What Do We Really Know About Translaminar Pressure?: Response

Letters to the Editor 5. Mindermann T. Pressure gradients within the central nervous system. J Clin Neurosci. 1999;6:464-466. 6. Killer HE, Mironow A, Flammer J. Optic neuritis with marked distension of the optic nerve sheath due to fluid congestion. Br J Ophthamol. 2003;87:249. 7. Killer HE, Flammer J. Unilateral papilledema in a patient with a fronto-parieto-occipital arachnoid cyst. Am J Ophthalmol. 2001;132:589-591. 8. Killer HE, Jaggi GP, Flammer J, Miller NR, Huber AR, Mironov A. Cerebrospinal fluid dynamics between the intracranial -and the subarachnoid space of the optic nerve. Is it always bidirectional? Brain. 2007;129:1027-1030. 9. Killer HE, Miller NR, Flammer J, Meyer P, Weinreb RN, Remonda L, Jaggi GP. Cerebrospinal fluid dynamics in normal-tension glaucoma. Br J Ophthalmol. 2012;96: 544-548. 10. Killer HE, Laeng HR, Flammer J, Groscurth P. The arachnoid trabeculae and septae in the subarachnoid space of the human optic nerve: Anatomy and clinical considerations. Br J Ophthalmol. 2003;87:777-781. What Do We Really Know About Translaminar Pressure?: Response nized and we, therefore, have no correlation between the two amplitudes" at lumbar and optic nerve regions. It is unlikely that such a resonant fluid pressure anticorrelation would develop based on fluid dynamics. So left to the static relationship between pressures throughout the cavity, it seems reasonable that there would be some correlations. Put another way, in an organ pipe, there are surely anticorrelated pressure fluctuations at the two ends of the pipe. However, if you force 1,000 psi of air into that pipe, both ends will have relatively high pressure: no need to quote Einstein! In conclusion, our understanding of the relationship between intraocular pressure, ICP, and glaucoma is in its infancy. Existing data are far from conclusive but are suggestive that there is validity to the seemingly reasonable hypothesis that ICP as a component of the translaminar pressure gradient impacts the development of glaucoma. Further investigation is needed. W e appreciate the interest in our recent review article and that despite recent efforts, there is much surrounding the relationship between intracranial pressure (ICP) and glaucoma that existing data fail to elucidate (1). It is a valid criticism that some investigations retrospectively used patients who underwent lumbar puncture for a variety of causes. However, one must recognize ethical issues surrounding prospectively performing lumbar punctures on healthy patients in whom such procedures are not otherwise indicated. We should also like to point out that not all published studies were retrospective (2). With regard to the lesson in physics from Killer and Piecher, we agree with the assertion that the pressure in these two widely separated regions (the lumbar spinal column and the optic nerve sheath) is not expected to be identical. This point is worthy of further consideration. A "Newtonian fluid" is basically one in which the pressure forces conduct freely through the system, like in a balloon. In the spinal fluid cavity, there are undoubtedly pressure differences owing to 1) the constrictions in the cavity (where surface effects could play a larger role), 2) the specific properties of spinal fluid, and 3) nonuniform motion of the spinal fluid. Regarding 1) and 2), it would seem that even accounting for these nonlinearities in the fluid or the cavity, there would be some correlations between these pressures. Regarding 3), this is much more complicated. There are indeed cases where there is an anticorrelation in the pressure variations in a Newtonian fluid. For example, in a closed organ pipe (the air supplied from an open end and the other end of the pipe closed), the pressure on one side is high (closed end), whereas the open end is low. But, this is a so-called resonant condition, and it requires a special relationship between the speed of sound and the length and shape of the pipe cavity. Calculating the volume of the cavity from lumbar to optic nerve would be very complicated, and there are probably certain fluid velocity distributions that would give rise to an anticorrelation. But, as Killer points out, the pressure waves of spinal fluid ". . . are not synchro- Letters to the Editor: J Neuro-Ophthalmol 2016; 36: 110-114 Timothy J. McCulley, MD Jessica R. Chang, MD The Wilmer Eye Institute, Johns Hopkins School of Medicine, Baltimore, MD Christopher R. Monroe, PhD Department of Physics, University of Maryland, College Park, MD The authors report no conflicts of interest. REFERENCES 1. McCulley TJ, Chang JR, Piluek WJ. Intracranial pressure and glaucoma. J Neuroophthalmol. 2015;35(suppl 1):S38- S44. 2. Ren R, Jonas JB, Tian G, Zhen Y, Ma K, Li S, Wang H, Li B, Zhang X, Wang N. Cerebrospinal fluid pressure in glaucoma: a prospective study. Ophthalmology. 2010;117:259-266. 113 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.