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]. The choroid plexus in such denuded of ependyma focall y [11 , 12]. The hippocampal gyru s is cases is displaced anterolaterall y with th e displaced lateral edge of attenuated to obliteration. The pia remains intact. fornix. The continuity of th e medial atrial wall is disru pted at the Infreq uentl y, th e attenuated alvear-crural sheet dehisces or dehiscences. The norm al posteromedial convexity of the medial shears away from th e splenium creati ng unilateral or bil ateral diver­ edge of the c rus is now concave where it arcs around th e herni a ticulu m ostia 5- 20 mm in diameter (fig. 2A) [8, 10]. Th ese ostia are ostium . bordered by the thinned splenium posteromedially and the dis- After th e ostium forms, CSF pushes through the remn ant mantle 260 NAIDICH ET AL. AJNR:3, May/ June 1982

E F G

Fig. 4. -Unilateral atrial diverticulu m, 2-month-old girl with congenital (arrowheads) believed to repre sent internal cerebral veins. E-G, Direct hydrocephalu s. A, Noncontrast scan. Hydrocephalu s with larger left lateral coronal MCTV from anterior to posterior. E, Anteriorly, diverticulum indented venlricle. Third ve ntricle (arrowhead) li es to right of midline. Large, round, by intern al cerebral ve in (arrowhead) . Medial part of diverticulum drapes over midli ne CSF-density atrial diverticulu m (arrows) was initially mistaken for internal cerebral vein into cistern of velum interpositum and displaces third dilated fourth ventricle. B, Higher contrast-enhanced scan. Asymmetricall y ventricle (3) inferiorl y and rightward . F, At hernia ostium. Wedgelike divertic­ shorter lenlorial band (arrowhead) on side of larger ventricle. Right atrial wall ulum arises from the left medial atrial wall , extends inferomedially into (arrow) intact. Left atrial wall not discernible. Continuity of CSF density posterior fossa, and displaces fourth ventricle (arrow) inferiorly (cf. 2C). Soft­ belween dilated left atrium and midline CSF-density cyst. C and D, Axial tissue density of tentorium and cerebral mantle (arrowhead) seen on contra­ MCTV scans displayed from below upward. Larg e opacified posterior fossa lateral side is absent at level of ostium of diverticulum. G, Behind ostium " cyst" (long arrow), rightward displacement of third ventricle (3), intact right diverticulum. Dilated atria, large midline, opacified space separated from atri a alrial wall (short arrow), dehiscenl left atrial wall , and conlinuity of opacified by fa lx and tentorium (arrowheads), and inferiorly displaced fourth ventricle CSF from dilated left atrium into midline cyst. Two linea r filling defects (arrow).

and bulges th e pia inferomediall y to form a pulsion diverticulum of displaces the vein of Galen and occipital lobes anterosuperiorly (fig. the inferomed ial atrial wall. The diverticulum protrudes into the 28) [8,10]. subarachnoid space, comes into contact with the arachnoid mater, Gas ventriculograms in such patients document that these diver­ and displaces the arachnoid before it as it grows [8-10]. The pi a ticula may be unilateral or bilateral and symmetrical or asymmetrical remains intact, preventing communication between th e ventricle (fig. 2C) [8]. They usuall y communicate with a single lateral ventricle and the subarachnoid space. With progressive enl argemen t, the but may rarely communicate with both lateral ventricles [13]. His­ pial diverticulum and displaced arachnoid surroundin g it bulge tologic examination of the cyst wall shows pia mater with tags of downward through the incisura alongside and behind the midbrain white matter adherent to it [10]. Ependymal lining cell s have not to form an incisural and subtentorial cyst within the ambient and been recognized [8, 10, 14]. Occasionally, the membrane appears superior vermian cistern s and the precentral cerebell ar fissure (fig . hemorrhagic [14]. In the single sample analyzed, the cyst fluid was 28) [8-12]. Such cysts range in size from 2 x 2 x 2 cm up to 7 similar to CSF but with substantiall y higher protein content than the X 5 X cm [1 0]. The cyst typicall y displaces the quadrigeminal plate lateral ventricle (1 38 mg / dl versus 20 mg / dl), implying partial and pineal gland anteroinferiorly, displaces the fourth ven tricle loculation [15]. directl y inferiorly, displaces the anterior vermis posteroinferiorly, Appreciation of the appearance of atrial diverticula on contrast­ displaces the straight sinus and tentorium posterosuperiorly, and enh anced CT req uires understanding of three additional anatomic AJNR:3, May/ June 1982 DIVERTICULATION OF MEDIAL ATRIAL WALL 261

