Effect of Positioning on Intracranial Pressure: Response

Letters to the Editor Effect of Positioning on Intracranial Pressure I read with interest the point–counterpoint between Drs. Lueck and McClelland regarding the effect of positioning on intracranial pressure (ICP) during lumbar puncture (1). Although I appreciate the concept that even a small difference in ICP measurement could make a big difference in the management of a patient with possible increased ICP who undergoes a lumbar puncture in the prone vs the lateral decubitus position, I would argue that an even bigger issue is the effect on ICP when a lumbar puncture is performed with the patient in the sitting position, rather than either the lateral decubitus or the prone position, which occurs routinely in emergency departments throughout the world. There seems to be a gap in our knowledge regarding the relationship between changes in body position and ICP specifically, although ICP is widely studied in the context of trauma and diseases that involve suspicion of disturbed cerebrospinal fluid (CSF) dynamics, such as idiopathic intracranial hypertension (IIH), it is studied almost exclusively in the supine or prone position. However, particularly for patients with IIH, it would seem appropriate to understand the effects on opening pressure when patients undergo a lumbar puncture in the sitting position. Under normal circumstances, the CSF compartment is a communicating fluid system and, as such, subject to hydrostatic pressure gradients. Although pressure is essentially the same in the entire system in the supine (and prone) position, gravity will give rise to hydrostatic pressure gradients in the seated position. In addition to the importance of knowing whether or not a patient with optic disc swelling has increased ICP, postural effects are of interest because of CSF shunts, where much care has been taken to develop antisiphon devices to negate gravitational effects. In addition, questions have been raised regarding the effect of weightlessness on ICP because astronauts have presented with symptoms and signs thought to be related to changes in ICP and CSF dynamics after long-duration spaceflight (spaceflightassociated neuro-ocular syndrome) (2). It seems most studies of the effects on ICP of the sitting vs the lateral decubitus position have been performed with the patient initially in the lateral Effect of Positioning on Intracranial Pressure: Response W 138 e thank Dr. Miller for his insightful comments on our Point Counter-Point article and can only agree position, in which the hydrostatic effects on ICP are equally distributed throughout the system, and then abruptly changing the patient's position to sitting (3). In this scenario, there seems to be a definite reduction in ICP when moving from the supine to the sitting position. Other investigators have studied the effects of gradually moving the patient from the supine position to the sitting position and have described a similar drop in ICP (4). For example, in the study by Ecklund et al (4), in which 11 healthy subjects were moved gradually from a supine to a sitting position with ICP measured along the way, there was a mean reduction of ICP from 10.5 6 1.5 mm Hg in the supine position vs 20.8 6 3.8 mm Hg in the sitting position (4). Unfortunately, none of these techniques mirror reality, in which a patient who generally has been seated (or standing) for some time remains seated during the lumbar puncture. If indeed the ICP is substantially lower in the sitting position than in the lateral decubitus or prone positions (due to reduction of jugular vein outflow when patients are upright?), only measurements of ICP that are abnormally high would be of value; “normal” ICP measured in the sitting position would have no meaning. I wonder if the authors or the section editors (Drs. Van Stavern and Lee) would comment on this issue. Neil R. Miller, MD Johns Hopkins University School of Medicine, The Wilmer Eye Institute, Baltimore, Maryland The author reports no conflicts of interest. REFERENCES 1. Lueck CJ, McClelland C. Is positioning during lumbar puncture clinically significant? J Neuroophthalmol. 2019;39:268–272. 2. Mader TH, Gibson CR, Miller NR, Subramanian S, Patel NB, Lee AG. An overview of spaceflight-associated neuro-ocular syndrome. Neurol India. 2019;67:206–211. 3. Qvarlander S, Sundström N, Malm J, Eklund A. Postural effects on intracranial pressure: modeling and clinical evaluation. J Appl Physiol. 2013;115:1474–1480. 4. Eklund A, Jóhannesson G, Johansson E, Holmlund A, Koskinen LOD, Malm J. The pressure differences between eye and brain changes with posture. Ann Neurol. 