A technique for noninvasive determination of the pressure in the cerebrospinal fluid (CSF) has been proposed. This technique would involve two main steps: First, an optical coherence tomographic scanner would be aimed into an eye and used to image the optical disk (see figure). Next, the resulting tomographic imagery would be digitized and processed to determine the thickness of the neural fiber layer, which thickness is known to increase with the CSF pressure.
Heretofore, the spinal tap has been the standard technique for measuring the CSF pressure (also known as the intracranial pressure or "ICP"). Spinal taps are painful, dangerous, and expensive. The proposed technique could make it unnecessary to perform spinal taps.
Papilledema — swelling of the optic-nerve axons in the optic disk — is known from previous studies to be associated with an increase in the intracranial pressure beyond the normal limit of approximately 14 mm of Hg (about 1.9 kPa). The neural fiber layer can swell to as much as 20 times its normal thickness within hours after the onset of elevated intracranial pressure.
Papilledema can be seen through an ophthalmoscope. Stereoscopic fundus photography and optic-nerve-head analysis also reveal aspects of papilledema. The common limitation of these previously developed optical techniques is that they depend on changes in surface topography. The proposed version of optical coherence tomography would measure the thickness of the neural fiber layer regardless of changes in surface topography, and should be capable of revealing swelling of axons earlier than do the previously developed optical techniques.
By use of a previously constructed optical coherence tomographic scanner, it has been found that the thickness of the neural fiber layer in normal individuals varies by no more than 3 µm. This instrument has also been found capable of revealing early thinning of the neural fiber layer associated with glaucoma. Thus, it has been reasoned, it should be possible to use optical coherence tomography to measure early neural-fiber-layer swelling associated with intracranial hypertension. If the instrument could be modified to achieve a tenfold refinement of its resolution, then the instrument would be correspondingly more sensitive as an indicator of intracranial pressure. The modified instrument could be used in research on the sequence of events in papilledema because it could also provide information on deep and surface changes in the optic disk, flows of blood in veins and arteries, and shifts in the spectral reflectance of the optic disk.
This work was done by Mark S. Borchert and James L. Lambert of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Bio-Medical category. NPO-20079