F Fig. 5 .- Unilateral rig ht atrial diverticulum in 5-day-old boy with severe congenital hydrocephalus. A-C, Contiguous contrast-enhanced axial scans at age 1 day. A, Lowest section. Marked hydroceph alus with large infraten­ torial CSF-density space. Tentorial leaves and mantles appea r intact. B and C, Next hi gher sec tions. Asymm etrical shortening of tentorial bands on right (arrowhead), dehiscence of right atrial wall , rightward displacement of ri ght choroid plexus (long white arrow), leftward displacement of two intern al cerebral ve in s (short white arrows) with increased se parati on between cho­ roid pl exus and intern al cerebral veins, and continuity of CSF density between dil ated right atrium and incisural " cyst. ·· D and E, Serial direct coronal contrast-enhanced scans from anteri or to posteri or and vi ewed from behind. D, At posteri or third ve ntricle (black arrow) , right choroid plexus (white arrow) is displ aced laterall y and separated from ipsilateral internal cerebral ve in . E, Behind third ventricle. Dehi scence of rig ht cerebral mantl e and tent orial blush (arrowhead) and continuity of CSF density between atrium and infratentorial " cyst. " Dehi scence is ostiu m of dive rticulum. F, Noncontrast axial scan at age 2 months, after placement of sing le shunt catheter into right lateral ventricle. Decreased size of lateral ventricles and small , residual outpouching D E of ri ght atrial wall (arrowhead) which disappeared on follow-up scans. Pos­ terior fossa cyst no longer detected.

relationships: 3. On contrast-enhanced CT, the anterior end s of th e tentorial 1. Figure 3 depicts the relation among the med ial atrial wall , the bands mark the free marg ins of th e tentorium at the leve l of th e choroid plexus, the internal cerebral vein s, and the vein of Galen in secti on [ 18). The gap between th e an terior ends of the tentorial an anatomic specimen (fig. 3 A) and a contrast-enh anced CT scan bands, therefore, marks the exact site of the in cisura at the level of (fi g. 3B) [16]. Unilateral atrial diverticula that extend downward into the secti on. Lesions si tuated lateral to the tentori al band s or an terior th e quadrigeminal pl ate cistern will displace th e intern al cerebral to th e anteri or ends of the tentorial bands are supratentorial. Lesions ve in s and the vein of Galen to the opposite side. Simultaneously, situated between the ten torial bands are infratentori al and / or inci­ th ey will displ ace th e c rus of fornix and the accompanying choroid sural. Th erefore, on contrast-enhanced scans, any lesion th at plexus ipsil aterally, widening the distance between the choroid drapes over the anterior end of the tentori al band on CT is crossing plexus and th e intern al cerebral vein s on the side of the diverti culum. through the in cisura in vivo [18]. Bilateral atrial diverticula will squeeze the two internal cerebral ve in s between them and displace the veins away from the larger diverticulum. Infrequently, medial extension to the cistern of the velum interpositum will cause th e divertic ulum to drape over the Results ipsil ateral intern al cerebral ve in (figs. 3A, 40, and 4E). Rarely, atrial Revi ew of th e numerous se ri al CT scans of each of the 60 distension may displace the ipsil ateral choroid pl exus asymmetri­ pati ents with extreme hydrocephalu s disclosed 15 (25%) call y closer to th e midline despite the presence of a diverticulum with CT signs of medial atrial dive rti culum (figs. 4- 8 ). Th ese ostium. signs in clude: (1) marked unilateral or bil ateral atrial dilata­ 2. Contrast-enhanced CT sections through the upper tentorium near the apex of incisura depict the left and right tentorial leaves as ti on; (2) focal dehi scence of th e medial atrial wall ; (3) char­ a V-shaped blush [17]. In properly positioned, norm al patients, the acteri sti c ipsil ateral shortening of the tentorial band in axial length and position of the two sides of the V are very symm etrical. section CT; (4) focal defect in the tentorial band in coronal Severe, asymmetric hydrocephalu s in utero would be expected to section CT; (5) draping of th e medial atri al wall over the free cause greater migration and lower insertion of th e tentorium on the margin of tentorium , with continuity of CSF densi ty around side of the larg er ventricle. Th e consequent tentori al asymmetry th e edge of tentorium in axial and / or coronal section CT; would cause corresponding asymmetry of the V blush with a shorter (6) separati on of forni x from spl enium , with vi suali zation of tentorial blush on the side of the larger ventricle and lower tentorial th e herni a ostium; (7) anterolateral bowin g of th e cru s (or leaf. Conceivably, pressure defl ection or pressure attenu ati on of crura) of forni x around the herni a ostium; (8) asymmetri cal th e tentorial edge could also cause an asymmetricall y shorter, position of the choroid pl exi th at attach to and define the ipsil ateral tentori al blush. Severe bilateral hydrocephalu s in utero would cause unusuall y low in sertion of th e tentorium with bilaterall y lateral borders of the fornices; (9) contralateral displace­ short and stubby arm s of th e V. ment of the intern al cerebral veins; and (10) septa separat- 262 NAIOICH ET AL. AJNR :3, May/ June 198 2

