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     Growing Evidence Demonstrating Significance of Minimal or no Herniation of Cerebellar Tonsils

                    The authors report four cases of syringomyelia thought to be idiopathic syringomyelia but treated by craniocervical
                    decompression with favorable results. Syringomyelia was present without hindbrain herniation. In such cases, the
                    subarachnoid space anterior to the brainstem at the level of the foramen magnum is usually open but the cisterna magna
                    is impacted by the tonsils, a condition the authors term "tight cisterna magna."
                    ... Syringomyelia associated with tight cisterna magna should not be classified as idiopathic syringomyelia;
                    rather, it belongs to the category of organic syringomyelia such as Chiari malformation. A possible pathogenesis of
                    cavitation is obstruction of the CSF outflow from the foramen of Magendie, and the cavity may be a communicating
                    dilation of the central canal.

                    Cases are presented that highlight the possible connection between familial syringomyelia and the Chiari 0 malformation.
                    Magnetic resonance imaging further revealed that one brother had Chiari I malformation and the other had Chiari 0
                    malformation. Both underwent posterior fossa decompression with radiologic improvement of their syringes. These case
                    reports lend credence to earlier reports of improvement in syringomyelia following posterior fossa decompression in
                    the absence of Chiari I malformation, the so-called Chiari 0 malformation.

                    COMMENTS

                    Milhorat and Bolognese report an excellent technique application. Dr. Milhorat is a member
                    of the scientific community who possibly performs more CMI operations than any other surgeon
                    in the United States. He shows that he meticulously studies each patient and each approach
                    so that he may improve on what has been done previously. ... This is an enormous advantage for
                    those surgeons who base their treatment on the correction of the anatomic and thus the
                    physiological conditions that exist in the posterior fossa. ... First, however, the authors must
                    show how this technique and the subsequent tailored surgical approach correlate with patient
                    outcome in terms of symptoms and radiological criteria such as magnetic resonance imaging.
                    In the next several years, I have little doubt that Milhorat and Bolognese will do precisely that.
                    ...Richard G. Ellenbogen, Seattle, Washington

                    Important comments:

                    "In recent years, the term CMI has been used synonymously with
                    tonsillar ectopia or chronic tonsillar herniation in a wide variety
                    of congenital and acquired disorders. The radiological definition
                    of CMI has been reported as tonsillar herniation of at least 3 mm
                    or at least 5 mm below the foramen magnum. However,
                    this definition is limited to a single criterion and makes no
                    reference to clinical symptoms or the presence or absence of
                    associated findings such as syringomyelia.

                    The radiological definition of CMI may be too restrictive. In
                    this study, there were 32 of 364 patients (9%) who exhibited
                    tonsillar herniation of less than 5 mm and symptoms that
                    were typical of CMI. Seventeen of those 32 patients (53%) had
                    syringomyelia. All patients showed MRI evidence of hindbrain
                    overcrowding, and CINE-MRI demonstrated
                    abnormalities of CSF velocity/flow that were similar
                    to those reported for patients with tonsillar herniation of at
                    least 5 mm. These observations indicate that the
                    extent of tonsillar herniation cannot be used as the sole criterion
                    for the diagnosis of CMI. We could not confirm reports
                    that the severity of symptoms is directly related
                    to the extent of tonsillar herniation. Because tonsillar herniation
                    of at least 5 mm can be encountered as an incidental
                    finding among asymptomatic patients, it is likely that the
                    position of the cerebellar tonsils, although providing a general
                    index of hindbrain overcrowding, is only one factor influencing
                    the clinical features of CMI."

                    "Minimal evidence of hindbrain
                    overcrowding consists of obliteration of the retrocerebellar CSF
                    spaces in association with a meniscus sign at the lower pole of
                    the cerebellar tonsils. CINE-MRI can be helpful in demonstrating
                    a disturbance of CSF velocity/flow at the foramen magnum in
                    patients with tonsillar herniation of less than 5 mm."

                    "Minimal evidence of hindbrain overcrowding consisted of obliteration of
                    the retrocerebellar CSF spaces in association with a meniscus
                    sign at the lower pole of the cerebellar tonsils. For 47 of 364
                    patients who underwent phase-contrast CINE-MRI, including
                    21 patients with tonsillar herniation of less than 5 mm,
                    there was evidence of decreased CSF velocity/flow in the
                    cisterna magna and subarachnoid space posterior to the cerebellum
                    (47 patients) and the premedullary and prepontine
                   spaces anterior to the brain stem (15 patients)."

