This article addresses the key features, clinical presentation, and radiographic findings associated with craniovertebral junction instability in the setting of Chiari I malformation. It further discusses surgical technique for treating patients with Chiari I malformation with concomitant craniovertebral junction instability, focusing on modern posterior rigid instrumentation and fusion techniques.
Key points
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Craniovertebral junction (CVJ) instability in the setting of Chiari I malformation (CMI) is a rare but clinically important finding, most often presenting as basilar invagination.
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The diagnosis of CVJ instability should be based on a combination of clinical and radiographic findings, including myelopathy, basilar invagination, ventral brainstem compression, and Chiari 1.5.
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In most cases, children found to have CVJ instability with basilar invagination should undergo attempts at preoperative or intraoperative reduction to minimize the need for anterior decompression.
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Most children with symptomatic CMI, even those presenting with signs of brainstem compression, do not have CVJ instability and will improve with a standard posterior fossa decompression alone.
Introduction
Almost all patients with symptomatic Chiari I malformation (CMI) can be successfully treated with standard posterior fossa decompression ( Fig. 1 ). Large studies have shown that symptoms, including those associated with basilar invagination and syringomyelia, are likely to improve with decompression alone, with or without duraplasty. However, a small subset of patients will have radiographic and clinical evidence of craniovertebral junction (CVJ) instability on initial presentation and will require stabilization ( Fig. 2 ). Furthermore, some patients will either fail to improve after an initial decompression or may progressively worsen over time. Some of these patients will be subsequently diagnosed with CVJ instability and will also require stabilization ( Fig. 3 ).
Brockmeyer recently defined the complex Chiari malformation as “cerebellar tonsil herniation combined with one or more of the following radiographic findings: brainstem herniation through the foramen magnum (Chiari 1.5 malformation), medullary kink, retroflexed odontoid, abnormal clival-cervical angle, occipitalization of the atlas, basilar invagination, syringomyelia or scoliosis.” The investigators further differentiated complex Chiari malformation patients from typical CMI patients as being more likely to require further surgical interventions beyond a standard suboccipital decompression, including odontoid resection and CVJ stabilization.
This article will:
- 1.
Define CVJ instability and indications for stabilization in the setting of CMI based on a constellation of clinical and radiographic signs, and
- 2.
Discuss the surgical technique for a posterior decompression and stabilization with rigid internal fixation and fusion.
Introduction
Almost all patients with symptomatic Chiari I malformation (CMI) can be successfully treated with standard posterior fossa decompression ( Fig. 1 ). Large studies have shown that symptoms, including those associated with basilar invagination and syringomyelia, are likely to improve with decompression alone, with or without duraplasty. However, a small subset of patients will have radiographic and clinical evidence of craniovertebral junction (CVJ) instability on initial presentation and will require stabilization ( Fig. 2 ). Furthermore, some patients will either fail to improve after an initial decompression or may progressively worsen over time. Some of these patients will be subsequently diagnosed with CVJ instability and will also require stabilization ( Fig. 3 ).
Brockmeyer recently defined the complex Chiari malformation as “cerebellar tonsil herniation combined with one or more of the following radiographic findings: brainstem herniation through the foramen magnum (Chiari 1.5 malformation), medullary kink, retroflexed odontoid, abnormal clival-cervical angle, occipitalization of the atlas, basilar invagination, syringomyelia or scoliosis.” The investigators further differentiated complex Chiari malformation patients from typical CMI patients as being more likely to require further surgical interventions beyond a standard suboccipital decompression, including odontoid resection and CVJ stabilization.
This article will:
- 1.
Define CVJ instability and indications for stabilization in the setting of CMI based on a constellation of clinical and radiographic signs, and
- 2.
Discuss the surgical technique for a posterior decompression and stabilization with rigid internal fixation and fusion.
Scope of the problem
Regardless of symptom presentation, multiple large studies looking at outcomes after CMI decompression have found posterior fossa decompression alone to successfully alleviate symptoms more than 80% of the time. Looking at their institutional experience with 500 patients who underwent surgical treatment of pediatric CMI, Tubbs and colleagues report a 3% reoperation rate for continued symptoms or persistent large syringomyelia, with only 4 out of 500 patients requiring transoral odontectomy and occipitocervical fusion.
The prevalence of CVJ instability and the need for stabilization is likely to be skewed by studies from centers known to specialize in CVJ abnormalities. For instance, Brockmeyer found that 10% of patients who underwent surgery for CMI required occipitocervical fusion. Menezes also reported a high incidence of CVJ instability, likely because he mainly focuses on patients who have known CVJ abnormalities, some in conjunction with CMI. In the population at large, however, the coincidence of CVJ instability with CMI is much less frequent.
Diagnosis
Defining CVJ instability in the setting of CMI is often challenging. Given the wide variety of causes and radiographic appearances, there is limited consensus among neurosurgeons about the exact clinical and radiographic criteria that should be used when evaluating patients with CMI to determine which patients should undergo CVJ stabilization at the same time as the initial decompression, or even which patients have a higher likelihood of requiring a stabilization procedure down the line. The diagnosis of CVJ instability should be based both on clinical and radiographic findings.
Clinical Findings
The clinical findings associated with CVJ instability in the setting of CMI are often not distinct from the symptoms attributable to the Chiari malformation alone, or CMI with associated syringomyelia. The most common presenting symptom of CMI is occipital headache, found in 28% to 63% of patients. Looking at 1700 children presenting with CVJ abnormalities, Menezes found cervical and occipital pain to be present in 85% of subjects. The most common neurologic deficit reported was myelopathy, ranging from monoparesis to quadriparesis, with central cord findings and posterior column dysfunction also commonly seen. Downbeat nystagmus was found to be present more often in patients with CVJ instability and an associated CMI, compared with patients with CVJ instability without a concomitant Chiari ( Box 1 ).
