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Intracranial hypertension literally means that the pressure of cerebrospinal fluid (CSF) within the skull is too high. “Intracranial” means “within the skull.” “Hypertension” means “high fluid pressure.” To understand how this happens, it’s helpful to look at the basic anatomy of the brain and skull, as well as the process in which cerebrospinal fluid is created and absorbed.
Cerebrospinal fluid is one of three major components inside the skull; the other two are the blood supply (the arteries and veins known as the vasculature) that the brain requires to function and the brain itself. Under normal circumstances, these components work together in a delicate balance. A pressure and volume relationship exists between CSF, the brain and the vasculature.
CSF has several important functions. It cushions the brain within the skull, transports nutrients to brain tissue and carries waste away. CSF is produced at a site within the brain called the choroid plexus, which generates about 400-500 ml. (one pint) of the fluid each day or approximately 0.3 cc per minute. (The total volume of CSF in the skull at any given time is around 140 ml. That means the body produces, absorbs and replenishes the total volume of CSF about 3-4 times daily.)
Cerebrospinal fluid flows from the choroid plexus through the brain’s four, interconnecting ventricles before finally entering the sub-arachnoid space, which surrounds the brain and spinal cord. The fluid then flows over the brain and spinal cord and is eventually absorbed into the venous blood system through tiny, one-way channels called arachnoid granulations or villi.
When this continuous cycle of CSF production, circulation and absorption functions normally, it regulates the volume of CSF in the skull and the fluid pressure remains at a constant level. In other words, the CSF production rate remains equal to the CSF absorption rate.
But when the body cannot effectively absorb or drain CSF, intracranial pressure increases within the skull, which is made of bone and cannot expand. And since the brain and the vasculature can only be compressed so far, intracranial pressure must rise. Intracranial hypertension in adults is generally defined as intracranial pressure that reaches 250mmH2O or above.
Acute IH vs. Chronic IH
Intracranial hypertension can be divided into two categories: acute IH and chronic IH.
Acute IH often occurs as the result of severe head injury or intracranial bleeding from an aneurysm or a stroke. It is characterized by a very rapid onset after the initial injury and extremely high intracranial pressure that can be fatal. The underlying cause of acute IH is brain-swelling or intracranial bleeding into the sub-arachnoid space that surrounds the brain. In many cases, a piece of skull is surgically removed to accommodate brain swelling and lower intracranial pressure. This can be life-saving.
In contrast, chronic intracranial hypertension is a neurological disorder in which the increased cerebrospinal fluid (CSF) pressure has generally arisen and remains elevated over a sustained period of time. It can either occur without a detectable cause (idiopathic intracranial hypertension) or be triggered by an identifiable cause such as an underlying disease or disorder, injury, drug or cerebral blood clot (secondary intracranial hypertension). It is frequently a life-long illness with significant physical, financial and emotional impact.
Chronic IH can cause both rapid and progressive changes in vision. Vision loss and blindness due to chronic IH are usually related to optic nerve swelling (papilledema), which is caused by high CSF pressure on the nerve and its blood supply.
In addition, individuals with this disorder often suffer severe pain. The most common form is a chronic headache, which is generally unresponsive to the most potent pain medication.
Anyone can develop chronic IH, regardless of age, gender, ethnicity, race or body type. While the chronic form of intracranial hypertension is not usually fatal, current treatments for the disorder can result in serious, sometimes life-threatening complications.
Researchers are eager to identify the mechanism that underlies chronic intracranial hypertension. While no one is sure why IH happens, some researchers believe that the answer may involve resistance or obstruction of CSF outflow through the exiting pathways from the brain. Our mission at IHRF is to discover exactly why chronic IH occurs and to foster medical research to find better ways to treat, prevent and ultimately cure this disorder. While our primary focus is on chronic IH, such research may also lead to improved understanding and treatment of acute IH.
Chronic IH: Idiopathic IH and Secondary IH
What’s the difference between pseudotumor cerebri, idiopathic intracranial hypertension and secondary intracranial hypertension?
