VALIDATION OF THE OPTIC-NERVE SHEATH RESPONSE TO CHANGING CEREBROSPINAL-FLUID PRESSURE - ULTRASOUND FINDINGS DURING INTRATHECAL INFUSION TESTS

Raised intracranial pressure leads to increased pressure around the op tic nerve (ON), which underlies the formation of papilledema and the e nlargement of the dural optic nerve sheath (ONS). In clinical practice , the presence of widened ONSs is demonstrable on neuroimaging. but th eir relationship to cerebrospinal fluid (CSF) pressure remains unknown . The authors investigated the ONS response to pressure during CSF abs orption studies in 12 patients undergoing neurological testing. The ON S diameter was evaluated by serial B-mode ultrasound scans of the ante rior ON near its, entry into the globe. All patients tested showed ONS diameter changes that exhibited covariance with the alteration of lum ber CSF pressure and were completely reversible during the infusion te sts. The maximum difference in ONS diameter between baseline and peak pressure conditions was 1.8 mm on average (range 0.7-3.1 mm). correspo nding to an average ONS diameter variation of 45% (range 15-89%). Regr ession analysis yielded a linear covariance between ONS diameter and C SF pressure with different slopes across subjects (0.019-0.071 mm/mm H g, mean r = 0.78). However, this linear relationship was only present within a CSF pressure interval. This interval differed between patient s: 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 10 mm Hg. With a single excep tion, 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 ba sis of the ONS response to intrathecal infusion alone. It is concluded that the human ONS has sufficient elasticity to allow a detectable di lation in response to intracranial hypertension. Because of a variable pressure-diameter relationship. the subarachnoid pressure cannot be p redicted exactly single scans, Therefore, the clinical relevance of th is method relies on the demonstration of pathologically enlarged ;heat hs or ongoing enlargement on serial ultrasonography studies.