_{BILATERAL TRANSVERSE SINUS STENOSIS IS A CRITICAL FINDING IN IDIOPATHIC INTRACRANIAL HYPERTENSION SYNDROME (IIT)}

INTRODUCTION: Idiopathic intracranial hypertension (IIH) or pseudotumor cerebri is defined by clinical criteria
that include:
• increased intracranial pressure (eg, headache, papilledema, vision loss)
• elevated intracranial pressure
• normal cerebrospinal fluid composition
• no other cause of intracranial hypertension evident on neuroimaging or other evaluations.

Previously it was wrongly called benign intracranial hypertension, to distinguish it from secondary intracranial
hypertension from a malignancy. However it is not a benign disorder. It can cause intractable headache and a risk
of severe, permanent vision loss.

EPIDEMIOLOGY: The incidence of IIH is 1 to 2 per 100,000 population/year. There is a higher incidence in obese
women between the ages of 15 and 44 years (4 to 21 per 100,000). With the increasing obesity epidemic in the
United States and in other parts of the world, the incidence and prevalence of IIH is rising.
Risk factors: IIH affects women of childbearing age who are overweight. In a prospective study of 50
consecutively-diagnosed IIH patients, 92 percent were women with a mean age of 31 years (range 11 to 58
years) and 94 percent were obese. Other case series in different geographic areas and ethnic groups report
consistent findings.

IIH can also occur in males, in children as young as four months, in older patients (up to 88 years), and in patients
who are not overweight. Risk factors are somewhat different in prepubertal children compared with adults.
Other conditions: Although IIH is, by definition, idiopathic, a number of systemic diseases, drugs, vitamin
deficiencies and excesses, and hereditary conditions have been reported to be associated with IIH. Most patients
with IIH do not have one of these conditions.

The true link between these conditions and IIH is uncertain in many cases. In case-control studies, the prevalence
of menstrual irregularities, pregnancy, antibiotic use, iron deficiency anemia, thyroid dysfunction, and oral
contraceptive use were no different among cases versus controls.

Systemic illnesses: In addition to obesity, systemic illnesses reportedly associated with IIH include:
• Addison disease
• Hypoparathyroidism
• Anemia, usually severe
• Sleep apnea
• Systemic lupus erythematosus (SLE)
• Behcet's syndrome
• Polycystic ovary syndrome
• Coagulation disorders
• Uremia

The pathogenic mechanisms associating these conditions with IIH are uncertain and in some cases the apparent
association may be indirect or spurious.

PATHOGENESIS: Although many theories for IIH have been proposed, the precise pathogenesis of IIH remains
unknown. Any theory must account for the high incidence of IIH in obese women of the childbearing years.
Proposed etiologies include cerebral venous outflow abnormalities (eg, venous stenoses and venous
hypertension); increased cerebrospinal fluid (CSF) outflow resistance at either the level of the arachnoid
granulations or CSF lymphatic drainage sites; obesity-related increased abdominal and intracranial venous
pressure; altered sodium and water retention mechanisms; and abnormalities of vitamin A metabolism.

Intracranial venous hypertension:
Elevated intracranial venous pressure is postulated both as a primary mechanism and as a "final common
pathway" for IIH. This theory is supported by the similar clinical appearances of IIH and secondary intracranial
hypertension due to cerebral venous thrombosis and other causes of obstructed venous outflow. Some patients
thought to have IIH have been later discovered to have one of these conditions. Although several authors have
reported cerebral venous outflow abnormalities on magnetic resonance imaging (MRI) and MR venography
(MRV) in patients with IIH, there is some disagreement as to the frequency or the significance of these findings.
Clear venous sinus thrombosis has definite clinicopathologic significance, but apparent venous sinus narrowing or
stenosis may not. Differentiation of venous stenosis from flow related abnormalities, especially on non-contrast
MRV, can be difficult as flow-related artifacts can mimic venous sinus stenoses or even be mistaken for sinus
thrombosis. These can sometimes be resolved with post-contrast MRV. Literature that suggests a high frequency
of venous sinus abnormalities in patients with IIH must be viewed with caution.

Cerebral venous sinus structural abnormalities were systematically identified in a series of 29 patients with IIH
using a specialized MR venography technique, auto-triggered elliptic-centric-ordered three-dimensional
gadolinium-enhanced MR venography (ATECO MRV). Blinded readers detected substantial bilateral sinovenous
stenoses in 27 of 29 patients compared with 4 of 59 controls, corresponding to a sensitivity and specificity of 93
percent. The investigators speculated that congenital narrowing of the venous sinus might be a primary cause of
IIH or instead may be a secondary, but potentially contributing factor. In the latter scenario, increased CSF
pressure compresses the transverse sinus leading to the stenosis, which in turn would exacerbate the increased
intracranial pressure (ICP), potentially bringing the patient to clinical presentation.

There is some evidence that the venous abnormalities are secondary to the intracranial hypertension. One study
documented high intracranial venous sinus pressures in patients with IIH that was reduced by removal of CSF.
This finding implies that increased cerebral venous sinus pressure and apparent stenosis in IIH is caused by the
elevated ICP and not the reverse. Other reports have also documented reversal of apparent transverse sinus
stenosis after CSF shunting. In contrast, one case series documented persistent transverse sinus stenosis in nine
patients with IIH whose CSF pressure normalized over six years with medical treatment.

Other theories: There is some evidence that central obesity increases intra-abdominal pressure, pleural pressure,
cardiac filling pressure, and central venous pressure and may lead to increased intracranial venous pressure and
IIH. However, this mechanism does not account for the gender difference in IIH, the lack of increased incidence in
pregnancy, and the cases of IIH in thin patients.

Cerebral edema was one of the earliest proposed mechanisms for IIH and had pathologic support in one case
series. However, subsequent pathologic and MRI studies have not found evidence of cerebral edema in patients
with IIH.

Other causes of impaired CSF absorption or increased CSF production have also been postulated as an etiology
of IIH. However, most known causes of impaired CSF absorption (after subarachnoid hemorrhage or infectious
meningitis) and CSF overproduction (choroid plexus papilloma) produce hydrocephalus, which is not seen in IIH.
Sleep apnea can be a complication of obesity, and may play a role in IIH, perhaps in men in particular.

Hypercarbia may produce elevated ICP through vasodilation. One study found that 14 of 37 patients with IIH had
a sleep disturbance, and of these, 13 had evidence of sleep apnea or upper airway resistance syndrome. In
another report, a patient with IIH underwent ICP monitoring with pulse oximetry to reveal that apneic episodes
were associated with marked elevations of ICP.

SUMMARY — Idiopathic intracranial hypertension (IIH) is a disorder defined by clinical criteria including
symptoms of increased intracranial pressure (eg, headache, papilledema, vision loss), elevated intracranial
pressure with normal cerebrospinal fluid composition, and no other cause of intracranial hypertension evident on
neuroimaging or other evaluations.