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 raise d intracranial pressure is transmitted to the perineural subarachnoid space (SAS). This phenomenon has gained interest because ultrasound me thods are able to quantify the optic nerve sheath diameter (ONSD) in-v ivo non-invasively with a resolution below 0.5 mm. In order to study t he normal variation and distensibility of the human ONS. Histologic te chniques and sonographic measurements were applied to 54 human optic n . specimens before and after exposure to pressure. In untreated postmo rtem 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 result s 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-inves tigator differences proved negligible. Our results suggest that indivi dual factors determine both baseline sheath diameter and distensibilit y. The different extent of pressure-induced sheath expansion along the nerve may be partly due to the non-uniform distribution of subarachno id trabecular fibers between nerve and sheath. In conclusion, measurem ents of the ONSD for clinical purposes should be targeted to the regio n immediately behind the globe. Under conditions of raised pressure ar ound the intraorbital optic n., bilateral ONSD measurements should giv e comparable findings.