Ocular sonography in pre-eclampsia: a simple technique to detect raised intracranial pressure?
2019; Elsevier BV; Volume: 41; Linguagem: Inglês
10.1016/j.ijoa.2019.09.002
ISSN1532-3374
AutoresClemens M. Ortner, P. Macias, Elmari Neethling, Vijay Krishnamoorthy, Brendan Carvalho, Justiaan Swanevelder, R.A. Dyer,
Tópico(s)Cerebral Venous Sinus Thrombosis
ResumoIn the past three decades, ultrasonographic images of the fibro-elastic sheath surrounding the optic nerve have become an important application of point-of-care ultrasound.1Weiniger C.F. Sharoni L. The use of ultrasound in obstetric anesthesia.Curr Opin Anaesthesiol. 2017; 30: 306-312Crossref PubMed Scopus (16) Google Scholar, 2Whitson M.R. Mayo P.H. Ultrasonography in the emergency department.Crit Care. 2016; 20: 227Crossref PubMed Scopus (106) Google Scholar There is a close correlation between optic nerve sheath diameter (ONSD) and intracranial pressure (ICP)3Liu D. Kahn M. Measurement and relationship of subarachnoid pressure of the optic nerve to intracranial pressures in fresh cadavers.Am J Ophthalmol. 1993; 116: 548-556Abstract Full Text PDF PubMed Scopus (150) Google Scholar that is sufficiently dynamic to reflect pressure changes.4Hansen H.C. Helmke K. Validation of the optic nerve sheath response to changing cerebrospinal fluid pressure: ultrasound findings during intrathecal infusion tests.J Neurosurg. 1997; 87: 34-40Crossref PubMed Scopus (317) Google Scholar, 5Hansen H.C. Lagreze W. Krueger O. Helmke K. Dependence of the optic nerve sheath diameter on acutely applied subarachnoidal pressure – an experimental ultrasound study.Acta Ophthalmol. 2011; 89: e528-e532Crossref PubMed Scopus (104) Google Scholar The ONSD has been validated as an accurate screening tool to detect raised ICP in patients with idiopathic intracranial hypertension (IIH),6Bauerle J. Nedelmann M. Sonographic assessment of the optic nerve sheath in idiopathic intracranial hypertension.J Neurol. 2011; 258: 2014-2019Crossref PubMed Scopus (121) Google Scholar traumatic brain injury,7Geeraerts T. Merceron S. Benhamou D. Vigue B. Duranteau J. Non-invasive assessment of intracranial pressure using ocular sonography in neurocritical care patients.Intensive Care Med. 2008; 34: 2062-2067Crossref PubMed Scopus (267) Google Scholar, 8Soldatos T. Karakitsos D. Chatzimichail K. Papathanasiou M. Gouliamos A. Karabinis A. Optic nerve sonography in the diagnostic evaluation of adult brain injury.Crit Care. 2008; 12: R67Crossref PubMed Scopus (196) Google Scholar and spontaneous intracranial hemorrhage.9Moretti R. Pizzi B. Optic nerve ultrasound for detection of intracranial hypertension in intracranial hemorrhage patients: confirmation of previous findings in a different patient population.J Neurosurg Anesthesiol. 2009; 21: 16-20Crossref PubMed Scopus (110) Google Scholar, 10Moretti R. Pizzi B. Cassini F. Vivaldi N. Reliability of optic nerve ultrasound for the evaluation of patients with spontaneous intracranial hemorrhage.Neurocrit Care. 2009; 11: 406-410Crossref PubMed Scopus (160) Google Scholar In the field of obstetric anesthesiology, ocular sonography has been applied as a tool to diagnose raised ICP in pre-eclamptic women11Dubost C. Le Gouez A. Jouffroy V. et al.Optic nerve sheath diameter used as ultrasonographic assessment of the incidence of raised intracranial pressure in preeclampsia: a pilot study.Anesthesiology. 2012; 116: 1066-1071Crossref PubMed Scopus (73) Google Scholar, 12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 13Ortner C.M. Krishnamoorthy V. Neethling E. et al.Point-of-care ultrasound abnormalities in late-onset severe preeclampsia: Prevalence and association with serum albumin and brain natriuretic peptide.Anesth Analg. 