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Can Ocular Ultrasonography Be Used to Assess Intracranial Pressure?

2016; Elsevier BV; Volume: 68; Issue: 3 Linguagem: Inglês

10.1016/j.annemergmed.2016.04.022

1097-6760

Michael Gottlieb, John Bailitz,

Cerebrovascular and Carotid Artery Diseases

Take-Home MessageMeasurement of the optic nerve sheath diameter demonstrates good diagnostic test accuracy for differentiating between elevated and normal intracranial pressure when compared with computed tomography (CT). Specifically, the high sensitivity allows the clinician to rule out increased intracranial pressure in low-risk patients, whereas the high specificity can confirm elevated intracranial pressure in those at high risk.MethodsData SourcesElectronic searches were performed with PubMed, EMBASE, and the Cochrane Library databases from January 1986 to August 4, 2013. A combination of key words and Medical Subject Heading terms was used in the search (eg, "idiopathic intracranial hypertension," "optic nerve sheath," "ultrasound"). Bibliographies of included studies were also reviewed for relevant articles. No language or age restrictions were placed on the searches. When data were unclear, the authors were contacted for clarification.Study SelectionIncluded trials evaluated adults or children recruited from an emergency department (ED) or intensive care unit (ICU) with clinical suspicion of increased intracranial pressure. Each study was required to assess the performance of optic nerve sheath diameter, using CT evidence of increased intracranial pressure as the criterion standard. Included studies had greater than 80% use of CT for confirmation of increased intracranial pressure. The systematic review authors defined increased intracranial pressure as evidence of shift, collapse of ventricles, or intracranial bleeding of greater than 2 mm. Abnormal optic nerve sheath diameter was defined as greater than 5 mm for adults, greater than 4.5 mm for pediatric patients (aged 1 to 17 years), and greater than 4 mm for infants (<1 year).Data Extraction and SynthesisTwo authors were involved in the review process. One author reviewed all of the titles and articles, whereas a second author performed an independent review of a random sample of 100 articles. Full-text articles were then reviewed by one author, whereas a second author performed an independent review of a random sample of 25% of the articles. Agreement for both reviews was measured with the κ statistic. Disagreements were resolved by discussion. Quality assessment was performed with the Quality Assessment of Diagnostic Accuracy Studies (QUADAS) and the Cochrane Risk of Bias tool. Disagreements were resolved by discussion with a third author. Data extraction was performed independently by 2 authors. Diagnostic accuracy was estimated from 2×2 tables extracted from the original articles. To allow heterogeneity beyond chance as a result of the clinical and methodological differences between studies, a bivariate random-effects model was used to compute the summary diagnostic sensitivity and specificity values. Heterogeneity was assessed with forest plots, variance of logit-transformed sensitivity and specificity, and prediction ellipses. Measurement of the optic nerve sheath diameter demonstrates good diagnostic test accuracy for differentiating between elevated and normal intracranial pressure when compared with computed tomography (CT). Specifically, the high sensitivity allows the clinician to rule out increased intracranial pressure in low-risk patients, whereas the high specificity can confirm elevated intracranial pressure in those at high risk. Electronic searches were performed with PubMed, EMBASE, and the Cochrane Library databases from January 1986 to August 4, 2013. A combination of key words and Medical Subject Heading terms was used in the search (eg, "idiopathic intracranial hypertension," "optic nerve sheath," "ultrasound"). Bibliographies of included studies were also reviewed for relevant articles. No language or age restrictions were placed on the searches. When data were unclear, the authors were contacted for clarification. Included trials evaluated adults or children recruited from an emergency department (ED) or intensive care unit (ICU) with clinical suspicion of increased intracranial pressure. Each study was required to assess the performance of optic nerve sheath diameter, using CT evidence of increased intracranial pressure as the criterion standard. Included studies had greater than 80% use of CT for confirmation of increased intracranial pressure. The systematic review authors defined increased intracranial pressure as evidence of shift, collapse of ventricles, or intracranial bleeding of greater than 2 mm. Abnormal optic nerve sheath diameter was defined as greater than 5 mm for adults, greater than 4.5 mm for pediatric patients (aged 1 to 17 years), and greater than 4 mm for infants ( 5 mm was used for adults, ONSD > 4.5 mm was used for pediatric patients (aged 1-17 years), and ONSD > 4 mm was used for infants (aged < 1 year).95.6 (87.7–98.5)92.3 (77.9–98.4)12.5 (4.2–37.5)0.05 (0.016–0.14)CI, Confidence interval; ONSD, optic nerve sheath diameter.