Spontaneous Intracranial Hypotension
Spontaneous = not due to trauma or an inherited disorder
Intracranial = inside the skull
Hypotension = pressure that is lower than normal
Spontaneous intracranial hypotension is a condition in which the fluid pressure inside the skull is lower than normal.
The brain and spinal cord are covered by a tough, watertight membrane called the dura. Inside the dura is the cerebrospinal fluid (CSF), a liquid that bathes and cushions the brain and spinal cord. Normally, the CSF circulates inside the dura, gradually drains, and is constantly replenished with new fluid. But a leak in the dura can allow too much CSF to escape too quickly. This reduces the amount of CSF in circulation, reducing the fluid pressure and causing intracranial hypotension.
The major symptom of intracranial hypotension is a severe headache that is worse when standing up, and better when lying down. Sometimes the headache increases over the course of the day.
Other symptoms can include nausea, vomiting, vertigo, tinnitus (ringing in the ears), change in hearing, blurred or double vision, facial numbness, and tingling of the arms.
A tentative diagnosis may be reached based on the symptoms alone, especially the postural dependency of the headache.
On magnetic resonance (MR) imaging, certain signs may lead a physician to suspect intracranial hypotension. For example, due to the lower pressure inside the skull, the brain may “sag” toward, or even partially sag out of, the skull base. (This can resemble a structural problem known as Chiari malformation.)
Once a diagnosis of spontaneous intracranial hypotension has been made, it still remains to locate the spinal fluid leak. This can be a challenge–sometimes the defect causing the leak cannot be located at all. The most useful test for locating the leak is usually the computed tomography plus myelogram (myelo-CT). Like a plain CT scan, this procedure uses a computer and a series of X-rays to construct images of structures inside the body. But unlike a plain CT, a myelo-CT uses an opaque dye that is injected into the CSF. The dye is visible to the CT scan, so the scan can often reveal where the CSF and dye are escaping from the dura.
Spontaneous intracranial hypotension affects women more frequently than men, and occurs most frequently in the fourth or fifth decade of life. It is a rare condition, estimated to occur in 1 in 50,000 people.
Intracranial hypotension has several causes. For example, a congenital (in-born) defect in the dura may allow CSF to escape too quickly. Trauma to the dura, as from spinal surgery or a lumbar puncture, can also allow CSF to leak. Implanted shunts (drains) that are surgically placed to treat an accumulation of CSF might drain too much of the fluid, causing hypotension.
But spontaneous intracranial hypotension arises for reasons unrelated to inherited disorder, major trauma, or medical intervention. The most common cause of spontaneous intracranial hypotension is spontaneous spinal fluid leak. (Other, extremely rare, causes of spontaneous intracranial hypotension include diabetic coma or a leakage of CSF through the ears or nose.)
The causes of spontaneous spinal fluid leak are not completely understood. It is thought that the condition can occur after a relatively minor trauma when there is already a slight weakness in the dura.
Some cases of spontaneous intracranial hypotension resolve with nonoperative measures such as strict bed rest, hydration, or a procedure called an epidural blood patch. (In this procedure, the patient’s own blood is injected into the dural sac. The blood circulates throughout the sac and can often patch up the defect until it heals on its own.)
In other cases, the condition does not respond to these measures, or responds for only a short time. In these cases, when a defect in the dura can be identified, surgical repair is a possibility.
The neurosurgeon will perform a laminectomy (the removal of a piece of bone) to expose the problematic area. Then the surgeon can patch the dura, ensuring a watertight seal.