The subarachnoid space surrounding the optic nerves. An ultrasound study of the optic nerve sheath.

The presence of enlarged optic nerve sheaths (ONS) suggests that raised intracranial pressure is transmitted to the perineural subarachnoid space (SAS). This phenomenon has gained interest because ultrasound methods are able to quantify the optic nerve sheath diameter (ONSD) in-vivo non-invasively with a resolution below 0.5 mm. In order to study the normal variation and distensibility of the human ONS. Histologic techniques and sonographic measurements were applied to 54 human optic n. specimens before and after exposure to pressure. In untreated postmortem specimens, the largest diameters were found 3 mm behind the globe (baseline range: 2.1 to 4.8 mm). Following volume injection into the orbital perineural SAS, all n. sheaths were enlarged (maximum ONSD 6.5 mm). The sheath expansion affected predominantly its anterior section (mean 1.6 mm, e.g. 50.2%); the posterior regions showed markedly less dilatation (31.6%). No relation was found between the change of ONSD and the baseline diameter. Variance analysis of the sonographic results showed that the observed ONSD change depends on (a) the position of measurement along the nerve, as well as on (b) the origin of the nerve (different/same subject), whereas lateral (left/right) or inter-investigator differences proved negligible. Our results suggest that individual factors determine both baseline sheath diameter and distensibility. The different extent of pressure-induced sheath expansion along the nerve may be partly due to the non-uniform distribution of subarachnoid trabecular fibers between nerve and sheath. In conclusion, measurements of the ONSD for clinical purposes should be targeted to the region immediately behind the globe. Under conditions of raised pressure around the intraorbital optic n., bilateral ONSD measurements should give comparable findings.