Effect of intracranial pressure on the diameter of the optic nerve sheath

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Object

The subarachnoid space around the optic nerve in the orbit can be visualized using T2-weighted MR imaging with the fat-saturation pulse sequence. The optic nerve sheath (ONS) diameter can be estimated by measuring the outer diameter of the subarachnoid space. Dilated ONS is associated with idiopathic intracranial hypertension and hydrocephalus, and is believed to reflect increased intracranial pressure (ICP). The relationship between dilated ONS and ICP is unclear because of the difficulty in obtaining noninvasive measurements of ICP. The authors investigated the relationship between subdural pressure measured at the time of surgery and ONS diameter measured on MR images in patients with chronic subdural fluid collection.

Methods

Twelve patients underwent bur-hole craniostomy with continuous drainage for chronic subdural hematoma or hygroma in 2006. Orbital thin-slice fat-saturated MR images were obtained before and after surgery, and the ONS diameters were measured just behind the optic globe. Subdural pressure was measured using a manometer before opening of the dura mater.

Results

A significant correlation was found between the ONS diameter and the subdural pressure (correlation coefficient 0.879, p = 0.0036). The ONS diameter before surgery (6.1 ± 0.7 mm) was significantly reduced after surgery (4.8 ± 0.9 mm, p = 0.003; measurements are expressed as the mean ± standard deviation).

Conclusions

Increased ONS diameter measured on coronal orbital thin-slice fat-saturated T2-weighted MR images is a strong indicator of increased ICP, and helps to differentiate between passive subdural fluid collection due to brain atrophy and subdural hygroma with increased ICP.

Abbreviations used in this paper: CSDH = chronic subdural hematoma; CSF = cerebrospinal fluid; ICP = intracranial pressure; ON = optic nerve; ONS = ON sheath.

Article Information

Address correspondence to: Arata Watanabe, M.D., Ph.D., Department of Neurosurgery, Faculty of Medicine, University of Yamanashi, 1110 Shimokato, Chuo, Yamanashi 409-3898, Japan. email: arata@yamanashi.ac.jp.

© AANS, except where prohibited by US copyright law.

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Figures

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    Graph showing the association between the subdural pressure and the diameter of the ONS.

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    Case 5. High-resolution, thin-section, T2-weighted MR images of the orbit obtained with the fat-saturated fast spin echo sequences. A: The diameter of the ONS (arrows) just behind the optic globe was 6 mm on the right and 6.1 mm on the left before surgery. B: Three months after the operation, the diameter of the ONS (arrows) had decreased to 4.9 mm on the right and 4.3 mm on the left.

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