Chiari I malformation (CM) is a common neurosurgical diagnosis and spinal cord syrinx is frequently found in patients with CM. Asymptomatic CM is a common imaging finding. Symptomatic CM is less common. Variation in prevalence estimates may be attributed to differences in sensitivity of CM detection between studies as well as differences in the populations being analyzed. The prevalence of low tonsil position and CM on MRI is higher in children and young adults compared with older adults. Studies that include a large number of older adults find a lower prevalence compared with analyses of children.
Key points
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Defining prevalence of Chiari I malformation is challenging. Existing studies are limited by detection bias.
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Asymptomatic Chiari I malformation is common.
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Symptomatic Chiari I malformation is less common.
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Prevalence of Chiari I malformation likely decreases with older age.
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Spinal cord syrinx in the context of Chiari I malformation tends to occur in the cervical spinal cord and is increasingly seen with progressively lower tonsil position.
Imaging prevalence of Chiari I malformation
Chiari I malformation (CM) is a common neurosurgical diagnosis and spinal cord syrinx is frequently found in patients with CM. However, it has been difficult to establish the true prevalence of these two associated conditions. Determining the population prevalence of CM and any associated spinal syringes would require evaluating every individual within a population of interest using a tool that is both perfectly sensitive and perfectly specific for this condition. Because such a study has so far proved to be impossible, clinicians must rely on imperfect estimates of CM and syrinx prevalence. The largest studies of CM prevalence retrospectively reviewed large numbers of patients selected for brain and spine imaging. Rather than defining the true population prevalence, these studies describe, by definition, the imaging prevalence of the condition. The main limitation of this approach is that individuals undergoing MRI are not representative of the general population. Because patients with neurologic symptoms are more likely to undergo neuroimaging, imaging prevalence estimates are influenced by detection bias when the condition is associated with symptoms. Conditions that are usually symptomatic are over-represented in imaging prevalence studies because of detection bias. For this reason, imaging prevalence estimates are expected to correlate more precisely with true population prevalence estimates for asymptomatic conditions compared with symptomatic diseases. Their limitations notwithstanding, these imaging prevalence studies have provided the best estimates of CM prevalence in the population, especially given the large numbers of asymptomatic individuals who meet imaging criteria for the diagnosis. In addition, they provide insight into the prevalence of spinal cord syrinx in the context of CM.
On imaging, CM is typically defined by cerebellar tonsil position 5 mm or more below the foramen magnum. Using this definition, imaging prevalence studies estimate CM prevalence at between 0.24% and 3.6% of the population. Variation in these estimates may be attributed to differences in sensitivity of CM detection between studies as well as differences in the populations being analyzed. For instance, some studies have focused exclusively on children, others exclusively on adults, and others on both children and adults. For CM, the age of the population being examined significantly influences prevalence estimates. In a retrospective review of 22,591 patients undergoing brain MRI at a single institution, Meadows and colleagues found that 175 patients (0.77%) met imaging criteria for CM. These investigators did not report age-specific or age group–specific prevalence. Strahle and colleagues focused on the pediatric population in a retrospective review of 14,116 consecutive patients aged 18 years or younger who had undergone brain or cervical spine MRI at a single institution. They identified 509 patients with CM, representing 3.6% of the imaged population. The discrepancy between these estimates is a result of the different age groups analyzed. The prevalence of low tonsil position as well as CM on MRI is substantially higher in children and young adults compared with older adults. Any study that includes a large number of older adults will find a lower prevalence compared with an analysis focused on children.
Although no lifelong natural history studies have been performed, the changes in age-specific imaging prevalence probably reflect changes that occur in individuals during their lifetimes. Tonsil position should not be expected to be static over the course of a lifetime. Several investigators have reported spontaneous resolution of childhood CM and associated syringes. Mikulis and colleagues were the first investigators to analyze and report on age-related changes in normal cerebellar tonsil position. They identified a trend toward more cranial tonsil position with increasing age. However, their analysis was limited by a small sample size. More recently, Smith and colleagues measured cerebellar tonsil position in a large number of patients who had undergone MRI for any reason, and they compared tonsil positions according to age ( Fig. 1 ). Mean cerebellar tonsil position descended with advancing age into young adulthood, then ascended with advancing age throughout adult life. In all age groups studied, cerebellar tonsil position follows a normal or near-normal distribution ( Fig. 2 ). Individuals with tonsil position at the low end of this distribution are within the group that is usually considered consistent with an imaging diagnosis of CM. This finding has implications for disease nosology as it relates to CM. Individuals with this imaging finding are at the low end of a continuous spectrum of tonsil positions found in humans. Female sex is associated with a higher prevalence of CM on imaging. Female patients seem to have lower cerebellar tonsil positions compared with male patients in all age groups. Obesity or increased body mass index does not seem to be related to the prevalence of CM on imaging.
