What are the causes and diagnostic methods for intracranial hypotension (low cerebrospinal fluid pressure)?

Intracranial Hypotension: Etiology and Diagnosis

Etiology

Intracranial hypotension results from cerebrospinal fluid (CSF) leakage, which occurs through three primary mechanisms: dural defects, leaking meningeal diverticula, and CSF-venous fistulas. 1

Primary (Spontaneous) Causes

• Dural defects are the most common mechanism, typically occurring in the lower cervical and upper thoracic spine where mechanical stress is greatest 1, 2
• Leaking meningeal diverticula represent outpouchings of the dura that can rupture or leak CSF 1
• CSF-venous fistulas are abnormal connections between the CSF space and venous structures 1

Predisposing Risk Factors

• Connective tissue disorders weaken the dura and create ectatic meningeal cysts 1
• Spinal osteophytes can perforate the dura, creating CSF leakage pathways 1
• Idiopathic intracranial hypertension is paradoxically associated with many spontaneous CSF leaks 1
• History of bariatric surgery may weaken the dura through rapid loss of epidural fat 1
• Medication-induced leaks can occur in prolactinoma patients treated with dopamine agonists like cabergoline 1

Secondary (Iatrogenic) Causes

• Lumbar puncture is a common iatrogenic cause 3, 2
• Spine surgery with accidental dural opening or excessive CSF drainage 3, 2
• Chiropractic manipulation 2
• Spine trauma 2
• Degenerative spine disorders rarely cause secondary intracranial hypotension 2

Anatomical Considerations

• The spine is the source of most symptomatic CSF leaks, not the intracranial compartment, with the lower cervical and upper thoracic regions being predilection sites 4, 2

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Diagnosis

Clinical Presentation

The hallmark symptom is orthostatic headache that worsens when upright and improves when lying down. 5

Cardinal Symptoms

• Orthostatic headache is present in most patients, though approximately 10% may present with continuous non-postural headache 5, 6
• Nausea and vomiting 5, 6
• Tinnitus and hearing changes 5, 6
• Photophobia 5
• Dizziness and imbalance 5

Atypical Presentations

• Neck pain or upper extremity radicular symptoms in a small proportion of patients 2
• Gait disturbance 5
• Rare severe manifestations include coma, frontotemporal dementia-like symptoms, and leptomeningeal hemosiderosis 3

Potential Complications

• Cerebral venous thrombosis occurs in approximately 2% of cases 5
• Seizures 5
• Brain herniation in extreme cases 5
• Subdural fluid collections or hematomas 5, 6

Diagnostic Imaging

MRI of the brain without and with IV contrast is the most useful initial evaluation for suspected intracranial hypotension. 4

Brain MRI Findings (Preferred Initial Study)

The American College of Radiology recommends looking for the following characteristic findings 5, 4:

• Pachymeningeal (dural) enhancement - the most specific finding 5, 4, 7
• Brain sagging or descent with downward displacement 5, 4
• Subdural fluid collections (hygromas or hematomas) 5, 4
• Engorgement of venous sinuses 5, 4
• Pituitary enlargement with convex superior surface 5, 4
• Midbrain descent and decreased pontomesencephalic angle 4
• Effacement of basal cisterns 5

Spine MRI for Leak Localization

Initial spine imaging should be MRI complete spine without IV contrast (or without and with contrast), optimized with fluid-sensitive sequences, especially 3D T2-weighted fat-saturated sequences. 4

Spinal imaging findings include 4, 2:

• Epidural fluid collections (extra-arachnoid or extradural)
• Meningeal diverticula
• Meningeal enhancement
• Engorgement of epidural venous plexus

CT Imaging (Less Sensitive)

CT without contrast is not recommended as an initial imaging study, and even CT with contrast is less sensitive than MRI. 4

CT may show 4:

• Subdural fluid collections or hematomas
• Brain sagging or descent
• Engorgement of venous sinuses
• Effacement of basal cisterns

Radioisotope Cisternography

• May identify CSF leakage sites in the cervico-thoracic region 6
• Shows limited ascent of tracer to cerebral convexity and early bladder appearance 6
• The imaging strategy for leak localization (CT myelography, MR myelography with gadolinium, digital subtraction myelography) remains debated 3

CSF Analysis

• Low CSF opening pressure is typical but not required for diagnosis 8, 6
• CSF pressure can be normal in some patients with intracranial hypotension, making imaging findings crucial 4, 8
• Increased CSF albumin may be present 6
• Pleocytosis can occur 6

Important Diagnostic Caveats

• Approximately 20% of initial brain MRIs and 46-67% of initial spine imaging may be normal in patients with clinically suspected intracranial hypotension, so negative initial imaging does not exclude the diagnosis 4
• Intracranial hypotension can mimic aseptic meningitis or pituitary adenomas, making differential diagnosis critical to avoid unnecessary procedures 7
• Imaging findings are highly specific when present, and proper imaging guides targeted therapy such as epidural blood patch 4, 3

Pathophysiologic Mechanisms Underlying Imaging Findings

The characteristic imaging findings result from 5:

• Compensatory venodilation and blood volume expansion (venous sinus engorgement)
• Dural sinus stretching (pachymeningeal enhancement)
• Downward traction on meninges, nerves, and brain parenchyma (brain sagging)
• Loss of brain buoyancy leading to descent and subdural collections