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CAMBRIDGE, England, Sept. 11 -- An MRI scan of the optic nerve sheath may provide an accurate, noninvasive way to screen for raised intracranial pressure, researchers found.

MRI-measured diameter of the optic nerve sheath could identify or rule out elevated intracranial pressure in at least 90% of traumatic brain injury cases, reported Thomas Geeraerts, M.D., of Addenbrooke's Hospital here, and colleagues online in Critical Care.

These findings from a small retrospective cohort study suggest that MRI could be clinically useful in determining which patients need invasive intracranial pressure monitoring, the current gold standard, and for screening patients when the invasive devices are unavailable, the researchers said.

However, the question remains whether the technique would be useful outside the acute setting in comparison with fundoscopic examination of the eye for papilloedema, commented Stephen D. Silberstein, M.D., of Thomas Jefferson University in Philadelphia, in an interview.

"If it shows increased pressure even in patients without papilloedema, then it would be extremely important," he said.

Intracranial hypertension is frequent in stroke, liver failure, meningitis, and other conditions, and the ischemic brain damage it causes raises the risk of mortality and poor neurological outcomes.

"Early detection and treatment of raised intracranial pressure is therefore critical but often challenging," the researchers said, "because invasive intracranial pressure monitoring is not routinely undertaken in these settings."

Also, the clinical signs -- such as headache, vomiting, and drowsiness -- are not specific or easy to interpret, particularly in sedated patients, Dr. Geeraerts' group said.

MRI is often done in these patients for other reasons, they noted.

For instance, at their institution, brain MRI is done for research purposes during acute traumatic brain injury, typically in patients who are sedated, mechanically ventilated, and on invasive intracranial pressure monitoring.

To see whether this MRI scan could pull double duty, the researchers compared the correlation between MRI measurement of the diameter of the optic nerve sheath and invasive testing for detection of intracranial pressure.

They retrospectively analyzed blinded brain MRIs from a prospective cohort of 38 patients on invasive parenchymal sensor intracranial pressure monitoring after severe traumatic brain injury and from 36 healthy volunteers.

Overall, measurement of the optic nerve sheath with MRI imaging was feasible for 95% of patients with significantly higher average diameter in those with traumatic brain injury than controls (5.72 versus 5.08 mm, P=0.0001).

MRI-measured nerve sheath diameter significantly predicted intracranial pressure measured by more invasive means (P<0.0001).

The diameter was significantly higher for brain-injured patients with intracranial pressure of at least 20 mm Hg than for those without intracranial hypertension (6.31 versus 5.29 mm, P<0.0001).

Compared with healthy volunteers, the difference was also significant for injured patients with elevated pressure (P<0.0001) but not for those without elevated pressure (P=0.12).

The best threshold for detecting elevated intracranial pressure with MRI was a nerve sheath diameter of 5.82 mm, which had a sensitivity of 90%, specificity of 92%, and negative predictive value of 92%.

A threshold of 5.30 mm had 100% sensitivity and negative predictive value but specificity of only 50%.

However, the diameter of the optic nerve itself was not a good predictor of intracranial pressure (P=0.05), suggesting distension of the nerve sheath during increases in cerebrospinal fluid pressure.

The major limitation of the MRI method is "probably related to MRI itself, with limited access, necessity of patient transfer in a magnetic area, and specific contraindications," the researchers said.

The study was supported by a grant from the British Medical Research Council Program. Dr. Geeraerts reported support by grants from the Société Française d'Anesthésie et de Réanimation and from Journées d'Enseignement Post-Universitaire d'Anesthésie-Réanimation-Novo Nordisk. Co-authors reported support from the Gates Cambridge Trust, an Overseas Research Studentship, fellowships from the British Academy of Medical Sciences, the Medical Research Council, Royal College of Anaesthetists, Wellcome Trust, the Evelyn Trust, and Queens' College Cambridge. The researchers reported no conflicts of interest. Dr. Silberstein reported no conflicts of interest.

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