2018;80:269–276. with him. We did not discuss cerebrospinal fluid opening pressure (CSF-OP) measured at lumbar puncture in the sitting position because any value obtained in this way is not clinically useful. The lack of normative CSF-OP data in the seated position renders these results impossible for clinicians to interpret reliably. Consequently, such results Letters to the Editor: J Neuro-Ophthalmol 2020; 40: 138-140 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Letters to the Editor FIG. 1. Drawing to demonstrate change in measured CSFOP as a function of spinal level when sitting. Reprinted from Krönig and Gauss (2). CSF-OP, cerebrospinal fluid opening pressure. are misleading and should be disregarded. Nevertheless, we entirely agree that the effects of change in position on intracerebral pressure (ICP), intraocular pressure, and CSF-OP, as well as the effects of any changes generated by alteration in gravity, are worthy of study for multiple reasons. There were, in fact, several studies at the beginning of the 20th century looking at the effect of positioning on CSF-OP (1). The technique of lumbar puncture was introduced at the end of the 19th century and, along with assessing its role in the management of neurological diseases, there was considerable interest around its use in spinal anaesthesia: as Dr. Miller states, the height of the hydrostatic column of CSF profoundly influences the net pressure recorded from the spine in the sitting position, and this variation in intraspinal pressure has the potential to influence the absorption of anaesthetic agents. The size of the effect depends on the spinal level at which an injection is administered (Fig. 1) (2). Where it has been measured, CSF pressure in the lumbar region is typically about 25 cmH2O higher in the sitting posi- Letters to the Editor: J Neuro-Ophthalmol 2020; 40: 138-140 tion than in the lying position (3). Parenthetically, it should be remembered that any pressure measurement is actually a comparison against pressure at a nominal reference point (e.g., the left atrium), and that this reference point can be varied depending on circumstances. The authors of the studies referred to by Dr. Miller used the external auditory meatus as their reference point for ICP measurement, not the lumbar spine; hence, the very large drop in ICP observed when subjects moved from supine to sitting (4,5). The measurement of CSF-OP measured at lumbar puncture would have risen, not fallen. Dr. Miller is quite correct in his assertion that both ICP and CSF-OP in the upright position will be dependent on a number of hydrodynamic effects, not just the change from supine to sitting. ICP and CSF-OP will vary as a function of how quickly any change in position has taken place and also as a function of how long the patient has been in any given position before a measurement is made. These factors mean that we do not fully understand what is actually happening to the pressures inside the head and at the lamina cribrosa during day-to-day activities when patients are upright or, alternatively, when they are exposed to zero-gravity conditions. There is considerable ongoing research into finding a way to measure ICP noninvasively. Possible candidate methodologies include assessment of tympanic membrane displacement or otoacoustic emissions (6). For the above reasons, any noninvasive technique must take account of patient positioning (and timing) if it is to be introduced into routine clinical practice. At the current time, there is no accurate way of converting lumbar CSFOP measured in the sitting position to an equivalent supine pressure. That being so, any CSF pressure measured at lumbar puncture with the patient sitting up should be disregarded and excluded from any diagnostic consideration. Collin M. McClelland, MD Department of Ophthalmology and Visual Neurosciences, University of Minnesota, Minneapolis, Minnesota Gregory P. Van Stavern, MD Department of Ophthalmology and Visual Sciences, Washington University School of Medicine, Saint Louis, Missouri Andrew G. Lee, MD Department of Ophthalmology, Houston Methodist Hospital, Blanton Eye Institute, Houston, Texas 139 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Letters to the Editor Christian J. Lueck, PhD, FRCP, FRACP Department of Neurology, Canberra Hospital, Garran, Australia 3. 4. The authors report no conflicts of interest. REFERENCES 1. Davson H. Physiology of the Cerebrospinal Fluid. London, United Kingdom: J & A Churchill Ltd; 1967:359–364. 2. Krönig B, Gauss CJ. Anatomische und physiologische Beobachtungen bei dem ersten Tausend 140 5. 6. Rückenmarksanästhesien. Münchener Medizinische Wochenschrift. 1907;54:1969–1970. Masserman JH. Cerebrospinal hydrodynamics. IV. Clinical experimental studies. Arch Neurol Psychiatr. 1934;32:523–553. Qvarlander S, Sundström N, Malm J, Eklund E. Postural effects on intracranial pressure: modeling and clinical evaluation. J Appl Physiol. 2013;115:1474–1480. Eklund A, Jóhannesson G, Johansson E, Holmlund P, Qvarlander S, Ambarki K, Wåhlin A, Koskinen LOD, Malm J. The pressure difference between eye and brain changes with posture. Ann Neurol. 2016;80:269–276. Bruce BB. Noninvasive assessment of cerebrospinal fluid pressure. J Neuroophthalmol. 2014;34:288–294. Letters to the Editor: J Neuro-Ophthalmol 2020; 40: 138-140 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.