Chiari I malformation
- Characterized by downward herniation of the cerebellar tonsils through the foramen magnum, with a normal 4th ventricle.
- Approximately 25-50%% of patients have syringohydromyelia, commonly in the cervical cord. The medulla may be flattened or displaced.
- The cerebellar findings are theorized to be secondary to overcrowding of the posterior fossa from underdeveloped occipital somites. The cerebellar tonsils herniate downward as a result of mechanical factors.
- Syringohydromyelia formation is likely secondary to pathologic CSF dynamics. Pulsatile systolic waves push subarachnoid CSF through perivascular and interstitial spaces into the spinal cord central canal, producing cystic dilatation.
- The malformation is typically congenital with a genetic basis, but can also be acquired after ventriculoperitoneal shunt placement
- Many patients remain asymptomatic throughout their lives. Clinical symptoms may occur when chronic tonislar rubbing produces arachnoid scarring and adhesions near the foramen magnum, resulting in interference of normal CSF flow.
- Symptoms include:
- Occipital Headaches
- Ocular symptoms: visual disturbances, retro-orbital pain, photophobia, diplopia
- Dizziness, vertigo, nystagmus, hearing abnormalities
- Symptoms of brainstem compression: weakness, ataxia, cranial nerve palsies, apnea, palpitations, syncope, and sudden death
- Syringohydromyelia symptoms: pain, altered sensation, impaired motor control, incontinence
- Symptoms may mimic an orbital pseudotumor cerebri, Ménière disease, or bulbar compression
- Associated with other malformation such as platybasia, basilar invagination, craniosynostosis, acromegaly, and the Klippel-Feil syndrome.
 Imaging Findings
- Radiographs may demonstrate associated abnormalities of the skull base.
- CT may demonstrate hydrocephalus, herniated cerebellar tonsils, and a flattened spinal cord. Rarely will CT show a syrinx.
- MRI is the imaging modality of choice to evaluate for a Chiari I malformation. MRI findings:
- Cerebellar tonsillar herniation
- Wedge shaped tonsils
- Small posterior fossa
- Obstructive hydrocephalus
- Brainstem anomalies
- Tonsillar displacement is measured from the basion-opisthion line on a sigittal image.
- Herniation is usually at least 5mm, though patients with 3-5mm herniation may also have the malformation.
- MRI CSF flow studies may prove to be helpful and are currently under investigation.
 See also
 External Links
- Barkovich AJ, Wippold FJ, Sherman JL, Citrin CM. Significance of cerebellar tonsillar position on MR. AJNR Am J Neuroradiol. 1986 Sep-Oct;7(5):795-9.
- Haughton VM, Korosec FR, Medow JE, Dolar MT, Iskandar BJ. Peak systolic and diastolic CSF velocity in the foramen magnum in adult patients with Chiari I malformations and in normal control participants. AJNR Am J Neuroradiol. 2003 Feb;24(2):169-76.
- Elster A, Chen M. Chiari I malformations: clinical and radiologic reappraisal. Radiology. 1992; 183:347-353.
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