Several groups have attempted to mitigate the detection bias inherent in imaging prevalence analyses by reporting on intracranial findings after MRI screening of healthy adult volunteers. Vernooij and colleagues screened 2000 healthy adults more than 45 years of age. Eighteen (0.9%) of the volunteers had CM. Each study that relies on volunteers is subject to negative selection bias, because individuals with symptoms are likely to be excluded. As well as specific age criteria, this limits reliable extrapolation of results to the general population. None of these studies of healthy volunteers have included children.
Imaging prevalence of Chiari I malformation
Chiari I malformation (CM) is a common neurosurgical diagnosis and spinal cord syrinx is frequently found in patients with CM. However, it has been difficult to establish the true prevalence of these two associated conditions. Determining the population prevalence of CM and any associated spinal syringes would require evaluating every individual within a population of interest using a tool that is both perfectly sensitive and perfectly specific for this condition. Because such a study has so far proved to be impossible, clinicians must rely on imperfect estimates of CM and syrinx prevalence. The largest studies of CM prevalence retrospectively reviewed large numbers of patients selected for brain and spine imaging. Rather than defining the true population prevalence, these studies describe, by definition, the imaging prevalence of the condition. The main limitation of this approach is that individuals undergoing MRI are not representative of the general population. Because patients with neurologic symptoms are more likely to undergo neuroimaging, imaging prevalence estimates are influenced by detection bias when the condition is associated with symptoms. Conditions that are usually symptomatic are over-represented in imaging prevalence studies because of detection bias. For this reason, imaging prevalence estimates are expected to correlate more precisely with true population prevalence estimates for asymptomatic conditions compared with symptomatic diseases. Their limitations notwithstanding, these imaging prevalence studies have provided the best estimates of CM prevalence in the population, especially given the large numbers of asymptomatic individuals who meet imaging criteria for the diagnosis. In addition, they provide insight into the prevalence of spinal cord syrinx in the context of CM.
On imaging, CM is typically defined by cerebellar tonsil position 5 mm or more below the foramen magnum. Using this definition, imaging prevalence studies estimate CM prevalence at between 0.24% and 3.6% of the population. Variation in these estimates may be attributed to differences in sensitivity of CM detection between studies as well as differences in the populations being analyzed. For instance, some studies have focused exclusively on children, others exclusively on adults, and others on both children and adults. For CM, the age of the population being examined significantly influences prevalence estimates. In a retrospective review of 22,591 patients undergoing brain MRI at a single institution, Meadows and colleagues found that 175 patients (0.77%) met imaging criteria for CM. These investigators did not report age-specific or age group–specific prevalence. Strahle and colleagues focused on the pediatric population in a retrospective review of 14,116 consecutive patients aged 18 years or younger who had undergone brain or cervical spine MRI at a single institution. They identified 509 patients with CM, representing 3.6% of the imaged population. The discrepancy between these estimates is a result of the different age groups analyzed. The prevalence of low tonsil position as well as CM on MRI is substantially higher in children and young adults compared with older adults. Any study that includes a large number of older adults will find a lower prevalence compared with an analysis focused on children.
Although no lifelong natural history studies have been performed, the changes in age-specific imaging prevalence probably reflect changes that occur in individuals during their lifetimes. Tonsil position should not be expected to be static over the course of a lifetime. Several investigators have reported spontaneous resolution of childhood CM and associated syringes. Mikulis and colleagues were the first investigators to analyze and report on age-related changes in normal cerebellar tonsil position. They identified a trend toward more cranial tonsil position with increasing age. However, their analysis was limited by a small sample size. More recently, Smith and colleagues measured cerebellar tonsil position in a large number of patients who had undergone MRI for any reason, and they compared tonsil positions according to age ( Fig. 1 ). Mean cerebellar tonsil position descended with advancing age into young adulthood, then ascended with advancing age throughout adult life. In all age groups studied, cerebellar tonsil position follows a normal or near-normal distribution ( Fig. 2 ). Individuals with tonsil position at the low end of this distribution are within the group that is usually considered consistent with an imaging diagnosis of CM. This finding has implications for disease nosology as it relates to CM. Individuals with this imaging finding are at the low end of a continuous spectrum of tonsil positions found in humans. Female sex is associated with a higher prevalence of CM on imaging. Female patients seem to have lower cerebellar tonsil positions compared with male patients in all age groups. Obesity or increased body mass index does not seem to be related to the prevalence of CM on imaging.
Related posts:
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Pathogenesis and Cerebrospinal Fluid Hydrodynamics of the Chiari I Malformation
Clinical Presentation of Chiari I Malformation and Syringomyelia in Children
Surgical History of Sleep Apnea in Pediatric Patients with Chiari Type 1 Malformation
Craniovertebral Junction Instability in the Setting of Chiari I Malformation
Complex Chiari Malformations in Children
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