Excerpt from Pseudotumor Cerebri

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Background

Pseudotumor cerebri, also known as idiopathic intracranial hypertension (IIH) is a disorder of unknown etiology. It affects predominantly obese women of childbearing age. The primary problem is chronically elevated intracranial pressure (ICP), and the most important neurologic manifestation is papilledema, which may lead to progressive optic atrophy and blindness.

Pathophysiology

A dominant early theory concerning the pathogenesis of elevated ICP in these patients was cerebral edema. Against this is the fact that no altered level of alertness, cognitive impairment, or focal neurological findings are associated with the elevated ICP. In addition, no pathologic signs of cerebral edema have been documented in these patients. Early reports describing edema were later considered to represent fixation artifact (ie, from tissue preparation) rather than in vivo edema.

Current theories include increased resistance to cerebrospinal fluid (CSF) outflow at the arachnoid granulations that line the dural venous sinuses and through which CSF reabsorption is thought to occur by bulk flow. Alternatively, occult cerebral venous outflow abnormalities may produce IIH.

Farb and colleagues have demonstrated that, in a series of 29 patients with IIH, narrowing of the transverse dural venous sinus was demonstrable on MR venography, while none of the 59 control subjects had this finding.¹ These authors suggest that the narrowing is a consequence of elevated intracranial pressure, and, when the narrowing develops, it exacerbates the pressure elevation by increasing venous pressure in the superior sagittal sinus.

• CSF production rate (mL/min) should be equal to the CSF reabsorption rate.
• If production exceeds absorption, ICP rises until it exceeds mean arterial pressure, which, if sustained, would be fatal.
• In IIH the production rate equals the reabsorption rate; however, a higher than normal pressure is required to achieve this owing to the increased resistance at the arachnoid granulations.

Bateman has shown that some patients with IIH with normal dural venous drainage have increased arterial inflow suggesting that collateral venous drainage occurs in addition to that provided by the superior sagittal sinus and transverse sinuses.² The same investigator measured MR venography and MR flow quantification in cerebral arteries and veins in a series of 40 patients with IIH, of which 21 patients had venous stenosis. The arterial inflow was 21% higher than normal and superior sagittal sinus outflow was normal, resulting in reduced percentage of venous outflow compared to inflow. The remainder of arterial inflow volume is presumed to have drained via collateral venous channels. With clinical remission of symptoms, the arterial inflow volumes returned to normal.³

Frequency

United States

• Annual incidence of pseudotumor cerebri in Iowa and Louisiana⁴ 
• • 0.9 case per 100,000 population
  • 13 cases per 100,000 (Iowa) and 14.85 per 100,000 (Louisiana) in women aged 20-44 years and 10% over ideal weight
  • 19.3 cases per 100,000 in women 20% over ideal weight
  • Female-to-male ratio 8:1 for mean weight 38% over ideal weight for height
• Annual incidence at Mayo Clinic (Rochester, MN) between 1976 and 1990⁵
• • 0.9 case per 100,000 population
  • 1.6 cases per 100,000 women
  • 3.3 cases per 100,000 females aged 15-44 years
  • 7.9 cases per 100,000 obese women aged 15-44 years

International

• Annual incidence of idiopathic intracranial hypertension in Benghazi, Libya, in a study conducted between 1982 and 1989 comprising 81 patients (76 females and 5 males) aged 8-55 years⁶
• • 2.2 cases per 100,000 population
  • 4.3 cases per 100,000 women of all ages
  • 12 cases per 100,000 women aged 15-44 years
  • 21.4 cases per 100,000 obese women aged 15-44 years

Mortality/Morbidity

• IIH is associated with no known specific mortality risk. The increased mortality rate associated with morbid obesity has a selective expression in this group because of the strong predilection of the disease to affect obese females.
• Vision loss
• • The only permanent morbidity in IIH is vision loss from decompensation of papilledema with progressive optic atrophy. The frequency and degree to which visual loss occurs in this disease is difficult to establish from the existing literature.
  • As outlined by Radhakrishnan et al in 1994⁷, the reported incidence of vision impairment is much higher in series published from referral centers (as many as 96% of cases with some degree of visual field loss) compared to population-based series (eg, 22% in Iowa⁴).
  • Two equally valid explanations for this discrepancy have been proposed.
  • • The referral centers perform more extensive vision testing, including Goldmann and computerized automated threshold perimetry, and thus discovered visual deficits that were not tested for in the community-based studies.
    • The worst cases are referred for tertiary care consultation and thus the referral center series are biased toward more severe vision loss cases than the community-based studies.

Race

No evidence exists to suggest predilection for any particular racial or ethnic group apart from variation in the prevalence of obesity in the different groups.

Sex

• Obese females of childbearing age are affected selectively by IIH.
• Specific numbers are available from the epidemiological studies.
  • Durcan et al⁴ - Female-to-male ratio 8:1
  • Radhakrishnan⁵ - Female-to-male ratio 8:1 (N = 9)
  • Radhakrishnan⁵ - Female-to-male ratio 8:1 (N = 76)

Age

Please refer to the incidence statistics in Frequency. The highest incidence is among obese women of childbearing age. For most of the epidemiological series, this was women aged 15-44 years.

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