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Idiopathic intracranial hypertension (IIH), sometimes called benign intracranial hypertension (BIH) or pseudotumor cerebri (PTC) is a neurological disorder that is characterized by increased intracranial pressure (ICP), in the absence of a tumor or other intracranial pathology.
Explanation of terms
The terms "benign" and "pseudotumor" have often been used for this disorder to make clear that the increased ICP is not caused by a tumor or malignancy. However, these terms belie the significance and potential morbidity of the disorder - Thus, it is most appropriately referred to as IIH, and not by its other names. Cases of increased ICP with a known cause can be called secondary intracranial hypertension (SIH), but for the purposes of this article the distinction is not entirely vital.
Diagnosis
The diagnosis of IIH is one of exclusion. The principle sign of IIH, papilledema, can occur because of brain tumors (hence the term "pseudotumor cerebri," which literally means "false brain tumor"), or in other conditions involving increased ICP. Thus, a thorough evaluation is essential to the diagnosis of IIH. Radiologic imaging scans are, as a rule, normal in IIH save for the finding of small or slit-like cerebral ventricles, and what may appear to be an 'empty' sella turcica (caused by flattening of the pituitary under pressure). Cerebrospinal fluid (CSF) is a clear fluid which surrounds and circulates through the brain and spinal cord - it is to the fluid pressure of the CSF that the concept of 'intracranial pressure' refers. The chronic pressure increase in IIH is, as the word "idiopathic" indicates, of uncertain etiology; Most researchers believe that the body's ability to absorb CSF is somehow impaired in those individuals with IIH. A less likely possibility (one that is now generally dismissed) is that of CSF overproduction. Many scientists and doctors believe that there is also some degree of brain swelling or engorgement.
Signs and symptoms
IIH most commonly affects women, particularly overweight women between ages 15 and 45. However, the disorder is not limited to women, and can affect people of all ages and races, both male and female, of all shapes and sizes. It is estimated there are about 12,000 people living in the United States with this disease. The 'cardinal sign' of IIH is papilledema (swelling of the optic nerves), although some atypical patients may not have papilledema. Occasionally patients may present with abducens or other cranial nerve palsies. Symptoms can include severe headache, pulsatile tinnitus, visual disturbances (e.g. diplopia), nausea/vomiting, etc. Most serious is the potential for permanent loss of vision or even blindness.
Risk factors include female gender, obesity, excess or deficiency of vitamin A or derivatives, certain medications (such as the tetracycline antibiotics, especially minocycline), and some other disorders. In cases linked to medication or other clear causes, the line between truly idiopathic IH and secondary IH (as mentioned above) may be difficult to draw.
A lumbar puncture (LP, spinal tap) must be done to measure ICP and confirm the diagnosis of IIH. Prior to the lumbar puncture, imaging of the brain, such as CT or MRI, is usually performed to rule out the possibility of an intracranial space-occupying mass lesion. Some authorities believe that a lumbar puncture can have potentially deadly consequences if performed when an intracranial mass lesion is present.
Most clinicians adhere to the 'modified Dandy criteria' in order to diagnose IIH, although experts disagree as to some of the finer points of said criteria. Generally, the patient should display papilledema, there should not be any structural abnormality of the brain in neuroimaging, and the lumbar puncture should confirm a high pressure in the absence of other abnormalities (such as infection, increased protein or malignancy). There are some exceptions to the criteria, most notably the small subset of patients with IIH who do not have papilledema (see article by Silberstein and Marcelis, "Idiopathic intracranial hypertension without papilledema," published April 1990 in the journal Neurology).
Treatment
The primary goal in treatment of IIH is the prevention of visual loss and blindness. Elevated CSF pressure generally causes the optic nerves to swell. The bulging of the optic disc (the round bundle of nerve fibers visible at the back of the eye) can be seen through an ophthalmoscope during a routine eye exam. This swelling can lead to irreversible loss of vision and, in the most severe cases, optic nerve atrophy and blindness.
IIH is treated mainly through the attempted reduction of CSF pressure and, where applicable, weight loss. The treatment of increased ICP of a known origin (SIH) consists of removing the inciting cause, if possible, and attempting to reduce CSF pressure. IIH may resolve after initial treatment, may go into remission and relapse, or may continue chronically.
CSF pressure may be temporarily decreased by repeated spinal taps (to remove excessive cerebrospinal fluid). The standard treatment of IIH includes drugs that reduce cerebrospinal fluid production- most notably acetazolamide. Acetazolamide is a carbonic anhydrase inhibitor, meaning it inhibits an enzyme that is key to the production of certain fluids, including CSF. Sold under the brand name Diamox, it is the most common drug used in treatment of IIH. Other drugs such as furosemide and various diuretics, topiramate and prednisone may be used in an attempt to reduce ICP.
Depending on the severity of the individual case, there are other treatment options available, such as optic nerve sheath fenestration, and shunts to remove excess CSF from the brain. In optic nerve sheath fenestration, surgery is undertaken to make a slit in the optic nerve sheath, which can alleviate swelling and slow or halt loss of vision.
Shunting is a neurosurgical procedure to facilitate the drainage of excess CSF (thereby reducing ICP). A shunt is essentially a silicone tube inserted somewhere in the fluid spaces of the central nervous system, which then drains CSF to the circulatory system or one of the body cavities. There are various types and configurations of shunts; lumboperitoneal shunts drain from the lumbar spine to the peritoneal cavity, while ventriculoatrial shunts run from the cerebral ventricles to the heart. Although shunts can dysfunction due to occlusion, infection, malfunction, etc., they are very effective in normalizing CSF pressures. Optic nerve sheath fenestration is less effective in controlling the CSF pressure (and in controlling most symptoms, such as headache), but is more effective in protecting the optic nerve from the effects of pressure. In cases of severe obesity, some clinicians advocate the use of gastric bypass surgery, though the efficacy of this procedure in the treatment of IIH is not fully established.
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