Spontaneous intracranial hypotension (SIH) was first reported by Schaltenbrand in 1938 and is characterized by intracranial hypotension due to cerebrospinal fluid (CSF) leakage from the spine.
Epidemiology:
Three main mechanisms of CSF leakage (Schievink classification):
The annual incidence is estimated at 5 cases per 100,000 people (1/20,000)1), but as high-resolution imaging becomes more widespread, the actual frequency may be even higher. It is thought that asymptomatic or mild cases are also included.
The frequency of symptoms according to Japanese guidelines (Japanese cohort) is shown below 3).
| Symptom | Frequency |
|---|---|
| Headache (orthostatic) | 98.5% |
| Dizziness/vertigo | 50.5% |
| Nausea | 49.0% |
| Balance disorder | 42.6% |
| Posterior neck pain | 34.2% |
Other symptoms may include horizontal diplopia (abducens nerve palsy), tinnitus/hearing changes (involvement of cranial nerve VIII), facial numbness, blurred vision, and visual field defects. In severe cases, parkinsonism, coma, and frontotemporal dementia-like symptoms have also been reported.
Characteristics of orthostatic headache: Worsens when standing, improves or resolves when lying down.
Ophthalmic involvement (ocular symptoms in 42% of SIH patients):
Trendelenburg test: Lying flat with the head lowered 10–20 degrees for 5 minutes relieves or improves orthostatic headache. Useful as an auxiliary diagnostic finding.
Ocular symptoms occur in 42% of SIH patients, with the most common being diplopia due to abducens nerve palsy. Next is trochlear nerve (CN IV) involvement, and rarely the optic nerve is affected. Importantly, 97% of these ocular symptoms improve with correction of CSF pressure (rest, epidural blood patch, etc.).
Causes of CSF Leak:
Main Risk Factors:
A Danish report observing 36 SIH patients found that 4 cases (11%) developed SIH after flying, with a temporal association2). Reduced cabin pressure (about 0.8 atm, equivalent to cruising altitude) may promote rupture of meningeal diverticula. However, large multicenter studies have not been conducted, and causality is not established.
Comparison of diagnostic criteria:
| Diagnostic criteria | Major criteria |
|---|---|
| 2008 AJNR criteria | ① Extrathecal CSF on spinal imaging ② Head MRI findings suggestive of SIH + low opening pressure (<60mmH₂O)/dural diverticulum/improvement after EBP ③ Typical orthostatic headache + ≥2 items from ② |
| ICHD-3 criteria | Orthostatic headache + CSF pressure <60mmH₂O or imaging evidence of CSF leak 3) |
| Japanese guideline criteria | Orthostatic headache + dural enhancement or CSF pressure <60 mmH₂O → “probable” 3) |
Head MRI findings (in order of sensitivity):
MR myelography (recommended by Japanese guidelines): Specific findings such as the floating dural sac sign and dinosaur tail sign. Useful for identifying the leakage site 3).
Ultrasound: In SIH patients, the optic nerve sheath diameter (ONSD) decreases by 0.5 mm from supine to standing position. No change in healthy controls.
Occult SIH (oSIH): Typical orthostatic headache but normal imaging findings. Meta-analysis shows imaging evidence of CSF leakage in only 48–76% of cases 5).
Differential diagnosis: Meningitis (bacterial, fungal, aseptic), autoimmune diseases (RA, SLE), sarcoidosis, tuberculosis, brain tumors (meningioma, en plaque lymphoma), Chiari type I malformation (differentiated by measuring the mamillopontine distance; shortened in SIH 6)).
Conservative Therapy
Rest and fluid intake: Japanese guidelines recommend 2 weeks of bed rest plus intravenous fluids such as 1500 mL of lactated Ringer’s solution 3).
Abdominal binder.
Medication: Caffeine, theophylline, analgesics, NSAIDs.
Kampo medicine (Goreisan): Inhibits aquaporin 4 channels for water intoxication 3).
Steroids: Prednisone 1 mg/kg/day for 5 days (tapered over 7 days). The overall success rate of conservative treatment is only about 28% 1).
Epidural Blood Patch (EBP)
Effectiveness: Improvement in 87% after targeted EBP. Blind EBP improves 52%.
Method: Inject 10–15 mL of autologous blood into the lumbar epidural space. For cervical leaks, inject 20–40 mL into the thoracolumbar epidural space.
Recurrence: 25% recur within 8 years after targeted EBP. Repeat EBP is successful.
Occult SIH (oSIH): Empirical EBP results in improvement at discharge in 66.7% and at 3 months in 90.5%5).
Surgical Treatment
Indications: After two or more failed EBP. Clear identification of the leakage site is required.
Surgical technique: Ligation of meningeal diverticulum, direct repair of dural tear, epidural fibrin glue injection, duroplasty.
Vascular malformation-related: Fibrin glue patch, liquid embolization (n-BCA), surgical ligation. Standard EBP is often insufficient for fistula closure6).
Targeted EBP improves symptoms in 87% of cases, while blind EBP improves in 52% 5). The improvement rate after the first EBP varies from 25% to 90% across studies. If there is no improvement after the first session or if symptoms recur, a repeat EBP is performed. For occult SIH (oSIH), empirical EBP led to improvement in 90.5% of cases at 3 months 5).
Normal CSF dynamics:
Monroe-Kellie hypothesis in SIH pathophysiology: Since intracranial volume is constant (the skull is rigid), a decrease in CSF volume is compensated by an increase in the volume of low-resistance structures (veins around the brain and pituitary, dural venous sinuses)4)8). This compensatory mechanism produces characteristic MRI findings.
Mechanisms of CSF leakage:
Mechanism of superficial CNS hemosiderosis: Brain sagging → bridging vein hemorrhage, or epidural venous plexus hemorrhage around the dural defect → repeated subarachnoid hemorrhage4).
Tonello et al. (2022) reported a 38-year-old male with C2-level CSF leakage who received prednisone 1 mg/kg/day for 5 days followed by a 7-day taper, and imaging after one month showed near-complete resolution of the CSF leak 1). Multiple mechanisms of action are hypothesized, including improvement of cerebral edema and inflammation, fluid retention, and promotion of CSF reabsorption. Currently, this is at the case report level, and prospective randomized studies are needed.
In 4 of 36 cases (11%), a temporal association between airplane travel and onset of SIH was confirmed 2). It is hypothesized that decreased cabin pressure promotes rupture of meningeal diverticula, but large-scale multicenter case-control studies are needed.
Accurate nomenclature and classification of CSF-venous malformation fistulas and CSF-lymphatic malformation fistulas have been proposed 6). For lymphatic malformations, mTOR inhibitors (e.g., sirolimus) have been shown to reduce their size, potentially offering a treatment for associated CSF leakage 7).
For pediatric oSIH presenting with typical orthostatic headache but no leakage detected on MRI or CT myelography, the efficacy of empirical EBP has been reported 5). New imaging techniques such as digital subtraction myelography (DSM) and dynamic CT myelography are advancing the visualization of previously undetectable leaks 5).
