MR Demonstration of Ectopic Fourth Ventricular Choroid Plexus in Chiari II Malformation

J ames E. Stark 1 and Charles M . Glasier 1

Summary: An enhancing nodule was seen on Tl-weighted typical for Chiari II malformation, intraspinal neurenteric enhanced scans at the tip of the caudally displaced cerebellar and neuroectodermal cysts were surgically excised from vermis in three patients with Chiari II malformation. The en­ the cervical and thoracic spine. In the third patient, a hancing fibrovascular nodule found at the base of the cerebellum cervical syrinx was shunted. Posterior fossa exploration in patients with Chiari II malformation represents ectopic cho­ was not performed in any of these three patients. roid plexus and should not be confused with a pathologic mass.

Index terms: Chiari malformations; Choroid plexus, magnetic Results resonance; Choroid plexus, abnormalities and anomalies; Spina bifida; Pediatric neuroradiology All three patients showed enhancing nodular tissue at the tip of the caudally displaced cere­ The magnetic resonance (MR) appearance of bellar vermis on postcontrast Tl-weighted im­ the hindbrain deformity characteristic of the ages. This tissue was isointense to cerebellum on Chiari II malformation has been well documented the precontrast images (see Figs. 2 and 3). (1 ). The cerebellum, with transverse growth con­ strained by a small posterior fossa, grows in a vertical fashion. T he superior vermis extends high Discussion up between the posterior cerebral hemispheres A unified theory to explain the embryologic while the inferior vermis is located in the cervical ongm and the gross pathologic abnormalities or even upper thoracic spinal canal (2, 3). Sur­ found in patients with spina bifida aperta and geons and pathologist s have noted the presence Chiari II malformation has recently been pro­ of fibrovascular tissue containing choroid plexus posed. The initial insult in spina bifida, which located between the distal tip of the vermis and occurs during the fourth week of development, m edulla (3 4). We have noted in three patients is the failure of normal closure of the neural tube that this tissue, si milar to choroid plexus in ortho­ at the level of the posterior neuropore, leading to topic locations enhances intensely after the ad­ leakage of cerebrospinal fluid from the central m inistration of intravenous Gd-DTPA and should canal of the spinal cord at the time that the not be confused with a pathologic mass. intracerebral and intracerebellar vesicles are forming. Collapse of the cerebellar vesicles leads Patients and Methods to decreased inductive effects on the mesen­ chyme of the posterior fossa and results in a Three children \ ith known spina bifida aperta under­ small posterior fossa. During the fifth week, the \ ent spinal MR e amination because of suspected syrin­ pontine flexure fails to form, and the growing goh drom elia or tethering of the spinal cord. T he patients hindbrain is then forced to grow in a vertical ranged in age from 2 to 11 ears. MR scans were performed on a 1.5-T tem. agittal and axial T1-weighted scans fashion, superiorly through a dysplastic falx and ' ere obtained both before and after the administration of caudally into the foramen magnum and spinal intra enous Gd-DPT (0. l mmoljkg). xial T2*-weighted canal (5). gradient-echo cans \ ere obtained before the administra­ Extension of the inferior cerebellar vermis into tion of intra enou contra t. In t\ o patients otherwise the spinal canal usually accompanies the caudal

Recei ed pril 13, 1992; re isi n requested June 10; revision received Jul 2 and accepted July 6. 'Department f Radiolog . Uni ersit of rkansas for edical Sciences. and rkansas Children's Hospita l, 800 Marshall Street. Little Rock, AR 7220 . ddre· reprint requests to Charle . Glasier. 1D.

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618 AJNR: 14, May/ June 1993 ECTOPIC CHOROID PLEXUS IN CHIAR I II 619

