Raised intracranial pressure leads to increased pressure around the optic nerve (ON), which underlies the formation of papilledema and the enlargement of the dural optic nerve sheath (ONS). In clinical practice, the presence of widened ONSs is demonstrable on neuroimaging, but their relationship to cerebrospinal fluid (CSF) pressure remains unknown. The authors investigated the ONS response to pressure during CSF absorption studies in 12 patients undergoing neurological testing. The ONS diameter was evaluated by serial B-mode ultrasound scans of the anterior ON near its entry into the globe. All patients tested showed ONS diameter changes that exhibited covariance with the alteration of lumbar CSF pressure and were completely reversible during the infusion tests. The maximum difference in ONS diameter between baseline and peak pressure conditions was 1.8 mm on average (range 0.7-3.1 mm), corresponding to an average ONS diameter variation of 45% (range 15-89%). Regression analysis yielded a linear covariance between ONS diameter and CSF pressure with different slopes across subjects (0.019-0.071 mm/mm Hg, mean r = 0.78). However, this linear relationship was only present within a CSF pressure interval. This interval differed between patients: ONS dilation commenced at pressure thresholds between 15 mm Hg and 30 mm Hg and in some patients saturation of the response (constant ONS diameter) occurred between 30 mm Hg and 40 mm Hg. With a single exception, definitely enlarged ONS diameters (> 5 mm) were present when CSF pressure exceeded levels of 30 mm Hg. Retrospectively, discrimination between normal and elevated outflow resistance was possible on the basis of the ONS response to intrathecal infusion alone. It is concluded that the human ONS has sufficient elasticity to allow a detectable dilation in response to intracranial hypertension. Because of a variable pressure-diameter relationship, the subarachnoid pressure cannot be predicted exactly by single scans. Therefore, the clinical relevance of this method relies on the demonstration of pathologically enlarged sheaths or ongoing enlargement on serial ultrasonography studies.

ABSTRACT.Purpose: To determine the distensibility and elastic characteristics of the optic nerve sheath for development of a basic understanding of ultrasound studies aimed to measure optic nerve sheath diameter (ONSD) for detection of acutely elevated intracranial pressure (ICP). Methods: Isolated human optic nerves preparations obtained from autopsies were submitted to predefined pressure alterations, and consecutive changes in ONSD were measured by B-scan ultrasound under defined conditions. Results: Following submission to pressure, the diameter of the nerve sheath increased up to 140% of its baseline value. The increase (mean 1.97 mm, SD 0.52 mm) corresponded to the magnitude of pressure steps measured in the perineural subarachnoidal space (SAS). Similarly, the ONSD declined in each of the preparations within a few minutes after the optic nerve was decompressed. However, it did not reach its baseline value again when pressure loads of 45-55 mmHg or more had been applied. Conclusions: The elasticity of the anterior sheath of the optic nerve is sufficient for the detection of pressure changes in the SAS especially for upward pressure steps. This is basically important for the application of clinical monitoring of the sheath diameter to facilitate the identification of patients with elevated ICP non-invasively (screening). However, further implementation of this procedure in neurointensive care and emergency medicine has to consider that the sheath diameter reversibility may be impaired after episodes of prolonged intracranial hypertension and a model for hysteresis is proposed.

The precision of magnetic resonance imaging exceeds that of ultrasonographic methods for determining optic nerve and nerve sheath diameters. HASTE sequences appear particularly appropriate for investigating the retrobulbar optic nerve complex and may be useful in future studies quantifying axonal loss within the optic nerve.

No abstract

Patients with poor prognosis related to raised ICP in pediatric liver failure can be identified by ultrasound measurement of ONSD without the disadvantages of invasive procedures. Although the exact intracranial pressure level cannot be deduced from single examinations, ONSD trends can reflect the evolution of ICP in hepatic encephalopathy.