硬膜间间隙内的。枕骨大孔下缘延续向下进入寰枕之间的脑膜层,因为背侧还覆盖了VVP和上述“部分性”骨膜层,故形成较厚的寰枕硬膜(dura)(图19左中图ms下的这部分硬膜,此图还展示了VA-V3段的四个袢loop),这也解释了上述“远环”的膜性构成(一层半)。另外,VA的远环与ICA的远环一样,融入附近的血管壁外膜层[31]。由此可见,Al-Mefty的VVP是狭义的概念,指紧贴椎管骨质内(internal VVP)和外(external VVP)的薄层硬膜间静脉间隙,是颅内静脉窦的向下延续,是颅内外静脉回流的重要“缓冲”途径[32],而非有些文献(包括下文Youssef文献)泛指的上述静脉丛的合称(错误)。2000年Parkinson[8]回顾了其历史,即“Batson's veins”,并将眼眶-海绵窦-斜坡基底静脉丛-枕窦-VVP这一完整空间称为“硬膜外神经轴间隙”(extradural neural axis compartment,ENAC),需注意,这里的extradural其实指的是硬膜的脑膜层外。回顾问题一(见《上篇》)中,我们也将颈静脉孔内的间隙划入ENAC中,但其并不位于中线区域。(图20)
图20:左,VVP示意图;中,VA远环融入血管壁外膜层;右,ENAC示意图
1、枕下肌群整个一层掀开,此步骤仅在中线处的C1后结节和C2棘突处需要锐性切开肌肉筋膜,随后可向外侧沿富含脂肪的“筋膜间/骨膜外间隙”剥离,避免使用单极以免打开SCS损伤静脉丛和VA。在这过程中,需要牺牲贯穿上述两层之间的肌肉静脉(muscular veins,即anastomotic vein)而不破坏硬膜间间隙背侧的骨膜层导致椎静脉丛出血。(图21 A-D)
2、随后,贴着C1后弓切开包绕C1背侧的骨膜,然后向外剥离至C1横突;贴着枕骨下端切开骨膜,然后向外剥离至枕髁。上述剥离犹如将枕骨和C1后弓“剜出”,因为骨质仍然是“硬膜外”结构,而此时依然没有进入硬膜间。此步骤还是常说的“骨膜下(subperiosteal)”分离技术,但必须注意,这里的“骨膜下”,并不是早早地广泛切开骨膜层而进入硬膜间,而是上述在枕骨下端和C1后弓处切开骨质外表面(背侧)的骨膜层,从骨膜下“剜出”骨质。
3、切开POAM,但又不破坏更深面的VVP间隙的背侧“部分性”骨膜层分隔,显露出寰枕区域的硬膜dura。向外剥离至V3段最内侧端,即VA进入“远环”处,此步骤实为进入了SCS和VVP之间的硬膜间间隙,刚好是静脉丛的“真空带”,故出血不多。接下来就是进入硬膜内了。(图21 G-H)
图21:筋膜间/骨膜外-骨膜下-硬膜间技术步骤,注意图中的VVP其实是泛指SCS+VVP+VAVP的硬膜间间隙。A-D约为步骤1;E-F展示切开部分SCS背侧界的骨膜层,进入SCS硬膜间间隙,暴露VA及周围静脉丛;G-H约为步骤3,切开POAM,显露较厚的寰枕硬膜(dura)
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