2019; 128: 1208-1216Crossref PubMed Scopus (20) Google Scholar, 14Singh S.K. Bhatia K. Ultrasonographic optic nerve sheath diameter as a surrogate measure of raised intracranial pressure in severe pregnancy-induced hypertension patients.Anesth Essays Res. 2018; 12: 42-46Crossref PubMed Google Scholar (Table 1), and to monitor ICP changes induced by epidural blood patch for the treatment of post-dural puncture headache.15Dubost C. Le Gouez A. Zetlaoui P.J. Benhamou D. Mercier F.J. Geeraerts T. Increase in optic nerve sheath diameter induced by epidural blood patch: a preliminary report.Br J Anaesth. 2011; 107: 627-630Abstract Full Text Full Text PDF PubMed Scopus (41) Google ScholarTable 1Studies relating optic nerve sheath diameter to raised intracranial pressure.StudyCohort size (N)(pre-eclamptics/eclamptics/controls)Disease severity(±severe features)Disease onset(early vs. late)cut-off(mm)Mean ONSD (mm)Prevalencen > cut-of/NFollow-upn (%)AssociationsCasesControlsDubost et al.11Dubost C. Le Gouez A. Jouffroy V. et al.Optic nerve sheath diameter used as ultrasonographic assessment of the incidence of raised intracranial pressure in preeclampsia: a pilot study.Anesthesiology. 2012; 116: 1066-1071Crossref PubMed Scopus (73) Google Scholar26/0/25Severe (n=13)Not severe (n=13)early + late5.85.4 mm(95%CI: 5.2 to 5.8)4.5 mm(95%CI: 4.3 to 4.8)5/26 (19%)vs. 0/30 (0%)Day 5: 2/26, day 7: 0/26ANo difference between severe and none-severe disease.NoneSimenc et al.12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar30/0/30Severe (n=30)early + late5.85.7 mm(Q25–Q75: 5.2–6.0, range: 3.8–7.5)4.5 mm(Q25–Q75: 4.3–4.6, range: 4.2–4.7)13/30 (43%)vs. 0/30 (0%)Day 4: 3/30 (10%)BNo association with headaches/visual disturbances (p=0.63).With lung ultrasound pattern (P <0.001)Ortner et al.13Ortner C.M. Krishnamoorthy V. Neethling E. et al.Point-of-care ultrasound abnormalities in late-onset severe preeclampsia: Prevalence and association with serum albumin and brain natriuretic peptide.Anesth Analg. 2019; 128: 1208-1216Crossref PubMed Scopus (20) Google Scholar95/0/0Severe (n=95)late5.85.4 mmSD (0.5)NA27/95 (28%)Not reportedCNo associations with clinical symptoms (blood pressures, headaches, fetal cardiotocography), abnormalities on cardiopulmonary ultrasound, or laboratory markers (brain natriuretic peptide, albumin).NoneSingh at al.14Singh S.K. Bhatia K. Ultrasonographic optic nerve sheath diameter as a surrogate measure of raised intracranial pressure in severe pregnancy-induced hypertension patients.Anesth Essays Res. 2018; 12: 42-46Crossref PubMed Google Scholar25/24/25Severe (n=25) and eclampsia (n=24)late5.75.6 mm(SD 0.37)5.8 mm*mean ONSD in eclamptic cohort(SD 0.36)4.7 (SD 0.46)11/25 (44%) vs. 16/24 (66%) vs. 0/0 (0%)Not reportedNot reportedONSD: optic nerve sheath diameter(A) No difference between severe and none-severe disease.(B) No association with headaches/visual disturbances (p=0.63).(C) No associations with clinical symptoms (blood pressures, headaches, fetal cardiotocography), abnormalities on cardiopulmonary ultrasound, or laboratory markers (brain natriuretic peptide, albumin).* mean ONSD in eclamptic cohort Open table in a new tab ONSD: optic nerve sheath diameter In a recent volume of this journal, Brzan Simenc et al.12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar provided further evidence that the ONSD, as measured by ultrasonographic techniques, is increased in a significant proportion of women with pre-eclampsia, compared with healthy parturients. Using a comparable technique, and by referring to a study investigating ONSD in 10 patients suffering from IIH,6Bauerle J. Nedelmann M. Sonographic assessment of the optic nerve sheath in idiopathic intracranial hypertension.J Neurol. 