Definitions and Anatomic Considerations in Chiari I Malformation and Associated Syringomyelia




Current understanding of the hindbrain hernias known as Chiari I malformations is based on more than 100 years of pathologic and clinical experience. Over time, the definition of this finding has been analyzed and altered. The term Chiari I malformation is currently used to describe tonsillar ectopia in a wide range of patients with varying embryonic derailments. This article discusses this malformation, its various definitions, and varied anatomic traits. In addition, the morphology of the commonly associated syringomyelia is reviewed.


Key points







  • •

    Understanding of the Chiari I malformation has evolved over time.


  • •

    The Chiari I malformation has many different known and unknown causes.


  • •

    The definition of the Chiari I malformation is in need of refinement based on continued clinical and surgical experience.






Introduction


Since the original description and classification of hindbrain hernias more than a century ago, the Chiari malformations have revealed much of their pathophysiology and have become easily diagnosed radiologically. With the availability of MRI, more and more patients are being diagnosed but without symptoms or appropriate symptoms. Time and clinical experience have shed light on the edges of this diagnosis, but indications for surgical intervention in some patient groups have become indistinct.


The 4 traditional types of Chiari malformations represent various clinical and anatomic processes that entail involvement of the rhombencephalon (hindbrain). Chiari types I, II, and III involve varying degrees of herniation of rhombencephalic derivatives out of the posterior fossa. Chiari type IV malformations involve cerebellar hypoplasia or aplasia, with no herniation of the hindbrain. At present, there is no consensus on how to define, treat, or label a causal process for the pathologic term Chiari malformation. Although the Chiari classification is usually helpful in categorizing patients, this scheme probably does not represent a precise continuum of the same disease and does not provide the classification needed to compartmentalize all forms of hindbrain hernias encountered. Moreover, with regard to this article, multiple causes are responsible for the Chiari I malformation (CIM) ( Fig. 1 ), which is an imprecise term, usually diagnosed on a single midline sagittal MRI image, that does not take into account respiration, presence or absence of Valsalva maneuver, shape or thickness of the foramen magnum, or that the cerebellar tonsils are paired parasagittal structures ( Figs. 2 and 3 ).




Fig. 1


Typical CIM on T2-weighted MRI.



Fig. 2


Sagittal MRI without evidence of tonsillar ectopia.



Fig. 3


Coronal MRI of same patient seen in Fig. 2 , noting mild right-sided tonsillar ectopia that was not evident on sagittal imaging.




Introduction


Since the original description and classification of hindbrain hernias more than a century ago, the Chiari malformations have revealed much of their pathophysiology and have become easily diagnosed radiologically. With the availability of MRI, more and more patients are being diagnosed but without symptoms or appropriate symptoms. Time and clinical experience have shed light on the edges of this diagnosis, but indications for surgical intervention in some patient groups have become indistinct.


The 4 traditional types of Chiari malformations represent various clinical and anatomic processes that entail involvement of the rhombencephalon (hindbrain). Chiari types I, II, and III involve varying degrees of herniation of rhombencephalic derivatives out of the posterior fossa. Chiari type IV malformations involve cerebellar hypoplasia or aplasia, with no herniation of the hindbrain. At present, there is no consensus on how to define, treat, or label a causal process for the pathologic term Chiari malformation. Although the Chiari classification is usually helpful in categorizing patients, this scheme probably does not represent a precise continuum of the same disease and does not provide the classification needed to compartmentalize all forms of hindbrain hernias encountered. Moreover, with regard to this article, multiple causes are responsible for the Chiari I malformation (CIM) ( Fig. 1 ), which is an imprecise term, usually diagnosed on a single midline sagittal MRI image, that does not take into account respiration, presence or absence of Valsalva maneuver, shape or thickness of the foramen magnum, or that the cerebellar tonsils are paired parasagittal structures ( Figs. 2 and 3 ).




Fig. 1


Typical CIM on T2-weighted MRI.



Fig. 2


Sagittal MRI without evidence of tonsillar ectopia.



Fig. 3


Coronal MRI of same patient seen in Fig. 2 , noting mild right-sided tonsillar ectopia that was not evident on sagittal imaging.




