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Atrial Diverticula in Severe Hydrocephalus

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Atrial Diverticula in Severe Hydrocephalus 257 Atrial Diverticula in Severe Hydrocephalus Thomas P. Naidich 1 Massive ventricular dilatation causes stretching and dehiscence of the fornix with David G. McLone2 formation of unilateral or bilateral pial pulsion diverticula of the inferior medial wall of Yoon S. Hahn2 the atrium. Enlargement of the pial pouch creates a dramatic subarachnoid cyst that Joseph Hanaway3 may herniate downward through the incisura into the lateral mesencephalic, precentral cerebellar, and superior vermian cisterns where it displaces the brain stem, vermis, and fourth ventricle. Lateral ventricular diverticula may be identified and distinguished from the dilated fourth ventricle and dilated supra pineal recess, with which they are so commonly confused, when all of the following signs are apparent on computed tomog­ raphy (CT): (1) marked unilateral or bilateral atrial dilatation; (2) focal dehiscence of the medial atrial wall; (3) ipsilateral shortening of the tentorial band in axial section; (4) focal defect in the tentorial band in coronal section; (5) draping of the medial atrial wall over the free margin of tentorium, with continuity of cerebrospinal fluid density around the edge of tentorium in axial and/ or coronal sections; (6) bowing of the crus (or crura) of fornix; (7) separation of fornix from splenium, with visualization of the hernia ostium; (8) asymmetrical position of the choroid plexi, which attach to and define the lateral borders of the fornices; (9) contralateral displacement of the internal cerebral veins; and (10) septa separating diverticulum from third ventricle. Proper surgical management of the patient with massive hydrocephalus and a midline, incisural, cerebrospinal fluid (CSF)-density "cyst" requires accurate preoperative identification of the nature of the " cyst" and its relation to the ventricular system. Primary arachnoid and ependymal cysts, which cause hydro­ cephalus, may be treated by extirpation or direct shunting of the cyst with consequent relief of hydrocephalus. Focal ventricular dilatations that result from hydrocephalus are treated most effectively by simple shunting of the lateral ventricles. Initial difficulty in differentiating among primary arachnoid cysts, Received June 26, 1981 ; accepted after revi­ enlargement of the suprapineal recess of the third ventricle, pulsion diverticulum sion January 25, 1982. of the medial atrial wall , and upward bulging of the dilated fourth ventricle led to Presented at the annual interim meeting of the Ameri can Association of Neurological Sciences, review of the relevant anatomy and pathology and elaboration of criteria for Section of Pediatric Neurological Surgery, New accurate computed tomographic (CT) diagnosis of the pulsion diverticulum of th e York, December 1980, and at the annual meetin g medial wall of the atrium. of the American Society of Neuroradiology, Chi­ cago, April 1981. , Department of Radiology, Children's Memo­ Materials and Methods rial Hospital and Northwestern University Medical School, Chicago, IL 60614. Address reprint re­ Serial CT scans of 300 pediatric and adult patients with ventricular dilatation were quests to T. P. Naidich, Children's Memorial Hos­ reviewed to select 60 patients satisfying the Meese et al. [1) criteria for extreme hydro­ pital, 2300 Children's Plaza, Chicago, IL 60614. cephalus. Patients believed to have atrophy on the basis of clinical or CT criteria were 2 Department of Neurological Surgery, Chil­ rigorously excluded. In 10 cases, where patient benefit justified the ri sk, CT diagnoses of dren's Memorial Hospital and Northwestern Uni­ atrial diverticula, arachnoid cyst, etc. were confirmed by metrizamide CT ventriculography versity Medical School, Chicago, IL 60614. (MCTV), metri zamide CT cisternography (MCTC), or surgical explorati on [2-6). 3 Department of Neurology, Incarnate Word Hospital, St. Louis, MO 63104. AJNR 3:257-266, May/ June 1982 Anatomic and Pathologic Basis for CT Signs 0195-6108/ 82/ 0303-0257 $00.00 © American Roentgen Ray Society The medial wall of atrium is formed by the splen ium above and behind, th e symmetric 258 NAIDICH ET AL. AJ NR :3 , May I Jun e 1982 A 8 c Fig. 1 .