A B D

F G Fi9 · 6. -Combined suprapineal recess and lateral ventricul ar diverti culum, 1 O-year-old girl. A, Axial contrast-enhanced scan. Dilatation of third and lateral ventricles; midline incisural CSF­ density "cyst" initially interpreted as dilated suprapinea l recess. Small soft-tissue density (arrows) crosses thi s space. B, Next higher level. Leftward displacement of both internal cerebral veins (arrow) and rightward displacement of right choroid plexus (arrowheads ). Interval between left choroid plexus and left internal cerebral vein marks width and position of left fornix. Greater interval on right indicates anterol ateral displ acement of right cru s of fornix. Right fornix seen faintly just medial to the ipsilateral choroid plexus. Concave posteromedial border of right crus defin es margin of diverticulum ostium . Continuity of CSF density through this ostium into incisural cyst (cf. 2A for shape and position of right cru s and ostium). Reconstructed coronal (C) and sagittal (D) noncontrast scans confirm wedge-shaped diverticulum (arrowheads) of inferomedial wall of right atrium and its in fratentorial extension. E, Lateral brow-up metrizamide ventriculogram. Small, densely opacified suprapinea l recess (arrow) and broader, less well-defined right atrial diverticulum (arrowhead). Gravitational layering of hyperbaric contrast prevents complete vi sualization of ventricles and diverticulum. F and G, Seri al 5 mm high-resolution axial scans during same metrizamide ventricul ogram (displayed from bottom upward). Small outpouching of right atrial wall H (arrowhead) leads directl y inferiorly along right side of dense r, displaced suprapineal recess (arrow) into diverticulum within quadrigeminal pl ate cistern . This collection flattened collicular pl ate. H, Computer-reformatted coronal MCTV, viewed from behind. Intact left atrial wall, displaced suprapineal recess (arrow), dehi scent right inferior medial atrial wall , and wedge-shaped ri ght atrial diverticulum (arrhowhead).