2011; 258: 2014-2019Crossref PubMed Scopus (121) Google Scholar Brzan Simenc et al. interpreted a cut-off value for ONSD of 5.8 mm as raised ICP. The optimal ONSD cut-off value for the definition of raised ICP is unknown, with values ranging from 5.0–6.2 mm.8Soldatos T. Karakitsos D. Chatzimichail K. Papathanasiou M. Gouliamos A. Karabinis A. Optic nerve sonography in the diagnostic evaluation of adult brain injury.Crit Care. 2008; 12: R67Crossref PubMed Scopus (196) Google Scholar, 16Dubourg J. Javouhey E. Geeraerts T. Messerer M. Kassai B. Ultrasonography of optic nerve sheath diameter for detection of raised intracranial pressure: a systematic review and meta-analysis.Intensive Care Med. 2011; 37: 1059-1068Crossref PubMed Scopus (333) Google Scholar, 17Tawfik E.A. Walker F.O. Cartwright M.S. Neuromuscular ultrasound of cranial nerves.J Clin Neurol. 2015; 11: 109-121Crossref PubMed Scopus (28) Google Scholar Brzan Simenc et al. interpreted their findings as representing raised ICP in 13/30 (43%) of women presenting with severe features of pre-eclampsia, compared with 5/26 (19%) as first described by Dubost et al.,11Dubost C. Le Gouez A. Jouffroy V. et al.Optic nerve sheath diameter used as ultrasonographic assessment of the incidence of raised intracranial pressure in preeclampsia: a pilot study.Anesthesiology. 2012; 116: 1066-1071Crossref PubMed Scopus (73) Google Scholar 28/95 (27%) as described by Ortner et al.,13Ortner C.M. Krishnamoorthy V. Neethling E. et al.Point-of-care ultrasound abnormalities in late-onset severe preeclampsia: Prevalence and association with serum albumin and brain natriuretic peptide.Anesth Analg. 2019; 128: 1208-1216Crossref PubMed Scopus (20) Google Scholar and 11/25 (44%) by Singh et al.,14Singh S.K. Bhatia K. Ultrasonographic optic nerve sheath diameter as a surrogate measure of raised intracranial pressure in severe pregnancy-induced hypertension patients.Anesth Essays Res. 2018; 12: 42-46Crossref PubMed Google Scholar (Table 1). The latter study cohort also included women with eclampsia, among whom 16/24 (66%) had increased ONSD. Brzan Simenc et al.,12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar also measured optic disc height (ODH), which they found to be ≥1 mm, and thus compatible with increased ICP,18Teismann N. Lenaghan P. Nolan R. Stein J. Green A. Point-of-care ocular ultrasound to detect optic disc swelling.Acad Emerg Med. 2013; 20: 920-925Crossref PubMed Scopus (33) Google Scholar in 77% of the women studied. An important limitation of these studies11Dubost C. Le Gouez A. Jouffroy V. et al.Optic nerve sheath diameter used as ultrasonographic assessment of the incidence of raised intracranial pressure in preeclampsia: a pilot study.Anesthesiology. 2012; 116: 1066-1071Crossref PubMed Scopus (73) Google Scholar, 12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 13Ortner C.M. Krishnamoorthy V. Neethling E. et al.Point-of-care ultrasound abnormalities in late-onset severe preeclampsia: Prevalence and association with serum albumin and brain natriuretic peptide.Anesth Analg. 2019; 128: 1208-1216Crossref PubMed Scopus (20) Google Scholar, 14Singh S.K. Bhatia K. Ultrasonographic optic nerve sheath diameter as a surrogate measure of raised intracranial pressure in severe pregnancy-induced hypertension patients.Anesth Essays Res. 2018; 12: 42-46Crossref PubMed Google Scholar is the lack of comparison with direct ICP measurements or indirect radiological techniques such as magnetic resonance imaging (MRI). Furthermore, a cut-off value for ONSD, such as above 5.8 mm, has never been validated in an obstetric cohort to predict increased ICP. It is also noteworthy that none of the observational studies examining ONSD in pre-eclamptic women11Dubost C. Le Gouez A. Jouffroy V. et al.Optic nerve sheath diameter used as ultrasonographic assessment of the incidence of raised intracranial pressure in preeclampsia: a pilot study.Anesthesiology. 