Increased and Decreased Intracranial Pressure

CHAPTER 15 INCREASED AND DECREASED INTRACRANIAL PRESSURE



Ronald Kanner, MD, FAAN, FACP




1. What is the normal range for intracranial pressure?


Intracranial pressure is generally measured by lumbar puncture. It is presumed that, because the spinal fluid at the lumbar level is continuous with spinal fluid throughout the brain, pressures are equal. The normal pressure on lumbar puncture is 65 to 195 mm of cerebrospinal fluid (CSF) or water. This is the equivalent of about 5 to 15 mmHg.



2. Does systemic hypertension usually cause an increased intracranial pressure headache?


No. Systemic hypertension is usually asymptomatic.



3. What is the Monro-Kellie doctrine?


The Monro-Kellie doctrine states that an increase in the volume of any of the calvarial contents (brain tissue, blood, CSF, or brain fluids) must be accompanied by a decrease in the volume of another component, or intracranial pressure will increase markedly because the bony calvarium rigidly fixes the total cranial volume. Under normal circumstances, brief increases in intracranial pressure are associated with the Valsalva maneuver, including coughing, sneezing, or straining at stool. Some of the increased intracranial pressure is mitigated by the fact that the cerebral vessels are somewhat elastic and can be compressed. In patients who already have increased intracranial pressure or irritated meninges, transient rises may produce severe pain.



4. Under what circumstances is the pressure measured by lumbar puncture not a true reflection of intracranial pressure?


If there is a block in CSF flow at a spinal level above the level of the lumbar puncture, but below the foramen magnum there may be a pressure gradient between the cerebral space and the lumbar space. Also, when the protein is extremely high, pressure may not be transmitted correctly through the thin needle.



5. How is cerebrospinal fluid formed?


CSF fills the four ventricles of the brain, is distributed over the convexity, and also fills the spinal canal. It is secreted by the choroid plexus, a series of capillaries surrounded by epithelial cells. A small amount of CSF is also formed directly by brain capillaries.


The direction of CSF flow is from the lateral ventricles (where the choroid plexuses are located) through the foramina of Monro into the third ventricle. From the third ventricle, CSF flows through the aqueduct of Sylvius to the fourth ventricle. The third and fourth ventricles are single, midline structures, whereas the lateral ventricles are bilateral. From the fourth ventricle, it exits laterally through the foramina of Luschka and medially through the foramen of Magendie. Then it goes down the spinal canal, bathing the spinal cord.


The spinal cord itself ends at about the level of the L1 or L2 vertebral body. The dural sac, however, extends to nearly the end of the spinal canal. Thus the space between L2 and the bottom of the canal is filled with some nerve roots and ample CSF. This is the area commonly used for lumbar puncture and measuring CSF pressure.



6. What is benign intracranial hypertension?


The clinical symptoms of benign intracranial hypertension, also known as pseudotumor cerebri, are headache and visual disturbance. No particular clinical characteristic of the headache is pathognomonic. Patients almost invariably demonstrate papilledema. Although it may occur at any age, most cases occur in the third and fourth decades. Women are much more commonly affected than men. Visual acuity is usually normal, but careful examination of the visual fields demonstrates enlarged blind spots. The neurologic examination reveals no other focal abnormalities. If focal abnormalities are present, the diagnosis of pseudotumor should not be entertained. A normal computed tomogram (CT) or magnetic resonance image (MRI) of the brain is mandatory for diagnosis. Pseudotumor cerebri is a diagnosis of exclusion; according to an old axiom, the most common cause of pseudotumor is a real tumor.


The prototype for the disease is an obese woman with chronic headaches. Papilledema is detected incidentally during routine examination. Sometimes the CT scan is rated as showing “slitlike” ventricles.



7. Why do patients with pseudotumor cerebri have enlarged blind spots?


On the normal visual fields, the “blind spot” is caused by the optic disk. There are no light receptors on the disk. In papilledema, the disk is swollen and enlarged, thereby causing an enlarged blind spot.



8. What are visual obscurations?


Visual obscurations are transient darkenings of vision that are sometimes seen in patients with increased intracranial pressure. In pseudotumor cerebri, there are two mechanisms that are theorized to cause these visual changes. The first is direct pressure on the optic nerve. This second is pressure on the posterior cerebral arteries, causing occipital blindness.



9. What studies are important if the diagnosis of pseudotumor is entertained?


For the diagnosis of pseudotumor to be entertained, the patient first must meet clinical criteria, including headache and papilledema with no other obvious cause. Second, an imaging procedure must rule out the presence of a structural lesion. Lumbar puncture is then performed to confirm a CSF pressure; with pseudotumor, CSF pressure is at least 200 mm (in most cases, it is well over 300 mm). Imaging studies may appear normal; that is, both the ventricles and the sulci appear quite small. An electroencephalogram (EEG) is not necessary. The vast majority of patients have normal EEGs, and even when the EEG is abnormal, it does not help with the diagnosis.

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Jun 14, 2016 | Posted by in PAIN MEDICINE | Comments Off on Increased and Decreased Intracranial Pressure

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