Cerebellar agenesis versus vanishing cerebellum in Chiari II malformation

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Medical Imaging Copyright

and Graphics, Vol. 19, No. 6, pp. 491-494, 1995 0 1996 Elsevier Science Ltd. All rights reserved Printed in the USA 0895411 l/95 $9.50 + .OO

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CEREBELLAR

AGENESIS CHIARI

VERSUS VANISHING II MALFORMATION

CEREBELLUM

IN

R. Nuri Sener’ Department of Radiology, Ege University Hospital, Bornova, Izmir, 35100, Turkey (Received

4 August

1995)

Abstract-The subjects in this study cover seven selected patients-four with Chiari Ii malformation associated witb a vanisbiug cerebellum and tbree with primary cerebellar agenesis-and the maiu radiological (MR imaging) diiferences between the two are discussed. The reason for this discussion is that some authors consider primary cerebellar agenesis as a type of Chiari malformations, and refer to it as Chiari IV malformation. Primary cerebellar agenesis can be distinguished from the Chiari II malformation associated with a vanishing cerebepum by presence of a relatively normal-sized posterior fossa, identification of symmetrical remnants of the anterior quadrangular lobules, by lack of scattered remnants of cerebellar tissue, and by presence of a normal brain and spine. The only common finding in both conditions is a small brainstem with loss of normal pontine conbguration. Considering the previous descriptions of anatomical changes related to the so-caped Chiari IV malformation which in&de absent cerebellum, and large posterior fossa cerebrospinal tiuid spaces, it is probable that many of the cases previously considered as Chiari IV malformation were unrelated to any type of Chiari malformations, and actually represented primary cerebellar agenesis. Copyright 0 1996 Elsevier Science Ltd. Key

Words:

Brain,

MR

studies,

Brain,

abnormalities,

Cerebellar

Primary cerebellar agenesis is a rare condition, and has previously been considered by some authors as the Chiari IV malformation. In these patients, however, associated changes of the Chiari malformations are usually lacking (l-3). On the other hand, in Chiari II malformation severe degeneration of the cerebellum may occur (46), which can be termed as and then which may be “vanishing cerebellum”, regarded as Chiari IV malformation. This article discusses the main differences between vanishing cerebellum in Chiari II malformation and primary cerebellar agenesis in an effort to explain that the socalled Chiari IV malformation actually represents primary cerebellar agenesis. AND

Cbiari

II malformation,

Cbiari

IV malformation

institutions. One of these patients with primary cerebellar agenesis, a 58-yr-old woman, was previously reported by us (3). All the patients had CT scans, and three (one with Chiari II malformation, two with primary cerebellar agenesis) were studied by MR imaging, MR imaging studies were performed at either 0.5, 1 or 1.5 T utilizing the conventional spinecho and inversion recovery pulse sequences.

INTRODUCTION

PATIENTS

agenesis,

RESULTS

All the patients with the Chiari II malformation (n = 4) had meningocele or meningomyelocele, beaked midbrain tectum and other cerebral changes, consistent with the anomaly. These patients had an apparently small posterior fossa evidenced by a low tentorial attachment and a steep straight sinus (the sinus oriented more vertical than usual). There were great amounts of cerebellar destruction, evidenced by presence of minute, scattered remnants of cerebellar tissue within the posterior fossa and upper cervical spinal canal (Fig. 1) The patients with primary cerebellar agenesis (n = 3) had a normal sized posterior fossa, and normal tentorial attachment and straight sinus. None of them had spinal and cerebral changes. In two of these patients, studied by MR imaging, remnants of the anterior quadrangular lobules of the cerebellum were identifiable (Fig. 2), indicating primary agenesis.

