Abstract
Idiopathic intracranial hypertension without papilloedema (IIHWOP) is uncommon, accounting for, at most, 5% of patients with IIH evaluated by neuro-ophthalmologists (1). In contrast to patients with IIH and papilloedema, patients with IIHWOP are less obese, have lower cerebrospinal fluid pressures, and are more likely to have non-organic visual loss (1). This entity is perhaps more readily diagnosed among headache specialists than neuro-ophthalmologists and is suspected in obese women with intractable or chronic daily headaches (2).
The integral relationship between the central venous system and intracranial pressure has been emphasised over the past several years with the discovery of venous sinus abnormalities in patients with IIH (3). Morphological irregularities and abnormal pressure gradients within the transverse sinuses are well described. In most cases, transverse venous sinus stenosis appears to be a result of high intracranial pressure rather than its cause (4). The transverse sinuses are asymmetric in most individuals and a unilateral hypoplastic sinus is considered a normal variant. However, bilateral transverse sinus stenosis (BTSS) may be associated with intracranial hypertension.
In this issue of Cephalalgia, Bono and colleagues studied 98 patients with chronic migraine and chronic tension type headache using magnetic resonance venography and 1-h continuous lumbar CSF pressure monitoring (5). Their findings are notable in several respects.
Strikingly, their patients were obese, regardless of CSF pressure status. Patients with normal CSF pressure (Group 1) were less overweight than in the groups with either intermittently (Group 2) or consistently (Group 3) elevated CSF pressure, and some patients in Group 1 had a normal body mass index (BMI), but overall this cohort was overweight. There was no different in BMI between subjects in Groups 2 and 3. Obesity is a risk factor for both IIH and the development of chronic daily headache (6).
Although there was a statistically significant difference in both mean CSF pressure and opening pressure between the three groups, only some of the patients in Group 3 met criteria for the diagnosis of IIH by current standards (7). Pressures between 201–249 mm CSF are non-diagnostic (8).
BTSS has been noted in a majority of patients with IIH by various investigators. It is less commonly encountered in a typical practice setting, even in patients with papilloedema. The high prevalence of BTSS in this cohort is perhaps attributable to the imaging technique used, which the investigators previously found to have a very high detection rate of transverse sinus stenosis (three dimensional phase contrast images with 15 cm/s velocity encoding). Conventional MR venography is often performed using two dimensional time of flight, and the images are likely subjected to less scrutiny in the practice setting than the research setting. All subjects in Groups 2 and 3 had BTSS, whereas only four of 54 patients in group 1 had BTSS. As previously noted, unilateral transverse sinus stenosis was not associated with abnormal intracranial pressure.
One-hour monitoring showed mean pressures that were consistently higher than the opening pressure in all groups. The importance of this finding cannot be overemphasised. An isolated CSF pressure measurement is but a ‘snapshot in time’ and may be spuriously high or low, depending on the situation and technique employed. Prolonged monitoring also revealed B-waves in Groups 2 and 3; B-waves are frequently recorded with increased intracranial pressure of various aetiologies, although there is far greater experience defining pressure wave abnormalities with intracranial pressure monitoring systems than with lumbar recording methods.
Digre and colleagues (1) found that treatment of patients with IIHWOP with agents conventionally used for IIH was less than satisfactory than in patients having IIH with papilloedema, and shunting was not always beneficial. The patients of Bono et al. were treated with a low dose of acetazolamide and topiramate which improved the headache in most subjects. However, the response to acetazolamide and topiramate is not specific, as both medications have a symptomatic effect on headache. Interestingly, some of the patients had been previously treated with topiramate for headaches without improvement (personal communication with Professor Bono); it is a bit surprising that adding a relatively low dose of acetazolamide would dramatically increase their therapeutic response. These patients were not re-imaged after treatment; paradoxically, another study by these investigators showed that the BTSS persisted in IIH patients who were successfully treated with medications rendering them asymptomatic with normal CSF pressures (9). These findings contrast with other reports demonstrating reversal of venous sinus stenosis acutely after measures to lower CSF pressure (4,10).
In summary, this paper raises intriguing questions about the pathophysiology of IIH, the contribution of both obesity and intracranial pressure to chronic headaches, and the possibility of a continuum of chronic daily headache to IIH. It underscores the involvement of the cerebral venous sinuses in the hydrodynamics of CSF pressure. BTSS may be a marker of increased intracranial pressure but it is not a consistent finding in patients with IIH and papilloedema in clinical practice. Whether BTSS is the cause or the effect of intracranial hypertension, its detection by magnetic resonance venography in patients with chronic headaches may provide important information into the nature of their headache disorder and subsequent management. Lumbar puncture, currently the ‘gold standard’ for diagnosing IIH and other disorders affecting intracranial pressure, may demonstrate misleading results at times but it is widely available and more practical than prolonged monitoring in the clinical setting. A high opening CSF pressure in and of itself is neither specific nor diagnostic and must to be used in context with other data from the history, examination, neuroimaging and laboratory to arrive at the correct diagnosis.