2012; 116: 1066-1071Crossref PubMed Scopus (73) Google Scholar, 12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 13Ortner C.M. Krishnamoorthy V. Neethling E. et al.Point-of-care ultrasound abnormalities in late-onset severe preeclampsia: Prevalence and association with serum albumin and brain natriuretic peptide.Anesth Analg. 2019; 128: 1208-1216Crossref PubMed Scopus (20) Google Scholar, 14Singh S.K. Bhatia K. Ultrasonographic optic nerve sheath diameter as a surrogate measure of raised intracranial pressure in severe pregnancy-induced hypertension patients.Anesth Essays Res. 2018; 12: 42-46Crossref PubMed Google Scholar found an association between ONSD and clinical neurological symptoms such as severe headache or visual disturbances. An MRI performed in women who presented with pre-eclampsia with severe features showed signs of focal cerebral swelling (but not necessarily raised ICP) and significantly lower incidences (0–14.5%) of raised ICP.19Morriss M.C. Twickler D.M. Hatab M.R. Clarke G.D. Peshock R.M. Cunningham F.G. Cerebral blood flow and cranial magnetic resonance imaging in eclampsia and severe preeclampsia.Obstet Gynecol. 1997; 89: 561-568Crossref PubMed Scopus (80) Google Scholar, 20Matsuda H. Sakaguchi K. Shibasaki T. et al.Cerebral edema on MRI in severe preeclamptic women developing eclampsia.J Perinat Med. 2005; 33: 199-205Crossref PubMed Scopus (23) Google Scholar, 21Osmanagaoglu M.A. Dinc G. Osmanagaoglu S. Dinc H. Bozkaya H. Comparison of cerebral magnetic resonance and electroencephalogram findings in pre-eclamptic and eclamptic women.Aust N Z J Obstet Gynaecol. 2005; 45: 384-390Crossref PubMed Scopus (13) Google Scholar These results suggest that it is unlikely that an increased ONSD in pre-eclamptic women only reflects raised ICP. Both Dubost et al.11Dubost C. Le Gouez A. Jouffroy V. et al.Optic nerve sheath diameter used as ultrasonographic assessment of the incidence of raised intracranial pressure in preeclampsia: a pilot study.Anesthesiology. 2012; 116: 1066-1071Crossref PubMed Scopus (73) Google Scholar and Brzan Simenc et al.12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar found a significant increase in the dimensions of the hypo-echogenic structure around the optic nerve on ultrasound, compared with healthy pregnant controls. In the population studied by Brzan Simenc et al., the size of this structure further correlated with the severity of interstitial pulmonary edema22Simenc G.B. Ambrozic J. Prokselj K. et al.Optic nerve ultrasound for fluid status assessment in patients with severe preeclampsia.Radiol Oncol. 2018; 52: 377-382Crossref PubMed Scopus (8) Google Scholar that was observed on lung ultrasound in the same cohort.19Morriss M.C. Twickler D.M. Hatab M.R. Clarke G.D. Peshock R.M. Cunningham F.G. Cerebral blood flow and cranial magnetic resonance imaging in eclampsia and severe preeclampsia.Obstet Gynecol. 1997; 89: 561-568Crossref PubMed Scopus (80) Google Scholar This is the only study in women with pre-eclampsia indicating that ocular ultrasonography may be used to assess disease severity. There is controversy regarding the optimal ONSD measurement techniques. Considerations include the visual axis versus the coronal axis technique, the A- versus the B-scan method,23De Bernardo M. Rosa N. Clarification on using ultrasonography to detect intracranial pressure.JAMA Ophthalmol. 2017; 135: 1004-1005Crossref PubMed Scopus (34) Google Scholar, 24De Bernardo M. Vitiello L. Rosa N. Optic nerve ultrasonography for evaluating increased intracranial pressure in severe preeclampsia.Int J Obstet Anesth. 