Intracranial Hypotension (IH)

Every human body cavity with fluid has a certain pressure. For example, the most taken blood pressure is the pressure in arteries. Other examples are the venous pressure in veins, or the ocular pressure in eyes. Intracranial pressure is the pressure inside the skull (outside of arteries and veins) of the cerebrospinal fluid or CSF. CSF is continuously produced, circulated and absorbed. Its pressure is maintained by a balance of these processes. Any significant alteration or defect of any of these processes may alter this pressure. Normal CSF pressure is between 6-20 cm of H2O. Any amount lower is called intracranial hypotension, and higher than 20 is called intracranial hypertension, the latter more common than the former.

CSF is produced by mushroom-looking structures called choroid plexus in the ventricles (central cavities) of brain. It passes through the ventricles (lateral, 3^rd, and 4^th) and then circulates on top of brain and the spinal cord, in a space called sub-arachnoid space. In this space, it gets reabsorbed by structures called arachnoid granulations. It seems that the low pressure of CSF is less of a problem than its low volume. This is the reason that a patient with this condition may have a normal or low normal CSF opening pressure on lumbar puncture.

What are symptoms of IH?

1. Positional headache: Headache that gets worse with sitting or standing and better with lying flat.
2. Chronic persistent headache, more so in the back of head and shoulder areas, especially during later part of the day.
3. Dizziness, vertigo, tinnitus, phonophobia
4. Diplopia, photophobia, blurred vision
5. Nausea

Some other symptoms may rarely happen in selective cases. This list included cranial nerve palsy, somnolence, cognitive dysfunction, incontinence, or movement disorder.

What usually causes IH?

Most common cause is a tear or a leak in the spinal dura (tissue enveloping the brain and the spinal cord). Sometimes, the nature or the reason for tear is obvious, like a recent lumbar puncture, spine surgery, or minor trauma.  If there is no such history, the condition is termed spontaneous intracranial hypotension.

How is IH diagnosed?

Traditionally, lumbar puncture is considered for diagnosis. CSF pressure less than 6cm is considered low. As stated above, in many cases, CSF pressure may be within normal range. It is important to know that mainstay of diagnosis is imaging, which in this case is not straight-forward. Following imaging techniques are used and they may not be available in all facilities:

1. CT: It may show some subtle findings such as sagging of brain in the posterior fossa or thickening of dura.
2. MRI: Plain MRI of brain is more sensitive than CT but reveals the same type of findings one may see on CT scan. It also rules out many other conditions that may cause headaches. MRI of spine is helpful to localize any related lesion such as signs of acute or chronic bleeding, presence of CSF outside of dural sac, or a fistula.
3. Radioisotope Cisternogram: This level of testing and some other described underneath are only done if there is strong case for this diagnosis made with the right clinical features and CT or MRI findings. It is done to find the CSF leak, but it is a rather invasive and non-sensitive test with low resolution.
4. Conventional CT Myelogram: As invasive as the cisternogram but may be little better with better sensitivity to reveal the location of leak.
5. Digital Subtraction Myelogram: Good resolution. It may be useful if the location of leak is already narrowed down: cervical, thoracic, or lumbar.
6. Dynamic CT Myelogram: May provide more detailed images, especially cross-sectional, but overall, about same as digital subtraction myelogram.
7. MR Myelogram with Gadolinium: May be useful for patients with CT contrast allergy.

Goal of imaging is to find the exact place of leak in dura or its communication with a space or cyst outside the dura. To do this, some kind of contrast is put in the dural space by doing a lumbar puncture and its movement is imaged by X-rays (CT scan) or by MRI scanning. Finding the exact location of the problem may be a tedious process, sometimes requiring multiple expensive tests.

How is IH treated?

As indicated above, IH is more about low volume of CSF than just low pressure. In acute settings, IV hydration may help. Caffeine for some reasons also helps. When the cause of IH or the location of the problem is obvious, like a recent lumbar puncture, one may try conservative treatment for 2-3 days with hydration, pain meds, and caffeine. If that does not work, a relatively simple procedure called the blood-patch is done, which usually is quite effective. In those cases, where the exact location of the problem is not known, investigations are needed to first find the exact cause. Treatment depends on the cause and the location and may include surgery.   There are no effective meds for this condition.

Where may I find more information about IH?

American Academy of Neurology