This case demonstrates the incidental finding of a Chiari I malformation and obstructive hydrocephalus, in a patient initially worked up for a pituitary tumor.

Hydrocephalus is uncommon in Chiari I malformation and may be either the cause of tonsillar descent or the result of an obstruction in the posterior fossa. Given that posterior fossa decompression failed to treat the hydrocephalus in this instance, the former may be favored.

Chiari I malformation is a congenital hindbrain dysgenesis characterized by caudal displacement of the cerebellar tonsils below the foramen magnum, and into the spinal canal ^1-2. The Chiari malformations are four separate anatomical entities with distinct clinical-anatomical features, of which Chiari I is the most common ³. It is found either incidentally or in patients asymptomatic or minimally symptomatic and is associated with a broad spectrum of clinical symptoms and features including ³:

• those related to hindbrain compression:  headache, neck pain, sensorimotor incontinence deficits, ataxia and cranial nerve palsies

• in the presence of syringomyelia, patients may additionally present with central cord syndrome, scoliosis, hyperreflexia, spasticity or torticollis

Chiari I malformations are often associated with descent and distortion of the brainstem-spinal cord junction, osseous abnormalities at the craniocervical junction, basilar invagination and scoliosis ^3. Hydrosyringomyleia and hydrocephalus often complicate it. These arise due to impaired CSF flow and dynamics. CSF across the foramen magnum is rhythmically synchronized with cardiac pulsation and respiratory variation ². In Chiari I malformations, CSF flow at the foramen magnum is impaired due to the abnormal pulsatile motion of the cerebellar tonsils ⁴. This produces selective obstruction CSF from the cranial cavity to the spine during cardiac systole. The increased systolic CSF waves in the spinal subarachnoid space drive the CSF into the central canal of the spinal cord through dilatated perivascular spaces.

MR imaging of the brain has revolutionized diagnostic evaluation of Chiari I malformation where until the past decade diagnosis has necessitated invasive CT myelography ¹. Furthermore, MRI can detect Chiari I malformation that previously remained unrecognized or was misdiagnosed.

Caudal displacement of the cerebellar tonsils, tonsillar configuration, and the aforementioned abnormalities are depicted on sagittal, coronal and axial T1- and T2-weighted MR images. Tonsillar displacement is measured on the midsagittal MRI, by drawing a line from the inner margins of the foramen magnum (basion to opisthion), and measuring the distance from this line to the tips of the cerebellar tonsils ¹. Thus, the definition of type I Chiari malformation varies across studies. However, the most widely used definitions include ^1-4:

• less than 3 mm cerebellar tonsillar displacement below the foramen magnum is normal

• greater than 5 mm (in adults) displacement and greater than 6 mm (in children) is consistent with type I Chiari malformation

• tonsillar displacement between 3 to 5 mm may be termed benign cerebellar ectopia or low-lying cerebellar tonsils and requires close correlation with other imaging findings and symptoms

Spine imaging is essential in the initial evaluation because of the high percentage of associated syrinx formation. Syrinx formation is most common at the C4 through C6 levels, but may occur anywhere along the spinal cord ⁵.

Treatment is reserved only for symptomatic patients or those with a syrinx and consists of a posterior fossa decompressive craniectomy with the removal of the posterior arch of C1 and duroplasty ^1-4.

Case courtesy of Associate Professor Pramit Phal