This article describes the diagnosis and management of patients with complex Chiari malformations. This group of patients cannot be categorized as a Chiari 1 or 1.5 because the pathology and progression of their disease is unique. The authors discuss how to diagnose and treat patients with complex Chiari malformations, particularly with regard to discerning which patients would benefit from a simple Chiari decompression versus Chiari decompression with occipitocervical fusion.
Key points
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Complex Chiari malformations may be a distinct category from other Chiari malformations.
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These patients are at increased risk for occipitocervical fusions after Chiari decompression or may need upfront decompression and fusion procedures.
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The decision-making algorithm for these patients needs to be modified to account for different outcomes for these patients. This algorithm includes clinical symptoms, extent of Chiari, evaluation of clival-cervical angle, retroflexed odontoid, and basilar invagination.
Definition of complex Chiari malformation
Chiari malformations were originally classified by Chiari in 1896 into three categories based on anatomic description :
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Chiari 1: Cerebellar tonsil and lower part of the medulla below the foramen magnum without displacement of the fourth ventricle.
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Chiari 2: Caudal migration of the lower part of the cerebellum associated with downward displacement of the fourth ventricle, which appears lengthened; foramina opens into the spinal subarachnoid space; associated with spina bifida.
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Chiari 3: Cerebellum and medulla displaced into the cervical spinal canal associated with an occipital meningocele.
Later, a fourth type was added that describes an incomplete or underdeveloped cerebellum.
These categories were clinically useful for almost 100 years; however, as neurosurgeons began studying outcome data for surgical decompression of Chiari 1 malformations (C1M), it became apparent that there was a subcategory of patients in whom the condition was more complex; these patients required more frequent surgical intervention than the others. Initial examination of this group by Grabb and colleagues in 1999 showed that odontoid retroflexion, manifested by a pBC2 distance (maximum perpendicular distance to the basion-inferoposterior point of the C2 body) greater than 9 mm, defined a patient group that frequently required craniocervical fusion procedures. Further experience with this patient population led to the observation that a subcategory of patients with Chiari malformation had caudal descent of the brainstem and tonsillar ectopia. Therefore, a new category, the Chiari 1.5 malformation (C1.5M), was proposed; it was defined as the presence of obex herniation below the foramen magnum as seen on MRI.
With a growing awareness of the complex nature of disease in some Chiari patients, Bollo and colleagues comprehensively analyzed their experience with patients that manifested a constellation of craniospinal radiographic findings aside from just tonsillar herniation. These findings included
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Brainstem herniation through the foramen magnum (C1.5M)
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Medullary kink
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Retroflexed odontoid
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Abnormal clival-cervical angle (CXA)
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Occipitalization of the atlas
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Basilar invagination (BI)
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Syringomyelia
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Scoliosis
The authors analyzed a group of patients with the previously mentioned radiographic findings and found that the presence of a C1.5M, a CXA less than 125°, and BI placed a patient at a higher risk for requiring a craniocervical fusion than those with a typical C1M. They proposed a new category of Chiari malformation, known as complex Chiari malformation (CCM), that encompasses these radiographic findings, all or in part. Further detail about these measurements is illustrated in Fig. 1 . Although there are several previous case series in the literature describing the management of complex-type Chiari patients, Bollo and colleagues were the first to analyze a large group of patients with CCM and define factors that place patients at a higher risk for craniocervical fusion. Here, we describe the diagnostic criteria for CCM and outline the decision-making process for the optimal treatment of this patient population.
Definition of complex Chiari malformation
Chiari malformations were originally classified by Chiari in 1896 into three categories based on anatomic description :
- •
Chiari 1: Cerebellar tonsil and lower part of the medulla below the foramen magnum without displacement of the fourth ventricle.
- •
Chiari 2: Caudal migration of the lower part of the cerebellum associated with downward displacement of the fourth ventricle, which appears lengthened; foramina opens into the spinal subarachnoid space; associated with spina bifida.
- •
Chiari 3: Cerebellum and medulla displaced into the cervical spinal canal associated with an occipital meningocele.
Later, a fourth type was added that describes an incomplete or underdeveloped cerebellum.
These categories were clinically useful for almost 100 years; however, as neurosurgeons began studying outcome data for surgical decompression of Chiari 1 malformations (C1M), it became apparent that there was a subcategory of patients in whom the condition was more complex; these patients required more frequent surgical intervention than the others. Initial examination of this group by Grabb and colleagues in 1999 showed that odontoid retroflexion, manifested by a pBC2 distance (maximum perpendicular distance to the basion-inferoposterior point of the C2 body) greater than 9 mm, defined a patient group that frequently required craniocervical fusion procedures. Further experience with this patient population led to the observation that a subcategory of patients with Chiari malformation had caudal descent of the brainstem and tonsillar ectopia. Therefore, a new category, the Chiari 1.5 malformation (C1.5M), was proposed; it was defined as the presence of obex herniation below the foramen magnum as seen on MRI.
With a growing awareness of the complex nature of disease in some Chiari patients, Bollo and colleagues comprehensively analyzed their experience with patients that manifested a constellation of craniospinal radiographic findings aside from just tonsillar herniation. These findings included
- •
Brainstem herniation through the foramen magnum (C1.5M)
- •
Medullary kink
- •
Retroflexed odontoid
- •
Abnormal clival-cervical angle (CXA)
- •
Occipitalization of the atlas
- •
Basilar invagination (BI)
- •
Syringomyelia
- •
Scoliosis
The authors analyzed a group of patients with the previously mentioned radiographic findings and found that the presence of a C1.5M, a CXA less than 125°, and BI placed a patient at a higher risk for requiring a craniocervical fusion than those with a typical C1M. They proposed a new category of Chiari malformation, known as complex Chiari malformation (CCM), that encompasses these radiographic findings, all or in part. Further detail about these measurements is illustrated in Fig. 1 . Although there are several previous case series in the literature describing the management of complex-type Chiari patients, Bollo and colleagues were the first to analyze a large group of patients with CCM and define factors that place patients at a higher risk for craniocervical fusion. Here, we describe the diagnostic criteria for CCM and outline the decision-making process for the optimal treatment of this patient population.
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