Besides a similar clinical presentation, idiopathic intracranial hypertension (IIH) and chronic migraine (CM) also share relevant risk factors, show a higher prevalence of allodynic symptoms and both respond to topiramate. Moreover, sinus stenosis, a radiological marker of IIH, in CM patients is much more prevalent than expected. As a consequence of these striking similarities, IIH without papilledema (IIHWOP) may be easily misdiagnosed as CM. Actually, IIHWOP has been found in up to 14 % of CM clinical series. Considering that, on one hand, an asymptomatic sinus stenosis-associated raised intracranial pressure (ICP) may be highly prevalent in the general population, and on the other, that IIH clinical presentation with chronic headache may require a migraine predisposition, we have proposed that an overlooked IIHWOP could represent a risk factor for migraine progression. This hypothesis prompted us to investigate the prevalence of IIHWOP and its possible role in the process of migraine chronification in a consecutive series of CM patients selected for unresponsiveness to medical treatment and evidence of significant sinus stenosis. The main finding of our study is that the large majority of such patients actually suffer from a chronic headache secondary to IIHWOP. This implies that an IIHWOP mimicking CM is much more prevalent than believed, is commonly misdiagnosed as CM on the basis of ICHD criteria and is strictly predicted by refractoriness to preventive treatments. However, our data fully comply with the alternative hypothesis that an overlooked IIHWOP, although highly prevalent amongst healthy individuals, in migraine-prone subjects is a powerful (and modifiable) risk factor for the progression and the refractoriness of pain. The normalization of ICP by even a single LP with CSF withdrawal may be effective in a significant proportion of patients with a long history of refractory chronic headache, who represent about one-fifth of the patients screened in our study. We suggest that IIHWOP should be considered in all patients with almost daily migraine pain, with evidence of sinus stenosis and unresponsive to medical treatment, referred to specialized headache clinics.
