Discussion

Abstract

T he previous chapters -and lectures they refer to -provided an update on what is new in the relationship between the brain and glaucoma and yielded information needed to plan future studies capable of translating this knowledge into clinical practice. During the second day of the Think Tank, our panel of scientists spent the time synthesizing and interpreting the material presented, examining new conceptual relationships, developing concepts for future directions and collaborations, discovering new areas for research, and elaborating on experiments that could be performed in the coming years to address unanswered questions and generate translational information. The following topics were merited special focus and a summary of their content is present below. HOW CAN WE MEASURE INTRACRANIAL CEREBROSPINAL FLUID (CSF) PRESSURE NONINVASIVELY?Given the strong arguments that the pressure gradient between the prelaminar optic nerve tissue and the subarachnoid CSF space may be more relevant than the 'estimated' intraocular pressure (IOP), the development of non-invasive methods to measure intracranial CSF pressure is warranted. Lumbar puncture is unsuitable as a routine procedure, particularly as an office procedure by ophthalmologists, given the risks of discomfort, infection and cerebral herniation. It is also possible that CSF pressure in the lumbar region may differ substantially from that in the subarachnoid space around the orbital portion of the optic nerve. Thus a technique for measurement of CSF pressure at the cranial level would be a valuable addition to our research and clinical armamentarium. Some of the proposed techniques are:Ophthalmodynamometry: since the central retinal venous pressure correlates with the subarachnoid space CSF pressure, the pressure exerted to the eye to cause cessation of venous pulsations seen during ophthalmoscopy could be a surrogate measure of the pressure in that compartiment. 1,2 One limitation is the lack of information on venous resistance, how it correlates with the pressure need to cease pulsation, and more importantly, what systemic variables (eg: systemic hypertension, use of medications) can interfere with venous resistance and its correlation with CSF pressure.Magnetic resonance imaging (MRI): recent studies have shown that the cross-sectional width of the optic nerveincluding its meningeal sheets -can provide information on the pressure level in the subarachnoid space. 3 Some of its disadvantages are cost and the fact that this technique does not provide numeric, objective measurements of CSF pressure, but rather a subjective dichotomized conclusion to be correlated with IOP measurements. Intracranial pressure measurement using tympanic membrane displacement (TMD) and infrasonic emissions: by measuring infrasonic emissions from the tympanic membrane researchers have be able to estimate in vivo and non-invasively whether the intracranial pressure is high or low. 4 Low intracranial pressure is associated with an initial high peak followed by peaks with smaller amplitudes ...

On the Cranial Nerves

“…Beyond the ocular globe, the optic nerve is categorized into three segments: intracranial; intracanalicular, located within the optic canal and the lesser wing of the sphenoid bone; and pre-chiasmal, before the optic chiasm formation [58]. At the optic chiasm, fibers from each eye intersect such that the optic tract comprises both ipsilateral temporal fibers and contralateral nasal fibers.…”