Radiology in the diagnosis of the Chiari I malformation
by Victor Haughton, MD, University of WI-Madison
For some patients with the Chiari I malformation CSF flow studies may be helpful in determining if surgical management is appropriate. Some patients with a Chiari malformation may not need the flow studies.
The position of the tonsils can be evaluated by a routine MR scan or even a CT scan, making the identification of the Chiari I malformation simple. The definition of the Chiari I malformation for radiologists is the descent of the tonsils 5 mm below the foramen magnum. Although this amount of tonsilar malposition is found once in approximately every 150 scans, it is not always a finding that needs to be evaluated further or treated. In some cases it is an incidental finding, that is not related to the conditions or symptoms for which the scan was requested. In other cases the finding may explain the patient's symptoms or conditions. For example, in patients with Valsalva-induced headaches (exertional headaches), syringomyelia (spinal cord cysts), apneic spells (temporary interruption of breathing pattern), the Chiari I malformation can be diagnosed as the cause of the symptoms with a high degree of confidence. In other patients, having MR because of a pituitary disorder for example, the low position of the tonsils does not likely relate to the patient’s symptoms. In many cases, the patient's signs and symptoms may not be easily attributed to one cause and not confidently explained by the tonsil position. Such patents might include patients with less distinctive types of headaches for example.
Therefore CSF flow studies have been developed to provide a functional test of the effect of the tonsils, to distinguish the "symptomatic Chiari I” (with symptoms resulting from the tonsil position) from the "asymptomatic Chiari I" (with symptoms or symptoms unrelated to the tonsil position). The usual test of CSF flow is the cardiac-gated Phase Contrast MR study (PC MR). This study shows the pattern and measures the velocity of CSF flow. The study may be viewed as a cine loop showing the changes in CSF flow during the cardiac cycle, or it may be viewed as a series of images representing different phases in the cardiac cycle. The images may be analyzed to measure the velocity of the CSF and, with sophisticated mathematical techniques, the pressures, pressure waves, flow directions, and flow structure. PC MR shows different CSF flow patterns in Chiari I patients than in normal subjects. It shows different patterns in "symptomatic" than in "asymptomatic" patients). In PC MR, the symptomatic patients display "synchronous bidirectional flow" and greater CSF velocity magnitudes compared to the "asymptomatic patients. PC MR has good but not perfect accuracy (1). The PC MR test for abnormal CSF flow should include axial images at the tip of the tonsils and perhaps sagittal images.
New research suggests that the accuracy of the test may be improved by obtaining axial PC MR images at multiple levels. The goal of research is to define the critical CSF flow parameters that result necessarily in signs and symptoms. Sophisticated techniques, Computation Flow Design, are used by some investigators in attempt to explain how obstruction of CSF flow by the tonsils results in a syrinx, spinal cord cyst, inches below the obstruction. These studies are enhancing our understanding of CSF flow and promising to improve our diagnosis and care of the Chiari I malformation.
- Hofkes SK, Iskandar BJ, Turski PA, Gentry LR, McCue JB, Haughton VM. Differentiation between symptomatic Chiari I malformation and asymptomatic tonsilar ectopia by using cerebrospinal fluid flow imaging: initial estimate of imaging accuracy. Radiology. 2007 Nov;245(2):532-40. Epub 2007 Sep 21.