METHODS

The patients in this study cover seven selected cases, four with Chiari II malformation (three men, one woman, ages ranging from 1 month to 4.5 yrs), and three with primary cerebellar agenesis (one man, two women, ages ranging from 6 to 58 yrs) who were seen by the author within 6 yrs in different ‘Correspondence should be addressed to R. Nuri Sener, Department of Radiology, Ege University Hospital, Bornova, Izmir, 35100, Turkey. Tel.: (90) 232 388 1390; Fax: (90) 323 342 0001. 491

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Fig. lax. Vanishing cerebellumin Chiari II malformation. 4.5yr-old boy. Tl-weighted sagittal (a, b) and axial (c) MR images.The posterior fossais very small.Minute remnants of degeneratedcerebellar tissuecan be identified in the posterior fossa (large arrows) and distal to the foramen magnum (small arrows). The brainstemis small, and the pons is narrow. Note that no remnant of the anterior quadrangular lobule is seenin the axial image (compare with Fig. 2).

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Fig. 2a-c. Primary cerebellaragenesis (subtotal agenesisof the cerebellum).6-yr-old girl. Tl-weighted sagittal (a), axial (b), and coronal (c) MR images.There is a normal-sized posterior fossa,however, the cerebellumis absent except for symmetrical remnants of the anterior quadrangular lobules (arrows), indicating primary agenesis(compare with Fig. 1). The brainstem is small, and the pons is narrow, which is the only common radiological finding with the vanishingcerebellumin Chiari II malformation.

Cerebellar agenesis versus vanishing cerebellum

The only common radiological finding seen in patients with Chiari II malformation and primary cerebellar agenesis was a small brainstem with apparent loss of normal pontine configuration (Figs 1 and 2). With respect to the clinical findings all the patients with Chiari II malformation had prominent cerebellar symptoms. They were mentally retarded. On the other hand, none of the patients with primary cerebellar agenesis had mental retardation, two had relatively moderate cerebellar symptoms, and one patient (a 5%yr-old woman) was totally asymptomatic (3). DISCUSSION

Primary cerebellar agenesis is defined as total or subtotal agenesis of the cerebellum, and is an extremely rare condition. Even in cases initially considered as total absence, small traces of the organ can be found, which usually are minute remnants of the anterior quadrangular lobules. Therefore the term subtotal agenesis is usually considered to be more accurate. In the past some authors regarded this condition as Chiari IV malformation. Previously reported cases of subtotal cerebellar agenesis on MR imaging, however, usually had a relatively normal sized posterior fossa without associated spinal or cerebral changes of Chiari II malformation. Identification of minute remnants of the anterior quadrangular lobules of the cerebellum was in favour of primary agenesis rather than secondary degeneration (l-3). This appears to be more than likely if the remnants of the anterior quadrangular lobules are symmetrically noted on an MR imaging examination (Fig. 2). CT may fail to identify these minute structures. In three cases discussed in this study the findings were consistent with primary cerebellar agenesis. None of the patients had associated myelomeningocele, virtually excluding Chiari II malformation. Chiari malformations have various types. The Chiari I malformation refers to tonsillar herniation not uncommonly associated with a syringomyelia. There is no associated brain anomaly. The Chiari II malformation includes a small posterior fossa, and various cerebellar, cerebral, dural, cranial and spinal changes. A spinal myelomeningocele is virtually seen in all cases. The Chiari III malformation covers cases with herniation of posterior fossa contents through a posterior spina bifida at Cl-C2 in combination with the features of the Chiari II malformation. On the other hand, severe cerebellar hypoplasia has sometimes been termed as Chiari IV malformation. This