2019; 38: 147Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar, 25Iaconetta G. De Bernardo M. Rosa N. Coronal axis measurement of the optic nerve sheath diameter.J Ultrasound Med. 2017; 36: 1073Crossref PubMed Scopus (34) Google Scholar and probe selection.26Shah S. Kimberly H. Marill K. Noble V.E. Ultrasound techniques to measure the optic nerve sheath: is a specialized probe necessary?.Med Sci Monit. 2009; 15 (MT63–68)Google Scholar Some authors have raised concerns that part of the structure measured with the approach applied in most studies that estimate ONSD11Dubost C. Le Gouez A. Jouffroy V. et al.Optic nerve sheath diameter used as ultrasonographic assessment of the incidence of raised intracranial pressure in preeclampsia: a pilot study.Anesthesiology. 2012; 116: 1066-1071Crossref PubMed Scopus (73) Google Scholar, 12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 13Ortner C.M. Krishnamoorthy V. Neethling E. et al.Point-of-care ultrasound abnormalities in late-onset severe preeclampsia: Prevalence and association with serum albumin and brain natriuretic peptide.Anesth Analg. 2019; 128: 1208-1216Crossref PubMed Scopus (20) Google Scholar might actually be artefact.27Blehar D.J. Gaspari R.J. Montoya A. Calderon R. Correlation of visual axis and coronal axis measurements of the optic nerve sheath diameter.J Ultrasound Med. 2008; 27: 407-411Crossref PubMed Scopus (33) Google Scholar, 28Copetti R. Cattarossi L. Optic nerve ultrasound: artifacts and real images.Intensive Care Med. 2009; 35: 1488-1489Crossref PubMed Scopus (50) Google Scholar, 29Teismann N.A. Lenaghan P. Stein J. Green A. Will the real optic nerve sheath please stand up?.J Ultrasound Med. 2012; 31: 130-131Crossref PubMed Scopus (7) Google Scholar In obstetric anesthesiology all studies measuring the ONSD have consistently used the traditional ONSD measurement technique, applying a visual axis technique and B-scan mode.11Dubost C. Le Gouez A. Jouffroy V. et al.Optic nerve sheath diameter used as ultrasonographic assessment of the incidence of raised intracranial pressure in preeclampsia: a pilot study.Anesthesiology. 2012; 116: 1066-1071Crossref PubMed Scopus (73) Google Scholar, 12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 13Ortner C.M. Krishnamoorthy V. Neethling E. et al.Point-of-care ultrasound abnormalities in late-onset severe preeclampsia: Prevalence and association with serum albumin and brain natriuretic peptide.Anesth Analg. 2019; 128: 1208-1216Crossref PubMed Scopus (20) Google Scholar, 14Singh S.K. Bhatia K. Ultrasonographic optic nerve sheath diameter as a surrogate measure of raised intracranial pressure in severe pregnancy-induced hypertension patients.Anesth Essays Res. 2018; 12: 42-46Crossref PubMed Google Scholar This involves placing a high-frequency linear array transducer horizontally onto a closed upper eyelid. Two distinct structures are visible immediately posterior to the optic disc. The anterior component of the optic nerve (ON) is seen as an anechoic to hypo-echoic crescent which tends to arc medially and is normally 2.5–3.5 mm in width (Fig. 1). The second thicker, linear or pyramid-shaped hypo-echoic structure that projects backward from the posterior surface of the eye to the end of the screen, is assumed to represent the optic nerve sheath. Using traditional methodology, false-positive images may be interpreted as the ONSD, however they are artefacts such as a shadow cast by the lamina cribrosa or a shadow generated by optic disc edema. Causes of these artefacts include raised ICP, systemic hypertension, ischemic optic neuropathy12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 13Ortner C.M. Krishnamoorthy V. Neethling E. et al.Point-of-care ultrasound abnormalities in late-onset severe preeclampsia: Prevalence and association with serum albumin and brain natriuretic peptide.Anesth Analg. 