Chiari I and Pseudotumor Cerebri (Idiopathic Intracranial Hypertension)

Definition of Pseudotumor Cerebri (Idiopathic Intracranial Hypertension)

Pseudotumor cerebri, sometimes known as Idiopathic Intracranial Hypertension (IIH), patients manifest clinical signs and symptoms of elevated intracranial pressure (ICP) and without ventricular enlargement or mass lesions. 5 There is evidence that Chiari I malformation and pseudotumor cerebri (Idiopathic Intracranial Hypertension) may coexist. Johnston et al 7 found that 6% of adult pseudotumor cerebri patients also have Chiari I, which is 8 times the incidence of Chiari I in the general population. Banik et al 1 retrospectively found that up to 24% of pseudotumor (Idiopathic Intracranial Hypertension) patients may demonstrate radiographic evidence of Chiari I. However, a cause-and-effect has yet to be proven.

For example, a Chiari pseudotumor cerebri syndrome is described in a group of patients who failed to improve after Chiari I decompression surgery, and who are found to have elevated ICP with small ventricles. 2,4,7 These patients’ symptoms may be relieved with CSF drainage or diversion (usually CSF shunt).

On the other hand, Fagan et al 4 discuss a group of patients treated for pseudotumor cerebri (Idiopathic Intracranial Hypertension), who later require Chiari decompression.

Chiari I is considered a disorder of the paraxial mesoderm with hindbrain maldevelopment and small posterior fossa volume. Pseudotumor cerebri has evidence of increased edema and altered CSF absorption with disordered intracranial compliance. 2,5 It is postulated that patients with coexistence of these two syndromes may have transient relief after Chiari decompression, which alters intracranial compliance. If an underlying or coexistent pseudotumor cerebri is left untreated, symptoms recur at an average of 9-10 months after surgery, and these patients then tend to need a CSF shunting procedure.

Signs and Symptoms of Pseudotumor Cerebri (Idiopathic Intracranial Hypertension)

There exists an overlap of the wide constellation of symptoms associated with Chiari I malformation and pseudotumor cerebri, including exertional headaches, visual changes, tinnitus, lower cranial nerve dysfunction, brainstem dysfunction, sensory changes, ataxia, abnormal reflexes, and fatigue. 1-7

Diagnostic Studies Regarding Chiari I Malformation & Pseudotumor Cerebri (Idiopathic Intracranial Hypertension)

Chiari I malformation is radiographically defined as tonsillar descent of 5mm or more below the level of the foramen magnum on MR imaging. 1,6 Pseudotumor cerebri (Idiopathic Intracranial Hypertension) is diagnosed based on clinical signs and symptoms of elevated ICP, documented high ICP (for example, by lumbar puncture or ICP monitoring), and commonly with papilledema but without focal neurologic signs, mass lesions, infections, or ventriculomegaly. It is associated with obesity, female sex, and menstrual irregularity. 1,4,5 In patients reported to have coexistence of Chiari I and pseudotumor cerebri, recurrence of Chiari I symptoms after Chiari decompression (failed Chiari) has been reported from 2 months to 3 years after surgery. 2,4 Case reports of acute presentation of increased intracranial pressure within a week of posterior fossa decompression have also been described, with a more fulminant clinical course likely due to multifactorial causes of edema and venous hypertension from surgery.

Treatment of Pseudotumor cerebri (Idiopathic Intracranial Hypertension)

In one retrospective review, it is claimed that up to 41.6% of failed Chiari patients may be diagnosed with pseudotumor cerebri syndrome. 4 If there is adequate decompression as noted on MRI, then a lumbar puncture may be used to measure opening pressures, and also to perform diagnostic and therapeutic CSF drainage to determine whether drainage improves symptoms. Both ventriculoperitoneal and lumboperitoneal shunts are options in patients who show symptomatic relief from the diagnostic lumbar puncture and CSF drainage in this setting.

Prognosis and Outcomes

In patients who experience symptomatic relief from a lumbar puncture, CSF shunt placement generally relieves signs of high ICP such as optic nerve swelling and to a lesser extent headaches. Of the 36 failed Chiari patients described by Fagan, et al, 4 15 were found to have pseudotumor cerebri (Idiopathic Intracranial Hypertension). Of those 14 underwent shunt placement (nine children and 5 adults). Of the 9 children, 7 experienced symptomatic improvement and 2 had moderate improvement. The 5 adults all showed decrease in optic nerve swelling with no improvement in the headaches. Bejjani, et al 3 reported symptomatic relief in 6 similar adult patients who underwent shunting.

Chiari I and pseudotumor cerebri may coexist, thus confusing the diagnosis, although the exact relationship is not understood. It may be that increased brain edema in pseudotumor cerebri may cause intracranial contents to herniate through the foramen magnum manifesting on imaging studies as a Chiari I malformation. Alternatively, it may also be that some Chiari I patients have abnormalities in CSF dynamics that predispose to the development of impaired CSF absorption after Chiari decompression. In such cases, a preexisting pseudotumor cerebri is unmasked by the decompression. 2,3,4 Because of the rarity of this relationship, it is rare that Chiari I patients have ICP investigations preoperatively; in the setting of failed Chiari patients, however, measuring ICP may be useful. If the ICP is elevated, then CSF diversion might be a viable treatment option.

The readers should again be cautioned that this is a rare relationship and the results remain controversial.


  1. Banik R, Lin D, Miller NR. Prevalence of Chiari I malformation and cerebellar ectopia in patients with pseudotumor cerebri. J Neurol Sci 2006;247:71-75.
  2. Bejjani GK. Association of the Adult Chiari Malformation and Idiopathic Intracranial Hypertension: more than a coincidence. Med Hypotheses 2003;60:859-863.
  3. Bejjani GK, Cockerham KP, Rothfus WE, Marroon JC, Maddock M. Treatment of failed adult Chiari malformation decompression with CSF drainage: observations in 10 patients. Acta Neurochir 2003;145:107-116.
  4. Fagan LH, Ferguson S, Yassari R, Frim DM. The Chiari pseudotumor syndrome: symptom recurrence after decompressive surgery for Chiari malformation type I. Pediatric Neurosurg 2006;42:14-19.
  5. Johnston I, Hawke S, Halmagyi M, Teo C. The pseudotumor syndrome. Disorders of cerebrospinal fluid circulation causing intracranial hypertension without ventriculomegaly. Arch Neurol 1991;48:740-747.
  6. Milhorat TH, Chou MW, Trinidad EM, Kula R, Mandell M, Wolpert C, Speer MC. Chiari I malformation redefined: clinical and radiographic findings for 364 symptomatic patients. Neurosurgery 1999;44:1005-1017.
  7. Sinclair N, Assaad N, Johnston I. Pseudotumor cerebri occurring in association with the Chiari malformation. J Clinical Neurosci 2002;9:99-101.

Revised 05/2010

Contact us:
Find us on:
View desktop version