IIH mechanisms clarified

Fort Lauderdale, FL-Several mechanisms of idiopathic intracranial hypertension (IIH), acting in isolation or coupled, have been proposed. Independent observations of increased cranial cerebrospinal fluid (CSF) volumes and altered venous outflow now provide strong evidence that the previously suggested mechanisms of impaired CSF homeostasis and venous outflow co-exist in obesity-related IIH.

Previous studies suggested that subarachnoid ventricular spaces are larger in patients with IIH, but measurements are highly variable. Also, limited or inconsistent reports of the normative values of extraventricular CSF volume confound the difficulty in determining whether total intracranial CSF volume increases in IIH, said Byron L. Lam, MD, professor of ophthalmology, Bascom Palmer Eye Institute and Department of Radiology, University of Miami.

To clarify these issues, Dr. Lam and colleagues prospectively studied MRI-derived measures of total cranial, ventricular, and extraventricular CSF volumes in patients with IIH. The rationale was that if IIH is associated with decreased CSF absorption, then increased CSF volume should be expected.

The investigative team studied the MRI images of 14 newly diagnosed, untreated female patients with IIH before a diagnostic spinal tap. The comprehensive imaging consisted of high-resolution T₁-weighted MRI and velocity-encoded MRI images of arterial inflow and venous outflow. The same comprehensive MRI imaging was performed on a control group of 11 healthy women of similar age and body mass index (BMI).

The patients with IIH had mild to moderate IIH, were relatively young (27 ± 9 years), had increased BMI, and had good visual acuity. Their visual fields were mildly to moderately decreased (up to –7.0 dB). Papilledema grading ranged from 1 to 4, and the opening CSF pressures ranged from 26 to 47 cm H₂O (mean 37.38 cm H₂O).

CSF volumetric measurements were performed using 3D T₁-weighted sequences with isotropic resolution of 1 mm and segmentation of intracranial space into gray matter, white matter, and CSF. Intracranial CSF volume was the total intracranial volume minus the volume of the gray and white matter. The ventricular CSF volume was the sum of the lateral third and fourth ventricles, and the extraventricular volume was calculated as the numerical difference between the intracranial and intraventricular CSF volumes.

Although the age and BMI of the patients with IIH and controls were similar, the extraventricular CSF space was significantly greater in the patient group versus the controls, 244.1 ± 36.4 ml and 208.8 ± 26.7 ml, respectively (p < 0.010), Dr. Lam said.

“This really accounts for the increase in intracranial CSF in the patients with IIH compared with the controls,” he added.

The measurements of intracranial CSF were 263.8 ± 41.8 ml in the patient group and 227.3 ± 28.3 ml in the controls (p < 0.017). The ventricular volume was the same between the two groups (19.7 ± 8.7 ml in the patients with IIH and 18.7 ± 6.2 ml in the controls).

Even when the data were normalized to reduce intra-individual variability, the difference in extraventricular CSF volume remained significant, Dr. Lam said.

Reporting cranial volumetric measurements, he noted that the gray matter volume in the patients with IIH was greater than in the control group (p = 0.38), while the white matter was similar between the groups. This suggests increased interstitial fluid or blood volume, or both, Dr. Lam said. The total intracranial volume was also higher in the patient arm (p = 0.029).

Results for the arterial and venous flow measurements showed that the total cranial blood flow was the same between groups, but the total internal jugular flow was lower in the patients with IIH (p = 0.011), indicating increased venous outflow resistance.

Dr. Lam presented these findings at the annual meeting of the Association for Research in Vision and Ophthalmology.

Dr. Lam did not report any relevant commercial relationships.

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