2019; 128: 1208-1216Crossref PubMed Scopus (20) Google Scholar, 14Singh S.K. Bhatia K. Ultrasonographic optic nerve sheath diameter as a surrogate measure of raised intracranial pressure in severe pregnancy-induced hypertension patients.Anesth Essays Res. 2018; 12: 42-46Crossref PubMed Google Scholar, 15Dubost C. Le Gouez A. Zetlaoui P.J. Benhamou D. Mercier F.J. Geeraerts T. Increase in optic nerve sheath diameter induced by epidural blood patch: a preliminary report.Br J Anaesth. 2011; 107: 627-630Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar or interstitial edema. In addition, the ON sheath as viewed in the visual axis is always straight, regardless of the orientation of the globe, which is inconsistent with the anatomical course of the ON. This incorrect assumption that the ON sheath runs in a straight line can lead to erroneously high measurements of the ONSD (see Fig. 2A–C for more details, including explanations in the legend). To mitigate these potential artefacts observed when imaging along the visual axis, it has been proposed that the ONSD should be measured from a position lateral to,30Agrawal A. Cheng R. Tang J. Madhok D.Y. Comparison of two techniques to measure optic nerve sheath diameter in patients at risk for increased intracranial pressure.Crit Care Med. 2019; 47: e495-e501Crossref PubMed Scopus (21) Google Scholar or below, the globe of the eye in the coronal axis.27Blehar D.J. Gaspari R.J. Montoya A. Calderon R. Correlation of visual axis and coronal axis measurements of the optic nerve sheath diameter.J Ultrasound Med. 2008; 27: 407-411Crossref PubMed Scopus (33) Google Scholar, 31Amini R. Stolz L.A. Patanwala A.E. Adhikari S. Coronal Axis Measurement of the optic nerve sheath diameter using a linear transducer.J Ultrasound Med. 2015; 34: 1607-1612Crossref PubMed Scopus (19) Google Scholar To bring the posterior aspect of the eye and the optic nerve into the scanning plane, volunteers are asked to lift their chin and direct their gaze downward (Fig. 3). Circular coronal sections are then measured from the lateral to the medial borders of the ONS. Measuring the ONSD along the coronal axis avoids the passage of the ultrasound beam through the lamina cribrosa or ocular papilla, potentially removing the measurement artefact (Fig. 3). Results of studies comparing measurements made in the visual axis with those in the coronal axis have shown significant differences when using a curvilinear probe,27Blehar D.J. Gaspari R.J. Montoya A. Calderon R. Correlation of visual axis and coronal axis measurements of the optic nerve sheath diameter.J Ultrasound Med. 2008; 27: 407-411Crossref PubMed Scopus (33) Google Scholar and decreased variability in ONSD compared with that found when using a linear probe.30Agrawal A. Cheng R. Tang J. Madhok D.Y. Comparison of two techniques to measure optic nerve sheath diameter in patients at risk for increased intracranial pressure.Crit Care Med. 2019; 47: e495-e501Crossref PubMed Scopus (21) Google Scholar Some authors, including De Bernado et al. who commented in this journal,24De Bernardo M. Vitiello L. Rosa N. Optic nerve ultrasonography for evaluating increased intracranial pressure in severe preeclampsia.Int J Obstet Anesth. 2019; 38: 147Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar argue that the measuring bias (and reason for cut-off variability) is not due to the axis chosen but to the propensity of the most widely applied B-scan technique to cause the so-called “blooming effect”.24De Bernardo M. Vitiello L. Rosa N. Optic nerve ultrasonography for evaluating increased intracranial pressure in severe preeclampsia.Int J Obstet Anesth. 2019; 38: 147Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar, 32De Bernardo M. Rosa N. Measuring optic nerve sheath diameter as a proxy for intracranial pressure.JAMA Ophthalmol. 