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Live at Annual Episode 7: Non-Invasive ICP Monitoring with Mohammad Hirzallah

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Mohammad Hirzallah joins Nick Morris to discuss improving the utility of optic nerve sheath diameter as a non-invasive intracranial pressure measurement through standardization and automation.


  • Nicholas A. Morris, MD

    Assistant Professor and Fellowship Director
    Division of Neurocritical Care and Emergency Neurology
    Department of Neurology
    University of Maryland School of Medicine Medicine

  • Mohammad Hirzallah, MD, MMSc

    Assistant Professor
    Departments of Neurology, Neurosurgery, and Center for Space Medicine
    Section of Neurocritical care and Vascular Neurology
    Baylor College of Medicine

  1. Hi, this is Nick Morris for the NCS
  2. podcast. We are live at the annual
  3. meeting and we just finished the
  4. non-invasive ICP session. I'm here with
  5. Dr. Mohamed Herzella from Baylor
  6. College of Medicine who really gave a
  7. wonderful talk on optic nerve sheath and
  8. it was clear to me that Dr. Herzella is
  9. probably thought more about optic nerve
  10. sheath than just about anyone I've ever
  11. met and I thought it would be wonderful
  12. to have him on the pod. So welcome Dr.
  13. Herzella. Thank you. Thank you for
  14. having me Now, you know, I think a lot
  15. of us employ optic nerve sheath diameter
  16. as a sort of easily obtainable
  17. non-invasive ICP measurement, but no
  18. one's quite clear on what the proper cut
  19. off or threshold should be for
  20. determining high ICP and you did a
  21. wonderful job in your talk and
  22. explaining some of the reasons why we
  23. don't understand this. Can you
  24. summarize those? Yeah, absolutely So
  25. to the first point, the cut off, the
  26. cut offs are tricky because the meta
  27. analysis often allows studies to use
  28. their own cutoffs. So you'll see
  29. cutoffs as low as four millimeters and
  30. others as high as 65 millimeters. So
  31. that's a huge variation. I would say a
  32. good rule of thumb is above five and
  33. someone that is unconscious, you likely
  34. have CT changes.
  35. Above six is a higher probability that
  36. you have ICP changes. So kind of use
  37. your pretest probability to adjust the
  38. cutoff. So if someone, if you have a
  39. high suspicion, use five millimeters as
  40. a cutoff. If your suspicion is low, I
  41. would use six millimeters as a cutoff.
  42. Great, and you pointed out in your talk
  43. some of the technical factors that vary
  44. from study to study. In fact, some
  45. studies you pointed out didn't even
  46. measure the optic nerve sheet at all.
  47. They actually measured just the optic
  48. nerve. Can you kind of go through some
  49. of these factors and what we should be
  50. thinking about and aware of when we're
  51. getting these measurements? Yeah,
  52. absolutely I think part of this is maybe
  53. we need to do this. to upgrade how we
  54. do this measurement, right? If I come
  55. to you and say I have this thing called
  56. echocardiography and I'm not exactly
  57. sure but anything from an ejection
  58. fraction of 30 to 60 might be abnormal,
  59. no one would use it, right? So we need
  60. to kind of start becoming more rigorous
  61. about how we do the measurement. So
  62. think about where you place your probe
  63. and really making those small
  64. adjustments with your probe until you
  65. get a nice cut with that thin interface
  66. between the nerve and the eyeball and
  67. the clear and atomic differentiation
  68. between the subarachnoid space, the
  69. nerve,
  70. the dura, and the retroorptophat, you
  71. need to see those clean lines. And when
  72. you're doing that, remember, you're
  73. scanning a four to six millimeter
  74. structure, right? So that's maybe
  75. thinner than a pencil. And so when you
  76. are moving your probe around, these are
  77. really small adjustments to get that
  78. longitudinal cut through the nerve. So,
  79. once you get that nice image, then
  80. understanding the anatomy and placing
  81. the markers correctly is going to be the
  82. most impactful thing to get an accurate
  83. measurement. And you mentioned some
  84. studies clearly are not all using a
  85. linear probe, but your recommendation
  86. is right to always use a linear probe.
  87. Yes, so classic ultrasound teaching is
  88. linear probe for more superficial
  89. structures where we need the resolution
  90. and then
  91. your phased array or your curvilinear
  92. probe for when you need deeper
  93. penetration with less resolution, right?
  94. So linear probe is for this use case,
  95. so we care about that accurate lateral
  96. resolution so you should use a linear
  97. probe at least 75 megahertz, probably
  98. closer to 10 as ideal Great, and my
  99. trainees often know that they should be
  100. measuring 3 millimeters back, but they
  101. don't always know back from what, so
  102. what is your recommendation for that?
  103. So that was. That was one of the big
  104. sorts of variations. So you see some
  105. studies measuring from the papella,
  106. even with papilladema, right? So if
  107. your papella is already one or two
  108. millimeters elevated, you're barely
  109. getting to the optic nerve sheath if you
  110. only go three millimeters from that.
  111. You have studies measuring from the
  112. retina, and you have studies measuring
  113. from the sclera or behind the eyeball.
  114. So the retina seems to be the most
  115. consistent structure that people agree
  116. on It's also easier to spot because of
  117. that equagenicity difference between the
  118. retina and the rest of the eyeball. So,
  119. I would say a retina, you place your
  120. line right at that
  121. equagenicity change between the
  122. hyperacolic retina and the hyperacolic
  123. bitters fluid glacial marker there, go
  124. three millimeters depth along the axis
  125. of the optic nerve, right? So if your
  126. image is tilted, you need to adjust
  127. that depth marker, and that's where you