∗ ONSD > 5 mm was used for adults, ONSD > 4.5 mm was used for pediatric patients (aged 1-17 years), and ONSD > 4 mm was used for infants (aged < 1 year). Open table in a new tab CI, Confidence interval; ONSD, optic nerve sheath diameter. The search strategy identified 1,214 potential studies. After review, 12 studies, comprising 478 participants, met the inclusion criteria. The κ value for inclusion was 0.89. Nine studies assessed adults and 3 assessed children. Seven studies were conducted in the ED setting, 4 studies took place in the ICU setting, and 1 study assessed both ED and ICU patients. There was wide variation of elevated intracranial pressure prevalence, ranging from 13.6% to 100%. Overall, the studies were of high quality, with a mean QUADAS score of 8.6 out of 11, with most studies losing 1 point for delay between optic ultrasonography and performance of the criterion standard. Moderate heterogeneity was identified on analysis of the forest plot, variance of logit-transformed sensitivity and specificity, and prediction ellipses. The measurement of optic nerve sheath diameter to assess for increased intracranial pressure has been increasing in popularity during the past decade. This technique is particularly valuable for rapid bedside assessments, as well as in resource-poor settings. The rationale for using optic nerve sheath diameter as a surrogate for intracranial pressure is based on the fact that the optic nerve sheath is contiguous with the intracranial dura mater and the subarachnoid space, which contains the cerebrospinal fluid. Therefore, elevation of the intracranial pressure leads to a direct increase in optic nerve sheath diameter.1Hansen H.C. Helmke K. The subarachnoid space surrounding the optic nerves. An ultrasound study of the optic nerve sheath.Surg Radiol Anat. 1996; 18: 323-328Crossref PubMed Scopus (180) Google Scholar Although direct funduscopy may also be used to assess for elevated intracranial pressure, many providers are uncomfortable with their funduscopic skills.2Wu E.H. Fagan M.J. Reinert S.E. Self-confidence in and perceived utility of the physical examination: a comparison of medical students, residents, and faculty internists.J Gen Intern Med. 2007; 22: 1725-1730Crossref PubMed Scopus (54) Google Scholar Furthermore, performing funduscopy on patients with suspected altered mental status or severe headache from elevated intracranial pressure can be challenging. Most studies measure the optic nerve sheath diameter 3 mm posterior to the optic globe, according to initial studies demonstrating that this area has the greatest potential for distention with elevated intracranial pressure.1Hansen H.C. Helmke K. The subarachnoid space surrounding the optic nerves. An ultrasound study of the optic nerve sheath.Surg Radiol Anat. 1996; 18: 323-328Crossref PubMed Scopus (180) Google Scholar, 3Helmke K. Hansen H.C. Fundamentals of transorbital sonographic evaluation of optic nerve sheath expansion under intracranial hypertension. I. Experimental study.Pediatr Radiol. 1996; 26: 701-705Crossref PubMed Scopus (195) Google Scholar Although a variety of optic nerve sheath diameter cutoffs have been studied in the literature, 5 mm in diameter is the most commonly accepted value for adults.4Blaivas M. Theodoro D. Sierzenski P.R. Elevated intracranial pressure detected by bedside emergency ultrasonography of the optic nerve sheath.Acad Emerg Med. 2003; 10: 376-381Crossref PubMed Scopus (0) Google Scholar, 5Tayal V.S. Neulander M. Norton H.J. et al.Emergency department sonographic measurement of optic nerve sheath diameter to detect findings of increased intracranial pressure in adult head injury patients.Ann Emerg Med. 2007; 49: 508-514Abstract Full Text Full Text PDF PubMed Scopus (301) Google Scholar This meta-analysis demonstrated excellent sensitivity and specificity of ocular ultrasonography for the diagnosis of elevated intracranial pressure, using the 5-mm cutoff. If used appropriately, ultrasonography has the potential to decrease the use of advanced imaging associated with radiation and allows rapid and repeated bedside assessments. However, this review had several limitations. There was moderate heterogeneity between studies, likely because of variations in study location, ultrasonographer