Intracranial hypotension

Intracranial hypotension is a condition in which there is negative pressure within the brain cavity.

Monro-Kellie Hypothesis
The Monro-Kellie hypothesis or doctrine holds that the skull is a closed space, that the volume of the brain, blood, and cerebral spinal fluid together is a fixed amount. "An increase in one should cause a decrease in one or both of the remaining two." In intracranial hypotension, the loss of pressure affecting the amount of cerebral spinal fluid in the cavity can in turn cause cerebral abnormalities, often seen in imaging.

In other words, decreased cerebrospinal fluid volume in intracranial hypotension is compensated for by increased blood volume and why intracranial hypotension is characterized by dilated blood vessels in the brain.

Cranial cerebrospinal fluid leaks
According to Cedars-Sinai, symptoms include:
 * CSF rhinorrhea, a "runny nose" where the drainage is cerebrospinal fluid
 * otorrhea, drainage from the ear
 * salty or metallic taste in the mouth
 * drainage down the back of the throat
 * Cutaneous sinus tract drainage
 * loss of sense of smell
 * change in hearing or ringing in the ears

Cerebrospinal fluid leaks located in the spine

 * positional or orthostatic headache, which may be intense. The headaches are worse when upright and get better lying down. "The headache usually is in the back of the head or base of the skull, but can also occur in the front, sides or all over the head. It is rarely on just one side of the head, and often is described as a 'pulling sensation' from the head to the neck. The severity of the headache can range from mild to very severe and disabling."
 * migraine
 * nausea
 * blurred vision
 * dementia––"[b]ehavioral variant frontotemporal dementia (bvFTD) is a devastating early onset dementia[,]... and in spontaneous intracranial hypotension, is rare and associated with brain sagging and hypersomnolence"
 * coma

Diagnosis
=== Diagnostic criteria for spontaneous intracranial hypotension with spinal CSF leaks ===
 * orthostatic headache
 * no recent history of dural puncture
 * not attributable to another disorder
 * the presence of at least 1 of:
 * low opening pressure (less than or equal to 60 mm of H20)
 * sustained improvement of symptoms after epidural blood patching
 * demonstration of an active spinal CSF leak
 * cranial MRI changes of intracranial hypotension (e.g. brain sagging or pachymeningeal enhancement)

Imaging signs

 * pachymeningeal enhancement
 * brain "sagging" or "sinking"
 * low cerebellar tonsils
 * brainstem distortion
 * Pontine enlargement
 * crowding of the posterior fossa
 * flattening of the optic chasm
 * subdural hygromas (cerebrospinal fluid-filled cysts) and/or subdural hematomas (blood leaking from vessels)
 * engorged venous sinuses
 * pituitary hyperemia

Radiologists' mnemonic for spinal CSF leaks: SEEPS
Radiologists use the mnemonic, SEEPS, to describe and remember the signs seen in imaging for CSF leaks located in the spine (not cranial CSF leaks):"Subdural fluid collection""Enhancement of pachymeninges (dura)""Enlargement of veins""Pituitary hyperemia (engorged pituitary)""Sagging of brain (including not limited to saggy tonsils)"

Causes
Causes of intracranial hypotension are grouped into 3 kinds of cerebrospinal fluid leaks:

iatrogenic, which is caused by a medical procedure;

traumatic, which is the result of an injury; and

spontaneous (idiopathic), which is "occurring with minimal or no clear precipitant".

Iatrogenic causes of intracranial hypotension and spinal CSF leaks

 * lumbar puncture (spinal tap). Spinal CSF leaks can develop after a lumbar puncture. Headaches that can develop are "often known as Post Dural Puncture Headache = PDPH. This is the most common cause of a spinal CSF leak."
 * epidural injections
 * spine surgery

Traumatic causes of intracranial hypotension and spinal CSF leaks

 * brachial plexus injuries (nerves extending from the spinal cord)
 * spinal injuries
 * sports injuries
 * falls

Spontaneous (idiopathic) intracranial hypotension and spinal CSF leaks
Potential causes may include: "lifting small or large items, straining, stretching, positional changes, sporting activities, roller coaster rides and falls. Some of these might be categorized as traumatic.

• Spontaneous spinal CSF leaks may be associated with spinal pathology such as calcified disc material or bone spurs. These leaks are usually ventral or in front of the spinal cord.

• There is a growing evidence base suggesting that a significant proportion of spontaneous spinal CSF leaks occur as a result of preexisting weakness of the dura mater. A range of dural defects have been reported at surgery. Electron microscopy of dura has revealed abnormalities in a substantial proportion of cases. Heritable Disorders of Connective Tissue (HDCT) occur at a higher frequency in affected individuals; intracranial hypotension may be the first noted manifestation. Marfan syndrome, Ehlers-Danlos syndrome (both classic and hypermobility type) and Unspecified Heritable Disorders of Connective Tissue have been reported. Patients with spontaneous spinal CSF leaks have been shown to have higher risk of intracranial aneurysms, bicuspid aortic valve and thoracic aortic aneurysms and should be evaluated for HDCT.

Spontaneous spinal CSF leaks are uncommon, but not rare. One estimate of annual incidence is 5 in 100,000, however this has not been well-studied. Unfortunately, misdiagnoses and delayed diagnoses are common in this subset."

Treatment for spinal CSF leaks and intracranial hypotension
According to the Spinal CSF leak foundation, conservative treatment includes "bedrest, oral and IV fluids, oral and IV caffeine" in the absence of severe symptoms.

Epidural blood patch

Epidural patch with fibrin glue +/- blood

Surgery

Notable Studies
2019, Intracranial Hypotension and Cerebrospinal Fluid Leak. (Abstract)

2016, Imaging Signs in Spontaneous Intracranial Hypotension: Prevalence and Relationship to CSF Pressure. (PDF of full study)