ing diverticulum from third ventricle. These signs are all The changes described do not reflect asymmetry of atrial present in each case, but must be sought specifical ly by size alone (fig , 9), Simple asymmetry may be associated closely spaced axial and direct coronal images through the with some dural deformities and midline shift but will not atrium and incisura, whenever the diverticulum is small. cause discontinuity of the atrial wall, draping of CSF density In difficult cases, MCTV and MCTC may be used to over the anterior end of the tentorial band, or wedgelike docum ent direct continuity between th e lateral ventricle and inferior extensions of CSF on coronal scans. the incisural cyst (figs. 4 and 6) [19]. The wedge-shaped In the group of 15 patients with ventricu lar diverticula, the density that arises at the inferomedial aspect of th e atrium diverticula were bil ateral in four and unilateral in 11. They and extends downward and medially into the posterior fossa were large in five cases and small in the rest. In almost is characteristic of the lateral ventricular diverticulum (fig. every case, the diverticula regressed completely after shunt­ 4), Once appreciated on MCTV, it can be recognized as a ing the ipsilateral lateral ventricle relieved the patient's wedge-shaped CSF space on direct or reformatted coronal hydrocephalus. OccaSionally, a small outpouching of the im ages obtain ed without metrizamide (figs, 6C and 60). medial atrial wall indicated the site of a once-large " cyst" AJNR:3, May/ June 1982 DIVERTICULATION OF MEDIAL ATRIAL WALL 263

Fig. 9 .-Markedly asymmetric atrial dil atation wi th n, o diverticulum . Absence of any CT signs described for diag nos!s of atrial diverticulum indicates that signs are specific for diverticulum rath er than simple atrial asymmetry.

A B

Fig . 7 .-Bilateral atrial diverticula. Contrast-enhanced axial scans in 15- yea r-old boy. Stubby and asymmetrical tentorial bands. Medial atrial walls drape over free edge of tentorium bilaterally. Pi al investments compressed to soft -ti ssue septum in midline.

A B c

Fig. 8. -Atrial diverticulum invaginates into Dandy-Walker cyst in 2-year­ arrowhead), dehiscent medial wall (short arrows), and leftward displacement old boy with Dandy-Walker malformation initially shunted directl y into cyst. of third ventricle (long arrow). Pial sac (white arrowheads) deli mits atrial Poor drainage of lateral ventricles caused dilatation of right atrium and dive rticulum, whi ch in vag inates in to, but remains isolated from, Dandy-Walker fo rm ation of right atrial diverticulum, which descended into posteri or fossa, cyst. Pn eumography confi rmed CT interpretation. C, Diverticulum (arrow­ necessitating shunting of right lateral ventricle. A and B, Serial noncontrast heads) persisted after shunt decompression of dilated right atrium. (Case scans from below upward . Posterior fossa shunt, dilated right atrium (black courtesy of Douglas Yock, Metropolitan Medical Center, Minneapolis, Minn.)

(fig. 5F). Rare diverticula persist after supposedly success­ and other tumors [8, 9, 12, 13, 20]. Granular ependymitis fu l shunt decompression of the lateral ventricle (fig . 8C). [14], aqueductal gliosis [10, 12], and external obstructi ve Because MCTV could not be justified in many of these hydrocephalus [10, 15] were considered infrequent causes cases, the accuracy of these diagnoses and, therefore, the of atrial diverticula. true incidence of atrial diverticulum remain presumptive. Every effort was made to count only " clear-cut" examples of diverticulum, so even the surprisingly high figure of 25% Differential Diagnosis is believed to be a conservative estimate of the incidence of The atri al diverti culum has been confused with: (1 ) the such diverticula in patients with massive hydrocephalus. dilated fourth ventri cle associated with outl et fou rth ventric­ In this series, no patient with atrial diverticulum was found ul ar obstructi on or isolated fourth ve ntricle [21 , 22], (2) the to have an obstructing neoplasm, although early reports of dilated suprapineal recess of an aqued uctal stenosis, and atrial diverticula emphasized the association of such diver­ (3) the primary arachnoid cyst of th e incisura: ti cula with hydrocephalus caused by brain stem gliomas, The midline incisural " cyst" caused by upward bul ging of hypothalamic gliomas, third ventricular epidermoidomas, a dil ated fourth ventricle, or Dandy-Walker cyst, may be 264 NAIDICH ET AL. AJNR:3. May/ June 1982

