Chiari I malformation

Findings

Figure 1 and Figure 2: Sagittal T1 and Sagittal T2 images showing tonsillar herniation, as well as the presence of syringomyelia at the level of the cervical cord.
Figure 3 and Figure 4: Axial imaging demonstrates the presence of a cervical syrinx on T1 and T2.


Diagnosis: Chiari I malformation


Chiari malformations are a spectrum of anomalies associated with a reduction of posterior cranial fossa volumes and CSF while conserving overall brain volume. Chiari I malformations refer to a greater than 5 mm herniation of the cerebellar tonsils down through the foramen magnum. The cerebellar vermis and ventricular system are typically unaffected, and the fourth ventricle is usually normal. In its pure form, the tonsils fall to the C1-C2 level, with the brain stem seen in normal position.

Hydromyelia (a dilatation of the central canal within the spinal cord) or syringomyelia (paracentral canal dilatation) is seen in 20% to 70% of Chiari I individuals. In symptomatic patients with herniation greater than 5 mm, syringomyelia has been seen in approximately 50% of the cases. Dilatation occurs, as there is backup of CSF and increased pressure within the affected canal. In addition, 20% of Chiari I patients have hydrocephalus.

Chiari I is also associated with numerous skeletal abnormalities. Unlike Chiari II malformations, Chiari I is not associated with myelomeningocele.

A genetic basis for Chiari I has been suggested by its association with achondroplasia, Hadju-Cheney syndrome (a very rare connective tissue and bone disorder affecting skull base, mandible, and fingers), and Klippel-Feil syndrome (C2-C3 fusion), along with reports of familial pattern of occurrence and concordance in monozygotic twins.

Patients typically present in the second or third decade of life with suboccipital headaches, retro-orbital pressure or pain, clumsiness, dizziness, vertigo, tinnitus, muscle weakness, and lower cranial nerve symptoms. Women are more frequently affected than men.

X-ray will show the associated skeletal anomalies, such as skull base invagination, scoliosis, kyphosis, cervical ribs, and many other skeletal anomalies. Axial images with CT may demonstrate an obliterated cisterna magna, tonsillar ectopia, and a normal fourth ventricle. CT may be useful in patients who have contraindications to MRI. MRI is the study of choice, as tonsillar position and configuration are easily identified along with other associated findings, such as syringomyelia. Most findings are best viewed in the sagittal plane.