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Show /. Clin. Neuro-ophth.l/mo/. 1: 101-167,1981. Neuroradiological Clinical Pathological Correlations Vertical Oscillopsia and Blurred Vision ROBERT M. QUENCER, M.D. Case History A 32-year-old normal-appearing, previously healthv female was seen by her ophthalmologist because of a 3-month history of blurred vision and a sensation of up and down movements of objects in both visual fields. On examination her visual acuity was 20/20 bilaterally, but a downbeating nystagmus, most prominent in down gaze, was noted. The remainder of the ophthalmological and neurological examinations were normal; specifically. there were no other ocular motor disturbances, visual field defects, headaches, diplopia, ataxia. dizziness, weakness, or cranial nerve deficits. All laboratory examinations were normal. The radiographic evaluation included computed tomography (eT) of the brain (Fig. 1), cervical spine and skull films (Figs. 2 and 3). cervical myelography (Fig. 4), and metrizamide computed tomography (MCT) (Fig. 5). Figures I.J -I d. Postcontr..st CT SCdn shows d slo~htly ('nl.H~('d but norm.. lJy positioned fourth ventricl" (drrows, J dnd b). Th('re is no evidence of ('ith", c",('bdldr or brdin-st('m dbnormdlities. The hi~her s"ctions (c dnd d I show hvdroceph.. lus with dil..ted third (open ",rows) dnd IdterJI v('ntrll!es (strJight .. rrows). Becdus(' there is dildtdti('n of the third, fourth dnd l.. ter..1ventricles without evid('nc(' of Jtrophy, In extrd ventri,uldr obstructive proc('ss should be SUSp(·c!('d. Note spl'(ifl("lly the norm..1inw.ud convex borders of the petrous bone (Jrrowhedds, d), the normdl sized post('rior fOSSJ. dnd the normdlly curved dors..1 border of the midbrdin (drrowheJds, l). Abnorm.. loties of these structures c.. n b(' found on the Ch,,,,i II mdlform.. tion (s('e Discussion). From the Dep",tment of R..diology. University of Mi,'mi S,hool of M"dicine, /dckson M('moridl HospitJl. Midml, F1oridJ. June 1981 Discussion Based on the presenting sign of downbeat nystagmus, a number of abnormalities at the level of the low posterior fossa/brain stem, or upper cer- 161 Vertic.11 Oscillopsi.1 Figure 1. (Continued.) vical cord must be considered. In a series of 39 patients I with downbeat nystagmus, spinocerebellar degeneration was present in IS, while in 14 a wide range of lesions including a nearly equal number with a Chiari malformation, basal meningitis, multiple sclerosis, and a variety of brain-stem abnormalities, including syringobulbia, ischemia, neoplasia and telangiectasia, were found. In the remaining 10 patients, a structural lesion could not be demonstrated and the cause of the downbeat nystagmus was classified as undetermined. Although these diagnostic possibilities indicate that well over half the cases are nonsurgical in nature, a search must be made for a surgic.llly curable lesion. CT of the brain should be the first radiographic study ordered in patients with downbeat nystagmus since it is capable of detecting spinocerebellar degeneration, multiple sclerosis, or a brain-stem neoplasm. If either of the former two entities are found, no further radiographic workup is necessary. Because CT is less reliable in visualizing syringobulbia. tumors at the foramen magnum, granulomatous meningitis, and brain-stem ischemia or telangiectasia, and because the least severe form of the Chiari malformation (Type I) cannot be diagnosed by CT alone, other radiological studies are necessary when the diagnosis is not apparent on the initial CT scan. In our case, because the initial CT scan (Fig. 1) showed significant hydrocephalus but no other abnormalities, further studies, specifically plain films and myelography of the craniovertebral junction, were necessary. There was no evidence of platybasia (i.e., flattening of the base of the skull), but basilar invagination (Fig. 2) and dysplasia of the occipital bone at the foramen magnum were present (Fig. 3). At myelography (Fig. 4) a mass at Journal of Clinical Neuro-ophthalmology Quencer (e) Figure 1. (Continued.) the C1-C2 level was identified and the effect of that mass on the adjacent lower medulla and upper spinal cord was clearly demonstrated with MCT (Fig. 5). The position and shape of the mass were classic for caudally displaced tonsillar tissue within the upper cervical canal. No other diagnostic workup in this case was necessary for diagnostic purposes. Occasionally a neurosurgeon, prior to surgery, may wish a vertebral angiogram to localize the position of the posterior inferior cerebellar arteries in relation to the low-lying tonsils, but in this case an arteriogram was not felt to be necessary. Based on this radiographic information, a diagnosis of a Chiari I malformation with basilar invagination was made. In the late 18oos, Chiari described and classified developmental anomalies of the hindbrain in which: 1) there was elongation of the tonsils and inferior lobes of the cerebellum into the cervical June 1981 canal along with normal coursing cervical nerve roots (Type I); 2) there was displacement of the cerebellar vermis, medulla and fourth ventricle into the cervical canal, caudal displacement of the pons, and a cephalic course of the cervical nerve roots (Type II); 3) there was displacement of the medulla, fourth ventricle and a large portion of the cerebellum into an occipital cervical encephalocele (Type III). The name of Arnold was added to the description of the Chiari Type II malformation in 1907 and, hence, the term "Arnold-Chiari malformation" came into common use. The Chiari Type IV malformation, i.e., cerebellar hypoplasia, with no inferior displacement of the hindbrain, is developmentally different from the other Chiari forms and probably should not be considered part of this disease spectrum. No single