Intracranial Hypotension

Introduction

Intracranial hypotension is a clinical manifestation of low cerebrospinal fluid (CSF) volume or pressure caused by a dural CSF leak. It is usually precipitated by dural injury from an iatrogenic event such as a lumbar puncture, recent surgery, overshunting, or trauma.

Spontaneous intracranial hypotension (SIH) was first described by Georg Schaltenbrand in 1938 and has been recognized as a rare but significantly underdiagnosed differential consideration for severe headache. Although its true incidence is unknown, a few observational studies have estimated its prevalence as 2 to 5 in 100,000 patients with primary headache. In these studies, female cases were twice as frequent as male cases, with a peak incidence in middle age.

The classic clinical presentation symptom of intracranial hypotension is orthostatic headache shortly after lumbar puncture, craniospinal surgery, or trauma. Orthostatic headache is also the most frequent presenting symptoms of SIH. The pattern of headache is usually diffuse, with varying degrees of severity and acuity. Other less sensitive and nonspecific symptoms include neck pain/stiffness, nausea, tinnitus, hyperacusis, and photophobia. In rare cases, severe brain displacement from intracranial hypotension can precipitate coma or stupor. Subdural hematoma and venous sinus thrombosis are sequelae of intracranial hypotension but can be the presenting symptoms in unsuspected SIH cases. Diagnostic criteria for SIH have been established by the International Headache Society ( Box 17.1 ).

Box 17.1

Modified from Schievink WI, Dodick DW, Mokri B, et al. Diagnostic criteria for headache due to spontaneous intracranial hypotension: a perspective. Headache . 2011;51:1442–1444.

Criteria for Diagnosing Spontaneous Intracranial Hypotension

A.

Orthostatic headache
B.

The presence of at least one of the following:
- •
  
  Low CSF opening pressure (<6 cm H ₂O)
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  Sustained improvement of symptoms after epidural blood patching
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  Demonstration of an active spinal CSF leak
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  Cranial MRI findings of intracranial hypotension
C.

No recent history of dural puncture
D.

Not attributable to another disorder

CSF , Cerebrospinal fluid; MRI , magnetic resonance imaging.

Evolution: Overview

Intracranial hypotension is a direct consequence of dural CSF leakage. The cause and location of CSF leakage may be obvious in iatrogenic or traumatic cases even without imaging investigations (e.g., CSF rhinorrhea or otorrhea). However, the specific location of the dural defect leading to SIH is often unknown. Computed tomography (CT) myelography, and intrathecal gadolinium magnetic resonance (MR) myelography are the most sensitive imaging modalities for localizing spinal CSF leaks and are preferred over radionuclide cisternography, given the latter’s low spatial resolution. Although there is a theoretical concern of cerebral herniation in performing a myelogram, there have been no reported cases of this and the expected low cerebral CSF pressure is also reassuring.

As can be inferred from the diagnostic criteria in Box 17.1 , magnetic resonance imaging (MRI) of the brain plays a major, noninvasive role in the evaluation of intracranial hypotension. Brain MRI can confirm suspected iatrogenic/traumatic cases by demonstrating positive imaging features while ruling out other etiologies to account for the patient’s symptoms. The characteristic MRI findings of intracranial hypotension ( Fig. 17.1 ) are listed in Box 17.2 and include diffuse smooth pachymeningeal enhancement (80%–100% sensitivity), subdural fluid collections/hematomas (27%–69% sensitivity), venous engorgement (75% sensitivity), pituitary hyperemia (unknown sensitivity), and brain sagging (10%–62% sensitivity). Brain sagging or downward displacement is a highly specific MRI feature, including flattening of the pons against the clivus, effacement of the prepontine or perichiasmatic cisterns, and descent of the cerebellar tonsils.

Box 17.2

Brain Magnetic Resonance Imaging Findings of Intracranial Hypotension

•

Diffuse, smooth pachymeningeal enhancement
•

Subdural hygroma/collection/hematoma
•

Intracranial venous engorgement
•

Pituitary hyperemia
•

Brain sagging or downward displacement
- •
  
  Flattening of the pons against the clivus
- •
  
  Effacement of prepontine or perichiasmatic cisterns
- •
  
  Cerebellar tonsil descent

In some severe or chronic cases of intracranial hypotension, pachymeningeal thickening/enhancement can be prominent and may even involve the spinal dura ( Fig. 17.2 ).