slightly prominent suprapineal recess, each component may be recognized independently (fig. 6). Primary arachnoid cysts of the incisura may be distin­ guished by the intact walls separating them from the sur­ rounding atria, third ventricle, and fourth ventricle. MCTV, MCTC, and/or air CT cisternography will show no direct, rapid communication of the cyst with the ventricles or cis­ terns. However, the cyst density may increase moderately over 8-1 2 hr from slow passage of metrizamide across the cyst wall [6, 23]. To our knowledge, complete loculation of an atrial diverticulum , a potential diagnostic pitfall, has not been reported as yet. Unusual arachnoid cysts of the posterior temporal fossa may extend infratentorially and mimic many of the signs of the atrial diverticulum (fig. 12). However, detection of the intact medial atrial wall and recognition that the lateral ventricles are separate, compressed, and displaced will usually indicate the correct diagnosis. Cyst puncture with metrizamide CT cystography will then confirm that the lesion is isolated from the ventricle and cisterns. Metrizamide CT cisternography may also confirm the diagnosis [19]. Cystic neoplasms of the deep structures, for example , cystic thalamic astrocytoma, may also mimic the appear­ ance of the lateral atrial diverticulum. Detection of the tumor c D mass distinguishes these lesions from the atrial diverticulum in most cases. Fi9 . 10 .-Larg e fourth ven tric te in newborn boy with obstructi on at exit foram ina of fourth ventric te. A and B. Seriat axiat contrast-enhanced scans. Tentoriat bands. atrial wall, and choroid plexi intact and symm etrical bilat­ Discussion erally. Internal cerebral vein s (not shown) were not displaced. C and D, Serial direct coronal contrast-en hanced scans. Intact atrial walls, intact tentorial Pulsion diverticula of the atria of the lateral ventricles bands. and symmetrical choroid plexi. were first reported by Penfield [13] in 1929, although Turn­ bull recognized the essential nature of the condition in 1923 (cited in [11, 12]). The anatomic derangements responsible recognized by the demonstration of symmetry and integ rity for the diverticulum were described by Sweet [9] (case 2) in of the tentori al bands and atrial walls, symmetrical choroid 1940. This description was amplified and correlated with plexi and internal cerebral veins, and lack of direct continuity the pneumographic appearance of th ese lesions in 1942 in between cyst and atrium (fig . 10). MCTV will confirm ab­ two independent reports by Childe and McNaughton [8] and sence of direct communication between "cyst" and lateral by Dyke [20]. Pennybacker and Russell [12] (1943) de­ ventricle. If th e fourth ventricle or Dandy-Walker cyst com­ scribed the first surgical approach to these lesions, and municates normally through th e aqueduct of Sylvius with MacFarlane and Falconer [10] (1947) described the first the lateral and third ventricles, the posterior fossa " cyst" successful treatment of hydrocephalus by resection of the will usually persist after shunting, but will become reduced pial and arachnoidal wall of the diverticulum to establish in size and more nearly normal in shape. If the fourth free communication between the atrium and the subarach­ ventricle does not communicate with the lateral and third noid space at the cisterna ambiens. Additional reports by ventricles, it will persist unchanged or bu lge further upward Perryman and Pendergrass [24] and others have been sum­ after shunting of th e lateral ventricles. Such isolated fourth marized by Kruyff [25]. ventricles may be difficult to distinguish from primary arach­ To our knowledge, the first angiographic description of noid cysts. A round or pearl ike shape, absence of a sepa­ these diverticula was by Alonso et al. [26] in 1979. Naidich rate, inferiorly displaced fourth ventricle, and extension of et al. [17] (1977) published the first CT im age of a lateral th e " cyst" into the region normally occupied by the fourth ventricular diverticulum. ventricle favor a diagnosis of a large fourth ventricle [21, The nomenclature of this lesion has been extremely con­ 22]. fusing. The same entity has been termed arachnoid cyst of The midline dilated suprapineal recess may be identified the cisterna ambiens (Noetzel, cited in [24]), spontaneous accurately by demonstration of continuity of CSF-density ventriculostomy [26], cerebral ventriculostium [9], sponta­ between the diverti culum and third ventricle; integrity and neous ventricular rupture [12, 14], and ventricular divertic­ symmetry of the medial atrial wall s with distinct septa sep­ ulum [8, 15, 20, 26]. In part, this confusion represented the arating atria from the midline recess; and symmetrical cho­ difficulty of distinguishing diverticulum formation from true roid plexi (fig. 11). Symmetrical internal cerebral veins may rupture by pneumography and oily contrast ventriculogra­ also be seen. When an atrial diverticulum coexists with a phy. Since the relations of tissue planes and spaces may AJNR:3. May/ June 1982 DIVERTICULATION OF MEDIAL ATRIAL WALL 265

