Ashley H. Aiken

Our manuscript is the first in the radiology literature to evaluate the incidence and morphology of cerebellar tonsillar ectopia (CTE) in patients with a clinical diagnosis of idiopathic intracranial hypertension (IIH). We first became interested in the appearance of the cerebellar tonsils as we witnessed the clinical dilemma the surgeons faced in patients with headache, CTE, and high intracranial pressures. In the Southeast, we care for many patients with IIH. At our institution, we are very careful to exclude high (or low intracranial pressure) before making a diagnosis of “Chiari I malformation” by imaging. We closely review the patient’s presentation, and we search for and report radiographic features that would support these alternative diagnoses. The surgical literature describes a subset of  patients with IIH with CTE meeting the criteria for Chiari I malformation (CM), but not responding to surgical decompression for CM. Fagan et al coined the phrase “Chiari pseudotumor cerebri syndrome” to described this subset of patients, in whom CM and IIH seem to co-exist. Neither the underlying pathophysiology explaining these diagnoses nor a cause and effect relationship is well understood. For this reason, treatment considerations, including decompression versus shunting, are complex. Radiologists need to be aware of these complexities when they are presented with a case of CTE.

Our study found that 21% of patients with IIH had CTE >5 mm, meeting the diagnostic criteria for CM. In addition patients with IIH had significantly lower tonsillar position (2.1 mm +/-2.8) than healthy controls (.7mm +/- 1.9). As our study shows, when CTE is identified, clinical and imaging consideration of IIH (looking for supportive findings such as empty sella, enlarged skull base foramina, transverse sinus stenosis, and papilledema) is warranted before surgical treatment of “CM.”

My co-author and colleague, Dr Amit Saindane, is continuing research into this area with studies that have evaluated the significance of other imaging findings that are typically associated with elevated intracranial pressure (ICP) including an “empty” sella turcica,¹ and other potential imaging

findings such as skull base and Meckel cave meningoceles that suggest elevated ICP,² as well as the importance of these types of findings on clinical outcomes in IIH.³ The imaging finding of transverse venous sinus stenosis (TSS) has also been highly associated with elevated ICP, and IIH in particular, and other studies by our group have explored its prevalence outside of IIH^4,5 and its prognostic value in IIH.⁶ A more recent study has shown that TSS is present in up to 30% of patients with Chiari I malformations and may be a sign of elevated ICP in Chiari I, misdiagnosis of IIH, or co-existent IIH in patients with surgically proven Chiari I malformations.⁷ The difficulties in differentiation of these entities makes it critical for the radiologist to appreciate the various orbital, skull base, and venous sinus abnormalities that can be suggestive of elevated ICP, and to be cognizant of the various possible diagnoses in the setting of tonsillar ectopia.

References

1. Saindane AM, Lim P, Aiken AH, et al. Factors determining the clinical significance of an "empty" sella turcica. AJR Am J Roentgenol 2013;200:1125–31, 10.2214/AJR.12.9013
2. Bialer OY, Rueda MP, Bruce BB, et al. Meningoceles in idiopathic intracranial hypertension. AJR Am J Roentgenol 2014;202:608–13, 10.2214/AJR.13.10874
3. Saindane AM, Riggeal B, Bruce BB, et al. Association of MRI findings and visual outcome in idiopathic intracranial hypertension. AJR Am J Roentgenol 2013;201:412–18, 10.2214/AJR.12.9638
4. Kelly L, Saindane AM, Bruce BB, et al. Does bilateral transverse cerebral venous sinus stenosis really exist in patients without increased intracranial pressure? Clin Neurol Neurosurg 2013;115:1215–19, 10.1016/j.clineuro.2012.11.004
5. Ridha M, Saindane AM, Bruce BB, et al. Magnetic resonance imaging findings of elevated intracranial pressure in cerebral venous thrombosis versus idiopathic intracranial hypertension with transverse sinus stenosis. Neuroophthalmology 2013;37:1–6, 10.3109/01658107.2012.738759
6. Riggeal B, Bruce BB, Saindane AM, et al. Clinical course of idiopathic intracranial hypertension with transverse sinus stenosis. Neurology 2013;80:289–95, 10.1212/WNL.0b013e31827debd6
7. Saindane AM, Bruce BB, Desai NK, et al. Transverse sinus stenosis in the adult Chiari I malformation. AJR Am J Roentgenol (in press)

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