theory has been accepted to explain the development of any of the forms of the Chiari 163 Vertic.l1 Oscillopsi<l (d! figure 1. (Continued.) malfonnation, although many have been proposed including theories of traction on the hindbrain by a spinal cord fixed at the site of a myelomeningocoele, a teratogenic growth of the cerebellum, hydrodynamic mechanisms with persistent hydrocephalus, and a primary overgrowth of the brain associated with a small bony posterior fossa. Likewise, the mechanism by which the Chiari malformations and other lesions at the cervical medullary junction cause ocular abnormalities as seen in this patient, are not understood. Radiologically the Chiari II malformation, or what is commonly but unjustifiably termed the "Arnold-Chiari" malformation, is quite distinct from the Chiari I malformation. In the Chiari II malformation abnormalities of the skull, dura, brain stem, cerebellum, ventricles, and cisternal spaces are present and can be identified by computed tomography.:I-r, These CT changes are related not only to the caudally displaced brain stem, fourth ventricle and cerebellum, but also reflect other congenital abnonnalities involving the skull and brain. Prominent features of the Chiari II malfonnation include abnonnal ossification of the calvarium resulting in an infant lacunar skull, distinct inward convexity of the posterior borders of the petrous bones, enlarged foramen magnum, small posterior fossa with a small or nonvisualized fourth ventricle, acutely angled dorsal border of the midbrain. abnormal growth of the cerebellum not only upwards through a hypoplastic tentorium, but also anterolaterally into the cerebellopontine angle cisterns, and moderate to marked asymmetric hydrocephalus. None of these findings are features of the Chiari I malformation with the exception of occasional mild to moderate hydrocephalus. The Chiari I malformation can be further distinguished from the Chiari II malformation by the fact that it usually presents with symptoms during adulthood rather Journal of Clinical Neuro-ophthalmology Queneer Figure 2. A lateral view of the craniovertebral junction sh,,,,,, basilar invagination, that is, over half of the odontoid process projects above a line drawn from the hard palate to the inferior portion of occipital bone (MacGregor's line). An arrow points to the base of the ,'dontoid and arrowheads point to the tip of the odontoid. There is no evidence of platyb..si.. since the angle formed between .. line dr..wn from the n..sion to the dorsum sella and .. lone from the dorsum sella to the anterior lip of the foramen magnum is 1300 " .. lues greater than 142 0 .. re abnormal and are diagnostic of a flat skull base or platybasi... Figure 3. A Towne's view of the skull show, J dy'pl."ti, f"r,.n1<'11 ",.lgIlU'" with "vcrgnlwth "I the occipital bOilI' forming the right later.. ) portion of the for..men m..gnum (arrow,). June 1981 165 Vertical Oscillopsi., Figures 4a and 4b. Cervical metrizamide myelography in the lateral (a) and AP (b) projections shows a mass (arro,,:s) within the spinal canal at the CI -C2 level which is thinning the metrizamide column dorsally and laterally. causing an incomplete block. (b) An AP tomographic section through the foramen magnum to C3 levels. The shape and location of this mass is typical for herniated cerebellar tonsils. than childhood, and unlike the spinal dysraphic state seen in the Chiari II malformation, the only skeletal abnormalities will be a frequent occurrence of platybasia, basilar inv,lgination, and osseous abnormalities at the foramen magnum. In the case presented here, caudally displaced tonsillar tissue surrounded by adhesions were found at surgery and a decompressive CI laminectomy was performed. Following the surgery the patient noted an improvement in her oscillopsia and blurred vision, while on examination the amplitudl? of her downbeat nystagmus had lessened rn,ukedly. References I. Sh.l\'l/, H. E.. and Smith, J. l.: Downbeat nystagmusA c1inicll update. In Neuro-ophthalmology Focus /080, J. l. Smith, Ed. Masson Publishing USA, Inc., New York, 19 79. pp. 433-430. 2. CHme), P. W., and Markesbery, W. R.: Early descriptions of the Arnold-Chiari malformation. The contribution of John Cleland. f. NeurDsurg. 37: 543547, 1979. 3. Maidich, T. P., Pudlowski, R. M., Maidich, J. B., Gornish, M., and Rodriquez, F. J.: Computed tomographic signs of the Chiari II malformation. Part 1. Skull and dural partitions. Radiology 134: 55-71, 1980. Journal of Clinical Neuro-ophthalmology Quencer Figures Sa and Sb. Metrizamide computed tomography (MCT) in the axial plane (a) shows the cervical spinal cord (SC) at the C1 level which is being distorted and compressed by a dorsally situated mass (M) outlim'd by arrowheads. The tip of the odontoid (0) and the anterior arch of CI are also labeled. Multiple axial sections reconstructed in the sagittal plane (bl clearly show the mass (M) extending through the foramen magnum (the anterior and posterior portions are labeled FM) and compressing the dorsal aspect of the cord (arrowheads). There is a clear continuity of the mass with cerebellar tissue above m"king the diagnosis of herniated cerebellar tonsils most likely. These radiographic features are characteristic of a Chiari I malformation. 4. Maidich, T. P., Pudlowski, R. M., and Maidich, I. B.: Computed tomographic signs of the Chiari II mdlformation. Part 2. Midbrain and cerebellum. l~Jdio/ogy 134: 391-398, 1980. 5. Maidich, T. P., Pudlowski, R. M., and Maidich, 1 B.: Computed tomographic signs of the Chiari II mdl- June 1981 form,lti(lfi. Pdrt 3. Ventricles dnd cisterns. Rddio/ogy 134: b57-bb3, 1980. Write for reprints to: Robert M. Quencer, M.D., [)ep,lrtment of R"diology (R-130), University of Miami School of Medicine, P.O. Box 01b9bO, Miami, Florida 33101 167 |