2018; 136: 1309-1310Crossref PubMed Scopus (34) Google Scholar Particularly for structures with dimensions below 0.5 mm, inappropriate sensitivity settings with ONSD-ultrasonography might lead to falsely elevated measurements. To increase precision, an A-scan technique has been suggested,23De Bernardo M. Rosa N. Clarification on using ultrasonography to detect intracranial pressure.JAMA Ophthalmol. 2017; 135: 1004-1005Crossref PubMed Scopus (34) Google Scholar, 25Iaconetta G. De Bernardo M. Rosa N. Coronal axis measurement of the optic nerve sheath diameter.J Ultrasound Med. 2017; 36: 1073Crossref PubMed Scopus (34) Google Scholar but this technique is significantly more challenging to learn, in particular for non-ophthalmologists.33Gans M.S. Byrne S.F. Glaser J.S. Standardized A-scan echography in optic nerve disease.Arch Ophthalmol. 1987; 105: 1232-1236Crossref PubMed Scopus (40) Google Scholar, 34Lucovnik M. Brzan Simenc G. Prokselj K. Cvijic M. Ambrozic J. In reply: optic nerve ultrasonography for evaluating increased intracranial pressure in severe preeclampsia.Int J Obstet Anesth. 2019; 38: 148Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar Data are lacking using the coronal axis or the A-scan technique in women with pre-eclampsia. Moreover, since mean values for ONSD depend on the ultrasound transducer used,26Shah S. Kimberly H. Marill K. Noble V.E. Ultrasound techniques to measure the optic nerve sheath: is a specialized probe necessary?.Med Sci Monit. 2009; 15 (MT63–68)Google Scholar and vary significantly among studies,26Shah S. Kimberly H. Marill K. Noble V.E. Ultrasound techniques to measure the optic nerve sheath: is a specialized probe necessary?.Med Sci Monit. 2009; 15 (MT63–68)Google Scholar, 35Bauerle J. Lochner P. Kaps M. Nedelmann M. Intra- and interobsever reliability of sonographic assessment of the optic nerve sheath diameter in healthy adults.J Neuroimaging. 2012; 22: 42-45Crossref PubMed Scopus (158) Google Scholar it has been proposed that individual centres need to establish their own normal range reference values.17Tawfik E.A. Walker F.O. Cartwright M.S. Neuromuscular ultrasound of cranial nerves.J Clin Neurol. 2015; 11: 109-121Crossref PubMed Scopus (28) Google Scholar Finally, considering the clinical application of ocular ultrasonography in critically-ill obstetric patients, the safety of ultrasound exposure should be addressed. Diagnostic ultrasound is generally considered safe but important thermal and mechanical biological effects should be acknowledged.36Shankar H. Pagel P.S. Potential adverse ultrasound-related biological effects: a critical review.Anesthesiology. 2011; 115: 1109-1124Crossref PubMed Scopus (84) Google Scholar Ultrasound may damage ocular structures to varying degrees, depending on the duration of exposure and the intensity applied.37Lizzi F.L. Coleman D.J. Driller J. Franzen L.A. Jakobiec F.A. Experimental, ultrasonically induced lesions in the retina, choroid, and sclera.Invest Ophthalmol Vis Sci. 1978; 17: 350-360PubMed Google Scholar Ultrasound results in the transmission of mechanical energy to the eye and may warm the intraocular fluid. The Food and Drug Administration (FDA) in the United States and the British Medical Ultrasound Society (BMUS) recommend limiting acoustic output level exposure for ophthalmic ultrasound to a thermal index (TI) of ≤1, and a mechanical index (MI) ≤0.23 (FDA) or 0.7 (BMUS).36Shankar H. Pagel P.S. Potential adverse ultrasound-related biological effects: a critical review.Anesthesiology. 