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reportedly is a very rare entity, and it includes absent or severely hypoplastic cerebellum, small brainstem, and large posterior fossa cerebrospinal fluid spaces (1, 2). It is highly probable that many of these cases represented primary cerebellar agenesis or hypogenesis, based on the fact that the posterior fossa was normal in size in all of the three cases in this study, and in previous studies (l-3). In contrast to this, in all of the four cases of Chiari II malformation discussed herein, the posterior fossa was apparently small. There were varying degrees of cerebellar destruction, evidenced by presence of scattered, minute remnants of cerebellar tissue within the posterior fossa and upper cervical spinal canal (Fig. l), which suggested an event which could be termed as vanishing cerebellum. No remnants of the anterior quadrangular lobules were identified in the cases studied by MR imaging, in contrast to the cases with primary cerebellar agenesis. According to McLone and Knepper the posterior fossa changes of Chiari II malformation are the consequence of a normal-sized cerebellum developing in an abnormally small posterior fossa with a low tentorial attachment which causes squeezing of the cerebellum as it grows (5). This may lead to an ischemic degeneration of especially the herniated parts of the cerebellum, superiorly through the tentorial hiatus, and inferiorly through the foramen magnum. This may be severe enough that virtually only small parts of the cerebellum may remain in the posterior fossa (4-6). This condition appears to occur in the late intrauterine and early infantile periods, and we suggest it could be termed as vanishing cerebellum in Chiari II malformation, whereas total (or subtotal) cerebellar agenesis probably represents an unrelated isolated primary anomaly. Considering the previous descriptions of anatomical changes related to the so-called Chiari IV malformation which include absent or severely hypoplastic cerebellum, small brainstem, and large posterior fossa cerebrospinal fluid spaces (1, 2), it is probable that many of the cases previously considered as the Chiari IV malformation were unrelated to any type of Chiari malformations, and actually represented primary cerebellar agenesis. SUMMARY

Primary cerebellar agenesis can be distinguished from the Chiari II malformation associated with a vanishing cerebellum by the presence of a relatively normal-sized posterior fossa, by identification of symmetrical remnants of the anterior quadrangular lobules, by lack of scattered remnants of cerebellar

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tissue, and by presence of a normal brain and spine. The Chiari II malformation is always associated with a myelomeningocele, whereas primary cerebellar agenesis is not. The only common finding to the both conditions appears to be a small brainstem with loss of normal pontine configuration. Previous descriptions of anatomical changes in the so-called Chiari IV malformation included similar posterior fossa changes to those seen in primary cerebellar agenesis as noted in this study. Therefore, it is highly probable that many of the cases previously considered as the Chiari IV malformation were unrelated to any type of Chiari malformations, and actually represented primary cerebellar agenesis.

REFERENCES 1. Naidich, T.P.; Radkowski, M.A.; Bernstein, R.A.; Tan, W.S. Congenital malformations of the posterior fossa. In: Taveras, J.M.; Ferruci, J.T.; eds. Radiology, vol. 3. Philadelphia: Lippincott; 1991: 1-17.

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2. Osborn, A.G. Diagnostic neuroradiology. St. Louis: Mosby; 1994: 15-24, 66. 3. Sener, R.N.; Jinkins, J.R. Subtotal agenesis of the cerebellum in an adult. MRI demonstration. Neuroradiology 35 : 286-287; 1993. 4. Barkovich, A.J. Pediatric neuroimaging. New York: Raven Press; 1995: 238-246. 5. McLone, D.G.; Knepper, P.A. The cause of Chiari II malformation: a unified theory. Pediatr. Neurosci. 15 : 1-12; 1989. 6. Naidich, T.P.; McLone, D.G.; Fulling, K.H. The Chiari I1 malformation: Part IV. The hindbrain deformity. Neuroradiology 25: 179-197; 1983. About the Author-R. NURI SENER serves as the Chief of the Pediatric Radiology and Neuroradiology Sections, and Magnetic Resonance unit, at the Department of Radiology of Ege University, Izmir, Turkey, where he holds the position of Professor of Radiology. He completed a Neuroradiology Research Fellowship during 1990-1991 at the Neuroradiology Section of the University of Texas, Health Science Center at San Antonio, Texas, USA. Dr Sener’s primary field of research is Pediatric Neuroradiology. Dr Sener has served as a visiting professor at the Radiology Departments in the Ullevaal Hospital of Oslo University, Oslo, Norway, Royal Alexandra Hospital for Children, Sydney, Australia, and Hospital of SSo Paulo University, %o Paulo, Brazil.