The mechanism of increased CSF pressure in the idiopathic form of the disorder has remained elusive, but some experience suggests that, in at least some cases, there is a functional obstruction to outflow in the venous sinuses. Karahalios and colleagues and others have found the cerebral venous pressure to be elevated in pseudotumor cerebri; in half of their patients, there was a venous outflow obstruction demonstrated by venography with a pressure gradient across the site of narrowing of the venous sinus. A related finding in some cases, pointed out to us some time ago by Fishman (1984), is one of partial obstruction of the lateral sinuses by enlarged pacchionian granulations (seen during the venous phase of conventional angiography). It is here that the evidence for a venous cause of pseudotumor has been most persuasive. Several authors have proposed that venous hypertension increases the resistance to CSF absorption and is the proximate mechanism underlying pseudotumor. Similarly, Farb and colleagues, using sophisticated MRI venography, found venous stenosis in 27 of 29 patients with pseudotumor (and in 4 of 59 control subjects). In both studies and in others similar ones, the nature of the obstruction was not clear, but the fact that in some cases it was bilateral and focal suggests that the stenosis was not simply the passive result of raised ICP. This issue is not yet resolved, as noted below.
Intervention by stenting of a venous sinus at the sites of apparent obstruction has resulted in clinical improvement and a reduction in CSF pressure. For example, 5 of 12 patients treated by Higgins and colleagues became asymptomatic, but this was a population selected by the demonstration of a focal pressure gradient in the lateral sinus during venography. Karahalios and colleagues had similar success in several patients. What is not clear is how prevalent these partial venous obstructions are and what precisely is their nature (if not simply enlarged granulations). In some series, the abnormality has been found in 10 to 25 percent of patients who lack any features of pseudotumor (Leach et al). We have seen an obese patient of short stature who presented with typical pseudotumor cerebri, but who was found to have, in addition, anticardiolipin antibodies that resulted in thrombosis of the right transverse sinus; lysis of the clot led to resolution of the pseudotumor syndrome.
In an attempt to settle the role of the venous stenosis, King and colleagues measured intracranial venous pressure while withdrawing spinal fluid from the cervical subarachnoid space in patients with idiopathic pseudotumor cerebri. Their observation that the intracranial venous pressure drops immediately upon the reduction in CSF pressure supports the notion that the increased venous pressure is secondary. Furthermore, they describe patients with pseudotumor and normal venous pressures in the sagittal and transverse sinuses. The unsatisfactory nature of all the currently offered theories of causation of pseudotumor cerebri are reviewed by Walker, but at the moment our reading of the literature suggests that venous stenosis from granulations or from some as yet undefined functional change, may account for a proportion of what had previously been considered to be idiopathic cases. How the venous changes relate to obesity and sex is also unclear. Perhaps some individuals have a congenital configuration of the venous sinuses that is exaggerated with obesity and elevated systemic venous pressures.