  128. perform the measurement And then you
  129. mentioned in your talk that the there
  130. can be great variability in the
  131. measurements from the interior aspect of
  132. the optic nerve sheath versus the
  133. exterior aspect. And so what do you
  134. recommend we measure? So the, and this
  135. is one of those things where I'll try to
  136. be descriptive to kind of convey an
  137. image with words. So the inner more
  138. structure, that hypoacobic line,
  139. that's the nerve, then you have two
  140. structures outside of that and people
  141. describe them as the stripped,
  142. hyperacolic bands, so they're like
  143. horizontal strips, they're hyperacolic
  144. and they surround the nerve. That's
  145. most likely the subarachnoid space.
  146. Then you have the outer black line
  147. outside that, so an outer hypoacolic
  148. band surrounding that, that's most
  149. likely the dura. Then you have the
  150. retroorbital fat that is hyperacolic.
  151. And that's the
  152. challenge where it's important to get
  153. that nice resolution because the
  154. equigenicity of the subarachnoid space
  155. and the retroorbital fat can be very
  156. similar, so you really don't get that
  157. nice line. in between. Now the
  158. difference between internal and external
  159. is whether you place that marker inside
  160. the black line, which is the dura, and
  161. you measure the inside diameter of the
  162. optic nerve sheet, or you place that
  163. line outside the dura, and then it's
  164. the external diameter of the optic nerve
  165. sheet. And the reason that's important
  166. is they vary by around 15 millimeters.
  167. So, and that's statistically
  168. significantly different in some studies
  169. that we showed today, right? So, that
  170. would be the challenge is if you're not
  171. measuring the same structure
  172. consistently, you will get variable
  173. results, and then your measurements are
  174. not accurate. Both measurements are
  175. sensitive and specific for elevated ICP.
  176. So, whichever measurement you do in
  177. your practice for now, until we have
  178. standardization, just make sure that
  179. you're making that same measurement
  180. Great. And lastly, I don't think you
  181. mentioned it in your talk, but we've
  182. seen papers looking at. What is the
  183. right plane to look at the optic nerve
  184. and some papers suggest that we should
  185. be looking in multiple planes and taking
  186. an average, what's your practice? So
  187. yeah, we didn't mention that in the
  188. talk, but we discussed it extensively
  189. in the panel. So there are, the planes
  190. that are published are, there is an
  191. infraorbital plane that almost tries to
  192. get a cross-section that is not feasible
  193. in all people, then the more common
  194. planes that we see is the axial or the
  195. coronal plane.
  196. There are studies that compared them,
  197. and a lot of the times they don't
  198. correlate, but those weren't very
  199. extensive studies. From a, most
  200. panelists prefer the coronal axis, also
  201. from just a logistic standpoint, unless
  202. you have a specialized small footprint
  203. probe, it's really hard to fit that
  204. over the eye if you're going coronally,
  205. so that's why most people prefer axial
  206. and
  207. the classical that were first performed
  208. or axial. A lot of our normative
  209. databases, the good studies that kind
  210. of give you the normal values also used
  211. coronal. So it seems like most people
  212. prefer coronal and that's the one I
  213. would encourage people to use for
  214. clinical applications. But if you have
  215. a research application where you want to
  216. investigate different axes against each
  217. other or average them for to see if that
  218. improves the measurement that's that's
  219. valid but the standard is coronal.
  220. Great. And then lastly you mentioned it
  221. but we haven't explicitly said it so the
  222. the panel that you're describing can you
  223. tell the listeners who that is and how
  224. you're working with this group in order
  225. to standardize these measurements? Yeah
  226. absolutely. So we the panel is right
  227. now 60 panelists we have 10 steering
  228. committee members and these are people
  229. that published on variability and how to
  230. improve the measurement and then
  231. together we reviewed the literature
  232. and we classified the sources of
  233. variation. recruited our expert
  234. panelists outside the steering committee
  235. so that's our 50 panelists and we had
  236. selection criteria for our panelists so
  237. we picked the most published up
  238. technology damter experts by number of
  239. publications we reached out to them we
  240. reached out to education experts
  241. basically ultrasound fellowship program
  242. directors and asked them if they teach
  243. up technology damter frequently and if
  244. they'll be interested in participating
  245. and then we asked them to nominate
  246. clinicians that use this measurement and
  247. their clinical practice so we tried to
  248. keep it balanced between clinicians
  249. educators and researchers we have 50
  250. panelists Canada US Latin America Europe
  251. Africa Middle East and China so it's a
  252. diverse panel great well thank you so
  253. much I'm really excited to see what this
  254. panel produces and I think it
  255. will go a long way into standardizing
  256. this practice so that we're all on the
  257. same page when we talk about optic nurse,
  258. she's diameter and it's relation to ICP.
  259. So thanks for your work and we look
  260. forward to seeing the eventual
  261. publications that will come from this.
  262. Awesome. Thank you for having me. All
  263. right. Thank you.
  264. Thanks everyone for joining us here on
  265. the neurocritical care podcast. It's
  266. available anywhere you get your podcast
  267. and it comes along with some continuing
  268. education credits so check it out. This
  269. is Nick Morris live from the annual
  270. meeting