Some additional comments about the physiologic changes in CSF flow and pressure in relation to alternative mechanisms of pseudotumor may be informative. Using the method of constant infusion manometrics, Mann and coworkers demonstrated an increased resistance to CSF outflow, in their view, caused by an impaired absorptive function of the arachnoid villi. Other authors have attributed intracranial venous hypertension to raised intraabdominal and cardiac filling pressures, the mechanical result of obesity (Sugerman et al, 1995). On inconclusive evidence, benign intracranial hypertension was in the past attributed to an increase in brain volume secondary to an excess of extracellular fluid or blood volume within the cranium (Sahs and Joynt, Raichle et al). An interesting related finding has been an elevated level of vasopressin in the CSF but not in the blood (Seckl and Lightman). In the goat, this peptide causes a rise in ICP and a reduction in CSF absorption, raising the possibility that the pseudotumor state is caused by an aberration of the transit of water in the cerebrum.
Finally, Jacobson and colleagues have made the observation that serum vitamin A levels (in the form of retinol) are 50 percent higher than expected, in patients with pseudotumor—a difference that is not explained by obesity. Because the levels were considerably lower than in cases of hypervitaminosis A with symptomatic forms pseudotumor (see in the following section), the meaning of these findings is uncertain.
Symptomatic Causes of Pseudotumor Cerebri (See Table 29-1)
The main considerations in cases of generalized elevation of ICP and papilledema in the absence of an intracranial mass are, foremost, covert occlusion of the dural venous sinuses and then, a list of less-common conditions, including gliomatosis cerebri, occult arteriovenous malformation, and carcinomatous, infectious, or granulomatous meningitis. Although occlusion of the dural venous sinuses and their large draining veins is sometimes equated with pseudotumor, these cases are not, of course, idiopathic. When papilledema occurs in the context of a persistent headache, particularly if the cranial pain is centered near the vertex or medial parietal areas or if there are seizures, venous occlusion is likely. Venous sinus thrombosis can be detected in most instances by careful attention to the appearance of the superior sagittal and transverse sinuses on the T1-weighted MRI or on the contrast-enhanced CT scan, as discussed in Chap. 33 under “Thrombosis of Cerebral Veins and Venous Sinuses.” Isolated cortical vein thrombosis on the cerebral convexity does not cause pseudotumor (but does induce seizures).
A large cerebral arteriovenous malformation (AVM), by causing an increase both of venous pressure and cerebral blood volume, can give rise to a pseudotumor syndrome. In a few of our cases, these changes in the physiology of the cerebral circulation were made evident by the appearance of early venous flow on the angiogram or by thrombosis of the superior sagittal sinus.