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Occipital headache
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Neck pain
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Myelopathy
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Hyperreflexia
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Central cord syndrome
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Posterior column dysfunction
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Downbeat nystagmus
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Decreased gag reflex
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Dysphagia
In a large case series of 234 subjects who presented with CMI requiring CVJ stabilization, Fenoy and colleagues found the most common presenting symptom was hyperreflexia in 61% of subjects, followed closely by headache in 58% of subjects. This is in stark contrast to the retrospective series from Tubbs and colleagues of 500 subjects who underwent posterior decompression alone for CMI, very few of whom required further surgical intervention. In this series, lower extremity hyperreflexia was found in only 2.4% of subjects. By far the most frequent presenting symptoms were headache and back pain, seen in 40% of subjects.
Other symptoms that may suggest CVJ instability include a decreased gag response, seen in 39% of subjects who required CVJ fusion in the Fenoy series; upper extremity numbness or paresthesia, in 37% of subjects; and upper-extremity weakness, in 24% of subjects. This again is significantly different from the series from Tubbs and colleagues in which only 8% of subjects presented with upper extremity pain, weakness, or numbness. Fenoy and colleagues also found neck pain, dysphagia, and C2 numbness to each be present in 20% of subjects requiring CVJ fusion, compared with only 4% of subjects who reported dysphagia in the Tubbs and colleagues series.
Radiographic Findings
If a patient presents with a classic CMI, CVJ stability can be assumed, even if there is an associated syringomyelia or scoliosis. In a retrospective study of 210 pediatric subjects who underwent surgical management of Chiari-related malformations, for instance, Brockmeyer found that none of the 173 subjects who had a classic CMI required surgical intervention beyond a suboccipital craniectomy and C1 laminectomy, with or without duraplasty.
There is no radiographic finding that is present in all CMI subjects with CVJ instability. However, several radiographic findings have been associated with CVJ instability and a higher likelihood of requiring CVJ fusion.
Basilar invagination
Basilar invagination, often defined by Chamberlain line or Wackenheim line, has been reported in 2% to 38% of CMI patients at the time of initial presentation. In a retrospective series of 101 pediatric subjects with CMI published by Bollo and colleagues, 100% of subjects with basilar invagination required CVJ stabilization, compared with only 12.8% of subjects without basilar invagination, suggesting that, although not the only contributor, the presence of basilar invagination itself is highly suggestive of CVJ instability.
Ventral brainstem compression
Traditional means of defining ventral brainstem compression in pediatric patients with CMI relied solely on the presence of basilar invagination. However, it is now known that some patients can have symptomatic ventral compression and CVJ instability in the absence of basilar invagination.
In an effort to better define and assess ventral brainstem compression in patients with CMI, Grabb and colleagues defined a line that extends from the basion to the inferoposterior aspect of the body of C2 (B-C2). They further defined the line as the distance extending perpendicularly from the B-C2 line through the odontoid tip to the ventral dura (pB-C2). Grabb and colleagues found a pB-C2 distance of greater than 9 mm is associated with symptomatic ventral brainstem compression and risk of CVJ instability. Although it has not been validated with large numbers of patients, this distance has subsequently been used as an objective measure of ventral brainstem compression and odontoid retroflexion.
Using a pB-C2 measurement of greater than 5 mm as a cutoff, Brockmeyer found odontoid retroflexion in 20% (32 of 210) of CMI subjects, 13 of whom had successful reduction following CVJ instrumentation and fusion alone. Bollo and colleagues further reported that 74% of subjects who required CVJ fusion had a pB-C2 measurement of greater than or equal to 9 mm, whereas only 24% of subjects who did not require fusion had a pB-C2 measurement of greater than or equal to 9 mm.
Craniocervical angulation
Craniocervical angulation, defined conservatively as a clivoaxial angle (CXA) of less than 125°, has been shown to be a significant predictive factor for requiring CVJ fusion. Defined by Smoker as the angle formed at the intersection of the Wackenheim clivus baseline, a line drawn along the clivus and continued inferiorly into the upper cervical spinal canal, with a line drawn along the posterior surface of the axis body and odontoid process, the craniovertebral, or craniocervical, angle normally ranges from 150° in flexion to 180° in extension. In some CMI patients, this angle may become abnormally acute, with resultant compression of the CVJ. Increased angulation is thought to be indicative of hypermobility, which is associated with CVJ instability.
Chiari 1.5
Chiari 1.5, one of the complex Chiari malformations defined as brainstem herniation through the foramen magnum, has also been reported to be a risk factor for requiring CVJ stabilization and fusion. Bollo and colleagues reported that among 101 children with CMI, 95% (18 out of 19) subjects who required CVJ fusion had Chiari 1.5. In subgroup analyses looking at subjects with Chiari 1.5, CVJ fusion was reported in more than 50% of subjects.
Genetic Syndromes
Certain genetic syndromes, namely disorders of connective tissue, may present in conjunction with CMI and are also associated with a predisposition for CVJ instability ( Box 2 ). Other genetic syndromes known to be associated with CVJ instability are rarely found in conjunction with CMI. However, when these syndromes do coexist, fusion at the time of the initial decompression may be indicated, given the higher risk for CVJ instability.
Related posts:
Definitions and Anatomic Considerations in Chiari I Malformation and Associated Syringomyelia
Pathogenesis and Cerebrospinal Fluid Hydrodynamics of the Chiari I Malformation
Prevalence of Chiari I Malformation and Syringomyelia
Clinical Outcome Measures in Chiari I Malformation
Surgical History of Sleep Apnea in Pediatric Patients with Chiari Type 1 Malformation
Complex Chiari Malformations in Children
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