In Chiari II malformation, the inferior medullary velum is usually absent and choroid plexus is only rarely present within the fourth ventricle except in those patients who have cystic prolon­ gation of the fourth ventricle and absent foramen of Magendie (2, 5, 8). Instead, a nodular fibrovas­ cular mass containing choroid plexus tissue is usually present protruding between the vermis and medullary kink (Figs. 1A and 1B). This tissue may lie in various positions including the junction of the vermian "peg" and medulla, extending from the distal vermis to the medullary kink or on the dorsal surface of the vermis (2, 9). This tissue appears both on gross pathologic examination and on sagittal MR as a "cluster of grapes." The fibrovascular tissue is often seen on noncontrast images and has MR signal characteristics similar to those on the adjacent vermis. Occasionally, the ectopic choroidal tissue may have increased signal relative to the cerebellar vermis when is­ chemia secondary to compression or vascular compromise at the foramen magnum causes de­ crease in the vermian signal intensity on precon­ trast T1-weighted scans (1). Intense enhancement is noted on T1-weighted images after the admin­ istration of intravenous contrast material (Figs. 2 and 3). Intravenous contrast is not routinely used in Chiari II patients, but was administered in two patients because of the presence of surgically proved neurenteric and neuroectodermal cysts and in the third patient because of unusual, nod­ ular-appearing neural tissue in the caudal thecal B sac. This aberrant location of fourth ventricular Fig. 1. A , Gross pathologic specimen of hindbrain and cervical cord from patient with Chiari II malformation. Note nodular tuft choroid plexus tissue apparently has its origin in of choroid plexus protruding beneath the cerebellar tail (white early fetal life. In the normal 6-week human arrows). Impression made on the herniated hindbrain by the embryo, a portion of the cerebellum is intraven­ foramen magnum is easily seen (black arrows). tricular with the tuft of fourth ventricular choroid 8 , Transverse section through the cerebellar tail and cervical plexus located caudally with respect to the cere­ spinal cord shows choroid plexus and fibrous tissue (black arrows) bellar tissue (Fig. 4). In patients with Chiari II dorsal and lateral to the cerebellar tissue which is covered by ependyma. (From Daniel and Strich (3).) malformation, the constricted fetal posterior fossa prevents formation of the normal pontine flexure and the intraventricular portion of the cerebellum enlarges and herniates inferiorly, pushing before displacement of the medulla and pons in Chiari II it the choroid plexus which comes to lie dorsal malformation. This may or may not be accom­ and caudal to the low-lying vermis (8, 10, 11 ). panied by herniation of the cerebellar tonsils. In Shortening of T1 relaxation time with increased 2% of cases, only the cerebellar tonsils protrude signal intensity on postcontrast T1-weighted im­ caudally (2, 6). The distal tip of the vermis may ages occurs normally in tissues with an absent lie at any level between the first cervical and blood-brain barrier, including choroid plexus, pi­ second thoracic vertebral body. In a minority of neal gland, and both the anterior pituitary gland cases, 25% in one series (7), the tip may even and infundibulum. Enhancement of the fibrovas­ extend below the level of the medullary kink. cular nodule found at the inferior border of the A 8 c Fig. 2. Five-year-old boy with spina bifida and periscapular pain. A, Sagittal Tl-weighted 600/20 (TR/TE) scan through the midline of the posterior fossa shows nodular tissue isointense to brain caudal to the cerebellar tail (white arrows). 8 , Contrast-enhanced Tl­ weighted sagittal 900/20 and C, axial 800/25 sca ns through the cervical spine show intense contrast enhancement of choroid plexus ca udal and lateral to the cerebellar tail and dorsal to the cervica l cord (black arrows, 8 and C). Surgically proved neurenteric cyst (white arrows, B).

Fig. 4. Midline sagittal section through the hindbrain of a 45- Fig. 3. Eleven-year-old girl with spina bifida underwent spinal mm human embryo. Cranial is to the left, and caudal to the right. MR prior to scoliosis surgery. Sagittal Tl-weighted 800/ 25 con­ Normal pontine flexure is present ventrally (black arrows). Note trast-enhanced MR shows an enhancing nodule of ectopic choroid the intraventricular location of cerebellar tissue (white arrows) and plexus just below the cerebellar tail (arrow). Also note midcervical the tuft of choroid plexus caudal to the cerebellum (small black syrinx. arrows). (From Daniel and Strich (3).) AJNR: 14, May/ June 1993 ECTOPIC CHOROID PLEXUS IN CHIARI II 621 cerebellar peg in patients with Chiari ll malfor­ cran iocervica l junction: Chiari II malformation. In: Newton TH, Potts mation should not be confused with a pathologic DG , eds. Computed tomography of the spine and spinal cord. San Anselmo, CA: Clavadel Press, 1983;355-366 mass, but indeed is an expected finding in pa­ 5. Mel one DG , Makahara S, Knepper PA. Chiari II malformation: path­ tients with spina bifida and the characteristic ogenesis and dynamics. In: Marl in AE, ed. Concepts in pediatric Chiari II hindbrain anomaly. neurosurgery. Basel: Ka rger, 1991; I : 1- 17 6. Variend S, Emery JL. Cervical dislocation of the cerebellum in children with meningomyelocele. Teratology 1976; 13:28 1-290 References 7. Peach B. Arnold Chiari malformation: anatomic features of 20 cases. Arch Neural 1965; 12:6 13- 621 1. Curnes JT , Oakes WJ, Boyko OB. MR imaging of hindbrain deformity 8. Peach B. Cystic prolongation of fourth ventricle. Arch Neural 1964; in Chiari II patients with and without symptoms of brainstem compres­ 11 :609- 612 sion. AJNR: Am J Neuroradiol 1989; 10:293-302 9. Ogryzlo MA. The Arnold-Chiari malformation. Arch Neural Psychiatry 2. Naidich T P, Mel one DG, Fulling KH. The Chiari malformations. IV. 1942;48:30-46 The hindbrain deformity. Neuroradiology 1983;25: 179-1 97 10. Hamil ton WJ, Mossman HW. Human embryology: prenatal develop­ 3. Daniel PM, Stri ch SJ. Some observations on the congenital deformity ment of form and function. Cambrid ge, UK : Heffer £, Sons, 1972: of the central nervous system known as the Arnold-Chiari malfor­ 465-466 mation. J Neurop athol Exp Neural 1957; 17:255- 266 11. Arey LB. Developmental anatomy: a textbook and laboratory manual 4. Naidich TP, Mel one DG, Harwood-Nash DG. Malformations of the of embryology. 7th ed. Philadelphia: WB Sa unders, 1965:280-284