Idiopathic intracranial hypertension (IIH) is a relatively common neurologic disease that has been reported from most parts of the world with an annual incidence of 1 to 2 per 100,000. In the 1980s, the annual incidence in the general population was reported to be 0.9 per 100,000 persons in Iowa and Louisiana in the United States (19) and 1.7 per 100,000 in northeastern Libya (51). The syndrome occurs in both children and adults. No sex difference is seen in children, whereas the female:male ratio in most series of adult patients is between 3:1 and 10:1 (16,19,51,71). The condition occurs most commonly in obese women of childbearing age, in whom the incidence may be at least 19 per 100,000 (19). Familial occurrence has been encountered in siblings and in mother and son (7,54,68). The incidence of IIH seems to parallel that of obesity in a given population (6) and is thus increasing in Western countries. Recent studies in the United States suggest that the incidence of IIH has doubled over the past decade (27,39).

The pathophysiology of intracranial hypertension is controversial. In the absence of a space-occupying lesion or hydrocephalus, a number of mechanisms have been suggested as possible explanations of increased ICP: increased brain volume caused by increased brain water content (30), increased blood volume (15), increased ICP transmitted from elevated intra-abdominal and intrathoracic pressure in obese persons (65), increased rate of CSF formation (17), or a decreased rate of CSF absorption at the site of the arachnoid villi (42). Only insignificant increments in cerebral blood volume have been reported by some investigators studying IIH (1), and a normal CSF production rate was found using magnetic resonance (MR) techniques for indirect calculations of CSF production (29).

The two prevailing hypotheses of the pathophysiologic mechanisms of IIH are increased brain water content or increased resistance to CSF outflow. While intracellular or interstitial edema is an attractive hypothesis to explain IIH phenomenology, no confirmatory evidence of brain edema exists by either MR imaging (MRI) or autopsy studies (3,70). Increased resistance to CSF outflow across the arachnoid villi is proposed as the underlying cause of IIH (42), and many investigators have convincingly demonstrated an increased resistance to CSF outflow using spinal infusion or perfusion tests (28,61). Unfortunately, abnormal spinal infusion tests do not differentiate clearly between impairment of CSF absorption and decreased intracranial compliance resulting from an increase in brain volume. These two hypotheses are not
necessarily mutually exclusive, however. Increased resistance to CSF outflow causes an excessive CSF volume in the brain, but no ventricular enlargement or increased subarachnoid space is found in IIH; an expanded brain extracellular space may simulate a form of brain edema. Indeed, increased cerebral venous volume, cerebral venous hypertension, and venous occlusion have been demonstrated in IIH (36,43,46,50). Moreover, venous sinus pressure, as measured by manometry, is proportional to cerebrospinal fluid pressure (46). Cerebrovenous sinus thrombosis may present as IIH, but in most cases, the venous stenosis visualized on neuroimaging studies is likely a secondary manifestation of increased ICP (10).

No common cause of intracranial hypertension has been found in patients with IIH. Many diseases and conditions have been associated with the syndrome, some of which have been claimed to be causal (Table 113-1). Many of these proposed causal factors are common conditions that probably occur by chance in patients with IIH. The only consistent finding seems to be a predilection for obese women of childbearing age (41). Many patients report a recent substantial weight gain just prior to symptom onset (31,56). These observations have led to speculations of hypothalamic-pituitary disturbances in patients with IIH, but conventional tests of endocrine functions have disclosed only inconsistent and insignificant abnormalities in the function of the pituitary or the peripheral target glands (52,60). A number of other conditions, which should not be included under the term IIH, can produce a clinical picture resembling IIH, that is, increased ICP in the absence of a mass lesion or hydrocephalus (Table 113-2).

Diagnostic criteria of idiopathic intracranial hypertension are as follows (25):

1. Increased ICP (≥250 mm H₂O) measured by epidural, intraventricular, or lumbar subarachnoid pressure monitoring or by lumbar puncture

2. Symptoms and signs of ICP without any localizing features

3. No mass lesion, ventricular enlargement, or abnormal contrast enhancement on neuroimaging

4. Normal CSF contents

5. No clinical or neuroimaging suspicion of venous sinus thrombosis

6. No other cause identified

The most prominent symptom of IIH is headache, which is a presenting symptom in 75 to 100% of patients (31,56,61,69). The headache varies in character and may
be gradual or acute in onset (61). In most patients, the headache occurs daily and is of moderate intensity, although a questionnaire-based study revealed that 93% of patients with IIH reported their headache as their severest ever (69). The headache can be holocranial or one-sided, and a few patients report bouts of lightening pain in a localized area of the scalp. The headache can be throbbing or pressing and often worsens with Valsalva maneuvers or postural changes (59). Retro-ocular pain aggravated by eye movement is described in some patients (69). The headache may be migrainous, and many patients with IIH continue to have migraine or tension-type headache even after their intracranial pressure is controlled (25). IIH without papilledema has been reported but is uncommon; most patients reported with this syndrome are obese women with refractory chronic daily headaches or medication-overuse headache (72).

The mechanism of headache in IIH probably reflects either cerebral venous distention or meningeal irritation from increased CSF pressure. Artificial production of elevated spinal fluid pressure using saline infusion usually produces frontal and temporal headaches but may be associated with unilateral or holocranial head pain of any kind. Some patients sense no pain despite rapid rises in CSF pressure during the infusion tests (23).

Nausea is an accompanying symptom of IIH in about half of the patients, and vomiting is less common (61,69). Neck stiffness has been recognized in a high proportion of patients by some investigators (55,69). In addition to headaches that are attributed to high CSF pressure in patients with IIH, other factors related either to chronicity of increased ICP or to treatment may confound the picture further. After lumbar puncture, the headache is frequently relieved but may be complicated by the addition of a headache due to CSF leak. Furthermore, after lumboperitoneal shunt, the patient may become shunt dependent or tonsillar herniation can occur with a new type of headache known as the hindbrain herniation headache (49).

Visual complaints are an important feature of IIH. Transient visual obscurations are seconds-long spells of loss of vision caused by transient ischemia of the visual pathways. Most patients with IIH (50 to 75%) experience transient visual obscurations (12,56,61,71) that occur several times a day and are provoked by postural changes. Blurred vision and noticeable visual field loss may occur. Double vision, sometimes intermittent, is usually horizontal because of a sixth-nerve palsy and occurs in 16 to 38% of patients with IIH as a false localizing sign (12,61,71).