-Coronal sec ti ons. A, Through atri a of normal brain . Medial atrial now overh ang post eri or third ven tri cle and incisura. Lateral edg es of fornices wa ll fo rm ed by sp lenium of corpus ca llos um (S) above, sy mm etrical crura of and associated choroid pl exi (arrowhead) disp laced anterolaterall y. Straight lorn ices (C) inferi orly, and int erve nin g alve i (A) of hi ppocampus. Cho roid plex i sinus (arrow) above aqu educt. C, Through at ri a of seve rely hydrocephalic (arrowhead) enter vent ri cle along lateral edges of forni x and mark it s laleral brain . Sp lenium (S) is very th inned. Extremely attenuated alvei and crura of edges. Distance between alve us (A) and choroid tuft in dica tes width of forn ix forn ices (F) form hollowed-out alveoc rural sheet that ove rhang s pineal gland and position of lateral edge. B, Through at ri a of moderately hydrocephali c (P) and corpora quadrigemina (CO) at incisu ra. 4V = fourth ve ntricle. (Re­ brain. Corpus call osum (S) thinned and eleva ted. Alvei st retched. Crura of printed from 18 ].) fornix (C), thinn ed, depressed, and displaced medially, are hollowed out and c rura of the fornices inferiorly, and th e alvei and fimbriae of hippo­ lateral edges of the thinned c rura are d isplaced laterally and ante­ campus th at connect th e med ial edges of the c rura to the under­ riorly. The c horoid plexi that mark the lateral edges of th e c rura are surface of splenium (fig. 1A) [7-10]. Ependyma li nes th e inner similarly displaced laterall y and anteriorly. The alvei are displaced aspect of th e atrial wall. Very th in hippocampal gyru s and pia mater mediall y and stretc hed between th e elevated splen ium and the line th e outer aspect of the atrial wall. The c horoid plexi li e sym­ depressed crura. metricall y along th e lateral edges of the fornices where the tela At this stage, th e medial wall of atrium has been d isplaced med ial c horoidea emerges from th e c horoidal fi ssure. The positions of the to the free edge of the tentorium and now overhangs the in c isura c horoid plexi are reli abl e landmarks for th e lateral edges of the w ithout support [8]. The med ial atrial wall is intact and can be fornices. traced , in continuity, from th e midline alvei along the thinned, With progressive hydrocephalu s and progressive expansion of inferiorly convex c rura to th e c horoid plexi at the lateral edges of th e atria, th e splenium is thinned, elevated, and displaced poste­ th e crura (fig. 1 B). riorly (fig. 1 B) [8-11]. Th e c rura of th e fornices are depressed and With greater atrial dilatation, the alveus and c rus of fornix become thinned . The medial edges of the c rura are displaced med iall y. The an extremely thinned alvear-c rural sheet th at may be focall y d impled AJNR:3, May/ June 1982 DIVERTICULATION OF MEDIAL ATRIAL WALL 259 A 8 c Fig . 2.- 7'i2-year-old girl with severe hydrocephalus and bilateral, asym­ INF = inferior horn of ventricle, T = tumor. B, Lateral view of left half of brain metrical atri al diverticula. Hydrocephalus secondary to cystic polar spongio­ sectioned through midline. Probe (arrowheads) passes from left lateral ven­ blastoma of hypothalamus and anterior third ventricle. A, Axial section tricle (LV) through ostium (seen in A) into precentral cerebellar fissure cyst through severely dilated lateral ventricles. Bilateral diverticulum ostia (long (arrow). Infratentorial part displaces cerebellum (CER) posteroinferiorly, arrows). Ostia are delimited posteromedially by thinned, elevated, posteriorly fourth ventricle (4V) directly inferiorl y, and corpora quadrigemina (CO) anter­ displaced splenium (S) and delimited anterolaterally by displaced crura of oinferiorl y. Splenium displaced superi orl y. T = tumor, C = cystic component fornix (F). Choroid plexi (CP, arrowheads) are displaced anterolaterally with of tumor, 3V = third ventricle. C, Di ag rams of gas ventriculog ram illustrate lateral edges of fornix, but sti ll mark position of fornix. Posteromedial borders cross-hatched area of atrial diverticula and sites of origin from dilaled lateral of crura are now concave (short arrow), where they arc around hernia ostium. ventricles. (A and B reprinted from [8].) Fig . 3. -Relati ons among crura, choroid plexi, and intern al cerebral veins. A, Coronal section through atri a fr om behind, aft er partial resection of spl enium (S). On right side, choroid plexus (large white arrowhead) parlly overlaps crus (C) of right forni x. Resection of left choroid plexus exposes fu ll crus (C) and origin of choroid plexus from tela choroidea at lateral edge of forni x (black arrowheads). Two intern al cerebral ve in s (short white arrows) join to form vein of Galen (small white arrowhead) in midline. Unilateral atrial diverticulum that extends in fratentorially would follow course of long white arrow to displace choroid plexus laterall y and intern al cerebral veins contralaterally, widening gap between them. Dashed sidearm of long white arrow indicates infreq uent medial extension of diverticulum over intern al cerebral vei n into cistern of ve lum interpositum. B, Contrast-enhanced axial CT in hydrocephali c patient without pulsion diverticulum. Two internal cerebral veins (white arrows), vein of Galen (black arrow), and symmetri cal glomera and bodies of choroid plexi (arrowheads). Di stance between glomera and internal cerebral ve in s estimates sizes and positions of crura of forn ices. or markedly translucent (fig . 1 C) [8, 1 1, 12]. This sheet rn ay be placed crura an terolaterall y [8-10].
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