FUNDAMENTALS OF TRANSORBITAL SONOGRAPHIC EVALUATION OF OPTIC-NERVE SHEATH EXPANSION UNDER INTRACRANIAL HYPERTENSION .1. EXPERIMENTAL-STUDY

The optic nerve, ontogenetically part of the central nervous system, i s surrounded by subarachnoidal cerebrospinal fluid (CSF) and dura mate r. Because of the connection with the intracranial subarachnoidal spac e, CSF pressure variations influence the optic nerve sheath (ONS) diam eter. Histologic studies revealed a segment of the optic nerve in whic h maximal diameter fluctuations could be expected, namely the bulging dura mater region approximately 3 mm behind the papilla. Twenty prepar ations of optic nerves obtained post mortem were examined sonographica lly before and after dilatation of the ONS, by means of measurement fr om three different projections. After gelatine-induced widening of the subarachnoidal space, the mean diameter increased by 60 % at 3 mm beh ind the optic nerve head, but only by 35 % at 10 mm distance. Independ ent measurements by two examiners correlated highly, which indicates e xcellent reproducibility of the sonographic measurements. The optimal experimental scanning position was at a right angle to the optic nerve (longitudinal section). Under clinical conditions, however, only axia l sections can be obtained using anterior probe positions with transbu lbar sound directions. Using such axial projections the 3 mm position proved reliably reproducible. The reduced resolution of the optic nerv e itself, allowing it to be distinguished from its surrounding sheath, proved to be somewhat disadvantageous from this projection angle.