A B c D Fig . 11.-lnfratentorial herni ation of suprapineal recess. 44-year-old plexi are symmetrical. D, Melrizamide ventriculogram. Continuity of opacified woman. A- C, Noncontrast scans. Superior vermian and Quadrigeminal plate CSF from third ventricle to incisural "cyst. " (Case courtesy of Norman Leeds cistern s occupied by CSF-density "cyst " directly continuous with third ven­ and Robert D. Zimmerman. Montefiore Hospital and Medical Center. Bronx. tricle and well separated from mildly dilated atria. Atrial walls and choroid N.Y.)

A B c D Fig. 12.-Posterior temporal arachnoid cyst with incisural extension, 4- Direct coronal noncontrast scan, viewed from behind. Compressiol, and year-old boy. A and B. Axial non contrast scans. Ovoid posterior temporal elevation of right atrium (long arrow) by large posterior temporal and incisural CSF-density cyst (arrowheads) compresses right lateral ventri cle and dis­ cyst. Notch in cyst wall at free edge of tentorium (short arrow) . D, Axial places temporal horn (long arrows) anteriorly. Cyst is notched posteriorly metrizamide CT cystog ram corresponds to B. CSF-density space is cysl that (short white arrow) where it drapes over free edge of right tentorial leaf. C, communicates with neither ventricular system nor subarachnoid cistern s.

now be established by CT and MCTV, we urge that the pial­ tion bias in assembling the cases for analysis. However, we lined cystic space that is walled off from the subarachnoid believe, instead, that awareness of th e phenomenon has led space and that communicates only with the ventricle be to careful examination of the in cisura by serial 3 - 5 mm axial designated " ventricular diverticulum. " This designation contrast-enhanced CT sections and to frequent use of direct should be modified to indicate the ventricle of origin, for coronal contrast-enhanced CT in th ese patients with con­ example, lateral (or third) ventricular diverticulum. The term sequent demonstration of changes previously overlooked. ventriculocisternostomy should be used to designate free To summarize, lateral ventricular diverticula may be dis­ communication between the ventricle and the entire sub­ tinguished from diverticula of the third ventricle, the bal­ arachnoid space [27-29]. This term may also be modified looned fourth ventricle, and primary incisural arachnoid to indicate the ventricle of origin and whether such com­ cysts by new CT criteria based firmly on th e anatomic munication arose spontaneously or postoperatively. Rare derangements peculiar to each group of lesions. It is to be ventriculosubdural communications [11, 14], ventriculo­ expected, however, that strategically situated cysts and subgaleal communications [30], and external fistulae lucent solid masses may cause mass effects identical with [11 , 31 ] may be described by similar designations. those of the atri al diverticulum. Failure to appreciate the thin The surprisingly high in cidence of atrial diverticu la in intervening wall could then lead to possible misdiagnosis of patients with massive hydrocephalus could represent selec- an atrial diverticulu m on rare occasions. 266 NAIDICH ET AL. AJNR:3, May/ June 1982

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