experience, scanning technique, equipment, and disease prevalence. This was also demonstrated by the wide confidence intervals surrounding the sensitivity and specificity values. Therefore, one may wish to consider the lower end of the 95% confidence interval when applying these tests, despite the overall high sensitivity and specificity. The authors accounted for some of the anticipated heterogeneity by using a bivariate random-effects model when computing the summary diagnostic test characteristics. The systematic review was limited to studies using CT as the criterion standard, rather than those using invasive monitoring. However, Eisenberg et al6Eisenberg H.M. Gary Jr., H.E. Aldrich E.F. et al.Initial CT findings in 753 patients with severe head injury. A report from the NIH Traumatic Coma Data Bank.J Neurosurg. 1990; 73: 688-698Crossref PubMed Scopus (451) Google Scholar demonstrated that CT findings consistent with elevated intracranial pressure have a strong correlation with elevated intracranial pressure measures based on invasive monitoring. Additionally, studies using invasive intracranial pressure monitoring have demonstrated similar test characteristics for CT.7Kimberly H.H. Shah S. Marill K. et al.Correlation of optic nerve sheath diameter with direct measurement of intracranial pressure.Acad Emerg Med. 2008; 15: 201-204Crossref PubMed Scopus (351) Google Scholar, 8Rajajee V. Vanaman M. Fletcher J.J. et al.Optic nerve ultrasound for the detection of raised intracranial pressure.Neurocrit Care. 2011; 15: 506-515Crossref PubMed Scopus (267) Google Scholar Although CT was used as the criterion standard, only half of the included studies used CT in 100% of cases and there was variation in the criteria used to define elevated intracranial pressure on CT in the individual studies. Another limitation was that many of the studies had a significant delay between the CT and ultrasonographic measurements, ranging from 15 minutes to several hours. This delay between measurements could have resulted in increases or decreases in actual intracranial pressure, which may have diminished the validity of the criterion standard measurements. Many of the included studies did not specify operator experience level or the training provided before the study, so it is difficult to determine how much training or experience may be necessary to attain the above sensitivity and specificity. However, Tayal et al5Tayal V.S. Neulander M. Norton H.J. et al.Emergency department sonographic measurement of optic nerve sheath diameter to detect findings of increased intracranial pressure in adult head injury patients.Ann Emerg Med. 2007; 49: 508-514Abstract Full Text Full Text PDF PubMed Scopus (301) Google Scholar recommended that with an experienced operator, 10 scans with 3 abnormal scan results may be sufficient, whereas less experienced users may require additional training. Overall, ocular ultrasonography demonstrated good test performance, with the likelihood ratios suggesting that this test would be particularly valuable to rule in elevated intracranial pressure in patients with a high pretest probability and would likely rule it out in patients with a low one. For example, if the pretest probability were 14% (as in the study by Tayal et al5Tayal V.S. Neulander M. Norton H.J. et al.Emergency department sonographic measurement of optic nerve sheath diameter to detect findings of increased intracranial pressure in adult head injury patients.Ann Emerg Med. 2007; 49: 508-514Abstract Full Text Full Text PDF PubMed Scopus (301) Google Scholar) and the optic nerve sheath diameter were less than 5 mm (corresponding to a negative likelihood ratio of 0.05), then the posttest probability would be less than 1%, suggesting a very low likelihood of increased intracranial pressure. Similarly, if the pretest probability were 67% (as in the study by Karakitsos et al9Karakitsos D. Soldatos T. Gouliamos A. et al.Transorbital sonographic monitoring of optic nerve diameter in patients with severe brain injury.Transplant Proc. 2006; 38: 3700-3706Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar) and the optic nerve sheath diameter were greater than 5 mm (corresponding to a positive likelihood ratio of 12.5), then the posttest probability would be greater than 96%, suggesting a very high likelihood of increased intracranial pressure. A low posttest probability could negate the need for a CT, depending on the risk tolerance of both the patient and provider, whereas a significantly elevated value could help determine the urgency of CT imaging. This information may be particularly valuable when CT time must be prioritized, such as during mass casualty events, or in settings without ready access to CT.