High intracranial pressure has been given several names through the years. Intracranial hypertension (IH) is the general medical term that encompasses all forms of high intracranial pressure.
In the 1890s, a German physician named Heinrich Quincke coined the term “pseudotumor cerebri” to describe a neurological disorder which he believed had all the symptoms of a brain tumor, but without the presence of an actual tumor. The “false brain tumor” that Quincke identified more than a century ago is known today as idiopathic intracranial hypertension (IIH).
IIH is one of two forms of chronic intracranial hypertension. Idiopathic intracranial hypertension occurs spontaneously, without warning. The term “idiopathic” refers to the fact that in IIH, there is no identifiable cause that triggers the raised intracranial pressure. Idiopathic intracranial hypertension is sometimes also called primary intracranial hypertension. (Benign intracranial hypertension, like pseudotumor cerebri, is another older term for IIH but it does not accurately describe the disorder or its consequences.)
The other form of chronic IH is known as secondary intracranial hypertension (SIH). In contrast to IIH, secondary intracranial hypertension always has an identifiable cause. Something–head trauma, an underlying disease, a reaction to a certain drug–directly causes the intracranial hypertension. An SIH cause can also be a physical obstruction (such as a blood clot in the cerebral venous sinus), which causes IH by literally blocking the normal flow of CSF.
Causes of Secondary IH
Intracranial hypertension was first documented in the sixteenth century by a Dutch explorer, Gerrit de Veer, who identified the toxic effects of polar bear liver on early Artic explorers. Several men in his expedition developed secondary intracranial hypertension (SIH) and nearly died after consuming polar bear liver, which contains lethal levels of Vitamin A. Excessive ingestion of vitamin A is now a recognized SIH cause.
Other examples of SIH causes include:
• Head trauma (including post-traumatic brain injury (TBI) )
• Stroke (subarachnoid hemorrhage )
• Cerebral blood clots (dural venous thrombosis)
• Kidney failure
• Liver failure
• Sleep apnea
• Drugs associated with IH:
o Tetracycline
o Minocycline
o Isotretinoin (Accutane)
o All-trans retinoic acid (used in the treatment of promyelocytic leukemia)
o Excessive ingestion of Vitamin A (hypervitaminosis A)
o Amiodarone
o Nitofurantoin
o Lithium
o Levonorgestral (Norplant)
o Growth hormone treatments
o Steroid withdrawal
• Underlying infectious diseases:
o Meningitis (bacterial or viral)
o Lyme disease
o Human immunodeficiency virus (HIV)
o Poliomyelitis
o Coxsackie B viral encephalitis
o Guillain-Barre syndrome
o Infectious mononucleosis
o Syphilis
o Malaria
• Other underlying diseases:
o Lupus
o Sarcoidosis
o Hypoparathyroidism
o Addison’s disease
o Behcet disease
More SIH causes are thought to exist, but research is needed to confirm whether there are true relationships. Both idiopathic and secondary intracranial hypertension can cause the same physical symptoms of raised intracranial pressure and can become chronic; the main difference between the two forms is the presence of a cause.
Who Gets Chronic IH?
Anyone can develop chronic IH, regardless of age, gender, ethnicity, race or body type.
Idiopathic intracranial hypertension is characterized as a disorder affecting overweight women of childbearing age. The relationship between weight and elevated intracranial pressure is not well-understood, though in many cases of IIH, losing weight helps reduce papilledema and sometimes, induces a remission. This is why weight reduction is important for those who are overweight and have IIH. However, in some people, weight loss does not make a difference. So while weight can be a significant factor in IIH, it is clearly not the only factor.
Additionally, if a person is not overweight or a young woman, an IIH diagnosis should not necessarily be dismissed. IIH has been found, though less frequently, in men and women of all body types, ages and races. In cases of IIH in children under ten, weight and gender are not significant factors.
It is also important to note that in secondary IH, unlike IIH, obesity, gender, age and race are not factors. Physicians should never rule out chronic IH—idiopathic or secondary—based on gender or body type.