2011; 115: 1109-1124Crossref PubMed Scopus (84) Google Scholar Reviewing observational studies and case reports on the use of ocular ultrasonography in pre-eclamptic women, the acoustic output exposure levels applied have only been presented in a single case report.38Ferhi F. Khlifi A. Hachani F. Tarmiz K. Benjazia K. Ultrasound assessment of visual loss during severe preeclampsia: a case report.Crit Ultrasound J. 2018; 10: 6Crossref PubMed Scopus (3) Google Scholar Using high-frequency linear ultrasound transducers such as those applied in the studies of ONSD in pre-eclampsia,11Dubost C. Le Gouez A. Jouffroy V. et al.Optic nerve sheath diameter used as ultrasonographic assessment of the incidence of raised intracranial pressure in preeclampsia: a pilot study.Anesthesiology. 2012; 116: 1066-1071Crossref PubMed Scopus (73) Google Scholar, 12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 13Ortner C.M. Krishnamoorthy V. Neethling E. et al.Point-of-care ultrasound abnormalities in late-onset severe preeclampsia: Prevalence and association with serum albumin and brain natriuretic peptide.Anesth Analg. 2019; 128: 1208-1216Crossref PubMed Scopus (20) Google Scholar, 14Singh S.K. Bhatia K. Ultrasonographic optic nerve sheath diameter as a surrogate measure of raised intracranial pressure in severe pregnancy-induced hypertension patients.Anesth Essays Res. 2018; 12: 42-46Crossref PubMed Google Scholar and limiting the acoustic output level to that associated with a maximum MI of 0.23, would make it very challenging to achieve accurate measurements of ONSD (Fig. 4). However, higher frequency linear array or “hockey stick” transducers with small (e.g. ≤25 mm) footprints may circumvent this limitation by providing adequate image resolution and gain at safe acoustic output levels. Both Dubost et al.11Dubost C. Le Gouez A. Jouffroy V. et al.Optic nerve sheath diameter used as ultrasonographic assessment of the incidence of raised intracranial pressure in preeclampsia: a pilot study.Anesthesiology. 2012; 116: 1066-1071Crossref PubMed Scopus (73) Google Scholar and Brzan Simenc et al.12Brzan Simenc G. Ambrozic J. Prokselj K. et al.Ocular ultrasonography for diagnosing increased intracranial pressure in patients with severe preeclampsia.Int J Obstet Anesth. 2018; 36: 49-55Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar found a significant increase in the dimensions of the hypo-echogenic structure around the optic nerve on ultrasound compared with healthy pregnant controls. In the population studied by Brzan Simenc et al., the size of this structure further correlated with the severity of interstitial pulmonary edema 22Simenc G.B. Ambrozic J. Prokselj K. et al.Optic nerve ultrasound for fluid status assessment in patients with severe preeclampsia.Radiol Oncol. 2018; 52: 377-382Crossref PubMed Scopus (8) Google Scholar observed on lung ultrasound in the same cohort.19Morriss M.C. Twickler D.M. Hatab M.R. Clarke G.D. Peshock R.M. Cunningham F.G. Cerebral blood flow and cranial magnetic resonance imaging in eclampsia and severe preeclampsia.Obstet Gynecol. 1997; 89: 561-568Crossref PubMed Scopus (80) Google Scholar This is the only study in pre-eclampsia indicating that ocular ultrasonography may be used to assess disease severity. In conclusion, it has been shown repeatedly that the ONSD, measured using ultrasound in women with pre-eclampsia, is significantly increased in comparison with healthy pregnant controls. However, no comparisons with direct ICP measurements are available and no cut-off value for ONSD to predict increased ICP has been validated in an obstetric cohort. Therefore, this technique should currently be regarded as experimental and is not recommended for clinical decision-making. Further studies are needed to define the optimal ocular ultrasonography methodology as, in particular, the coronal axis approach may have advantages over the traditionally applied method. Although diagnostic ultrasound is generally considered safe, investigators should be mindful of the recommended acoustic output level exposure when performing ocular ultrasound.
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