Definition


In Chiari’s original definition, the type I malformation was considered herniation of the cerebellar tonsils below the plane of the foramen magnum but the degree of herniation was not defined. Therefore, the precise definition of the type I malformation has been debated. In addition, Chiari malformations represent a wide range of anatomy that may or may not have common embryologic origins. For example, the CIMs seen in patients with bony overgrowth of the posterior fossa, growth hormone deficiency, or craniosynostosis represent the same effect from varied causes. Therefore, the term CIM should be considered a generic appellation for a physical finding and should not imply a specific congenital cause.


In an attempt to better categorize tonsillar ectopia, Aboulezz and colleagues retrospectively examined 95 cases. In normal controls, the cerebellar tonsils did not extend more than 3 mm below the plane of the foramen magnum. These investigators considered herniation below 5 mm to be pathologic. Barkovich and colleagues measured the position of the cerebellar tonsils with respect to the inferior aspect of the foramen magnum in 200 normal patients and in 25 patients with a diagnosis of CIM. In the normal group, the mean position of the tonsils was 1 mm above the foramen magnum with a range from 8 mm above the foramen magnum to 5 mm below. In the patients with CIM, the mean position was 13 mm below the foramen magnum with a range from 3 mm below the foramen magnum to 29 mm below. Fourteen percent of normal patients had tonsils extending slightly below the foramen magnum. With 2 mm below the foramen magnum taken as the lowest extent for tonsils in normal patients, the sensitivity in predicting symptomatic patients was 100% and the specificity was 98.5% (3 false-positives). When 3 mm below the foramen magnum was used as the lowest normal tonsillar position, the sensitivity was 96% and the specificity was 99.5%. In addition, Mikulis and colleagues found that normal tonsils may prolapse up to 6 mm during the first 10 years of life and ascend gradually with increasing age.




Anatomic findings of the Chiari I malformation


The CIM was originally described as caudal displacement of the cerebellar tonsils to a level below the plane of the foramen magnum. In general, 3 to 5 mm of caudal descent of the tonsils is considered a CIM. There are other associated findings seen surgically, radiologically, and at postmortem in patients with CIM. These findings are described and categorized here according to their anatomic locations.


Skull


Basilar skull and craniocervical junction anomalies (eg, third occipital condyle) are seen in many patients with CIMs. It is now appreciated that the supraocciput and exocciput are underdeveloped in many patients with CIMs, and there is shortening of the supraocciput. The clivus is also often shorter, with a larger than normal foramen magnum, although in some diseases involving CIM, the foramen magnum may be smaller than normal. The midline sagittal diameter of the foramen magnum, measured from basion to opisthion, ranged from 30 to 40 mm (mean, 36.7 mm) in an earlier study from our group. On axial imaging, the maximal width of the foramen magnum was measured from 12 to 31 mm (mean, 26 mm). The foramen magnum may be narrowed in the coronal plane (ie, lateral compression).


The posterior fossa is shallower in many patients with this type of malformation. It is now known that, like Chiari type II malformations, type I malformations often have smaller than normal posterior cranial fossa volumes. Also, various forms of craniosynostosis are associated with CIM ostensibly because of reduced intracranial volume.


Other basilar skull abnormalities seen with the CIM include empty sellae, clival concavities, basilar impression, large midline occipital keels, and remnants of the proatlas, such as an accessory occipital condyle. The midline sagittal length of the basiocciput in this group has been measured with a range from 20 to 29 mm (mean, 24 mm).


Spine


Klippel-Feil deformity and atlanto-occipital assimilation are two of the most common spinal defects in type I malformations. Other occasional findings are retroflexion of the odontoid process ( Fig. 4 ) and thickening of the ligamentum flavum. The posterior atlanto-occipital membrane may also be thickened. Another common entity seen in CIMs is scoliosis, which is usually caused by an underlying syrinx and is most likely to be a single curve with a convexity to the left. Scoliosis may be present in the absence of CIM.


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Oct 12, 2017 | Posted by in NEUROSURGERY | Comments Off on Definitions and Anatomic Considerations in Chiari I Malformation and Associated Syringomyelia

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