Obstructive hydrocephalus and Chiari 1 malformation

Case contributed by Rajalakshmi Ramesh
Diagnosis certain


Six-month history of amenorrhea; Referred by a general practitioner for MR imaging of the brain to exclude a pituitary tumor.

Patient Data

Age: 30 years
Gender: Female

No MRI evidence of a pituitary mass lesion.

The ventricles are enlarged consistent with obstructive hydrocephalus with marked lateral and third ventriculomegaly. The floor of the third ventricle is bowed into the suprasellar region. No evidence of transependymal edema, however, the cerebral cortical sulci appear mildly effaced. The sella appears "empty" with a prominent suprasellar cistern and pituitary tissue compressed along the floor of the sella. No pituitary mass lesion is identified.

The cerebellar tonsils extend 20 mm below the foramen magnum with marked crowding at the foramen magnum with mass effect on the upper cervical cord and brainstem. No evidence of syrinx within the upper cervical cord.


Obstructive hydrocephalus. Chiari 1 malformation. It is unclear if the tonsillar descent is primary or secondary to hydrocephalus.

Given the findings of Chiari I malformation, the patient underwent further imaging to investigate for syrinx. CT and MRI images of the cervical spine are shown below.

CT Cervical spine


C-Spine imaged only down to C5. Anatomical alignment. No fracture/dislocation identified. No central canal or intervertebral foraminal stenosis. Hydrocephalus and low lying cerebellar tonsils within moderately enlarged foramen magnum noted. Enlarged, remodeled sella with dilated anterior the third ventricle within it. No aqueduct stenosis. Obliteration of pontine cistern and fourth ventricular outlet syndicates obstruction at the level of the fourth ventricular outlets. No associated syrinx seen.

MRI Cervical spine


Standard T1 and T2 weighted sagittal and axial scans were performed through the cervical spine. A typical Chiari I malformation is noted at the foramen magnum. The cerebellar tonsils extend inferiorly as far as the superior aspect of the arch of C2. The combination of the ectopic tonsils and the medulla obliterate almost all the subarachnoid space at the foramen magnum level. The dorsal columns are convex posteriorly at the proximal end of the cervical cord. The obex is prominent, but the central canal quickly resumes normal diameter and there is no evidence of a syrinx. Incidental hemangioma noted in the body of C6. Enlarged, "empty" sella also noted.

Conclusion: Chiari I malformation, but no syrinx.

It was the clinical belief of the treating team that the Chiari 1 malformation was the primary problem with secondary hydrocephalus. The patient, therefore, underwent a posterior fossa decompressive craniectomy with a C1 and C2 cervical laminectomy.

Two months following this initial surgery, the patient underwent further operative management with the insertion of a ventriculoperitoneal shunt for persisting obstructive hydrocephalus. No cause for this patient's amenorrhea was established and she was referred for further endocrinology review.

With regards to her intracranial pathology, the patient has remained stable and under radiological surveillance.

Case Discussion

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

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