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Intracranial hypotension means low pressure inside the head, referring to low cerebrospinal fluid (CSF) pressure which results from the loss of CSF volume, almost always from a CSF leak at the level of the spine. BUT, normal cerebrospinal fluid (CSF) pressures have long been known to occur in patients with a proven spinal CSF leak.

Peter Kranz and colleagues at Duke University published this paper to examine how common normal pressures occurred in patients they were seeing with spontaneous intracranial hypotension. In their study of 106 patients, only 34 % had low pressures, defined as less than or equal to 6 cm H2O. 61 % had normal pressures.

How common is normal cerebrospinal fluid pressure in spontaneous intracranial hypotension?
Kranz PG, Tanpitukpongse TP, Choudhury KR, Amrhein TJ, Gray L.
Cephalalgia. 2015 Dec 17. pii: 0333102415623071. [Epub ahead of print]
Abstract
OBJECTIVES:
To determine the proportion of patients with spontaneous intracranial hypotension (SIH) who had a cerebrospinal fluid (CSF) pressure >6 cm H2O and to investigate the clinical and imaging variables associated with CSF pressure (PCSF) in this condition.
METHODS:
We retrospectively reviewed 106 patients with SIH. PCSF was measured by lumbar puncture prior to treatment. Clinical and imaging variables – including demographic data, brain imaging results, symptom duration, and abdominal circumference – were collected. Univariate and multivariate analyses were performed to determine the correlation of these variables with PCSF.
RESULTS:
Sixty-one percent of patients had a PCSF between 6 and 20 cm H2O; only 34% had a PCSF ≤6 cm H2O. The factors associated with increased PCSF included abdominal circumference (p < 0.001), symptom duration (p = 0.015), and the absence of brain magnetic resonance imaging findings of SIH (p = 0.003). A wide variability in PCSF was observed among all patients, which was not completely accounted for by the variables included in the model.
CONCLUSIONS:
Normal CSF pressure is common in patients with SIH; the absence of a low opening pressure should not exclude this condition. Body habitus, symptom duration, and brain imaging are correlated with PCSF measurements, but these factors alone do not entirely explain the wide variability in observed pressures in this condition and this suggests the influence of other factors.
PMID: 26682575

Recently, another study examined the proportion of patients with normal CSF pressures along with factors that might affect these pressures. Their study of 206 patients revealed low pressures in 55 % (less than or equal to 6 cm H2O), normal pressures in 44 % and high pressures in 1 %.

Factors affecting cerebrospinal fluid opening pressure in patients with spontaneous intracranial hypotension.
Yao LL, Hu XY.
J Zhejiang Univ Sci B. 2017 Jul;18(7):577-585. doi: 10.1631/jzus.B1600343.
Abstract
OBJECTIVE:
Spontaneous intracranial hypotension (SIH) is recognized far more commonly than ever before. Though usually characterized by low cerebrospinal fluid (CSF) pressure, some patients with SIH are observed to have normal pressure values. In this study, we aimed to confirm the proportion of patients with normal CSF opening pressure (CSF OP) and explore the factors affecting CSF OP in SIH patients.
METHODS:
We retrospectively reviewed 206 consecutive SIH patients and analyzed their clinical and imaging variables (including demographic data, body mass index (BMI), duration of symptoms, and brain imaging findings). Univariate and multivariate analyses were performed to identify the potential factors affecting CSF OP.
RESULTS:
In a total of 114 (55.3%) cases the CSF OP was ≤60 mmH2O (1 mmH2O=9.806 65 Pa), in 90 (43.7%) cases it was between 60 and 200 mmH2O, and in 2 (1.0%) cases it was >200 mmH2O. Univariate analysis showed that the duration of symptoms (P<0.001), BMI (P<0.001), and age (P=0.024) were positively correlated with CSF OP. However, multivariate analysis suggested that only the duration of symptoms (P<0.001) and BMI (P<0.001) were strongly correlated with CSF OP. A relatively high R2 of 0.681 was obtained for the multivariate model.
CONCLUSIONS:
Our study indicated that in patients without a low CSF OP, a diagnosis of SIH should not be excluded. BMI and the duration of symptoms can influence CSF OP in SIH patients, and other potential factors need further investigation.
PMID: 28681582
PMCID: PMC5498838

Both of these authors found that:
1) normal pressures are common in spontaneous intracranial hypotension;
2) body habitus (abdominal circumference) or BMI correlated with pressure measurements;
3) duration of symptoms correlated with pressure measurements.

Normal CSF opening pressure cannot be used to rule out spontaneous intracranial hypotension. This is a common diagnostic error leading to under-diagnosis and additional suffering.