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The cause of idiopathic intracranial hypertension (IIH) remains unknown but catheter venography has shown that many patients have intracranial venous hypertension proximal to transverse sinus stenoses. These stenoses have subsequently been demonstrated on magnetic resonance and CT venography and it has been proposed that by reducing the passive resorption of CSF, intracranial venous hypertension due to these stenoses might be the cause of IIH. However, CSF withdrawal reduces venous sinus pressures, implying that venous hypertension is a secondary phenomenon.1 Moreover, resolution of transverse sinus stenoses has been reported in three patients with IIH treated by CSF diversion procedures.2,3 We report a patient with IIH in whom catheter and CT venography showed transverse sinus stenoses which resolved immediately after CSF withdrawal by lumbar puncture (LP).
A 35 year old woman presented with a 2 year history of headache and transient visual obscurations. She had papilloedema and constricted visual fields but no focal neurology. MRI of the brain was normal. At LP, the opening pressure was 35 cm H2O with normal CSF constituents. IIH was diagnosed and treatment started with acetazolamide 250 mg three times daily.
One year later the patient was referred to our institution with persistent headaches and papilloedema for consideration of a CSF diversion procedure. Magnetic resonance venography at this time suggested stenoses in the anterior part of both transverse sinuses and she was further investigated to establish the degree and reversibility of these stenoses with reference to possible stenting.
Direct retrograde cerebral venography and CT venography were performed before and after drainage of 45 ml of CSF by LP. The opening pressure was 26 cm H2O and closing pressure 2 cm H2O. The baseline catheter venogram recorded a pressure of 30 mm Hg in the superior sagittal sinus and gradients of 19 mm Hg across stenoses at the anterior ends of both transverse sinuses (fig 1A). The right transverse sinus was dominant. Immediately after CSF withdrawal, sagittal sinus pressure fell to 7 mm Hg, the right transverse sinus stenosis resolved (fig 1B) and the pressure gradient resolved. The left transverse sinus was not re-examined. CT venography confirmed the morphological changes, showing expansion of both transverse sinuses (figs 1C, D). With these results the patient was considered unsuitable for stenting.
Her symptoms improved for 2 weeks after CSF withdrawal but subsequently regressed. A ventriculoperitoneal shunt with an adjustable flow control valve was inserted 2 months later. She remained asymptomatic for 2 weeks after shunt insertion but then relapsed with headaches. The performance level of the valve was reduced from 2.0 to 1.5, and at the most recent follow-up, 1 year post-shunting, she was asymptomatic with normal visual acuity and no papilloedema.
Although many patients with IIH have transverse sinus stenoses, there has been debate over whether these are the cause or effect of raised intracranial pressure (ICP), and if they are an effect, whether they are an epiphenomenon or an exacerbating factor. Patients with IIH may constitute a heterogeneous group with respect to this issue. Increases in ICP can compress the venous sinuses, and secondary compression of the venous sinuses has been demonstrated in a number of patients with IIH. King et al showed that the pressure gradients across transverse sinus stenoses in IIH resolved after CSF drainage.1 Subsequently, investigators using magnetic resonance and CT venography have shown that transverse sinus stenoses in IIH can resolve after CSF diversion procedures. The case we describe implies that in some patients with IIH the calibre of the transverse sinuses responds immediately to changes in ICP.
Unlike CSF diversion, non-invasive reduction of CSF pressure does not seem to effect transverse sinus stenoses. A series of 14 patients with IIH and transverse sinus stenoses were treated with acetazolamide 250 mg twice daily. This normalised CSF pressure in nine patients but in all cases the transverse sinus stenoses persisted unchanged on MR venography.4 The authors suggested that CSF hypovolaemia induced by CSF diversion procedures causes the stenoses to resolve rather than a direct relationship between sinus calibre and CSF pressure alone. Our patient had a closing pressure of 2 cm H2O after 45 ml of CSF was withdrawn, which supports this view. This phenomenon might be expected from the Monro–Kellie doctrine whereby a decrease in the volume of CSF within the rigid space of the skull is compensated for by an increase in the volume of blood in the venous sinuses.
Stenting of the transverse sinuses has been proposed as an alternative to CSF diversion procedures in patients with IIH on the assumption that transverse sinus stenoses play some part in causing symptoms.5 Intuitively, however, patients whose sinuses dilate in response to CSF withdrawal would seem to be inappropriate candidates for stenting. This, in turn, invites questions about what proportion of patients with IIH have sinuses that respond in this way and what would be the role of stenting in these cases. Stenting seems to have been successful in some patients but not in others.5 It is not known whether the sinuses responded differently to changes in CSF pressure in patients who did well with stenting compared with those who did not because the CSF withdrawal test was not a part of their diagnostic workup. For the same reason it is also not known whether, in patients in whom stenting was successful, the stent addressed the primary cause of raised ICP or simply mitigated an exacerbating factor.
The rapidity with which the calibre of the venous sinuses in these patients can respond to changes in ICP means that imaging should be interpreted with caution. It is now recognised that most patients with IIH have narrowing of the venous sinuses even to the point where those patients with IIH who do not have venous narrowing require some explanation. Perhaps in some of these patients ICP was not raised at the time of the investigation—for example, because of recent LP and CSF drainage. The appearances of the venous sinuses in patients with IIH should be interpreted with reference to the CSF pressure at the time of the examination.
Competing interests: None.
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