How is Diagnosis Made?
The Modified Dandy Criteria for Idiopathic Intracranial Hypertension is the official criteria used to diagnose IIH. According to the Dandy criteria, an IIH diagnosis is appropriate if a person:
- has signs and symptoms of increased intracranial pressure, such as papilledema and headache;
- has no localizing findings on neurological examination (Localizing findings are findings that point to injury of specific brain areas. For instance, a localizing finding could be the inability to move a certain muscle.);
- has a normal MRI/CT scan with no evidence of venous obstructive disease;
- has high intracranial pressure of 250mm/H2O or above on a spinal tap, with no abnormalities of cerebrospinal fluid;
- is awake and alert;
- has no other cause of increased intracranial pressure found.
A secondary IH (SIH) diagnosis generally pinpoints the cause of the elevated intracranial pressure. Consequently, there may be abnormal findings, in addition to the signs and symptoms of increased intracranial pressure and a high opening pressure on a spinal tap.
What is a spinal tap?
A lumbar puncture (or spinal tap) is considered the gold standard when it comes to an IH diagnosis. This test measures the pressure of the cerebrospinal fluid that circulates in the sub-arachnoid space, which surrounds the brain and the spinal cord. When intracranial pressure is high, that pressure is transmitted through out the entire sub-arachnoid space. Diagnosis is usually confirmed after a spinal tap reveals a high opening CSF pressure.
How is a spinal tap performed?
The typical position for a spinal tap is to lie on your side, with knees slightly bent forward. Local anesthesia is given to numb the lower back. Sometimes, an x-ray (fluoroscopy) may be used to help guide the needle connected to the manometer, the device used to measure intracranial pressure, into the subarachnoid space. Once the spinal needle is inserted, the physician may have you extend your legs and ask you to relax in order to obtain an accurate reading. Holding your breath, bearing down (Valsalva maneuver), or sitting, rather than lying on your side, can produce an inaccurate reading. The opening pressure must be measured before any CSF is removed. (The amount of CSF drained during a spinal tap depends on several factors: the opening pressure, the desired closing pressure and the amount of CSF the physician requires for lab studies.
Intracranial pressure can be measured in millimeters (mm)/water or centimeters (cm)/water. For example, a reading of 200mm/H2O is equal to 20cm/H2O.
For adults:
Normal pressure readings are generally below 200mm/H2O.
Borderline high pressure readings are between 200-250mm/H2O.
Anything above 250mm/H2O is considered a high pressure reading.
For young children:
Anything above 200 mm/H2O is considered a high pressure reading.
Many find that the headache and pulse synchronous tinnitus associated with IH disappears during a spinal tap. While spinal taps can provide important diagnostic information, their therapeutic effects are temporary since spinal fluid regenerates quickly.
After the tap…
It’s important to take it easy after having a spinal tap. If possible, try to remain lying down for at least 30 minutes to an hour immediately after the tap. Some people may experience a low pressure headache after a spinal tap. This headache is usually better when lying completely flat and worse when standing up. If you experience multiple headaches like this, it’s important to tell your physician. Sometimes these headaches are caused by a small CSF leak at the site where the spinal needle entered the sub-arachnoid space. These leaks can be repaired with a minor procedure known as a blood patch.