                    "The most constant abnormality was compression
                    of the CSF spaces posterior and lateral to the cerebellum
                    (364 patients, 100%). Tonsillar herniation of at least 5 mm
                    below the foramen magnum was present in 332 patients (91%)."

                    COMMENTS

                    "Milhorat et al. provide an extensive amount of information
                    on the so-called CMI. On the basis of their very large case
                    series, they have been able to define the clinical syndrome in
                    a more comprehensive way than was previously possible.
                    They also have related this condition to the volume of the PCF
                    and have provided some evidence that CMI is a disorder of
                    the para-axial mesoderm. The data provided will facilitate
                    evaluation of patients in the future. CMI, more than ever,
                    becomes only a useful shorthand term for a disorder that is
                    even more complex than was previously realized."
                    ...Ulrich Batzdorf, Los Angeles, California

                    "Conclusion: A minor degree of tonsillar ectopia may produce signs and symptoms of
                    cervicomedullary compression which can improve following surgical decompression. The
                    current MRI criteria for the diagnosis of symptomatic Chiari type1 malformation should
                    be re-examined as they may be too restrictive."

                    "Only patients with caudal displacement of the cerebellar tonsils of less than 3.0 mm were included in this series."

                    "In our experience, limiting the diagnosis of the Chiari
                    type 1 malformation to cerebellar tonsil ectopia of 3-5 mm or greater below the rim of
                    the foramen magnum may be too restrictive. Patients with lesser
                    degrees of tonsillar ectopia who complain of symptoms consistent with compression of
                    the structures coursing through the foramen magnum could potentially be denied the
                    benefits of decompressive surgery."

                    "Despite the minimal degree of tonsillar ectopia, our patients reported improvement in symptoms
                    in response to surgical enlargement of the foramen magnum. An improvement in
                    objective neurological signs was also noted."

                    "We suggest that the Chiari type 1
                    malformation should be viewed as a relative disproportion between the cross sectional
                    area of the foramen magnum and the structures coursing through it rather than strictly as a
                    measure of tonsillar ectopia. If the structures within the foramen magnum are crowded,
                    then the patient may be symptomatic in the face of a minor degree of tonsillar ectopia.
                    Surgical decompression may be appropriate in selected cases."

                    "This series is the largest reported to date, and the prevalence of CM1
                    (defined conservatively as cerebellar tonsillar herniation >= 5mm) was estimated as 1/1280."                     "Idiopathic syringomyelia and Chiari I-type syringomyelia manifested central cervical myelopathy
                    and a small PF with narrow CSF spaces, suggesting that they develop by the same mechanism."                     "Today's routine clinical use of neuroradiological imaging facilitates determination of
                    malformation type without the need for invasive techniques. With this technological advance,
                    however, type 1 malformations with varying degrees of tonsillar ectopia are increasingly
                    recognized on midsagittal magnetic resonance images in asymptomatic subjects and in some
                    patients with a wide variety of clinical symptoms."

                    "There is continued debate regarding the significance of cerebellar ectopic changes
                    incidentally observed on magnetic resonance images. Neurologists face a challenge
                    that Chiari did not consider, and must determine the significance of incidental
                    Chiari malformations in the context of vague and difficult to localize symptoms."                     "Exclusion criteria were enforced, and the descent of tonsils beyond the foramen magnum line was
                    measured (>=5mm =CM [Chiari I malformations] group and <5mm =LT [low-lying tonsils] group)."

                    "Unilateral and bifrontal headaches were also prominent symptoms for the LT group
                    and showed no significant difference from the CM group."

                    "We offer that LT is part of a continuum of the Chiari malformations and can present
                    similarly to Chiari I malformations, but without central brainstem or cervical cord pathology."                     "All these findings could suggest that the current quoted reference level for diagnosis of
                     Chiari I malformation in AIS (Adolescents with Idiopathic Scoliosis) patients could be
                     inadequate and inaccurate. In conclusion, the current study suggested that using the current
                     MRI reference standards, the incidence of tonsillar herniation could be significantly
                     underestimated in AIS patients. In our experience, inferior displacement of the tonsil
                     below the BO (Basion-Opisthion) line in adolescents should be regarded as abnormal.
                     Scoliosis could be an important manifestation of subclinical tonsillar herniation."                 "Occasionally, even a small amount of tonsillar herniation can be symptomatic, and the
                 CSF flow studies will aid in making the diagnosis in this situation."

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